Singles v. Twins
Guest
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Scattercat, Hear Hear!!!
Widgeom,
Many years ago I read in a BASI report about one of the first 214st operating in Australia. This case was where a good engine was shut down, one of you guys out there will have the item, excellent reading. I stand to be corrected on this, but as i recall , the capt noticed an engine overspeed, I recall that the splits between the N1s were significant, in the process of working out which one was the runaway, I beleive they pulled collective to arrest their RRPM, somewhere in this process the good engine was doing what it was supposed to,somehow the indications in the cockpit were that it was the good engine running away, so it was shut down. There were no crosswires it was crew interpretation. The BASI had a CVR of the crew and it all happened 15 to 20 seconds till they ditched. If any of you guys find it, its an excellent accident investigation and if you do find it correct any discrepancies i have made, it was a long time ago when i read it.
Widgeom,
Many years ago I read in a BASI report about one of the first 214st operating in Australia. This case was where a good engine was shut down, one of you guys out there will have the item, excellent reading. I stand to be corrected on this, but as i recall , the capt noticed an engine overspeed, I recall that the splits between the N1s were significant, in the process of working out which one was the runaway, I beleive they pulled collective to arrest their RRPM, somewhere in this process the good engine was doing what it was supposed to,somehow the indications in the cockpit were that it was the good engine running away, so it was shut down. There were no crosswires it was crew interpretation. The BASI had a CVR of the crew and it all happened 15 to 20 seconds till they ditched. If any of you guys find it, its an excellent accident investigation and if you do find it correct any discrepancies i have made, it was a long time ago when i read it.
Guest
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With regards to closing down the wrong engine, the following maybe of some interest.
In the mid 90s a US Army UH 60 Blackhawk crashed killing 4 crew. The aircraft had suffered an engine fire, and despite being told by an accompanying aircraft that the No 2 engine was on fire, the crew shut down the No 1. The US Army conducted a study to see if pilot's reactions to single engine emergencies were a systemic problem and whether such risks could be reduced.
Questionnaires were sent to over 4000 Army aviators. Two questions provided particularly important insight into the problem:
Do you believe there is a potential problem of shutting down the operating engine during a single-engine malfunction.
Have you ever moved or started to move the wrong engine control lever during a simulated or actual emergency.
Over 70 percent believed there was potential for closing down the wrong engine, and 40 percent confirmed that they had confused ECLs during real or simulated emergencies. In addition, 50 percent of those who recounted confusion confirmed that they had shut down the "good" engine or had moved it's ECL.
In a simulator study, 47 crews were asked to fly a 2 hour pre-briefed, pre-planned mission. During the mission they encountered at least one of 6 randomly selected malfunctions that would require an engine to be shut down. In addition false indications of engine failures were also randomly included.
15 percent of the crews reacted incorrectly to the emergencies, and in the case of the false indications 25 percent of the crews reactions resulted in dual engine power losses with the associated fatalities! 22 of 47 had problems with the initial diagnosis and 17 of 47 made errors in the action taken.
The study indicated that in a malfunction that required an engine to be shutdown, there was a 1 in 6 chance that the crew would respond incorrectly.
The Army instigated several changes in training in order to overcome this problem and also an ergonomics study of the cockpit to help minimise confusion.
In the mid 90s a US Army UH 60 Blackhawk crashed killing 4 crew. The aircraft had suffered an engine fire, and despite being told by an accompanying aircraft that the No 2 engine was on fire, the crew shut down the No 1. The US Army conducted a study to see if pilot's reactions to single engine emergencies were a systemic problem and whether such risks could be reduced.
Questionnaires were sent to over 4000 Army aviators. Two questions provided particularly important insight into the problem:
Do you believe there is a potential problem of shutting down the operating engine during a single-engine malfunction.
Have you ever moved or started to move the wrong engine control lever during a simulated or actual emergency.
Over 70 percent believed there was potential for closing down the wrong engine, and 40 percent confirmed that they had confused ECLs during real or simulated emergencies. In addition, 50 percent of those who recounted confusion confirmed that they had shut down the "good" engine or had moved it's ECL.
