Black Hawk Accident was NOT settling with Power
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Nick,
HOoooly Cooow! This sure has been an enlightening post! At least for me, this VRS topic is very interesting, and I can now see how it could easily be confused with "settling with too little power".
I read all the info on the website and it seems to make sense, in the case of Bell 206 I was hovering and started to settle, I had extra power to climb, the L4 (with "high altitude kit") does pretty good at altitude in terms of engine power, TOT or N1 will not be reached before TQ, but for some reason applying the extra 15 or 20 % TQ I had did nothing, I had to dive and get out of it. I just don't know what happened.
(although the real limiting factor, in L4's without "high altitude TR kits" is the TR authoritiy, you will hit the left pedal stop at about 85% TQ, with 9,000 HD, so if you keep pulling TQ, you'll keep turning, unfortunately few L4's have this kit)
Trust me I know the difference between hovering with too little power and developed VRS, I have flown most of my 2,500 hrs. at HD's above 9,000 FT and I have rarely taken off with enough power to hover OGE, then sometime after burning some fuel I always end up doing it, I understand the line is thin and sometimes the helicopter might just be a bit heavy and the helicopter just sinks due to a lack of power but then again it just sometimes sinks unexplicably even with extra power!
Now there is still some strange things I don't fully understand, according to the chart you posted "VRS Cond. for Bell 206" its actually easier to get VRS with a little forward airspeed, and it is not possible to get the worst cases if you are descending truly vertical, this is strange.
Now in the case of the Canadian Sea King, I used to think of that as the perfect example of settling with power, I have the video, it hovered level for several minutes OGE, doing pedal turns to show off, then all of the suddenly it just started sinking slowly and then the rate of descent increased tremendously and with a slight yaw of about 80 degrees it slammed into the ground.
Correct me if I am wrong, the RoD requiered to encounter VRS is directy related to disk loading right?, and disk loading is related to GW? Could it be possible that HD was low, the helicopter really light, which in turn gives a low downdraft velocity, and as result the RoD required to meet VRS was met with a much lower than expected RoD? I read somewhere that the official cause for the crash was VRS, do you have any more info on this particular accident?
Now pulling out of VRS by just going thru it and into autorotation, that must be a ride! You say you have done it several times in an S-76, that's even more interesting, I have only flown the 76 (arriel powered) once, here at 7,500 ft, and it was scary, with an 1 1/2 hours fuel, 4 on board, it was over the edge, a real pig, my boss wanted to trade it for our Bell 230 which with its small 250's was a lot better, actually that same 76 ended up having one incident and afterwards finally crash landed one day after an engine failure during cruise flight, after that the remaining engine had enough power to sustain a very negative rate of descent..... I have some friends who've flown the 76 offshore a lot and they talk positively about it, except for the time when the 250 engines on 6 brand new helicopters started blowing up after a while, they told me Sikorskys solution to the problem for the moment was to somehow wrap or surround the turbine sections of the engines with "armor" so when/if they blew up they wouldn't throw the wheel thru the fuselage or have the same wheel mess something up.
Well and now I am now wandering aimlessly, I want to say thanks for the time, it has been enlightening and interesting, it has changed my view of VRS and hopefully will help me in not getting it mixed up in the future. I will have to dig deeper into the subject to fully understand it.
Also I remember calling someone at the beginning "ignorant", and I am sorry for that.
HOoooly Cooow! This sure has been an enlightening post! At least for me, this VRS topic is very interesting, and I can now see how it could easily be confused with "settling with too little power".
I read all the info on the website and it seems to make sense, in the case of Bell 206 I was hovering and started to settle, I had extra power to climb, the L4 (with "high altitude kit") does pretty good at altitude in terms of engine power, TOT or N1 will not be reached before TQ, but for some reason applying the extra 15 or 20 % TQ I had did nothing, I had to dive and get out of it. I just don't know what happened.
(although the real limiting factor, in L4's without "high altitude TR kits" is the TR authoritiy, you will hit the left pedal stop at about 85% TQ, with 9,000 HD, so if you keep pulling TQ, you'll keep turning, unfortunately few L4's have this kit)
Trust me I know the difference between hovering with too little power and developed VRS, I have flown most of my 2,500 hrs. at HD's above 9,000 FT and I have rarely taken off with enough power to hover OGE, then sometime after burning some fuel I always end up doing it, I understand the line is thin and sometimes the helicopter might just be a bit heavy and the helicopter just sinks due to a lack of power but then again it just sometimes sinks unexplicably even with extra power!
Now there is still some strange things I don't fully understand, according to the chart you posted "VRS Cond. for Bell 206" its actually easier to get VRS with a little forward airspeed, and it is not possible to get the worst cases if you are descending truly vertical, this is strange.
Now in the case of the Canadian Sea King, I used to think of that as the perfect example of settling with power, I have the video, it hovered level for several minutes OGE, doing pedal turns to show off, then all of the suddenly it just started sinking slowly and then the rate of descent increased tremendously and with a slight yaw of about 80 degrees it slammed into the ground.
Correct me if I am wrong, the RoD requiered to encounter VRS is directy related to disk loading right?, and disk loading is related to GW? Could it be possible that HD was low, the helicopter really light, which in turn gives a low downdraft velocity, and as result the RoD required to meet VRS was met with a much lower than expected RoD? I read somewhere that the official cause for the crash was VRS, do you have any more info on this particular accident?
Now pulling out of VRS by just going thru it and into autorotation, that must be a ride! You say you have done it several times in an S-76, that's even more interesting, I have only flown the 76 (arriel powered) once, here at 7,500 ft, and it was scary, with an 1 1/2 hours fuel, 4 on board, it was over the edge, a real pig, my boss wanted to trade it for our Bell 230 which with its small 250's was a lot better, actually that same 76 ended up having one incident and afterwards finally crash landed one day after an engine failure during cruise flight, after that the remaining engine had enough power to sustain a very negative rate of descent..... I have some friends who've flown the 76 offshore a lot and they talk positively about it, except for the time when the 250 engines on 6 brand new helicopters started blowing up after a while, they told me Sikorskys solution to the problem for the moment was to somehow wrap or surround the turbine sections of the engines with "armor" so when/if they blew up they wouldn't throw the wheel thru the fuselage or have the same wheel mess something up.
Well and now I am now wandering aimlessly, I want to say thanks for the time, it has been enlightening and interesting, it has changed my view of VRS and hopefully will help me in not getting it mixed up in the future. I will have to dig deeper into the subject to fully understand it.
Also I remember calling someone at the beginning "ignorant", and I am sorry for that.
we seem to get into heaps of discussion about VRS dont we?
My two bobs:
The Pave Hawk was not a VRS accident. See my verbal dihorea on the Black Hawk accident thread. An updraft causes you to reduce power to stay at the same hieght, thus it would REDUCE your likelyhood of VRS wouldn't it? Also, (see my other thread) the conning angle DOES not appear to get excessive despite claims above.
I agree with Nick - the Black Hawk is extremely difficult - but not impossible - to get into VRS. There was an incident in an Australian one practicing unusual attitude recovery in IMC when it got into VRS (or incipient). It fell thousands of feet before recovery (through LSALT too
).
I believe way too many accidents are blamed on VRS, particularly in the USA. I agree with Nick on the Sea King. Too many pedal turns, not enough power. Rotor bleed, tail rotor looses effectiveness, aircraft goes in. Another two video accidents that always stick in my mind for VRS blamming is the Skycrane that landed on a road along the side of a wide valley. As it takes off, it drops of the side of the road and tries to climb out of the valley. IMHO This too is a lack of power accident, not a VRS accident. Lastly there was the US Army (Gaurd I think) Black Hawk giving the rappel demo doing a quick stop, falling through, bouncing off the ground and rolling into the tree line. Again, IMHO, not a VRS accident, but a merely rotor droop issue.
I have believed the gospel according to my instructors - when below 30 KIAS and above 300 fpm RoD BE CAREFUL OF VRS. And so I try to be.
My two bobs:
The Pave Hawk was not a VRS accident. See my verbal dihorea on the Black Hawk accident thread. An updraft causes you to reduce power to stay at the same hieght, thus it would REDUCE your likelyhood of VRS wouldn't it? Also, (see my other thread) the conning angle DOES not appear to get excessive despite claims above.
I agree with Nick - the Black Hawk is extremely difficult - but not impossible - to get into VRS. There was an incident in an Australian one practicing unusual attitude recovery in IMC when it got into VRS (or incipient). It fell thousands of feet before recovery (through LSALT too
). I believe way too many accidents are blamed on VRS, particularly in the USA. I agree with Nick on the Sea King. Too many pedal turns, not enough power. Rotor bleed, tail rotor looses effectiveness, aircraft goes in. Another two video accidents that always stick in my mind for VRS blamming is the Skycrane that landed on a road along the side of a wide valley. As it takes off, it drops of the side of the road and tries to climb out of the valley. IMHO This too is a lack of power accident, not a VRS accident. Lastly there was the US Army (Gaurd I think) Black Hawk giving the rappel demo doing a quick stop, falling through, bouncing off the ground and rolling into the tree line. Again, IMHO, not a VRS accident, but a merely rotor droop issue.
I have believed the gospel according to my instructors - when below 30 KIAS and above 300 fpm RoD BE CAREFUL OF VRS. And so I try to be.
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This is a good post, I learn from each one.
I found that Sea King video, at least the last few seconds of it. It is certainly possible that the thing started out as a low power event, and it could have become VRS, but that is not necessary to explain the mess. I would guess that it hit at about 10 feet per second, maybe 12, which is only about 600 to 700 ft/smin. That could easily be explained if the engines were topped out and the rotor drooped down to the low 90's or less, where the tail rotor would run out of poop as well as the main rotor. Once on a torque limit (does the Sea King in UK have a dreaded torque limiter? I seem to remember a fancy fuel control with torque matching was developed and fitted) then the actual power of the engine is being reduced as the rotor droops and torque is held constant, so the situation goes to worms real fast, rate of descent builds and the crowd screams "VRS!" Is my memory wrong, or was there a more full version with lots of hover turns before the dramatic thump?
BlenderPilot - The forward speed comment is quite right. You need some forward velocity, or else the VRS bubble doesn't form and stay. The smoke picture on my web site shows that perfectly. I did get some real VRS on a single engine landing once, with 8 knots airspeed and about 35 feet per second rate of descent (yea that's per second!) It was a real eye opener (and landing gear spreader) I try not to spout theory here, most of my observations are based on practical experience, with some theory to explain it (got that, Flare Dammit!? No Christmas cards.)
Your description sure sounds like VRS, if you had 15% torque margin and applied it and nothing happened. What was your ROD when that was happening?
I did the shakedown and structural flying on the S-76, and we had to do zero knot auto entries, increase the ROD and keep going down to full auto. About 6000 ft/min descent when trimmed. The A and A+ model are not altitude machines, they are optimized for sea level and do nicely there. The engine bursts are (thankfully) ancient history. They seemed to be associated with rebuild techniques, IIRC.
Regarding "Ignorant" - we all are, we each have a piece of this puzzle, and when we all listen, we all learn. I do, every time.
helmetfire - The 300 fpm rule of thumb is not bad, especially for lightly loaded helos. A tad conservative for heavy big turbines, but what's the harm? I remember the Black Hawk TV demo where he screamed in, flared, landed hard and went into the bushes! Dramatic footage, as they say.
RJSquirrel, what say you? You started this mess. I don't agree with your 2500 feet per minute, that's the theoretical VRS at 1.0 times the downwash speed. I think the UH-OH point is half the downwash velocity. For a Hawk, that's about 1500 fpm as the max hover descent without creating a smoking hole.