In a simulator study, 47 crews were asked to fly a 2 hour pre-briefed, pre-planned mission. During the mission they encountered at least one of 6 randomly selected malfunctions that would require an engine to be shut down. In addition false indications of engine failures were also randomly included.
15 percent of the crews reacted incorrectly to the emergencies, and in the case of the false indications 25 percent of the crews reactions resulted in dual engine power losses with the associated fatalities! 22 of 47 had problems with the initial diagnosis and 17 of 47 made errors in the action taken.
The study indicated that in a malfunction that required an engine to be shutdown, there was a 1 in 6 chance that the crew would respond incorrectly.
The Army instigated several changes in training in order to overcome this problem and also an ergonomics study of the cockpit to help minimise confusion.
Guest
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I think we are missing the point here. Two engines double your chances of a engine failure. During flight below ETL (take off, landing, slings, etc) most helos cannot keep flying in some of those regimes. 2 engines are great for long flights over inhospitable terrain but for short flights involving lots of takeoffs and landings, go with the single. All else being equal of course.
j
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Here's to cheating, stealing, fighting, and drinking.
If you cheat, may you cheat death.
If you steal, may you steal a woman's heart.
If you fight, may you fight for a brother.
And if you drink, may you drink with me.
j
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Here's to cheating, stealing, fighting, and drinking.
If you cheat, may you cheat death.
If you steal, may you steal a woman's heart.
If you fight, may you fight for a brother.
And if you drink, may you drink with me.
Guest
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Before Landing Checklist
I hope you work in real estate cause id like to buy some land off you, check how many helos have had double engine failures, and even the ones who have, still the same result as a single. Two engines half your chance of an engine failure, in increase your chance of living.
I hope you work in real estate cause id like to buy some land off you, check how many helos have had double engine failures, and even the ones who have, still the same result as a single. Two engines half your chance of an engine failure, in increase your chance of living.
Guest
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Before Landing Checklist - "Two engines double your chance of an engine failure". I hope you're kidding. Using that logic two generators double your chance of a generator failure, two hydraulic systems double your chance of a hydraulic failure, etc. The point is,if you've got two of everything you'll still have something left if one of those engines/systems/instruments do fail. It's true most twins won't fly away if they lose an engine on takeoff but you'll still come down a lot more gently on the remaining engine than on no engine at all.
Guest
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Extrapolating - a favourite Sunday evening activity of mine - the thread's question somewhat, does anyone from the 53E/Merlin world know whether 3 are safer than 2 or 1?
I like the comment ascribed to the latter, engineering half of the Rolls Royce duo. When asked why he would only fly across the Atlantic in aircraft fitted with 4 engines, he is said to have replied along the lines of "because no-one makes them with 5.."
I like the comment ascribed to the latter, engineering half of the Rolls Royce duo. When asked why he would only fly across the Atlantic in aircraft fitted with 4 engines, he is said to have replied along the lines of "because no-one makes them with 5.."
Guest
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Before landing check list is absolutely correct in his assertion that 2 engines doubles your likelihood of an engine failure in any given aircraft. The statement relates to engine failures - not dual engine failures.
These are simple statistical interpretations of empirical data:
If the Mean Time Between Failure is (say) 100,000 hours, then in a single engine machine, the statistical likelihood of a failure is 1 in every 100,000 flight or engine hours.
In a twin the same likelihood is the same 1 in 100,000 engine hours, which occurs every 50,000 flight hours, as you have two motors!
Engine MTBF's are not calculated by application, but by individual engine hours, thus providing the basis for comparison.
There are a number of twin helicopters (and specific applications) that are quite capable of single engine operation, but there are a large number that are not. Some airframe/engine/system combinations are more reliable than others and consequently enjoy a superior accident record to others.
In a single engine aircraft, the loss of a powerplant demands immediate and specific action. In a twin engine aircraft, the loss of an engine will demand an immediate decision of appropriate choice of action to be taken, appropriate to the performance and flight phase of the machine. In many twins, dependent upon individual performance and phase of flight, the action is either to initiate immediate recovery to sustainable flight, or maintaining flight within the single engine performance capabilities.