I found that Sea King video, at least the last few seconds of it. It is certainly possible that the thing started out as a low power event, and it could have become VRS, but that is not necessary to explain the mess. I would guess that it hit at about 10 feet per second, maybe 12, which is only about 600 to 700 ft/smin. That could easily be explained if the engines were topped out and the rotor drooped down to the low 90's or less, where the tail rotor would run out of poop as well as the main rotor. Once on a torque limit (does the Sea King in UK have a dreaded torque limiter? I seem to remember a fancy fuel control with torque matching was developed and fitted) then the actual power of the engine is being reduced as the rotor droops and torque is held constant, so the situation goes to worms real fast, rate of descent builds and the crowd screams "VRS!" Is my memory wrong, or was there a more full version with lots of hover turns before the dramatic thump?
BlenderPilot - The forward speed comment is quite right. You need some forward velocity, or else the VRS bubble doesn't form and stay. The smoke picture on my web site shows that perfectly. I did get some real VRS on a single engine landing once, with 8 knots airspeed and about 35 feet per second rate of descent (yea that's per second!) It was a real eye opener (and landing gear spreader) I try not to spout theory here, most of my observations are based on practical experience, with some theory to explain it (got that, Flare Dammit!? No Christmas cards.)
Your description sure sounds like VRS, if you had 15% torque margin and applied it and nothing happened. What was your ROD when that was happening?
I did the shakedown and structural flying on the S-76, and we had to do zero knot auto entries, increase the ROD and keep going down to full auto. About 6000 ft/min descent when trimmed. The A and A+ model are not altitude machines, they are optimized for sea level and do nicely there. The engine bursts are (thankfully) ancient history. They seemed to be associated with rebuild techniques, IIRC.
Regarding "Ignorant" - we all are, we each have a piece of this puzzle, and when we all listen, we all learn. I do, every time.
helmetfire - The 300 fpm rule of thumb is not bad, especially for lightly loaded helos. A tad conservative for heavy big turbines, but what's the harm? I remember the Black Hawk TV demo where he screamed in, flared, landed hard and went into the bushes! Dramatic footage, as they say.
RJSquirrel, what say you? You started this mess. I don't agree with your 2500 feet per minute, that's the theoretical VRS at 1.0 times the downwash speed. I think the UH-OH point is half the downwash velocity. For a Hawk, that's about 1500 fpm as the max hover descent without creating a smoking hole.
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VRS. Veryemotive subject. I think one problem could be that many years ago, the British called it Vortex Ring and the Americans called it Settling with power. In the same way, the British called a fast stop into wind a"into wind fast stop", whereas Bell called it a "Whoa boy"; bit more expressive than the Brits!
Coming in to land, pulling full power and thudding in ,is not being in VRS but a combination of not enough power available, bad planning and bad flying. VRS has specific requirements:-
1. Very low airspeed, around 8-10 knots, less than transational lift for that helicopter.
2. ROD of at least 300 fpm. This has many variables; altitude,oat, type of helicopter, weight,etc.
3. Power applied. Doesn't have to be full power.
Getting out of VRS involves removing those conditions. Increase speed, remove power. This can cause large loss of altitude, ROD of 6000 fpm is common. A Puma ditched and the last 500 feet took 16 seconds.
Before VRS is fully established, you will get some vibration warning while in the insipient stages. Once established, the main rotor is basically stalled but the tail rotor isn't and I have found in demos that a boot of pedal can speed up the recovery process.
Oh sh@t, I think I might have disagreed with Nick! 
Coming in to land, pulling full power and thudding in ,is not being in VRS but a combination of not enough power available, bad planning and bad flying. VRS has specific requirements:-
1. Very low airspeed, around 8-10 knots, less than transational lift for that helicopter.
2. ROD of at least 300 fpm. This has many variables; altitude,oat, type of helicopter, weight,etc.
3. Power applied. Doesn't have to be full power.
Getting out of VRS involves removing those conditions. Increase speed, remove power. This can cause large loss of altitude, ROD of 6000 fpm is common. A Puma ditched and the last 500 feet took 16 seconds.
Before VRS is fully established, you will get some vibration warning while in the insipient stages. Once established, the main rotor is basically stalled but the tail rotor isn't and I have found in demos that a boot of pedal can speed up the recovery process.
Oh sh@t, I think I might have disagreed with Nick!
Yeah - I dont want to mess with him either....
In fact I agree with you Nigel. I think VRS is poorly named in the US, although settling with power may descibe the end result, I would hardly call it "settling" - perhaps "unsettling" is closer? Either way, I believe the terminology has always caused confusion.
Nick - I agree about the 300 fpm being conservative. I use that for lighties such as 206, EC120, etc, and I use 40 KIAS/500 fpm for the heavier aircraft such as 205, 412, UH-60, etc.
Me LIKE conservative though!!
Remembering that UH-60 demonstration, the Black Hawk again showed what an incredible aircraft it is by plowing into the tree line and staying perfectly intact, and it probably could have been backed out again for a drive train change. It was only when the pilots pulled off the engines and the un-powered blades drooped into the uncut trees that it started coming apart. Even so, I dont think anyone was seriously hurt.
In fact I agree with you Nigel. I think VRS is poorly named in the US, although settling with power may descibe the end result, I would hardly call it "settling" - perhaps "unsettling" is closer? Either way, I believe the terminology has always caused confusion.
Nick - I agree about the 300 fpm being conservative. I use that for lighties such as 206, EC120, etc, and I use 40 KIAS/500 fpm for the heavier aircraft such as 205, 412, UH-60, etc.
Me LIKE conservative though!!
Remembering that UH-60 demonstration, the Black Hawk again showed what an incredible aircraft it is by plowing into the tree line and staying perfectly intact, and it probably could have been backed out again for a drive train change. It was only when the pilots pulled off the engines and the un-powered blades drooped into the uncut trees that it started coming apart. Even so, I dont think anyone was seriously hurt.