This information can be verified by reviewing the accident statistics for both single and twin engined helicopters and fixed-wing. There is an obvious connection between the conclusion that the most dangerous flight regimes are take-off, landing and manouevering, and the performance characteristics of the aircraft involved.
I would much rather fly in a GOOD single engined machine than a twin with no single engine performance (I would still rather fly in a GOOD twin). I include in this group most Allison 250 powered helicopters, especially at high-gross weights. By choice I would never fly in a Twin-star, or for that matter a 300 or 400 series Cessna (amongst others).
The (natural, or instinctive) tendency to attempt a recovery on the remaining engine has led to a number of non-survivable accidents, which might have been quite survivable if the machine had simply made a forced landing whilst still under control. This is equally true in fixed-wing and helicopters.
Regarding the increased likelihood of failure with any dual systems, statistically the same stands true. Hence the requirements for redundancy in complex machines. I think that system failures however are much more dependent upon the manufacturer, quality of the maintenance, components and design. Some machines have abyssmal sytems, others have systems that just run perfectly. This is why there are multiple power sources, hydraulic systems, etc. How many modern helicopters can fly with NO hydraulics? Most of these singular system failures are non-events, simply from the design and regulatory requirements learned from past, bad, experience.
Dual engine failures do occur in twins. Considering their effect, perhaps a little more often than anyone might care to think (consider fixed-wing as well in this equation). They occur for a variety of reasons, not necessarily directly related to the powerplant itself, but also to systems and fuel and air management. A number of twin engine helicopter accidents have occured as a direct result of a single engine failure leading to a drivetrain failure, causing the second engine to quit! Historically, a catastrophic (uncontained)engine failure has resulted in a number of dual engine failures in twin helicopters, as the first, (exploding) motor takes out the second one installed along side it.
Over 25 years, I have lost 1 turbine helicopter to an engine failure. I have experienced at least 12 engine failures on turbine twins, none of which even caused any damage. This says a lot for the twin engine machines, and I believe the statistics bear this out. If my twins had been singles, we would be talking about a lot of accidents here!
The argument concerning Single v Twins is indeed complex, as the majority of accidents occur for reasons wholly unrelated to the number of engines. This is true for both categories, and is unlikely to ever change. The selection of a helicopter, appropriate for a specific application, should be made upon that basis, not upon whether it has 2 motors or not!
I wouldn't want to operate a single engine helicopter 100 miles out in the North Sea, but that is not to say that it is not an appropriate choice for the Gulf of Mexico (obviously that seems to be the choice of a large number of people). This has been one of the big arguments over JAR Ops 3 in the States, as these are regulations designed as one size fits all. For anyone flying around in a twin Allison powered Twin-star thinking they are safer, I beg to differ - give me one Arriel any day.
These are simple statistical interpretations of empirical data:
If the Mean Time Between Failure is (say) 100,000 hours, then in a single engine machine, the statistical likelihood of a failure is 1 in every 100,000 flight or engine hours.
In a twin the same likelihood is the same 1 in 100,000 engine hours, which occurs every 50,000 flight hours, as you have two motors!
Engine MTBF's are not calculated by application, but by individual engine hours, thus providing the basis for comparison.
There are a number of twin helicopters (and specific applications) that are quite capable of single engine operation, but there are a large number that are not. Some airframe/engine/system combinations are more reliable than others and consequently enjoy a superior accident record to others.
In a single engine aircraft, the loss of a powerplant demands immediate and specific action. In a twin engine aircraft, the loss of an engine will demand an immediate decision of appropriate choice of action to be taken, appropriate to the performance and flight phase of the machine. In many twins, dependent upon individual performance and phase of flight, the action is either to initiate immediate recovery to sustainable flight, or maintaining flight within the single engine performance capabilities.
This information can be verified by reviewing the accident statistics for both single and twin engined helicopters and fixed-wing. There is an obvious connection between the conclusion that the most dangerous flight regimes are take-off, landing and manouevering, and the performance characteristics of the aircraft involved.