Last edited by helmet fire; 3rd Jun 2002 at 05:19.
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Nick Lappos writes:
"This is a good post..."
Well, we all are our own harshest critics, eh what?
"I found that Sea King video, at least the last few seconds of it. It is certainly possible that the thing started out as a low power event, and it could have become VRS, but that is not necessary to explain the mess. I would guess that it hit at about 10 feet per second, maybe 12, which is only about 600 to 700 ft/smin. "
Do you mean 600 to 700 feet per minute? I find it amazing that a Sea King - a bird designed for shipboard duty - can hit the ground at a paltry 600-700 fpm and have the landing gear collapse. Blimey! Doesn't Sikorsky design them any better than that? Heh- some of my 206 landings have probably been that hard
I can't imagine that the Sea King was all that heavily loaded (it only had crew aboard). It would be troubling if a bird like that had such marginal power for HOGE at such a relatively low DA.
Finally, if you watch the video of the Black Hawk that went into the bushes after the hard landing, as the camera pulls back you can see some nearby flags - all pointing directly away from the camera. Downwind approach! Even the mighty Black Hawk is evidently not immune.
And as for the Christmas card - well, my mate Lou Zuckermann and I will just have to somehow survive.
Interesting thread indeed!
"This is a good post..."
Well, we all are our own harshest critics, eh what?
"I found that Sea King video, at least the last few seconds of it. It is certainly possible that the thing started out as a low power event, and it could have become VRS, but that is not necessary to explain the mess. I would guess that it hit at about 10 feet per second, maybe 12, which is only about 600 to 700 ft/smin. "
Do you mean 600 to 700 feet per minute? I find it amazing that a Sea King - a bird designed for shipboard duty - can hit the ground at a paltry 600-700 fpm and have the landing gear collapse. Blimey! Doesn't Sikorsky design them any better than that? Heh- some of my 206 landings have probably been that hard
I can't imagine that the Sea King was all that heavily loaded (it only had crew aboard). It would be troubling if a bird like that had such marginal power for HOGE at such a relatively low DA.Finally, if you watch the video of the Black Hawk that went into the bushes after the hard landing, as the camera pulls back you can see some nearby flags - all pointing directly away from the camera. Downwind approach! Even the mighty Black Hawk is evidently not immune.
And as for the Christmas card - well, my mate Lou Zuckermann and I will just have to somehow survive.
Interesting thread indeed!
Flaredamnit! - Is it a possibility that the landing gear collapsed due to the twisting moments applied as it touched down?
Also, I conceed that that there is a possibility that the Sea King, and the UH-60 demo, may have been caught in turbulent air that they had created themselves (ie their own downwash), but that is NOT VRS. Nor is it incipient VRS. It is a lack of power in trying to outclimb the turbulence. having done downwind approaches in the UH-60, I still cannot attribute that accident to VRS. I still believe it was excessive NR droop due the power required to terminate BECAUSE it was downwind quickstop. I think the pilots even claimed they had a stabilator failure which would make sense as the NR can droop to the point that the generators pop off line momentarily, tripping the stabilator offline and activating the audio.
Also, I conceed that that there is a possibility that the Sea King, and the UH-60 demo, may have been caught in turbulent air that they had created themselves (ie their own downwash), but that is NOT VRS. Nor is it incipient VRS. It is a lack of power in trying to outclimb the turbulence. having done downwind approaches in the UH-60, I still cannot attribute that accident to VRS. I still believe it was excessive NR droop due the power required to terminate BECAUSE it was downwind quickstop. I think the pilots even claimed they had a stabilator failure which would make sense as the NR can droop to the point that the generators pop off line momentarily, tripping the stabilator offline and activating the audio.
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TGT Limiting
HI all,
The usual disclaimers here, speculation is bad....
I have a few hours in the 60 and based on what I could make out I wonder if the crew was a victim of the 60 protecting itself from them.
The crew was hovering in an area of very limited visual reference, OGE at 11000 or so feet. Pave Hawks are, as a rule, heavier than UH's given the blue suit propensity for ordering all the options.
If memory serves me correctly the chances of doing immediate and fatal damage to a 60 by over-torque are nil but an engine over-temp or over-speed are both possible and bad so the aircraft prevents you from doing both.
Given the conditions of the rescue I find it likely that the crew got into TGT limiting and weren't looking inside so they did not know of this until they got a low rotor audio and light. If they failed to react appropriately, immediately and the droop worsened they would lose most cockpit indications when the GCU's dropped the generators from the AC busses.
Rather confusing in the best of circumstances when you have time to figure out what's going on.
In the simulator when doing autorotation training most crews turn on the APU to preclude this loss of AC power should they allow the RPM to droop. The caveat is the APU isn't certified for normal flight and may only be used in flight during an emergency.
These folks didn't know they had an emergency till it was too late and even so the appropriate procedure does not call for the APU.
Sorry about the APU tangent.
Um..... the issue is TGT limiting. Most folks have never seen it in flight because they don't get that high and the engines are marvellous. Most crews haven't seen rotor droop either. I'm speculating on this as the cause because of the gradual onset of both the descent and the right yaw, both of which I have experienced in a 60 in rather unusual circumstances and I was very surprised because you just don't expect to run out of anything in a hawk and because the onset is very benign. If you don't pay attention you'll miss what the aircraft is telling you till it really needs you to aviate.
The result of bumping a limitation in a 60 is usually bad because the pilot's skill level is often far below that needed to extract all the helicopter can deliver and when it needs help he's out of options.
Fortunately, no matter whether of pilot, mechanical or environmental cause, the airframe will keep you alive through impacts fatal in any other craft.
Thanks to Nick and the rest of the fine Sikorsky team for a fantastic machine.
My prayers and wish for a speedy recovery to the folks involved.
Best regards,
Brian
The usual disclaimers here, speculation is bad....
I have a few hours in the 60 and based on what I could make out I wonder if the crew was a victim of the 60 protecting itself from them.
The crew was hovering in an area of very limited visual reference, OGE at 11000 or so feet. Pave Hawks are, as a rule, heavier than UH's given the blue suit propensity for ordering all the options.
If memory serves me correctly the chances of doing immediate and fatal damage to a 60 by over-torque are nil but an engine over-temp or over-speed are both possible and bad so the aircraft prevents you from doing both.
Given the conditions of the rescue I find it likely that the crew got into TGT limiting and weren't looking inside so they did not know of this until they got a low rotor audio and light. If they failed to react appropriately, immediately and the droop worsened they would lose most cockpit indications when the GCU's dropped the generators from the AC busses.
Rather confusing in the best of circumstances when you have time to figure out what's going on.
In the simulator when doing autorotation training most crews turn on the APU to preclude this loss of AC power should they allow the RPM to droop. The caveat is the APU isn't certified for normal flight and may only be used in flight during an emergency.
These folks didn't know they had an emergency till it was too late and even so the appropriate procedure does not call for the APU.
Sorry about the APU tangent.
Um..... the issue is TGT limiting. Most folks have never seen it in flight because they don't get that high and the engines are marvellous. Most crews haven't seen rotor droop either. I'm speculating on this as the cause because of the gradual onset of both the descent and the right yaw, both of which I have experienced in a 60 in rather unusual circumstances and I was very surprised because you just don't expect to run out of anything in a hawk and because the onset is very benign. If you don't pay attention you'll miss what the aircraft is telling you till it really needs you to aviate.
The result of bumping a limitation in a 60 is usually bad because the pilot's skill level is often far below that needed to extract all the helicopter can deliver and when it needs help he's out of options.
Fortunately, no matter whether of pilot, mechanical or environmental cause, the airframe will keep you alive through impacts fatal in any other craft.
Thanks to Nick and the rest of the fine Sikorsky team for a fantastic machine.
My prayers and wish for a speedy recovery to the folks involved.
Best regards,
Brian
For my sins, I had a part in training the pilot who commanded that CH-124 in NY. I also subsequently flew with the observer who was thrown out of the passenger door in the crash.
The Canadians prefer to call the mishap: SWP as the phrase VRS does not appear as regularly in their literature, as it does here in the UK. For that matter, the reverse is evident here (VRS common, SWP not).
Very few people have experienced VRS, unless they have been demo'd it. It takes some considerable time to manifest itself. When established, the pilot has no control over the a/c, if it comes out of VRS it will do so of its own volition. What most pilots experience (should they be so unlucky), is Incipient VRS where there is still room for manouevre
The Canadians prefer to call the mishap: SWP as the phrase VRS does not appear as regularly in their literature, as it does here in the UK. For that matter, the reverse is evident here (VRS common, SWP not).
Very few people have experienced VRS, unless they have been demo'd it. It takes some considerable time to manifest itself. When established, the pilot has no control over the a/c, if it comes out of VRS it will do so of its own volition. What most pilots experience (should they be so unlucky), is Incipient VRS where there is still room for manouevre
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Nick,
I have to say that I agree that it doesn't look like VRS. I can't see any reason why the disk should cone that much unless they had run out of power and the Nr had drooped (I've done that, it's not fun).
Thanks for the excellent page explaining VRS. I couldn't get the formula to work though, it may have something to do with the RAF being metric. I tried to work out the downwash velocity for a heavy CH47 - 50000lbs with a rotor area of 5600 square feet and came up with 43. I was expecting something more like 100mph or 10000fps, what am I doing wrong?
I have to say that I agree that it doesn't look like VRS. I can't see any reason why the disk should cone that much unless they had run out of power and the Nr had drooped (I've done that, it's not fun).
Thanks for the excellent page explaining VRS. I couldn't get the formula to work though, it may have something to do with the RAF being metric. I tried to work out the downwash velocity for a heavy CH47 - 50000lbs with a rotor area of 5600 square feet and came up with 43. I was expecting something more like 100mph or 10000fps, what am I doing wrong?
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Per Ordure Ad Asti,
You got it right, the answer is 43 feet per second, or about 2580 feet per minute rate of descent. At half that rate, the rotor begins to get substantial VRS (1290 ft/min). The downwash velocity depends on the disk loading, and the Chinook is quite kindly in that way, even for its size. Pilots tend to believe that helicopter size alone drives downwash speed, and that is a common misconception.
I will put the units on my web page ASAP. Thanks!
My wife is horrified that I loaded up the web page with helicopter crap, and not pictures of the family and such. Oh well!
One of the reasons for the skeptisism of some of the posters on this thread (where experienced, knowledgable pilots insist that any descent could cause VRS) is probably due to their vast experience flying heavily loaded machines where OGE hover was all that is possible (or even IGE only).
It is different for those of us who fly and test aircraft where the maneuverability is the driving parameter, and the load is reduced to allow vertical maneuvering.
The shame of it is that an entire generation of helicopter pilots has grown to believe that when you get in trouble OGE it must be VRS, and so they feel that VRS is always with us, and it is a fundamental helicopter limitation in any OGE situation.
This vastly restricts their ability to use their machines as they can be used, and it affects their understanding of what it is that gets them in trouble OGE.
You got it right, the answer is 43 feet per second, or about 2580 feet per minute rate of descent. At half that rate, the rotor begins to get substantial VRS (1290 ft/min). The downwash velocity depends on the disk loading, and the Chinook is quite kindly in that way, even for its size. Pilots tend to believe that helicopter size alone drives downwash speed, and that is a common misconception.
I will put the units on my web page ASAP. Thanks!
My wife is horrified that I loaded up the web page with helicopter crap, and not pictures of the family and such. Oh well!
One of the reasons for the skeptisism of some of the posters on this thread (where experienced, knowledgable pilots insist that any descent could cause VRS) is probably due to their vast experience flying heavily loaded machines where OGE hover was all that is possible (or even IGE only).
It is different for those of us who fly and test aircraft where the maneuverability is the driving parameter, and the load is reduced to allow vertical maneuvering.
The shame of it is that an entire generation of helicopter pilots has grown to believe that when you get in trouble OGE it must be VRS, and so they feel that VRS is always with us, and it is a fundamental helicopter limitation in any OGE situation.
This vastly restricts their ability to use their machines as they can be used, and it affects their understanding of what it is that gets them in trouble OGE.
Per Ordue,
I have a copy of the footage and have watched the clip very carefully, but I dont see any sudden or large increase in the conning angle until after its begins striking the mountain - which may be expected (I'd be pulling like hell to get away from the slope too!!).
As I said in the other thread, the aircraft does not appear (appear is the critical word here) to strike the mountain until it has turned GENTLY right toward the mountain, pitched slightly nose down (only a few degrees if at all) whilst the same hover height. It then pitches rapidly nose up to what looks like about 40 odd degrees before sliding backward and begining a descent. At about a 50 ft height loss, it then pitches rapidly forward and only now does it appear to have the first strike (fuel probe followed by main rotor) off the nose of the aircraft as it is pointing into the hill (Did anyone see any evidence of a strike before this?) and THEN the conning angle can be seen to increase.
I have a copy of the footage and have watched the clip very carefully, but I dont see any sudden or large increase in the conning angle until after its begins striking the mountain - which may be expected (I'd be pulling like hell to get away from the slope too!!).
As I said in the other thread, the aircraft does not appear (appear is the critical word here) to strike the mountain until it has turned GENTLY right toward the mountain, pitched slightly nose down (only a few degrees if at all) whilst the same hover height. It then pitches rapidly nose up to what looks like about 40 odd degrees before sliding backward and begining a descent. At about a 50 ft height loss, it then pitches rapidly forward and only now does it appear to have the first strike (fuel probe followed by main rotor) off the nose of the aircraft as it is pointing into the hill (Did anyone see any evidence of a strike before this?) and THEN the conning angle can be seen to increase.
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For Per Ordure Ad Asti,
I have posted some data from the V-22 tests for those doubters who question the rate of descent estimate of 1200 feet per minuteas the trigger point for VRS in OGE flight:
http://mywebpages.comcast.net/llappos/
Let the debate rage!
I have posted some data from the V-22 tests for those doubters who question the rate of descent estimate of 1200 feet per minuteas the trigger point for VRS in OGE flight:
http://mywebpages.comcast.net/llappos/
Let the debate rage!
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I finally found some time to look at this accident. I looked at the video, and I looked at some performance data for the Pave Hawk.
First the video, the only one I've seen is the Sky News video, which appears to be a CBS news video. That video has a cutaway scene to the climbers in trouble, that perhaps seems to miss the very beginning of the problem as it developed. Anyway, the helo is hovering nicely over the climbers, winching up the 3rd climber, when suddenly the hoist cable is cut (stated in the news articles) and the pilot does an abrupt left cyclic manuver to both steer the helo away from the climbers and to position the helo over a ravine. I've looked at the video a number of times, and it's apparent that the altitude loss begins to occur at the start of the event, even before the pilot makes the abrupt left banking manuver (which also includes the right yaw). The altitude loss continues until ground impact. It looks like the pilot tries to keep the helo in the center of the ravine (presumably to try and keep the ground away for as long as possible), but he seems unable to get the nose turned around away from the mountainside. He contacts the ground just left of the center of the ravine after trying to decend down the center of the ravine. The helo then rolls more or less down the center of the ravine after the impact.
Regarding the yaw, maybe the pilot was trying to perform a right yaw (easier with reduced power) to get the nose turned away from the mountain. When the event started, the nose was left of directly facing the mountian, but he turned through directly facing the mountain, and managed to turn about 120 degrees total (more or less) to the right before impact. From the start of the event until impact seems to have taken about 6 seconds (from the video), so that's a yaw rate of about 20 degrees per second (give or take).
Now for the performance data. Pave Hawks have a standard empty weight of 12,330 lbs. The news reports indicate that 6 people were on board, so add about 1,200 lbs. Then there could have been a maximum of 2,433 lbs. of normal internal fuel on board (I'm assuming that the Pave Hawk's cabin aux fuel tanks were empty for a high altitude mission), minus what was burned on the trip to the mountain. So we could have had a gross weight of anywhere from about 14,000 lbs, to maybe a max of 15,500 lbs.
I have to make assumptions about weather, as I cannot find any data on the net regarding weather on the mountain at the time of the crash (1:40PM PST 5/30/02). So I'll assume standard baro, and a temp of 0 degrees C (unless someone else knows). The altitude of the climbers was about 10,430 ft (they were about 800 ft below the 11,230 ft peak), so the hover height before the problem developed should be about 10,500 ft (in round numbers).
For these conditions, the HOGE for the Pave Hawk at 14,000 lbs is about 16,200 ft, and at 15,500 lbs it's about 13,600 ft (10 minute ratings). I could not find a chart indicating the HOGE for OEI, but the service ceiling OEI at these weights is about 14,900 ft (14,000 lbs) and about 13,000 ft (15,500 lbs). I wish I could see a chart for OEI HOGE, but I'm pretty sure the helo could not hover at 10,500 ft on just one engine.
Based on the fact that the hoist cable was cut immediately, and the pilot's abrupt left cyclic move away from the climbers and towards the center of a ravine, seem to indicate to me that the crew of the H-60 knew they were going to loss altitude as soon as the event started. I don't know what that means, but I guess I'm still leaning towards the idea that an engine failure caused this accident. I also agree fully with Nick, no way was this a VRS or LTE accident.
First the video, the only one I've seen is the Sky News video, which appears to be a CBS news video. That video has a cutaway scene to the climbers in trouble, that perhaps seems to miss the very beginning of the problem as it developed. Anyway, the helo is hovering nicely over the climbers, winching up the 3rd climber, when suddenly the hoist cable is cut (stated in the news articles) and the pilot does an abrupt left cyclic manuver to both steer the helo away from the climbers and to position the helo over a ravine. I've looked at the video a number of times, and it's apparent that the altitude loss begins to occur at the start of the event, even before the pilot makes the abrupt left banking manuver (which also includes the right yaw). The altitude loss continues until ground impact. It looks like the pilot tries to keep the helo in the center of the ravine (presumably to try and keep the ground away for as long as possible), but he seems unable to get the nose turned around away from the mountainside. He contacts the ground just left of the center of the ravine after trying to decend down the center of the ravine. The helo then rolls more or less down the center of the ravine after the impact.
Regarding the yaw, maybe the pilot was trying to perform a right yaw (easier with reduced power) to get the nose turned away from the mountain. When the event started, the nose was left of directly facing the mountian, but he turned through directly facing the mountain, and managed to turn about 120 degrees total (more or less) to the right before impact. From the start of the event until impact seems to have taken about 6 seconds (from the video), so that's a yaw rate of about 20 degrees per second (give or take).
Now for the performance data. Pave Hawks have a standard empty weight of 12,330 lbs. The news reports indicate that 6 people were on board, so add about 1,200 lbs. Then there could have been a maximum of 2,433 lbs. of normal internal fuel on board (I'm assuming that the Pave Hawk's cabin aux fuel tanks were empty for a high altitude mission), minus what was burned on the trip to the mountain. So we could have had a gross weight of anywhere from about 14,000 lbs, to maybe a max of 15,500 lbs.
I have to make assumptions about weather, as I cannot find any data on the net regarding weather on the mountain at the time of the crash (1:40PM PST 5/30/02). So I'll assume standard baro, and a temp of 0 degrees C (unless someone else knows). The altitude of the climbers was about 10,430 ft (they were about 800 ft below the 11,230 ft peak), so the hover height before the problem developed should be about 10,500 ft (in round numbers).
For these conditions, the HOGE for the Pave Hawk at 14,000 lbs is about 16,200 ft, and at 15,500 lbs it's about 13,600 ft (10 minute ratings). I could not find a chart indicating the HOGE for OEI, but the service ceiling OEI at these weights is about 14,900 ft (14,000 lbs) and about 13,000 ft (15,500 lbs). I wish I could see a chart for OEI HOGE, but I'm pretty sure the helo could not hover at 10,500 ft on just one engine.
Based on the fact that the hoist cable was cut immediately, and the pilot's abrupt left cyclic move away from the climbers and towards the center of a ravine, seem to indicate to me that the crew of the H-60 knew they were going to loss altitude as soon as the event started. I don't know what that means, but I guess I'm still leaning towards the idea that an engine failure caused this accident. I also agree fully with Nick, no way was this a VRS or LTE accident.
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Nick,
Thanks for the update, the explanation of VRS given in groundschool was sketchy to say the least and we were never given any equations to play with. We are taught to use 30kts (the lowest reliable ASI reading) and 500fpm as a safety margin for the Chinook, it's nice to know that there is plenty of room for error.
An old and bold pilot once told me that, many moons ago, he put a Chinook into VRS on purpose (with difficulty) and managed to power out of it. I was taught that this couldn't happen but your explanation is much better.
I dont doubt your maths at all, but I find a 2500fps (25mph) downwash figure for a heavy Chinook to be a bit surprising. On the conversion course they make you hook up loads for the experience. I have seen strong men blown flat by the downwash and stories of destruction and mayhem to vehicles and buildings are common.
Thanks for the update, the explanation of VRS given in groundschool was sketchy to say the least and we were never given any equations to play with. We are taught to use 30kts (the lowest reliable ASI reading) and 500fpm as a safety margin for the Chinook, it's nice to know that there is plenty of room for error.
An old and bold pilot once told me that, many moons ago, he put a Chinook into VRS on purpose (with difficulty) and managed to power out of it. I was taught that this couldn't happen but your explanation is much better.
I dont doubt your maths at all, but I find a 2500fps (25mph) downwash figure for a heavy Chinook to be a bit surprising. On the conversion course they make you hook up loads for the experience. I have seen strong men blown flat by the downwash and stories of destruction and mayhem to vehicles and buildings are common.
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Per Ordure Ad Asti,
The discussion on the web site is the first time I have ever seen it all put together in one place, a reason for doing it. Lots of tribal knowledge, but little explained factually, I think. More to go, I am doing some correlation with low speed descent power increases to help explain the legitimate points raised by several posters (Flare Dammit! for example) that something is happening in descent that is not healthy. They are right, the descent power is rising even at low rates. More to follow.
The downwash velocity we are calculating is for the disk average, and assumes a constant velocity stream the diameter of the rotor. It is quite reasonable to assume that there are higher velocities at the tips, so the crew will feel gusts and peaks at perhaps 50% higher than that average (maybe more, I am looking for some papers of measured wake velocities). If 50% higher, than the 40 mph gets substantial enough to toss stuff around, I think. Also remember that the total energy is awesome, so the sheet of wind is a large one that reaches far.
The Chinook is particularly blessed with low wake velocities as it is a very low disk loading machine, a virtue of the tandem design. That is why it outlifts the tilt rotor with so very much less power. It has so very much less empty weight. At 50000 pounds total gross weight, it has an empty weight of 23,000 which is 46% empty weight fraction (the same as that of the 53E, 32000 empty for 69000 gross). Compare those excellent numbers to the V-22's 69% empty weight fraction is a real eye opener - 33,300 empty for HOGE of 47500 at operating altitude).
The discussion on the web site is the first time I have ever seen it all put together in one place, a reason for doing it. Lots of tribal knowledge, but little explained factually, I think. More to go, I am doing some correlation with low speed descent power increases to help explain the legitimate points raised by several posters (Flare Dammit! for example) that something is happening in descent that is not healthy. They are right, the descent power is rising even at low rates. More to follow.
The downwash velocity we are calculating is for the disk average, and assumes a constant velocity stream the diameter of the rotor. It is quite reasonable to assume that there are higher velocities at the tips, so the crew will feel gusts and peaks at perhaps 50% higher than that average (maybe more, I am looking for some papers of measured wake velocities). If 50% higher, than the 40 mph gets substantial enough to toss stuff around, I think. Also remember that the total energy is awesome, so the sheet of wind is a large one that reaches far.
The Chinook is particularly blessed with low wake velocities as it is a very low disk loading machine, a virtue of the tandem design. That is why it outlifts the tilt rotor with so very much less power. It has so very much less empty weight. At 50000 pounds total gross weight, it has an empty weight of 23,000 which is 46% empty weight fraction (the same as that of the 53E, 32000 empty for 69000 gross). Compare those excellent numbers to the V-22's 69% empty weight fraction is a real eye opener - 33,300 empty for HOGE of 47500 at operating altitude).
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Interesting stories all of them.
Funny on how no-one has touched on the APPARENT or RELATIVE ROD that the aircraft could have experienced. I have no idea of the geography or the met conditions that were experienced but if you are in a situation approaching the side of a mountain or hovering there with the wind coming up from below and it seems to be hovering for "free" get that s***t eating grin off your face and watch your arse. Ask a Llama pilot. The Llama being so lightly disc loaded sometimes and being able to easily get it self into places where many other machines can only dream about is a common candidate. If it looks like you are getting something for nothing - you probably will. I dont know how many times as a sprog pilot in a H300 I charged into a sloping face with a minimal tail wind and holy s***t. The 30 knots BS and 300 fpm BS is fine and only relavent if you are operating without any existing vertical flow components - (read wind) but if the wind is from behind and as you approach the slope the vertical component increases to maintain these relative figures you would likely find yourself climbing. Most pilot's only think about downdrafts unfortunately. Think about it as a normal approach and add the vectors for a tail wind that is coming UP behind you. The comparison of IAS versus groundspeed is going to indicate the tailwind, provided you have taken into account the TAS at high altitudes. The vertical component will be apparent (possibly) in the fact that the VSI is telling the same story but where did all that excess power come from? 2500 fpm vertical gust is not much but if you are coming down at 500 fpm you can subtract that from the figure - 2000 fpm or 20 knots vertical component is not much and it increases as you get closer the slope. Trust me you can get into OGE hover situations next to a slope where you are just sitting in an updraught that is going to eat you alive with VRS. This make sense to anyone yet?
As for these air velocities being able to be achieved in the natural world - I have had the needles split in effectively zero forward speed conditions in a B 205 at 10500lbs and been climbing. Dumped the water load and it went up after it left the tank. Corsica - top spot. The FW guys all stayed in the pub, joined them soon after.
Funny on how no-one has touched on the APPARENT or RELATIVE ROD that the aircraft could have experienced. I have no idea of the geography or the met conditions that were experienced but if you are in a situation approaching the side of a mountain or hovering there with the wind coming up from below and it seems to be hovering for "free" get that s***t eating grin off your face and watch your arse. Ask a Llama pilot. The Llama being so lightly disc loaded sometimes and being able to easily get it self into places where many other machines can only dream about is a common candidate. If it looks like you are getting something for nothing - you probably will. I dont know how many times as a sprog pilot in a H300 I charged into a sloping face with a minimal tail wind and holy s***t. The 30 knots BS and 300 fpm BS is fine and only relavent if you are operating without any existing vertical flow components - (read wind) but if the wind is from behind and as you approach the slope the vertical component increases to maintain these relative figures you would likely find yourself climbing. Most pilot's only think about downdrafts unfortunately. Think about it as a normal approach and add the vectors for a tail wind that is coming UP behind you. The comparison of IAS versus groundspeed is going to indicate the tailwind, provided you have taken into account the TAS at high altitudes. The vertical component will be apparent (possibly) in the fact that the VSI is telling the same story but where did all that excess power come from? 2500 fpm vertical gust is not much but if you are coming down at 500 fpm you can subtract that from the figure - 2000 fpm or 20 knots vertical component is not much and it increases as you get closer the slope. Trust me you can get into OGE hover situations next to a slope where you are just sitting in an updraught that is going to eat you alive with VRS. This make sense to anyone yet?
As for these air velocities being able to be achieved in the natural world - I have had the needles split in effectively zero forward speed conditions in a B 205 at 10500lbs and been climbing. Dumped the water load and it went up after it left the tank. Corsica - top spot. The FW guys all stayed in the pub, joined them soon after.
John,
there are several cliff faces in Snowdonia (Wales UK) where it was possible to demonstrate incipient vortex ring consistently time after time.
A 20kt+wind blowing straight up the cliff face was all that was required. The rising air could be made visual by using smoke cartridges fired from the helo (RAF SAR Wessex).
Hovering in this air stream along side the cliff was possible for about 30 to 60 seconds.To maintain relative position against the cliff face the collective would have to be raised slowly until max power was reached, then the nominated escape route HAD to be flown!
It was possible to repeat this exercise consistently, making it an excellent teaching point about safe mountain flying.
there are several cliff faces in Snowdonia (Wales UK) where it was possible to demonstrate incipient vortex ring consistently time after time.
A 20kt+wind blowing straight up the cliff face was all that was required. The rising air could be made visual by using smoke cartridges fired from the helo (RAF SAR Wessex).
Hovering in this air stream along side the cliff was possible for about 30 to 60 seconds.To maintain relative position against the cliff face the collective would have to be raised slowly until max power was reached, then the nominated escape route HAD to be flown!
It was possible to repeat this exercise consistently, making it an excellent teaching point about safe mountain flying.