I would much rather fly in a GOOD single engined machine than a twin with no single engine performance (I would still rather fly in a GOOD twin). I include in this group most Allison 250 powered helicopters, especially at high-gross weights. By choice I would never fly in a Twin-star, or for that matter a 300 or 400 series Cessna (amongst others).
The (natural, or instinctive) tendency to attempt a recovery on the remaining engine has led to a number of non-survivable accidents, which might have been quite survivable if the machine had simply made a forced landing whilst still under control. This is equally true in fixed-wing and helicopters.
Regarding the increased likelihood of failure with any dual systems, statistically the same stands true. Hence the requirements for redundancy in complex machines. I think that system failures however are much more dependent upon the manufacturer, quality of the maintenance, components and design. Some machines have abyssmal sytems, others have systems that just run perfectly. This is why there are multiple power sources, hydraulic systems, etc. How many modern helicopters can fly with NO hydraulics? Most of these singular system failures are non-events, simply from the design and regulatory requirements learned from past, bad, experience.
Dual engine failures do occur in twins. Considering their effect, perhaps a little more often than anyone might care to think (consider fixed-wing as well in this equation). They occur for a variety of reasons, not necessarily directly related to the powerplant itself, but also to systems and fuel and air management. A number of twin engine helicopter accidents have occured as a direct result of a single engine failure leading to a drivetrain failure, causing the second engine to quit! Historically, a catastrophic (uncontained)engine failure has resulted in a number of dual engine failures in twin helicopters, as the first, (exploding) motor takes out the second one installed along side it.
Over 25 years, I have lost 1 turbine helicopter to an engine failure. I have experienced at least 12 engine failures on turbine twins, none of which even caused any damage. This says a lot for the twin engine machines, and I believe the statistics bear this out. If my twins had been singles, we would be talking about a lot of accidents here!
The argument concerning Single v Twins is indeed complex, as the majority of accidents occur for reasons wholly unrelated to the number of engines. This is true for both categories, and is unlikely to ever change. The selection of a helicopter, appropriate for a specific application, should be made upon that basis, not upon whether it has 2 motors or not!
I wouldn't want to operate a single engine helicopter 100 miles out in the North Sea, but that is not to say that it is not an appropriate choice for the Gulf of Mexico (obviously that seems to be the choice of a large number of people). This has been one of the big arguments over JAR Ops 3 in the States, as these are regulations designed as one size fits all. For anyone flying around in a twin Allison powered Twin-star thinking they are safer, I beg to differ - give me one Arriel any day.
Guest
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Apart from the statistics, there is another important consideration on the twin/single debate; the psychological factor. It's all very well being aware of arguments for similar stats on accident rates, but what about the levels of pilot stress and anxiety, particularly flying single engine at night for example? Regulations and statistics apart, I know what I'm more relaxed in, and that is likely to make for better decision making. And of course pax are going to even more affected by the relative levels of confidence of 1 or 2 engines, assuming they actually know what they're flying in and understand the difference - which they often don't!
Guest
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No, I have not been every where and done everything. But I do have 4000+ just in helos, over 2/3 in dual engine helos. Most with the Army flying the magical Hawk. The balance in BO105, BH212, B205,MD500/530,BH 206 and some AS350(150 hrs)The hawk is powerfull, good T/R control but it is still a helo and will bite you in the ass if you do downwind aproaches and Takeoff as a rule and not a exception. And when is is heavy a engine failure < ETL will hurt you. If coming off of a platform as in oil platform a engine failure with half the helo off the side will not be pretty. Especially with a low rotor inertia machine like the H60 and BO105. If you read a litle closer you would have seen where I put in stipulations such as terrain you are flying over and if numerouse trips <ETL will be the norm. A BH205(one engine, civ model of the Huey but with added hyd and better tail rotor) will outlift a a BH212 when you are high and hot.
I know that flt time a ATP and CFII does not mean all but I do have a litle bit of experiance and the insurance companies rated a BH206 the worlds safest aircraft. I think it has only one engine. Also more fatal accidents in airplanes occur in twins even though more singles fly. That is because a failure in a single makes up yuour mind, in a twin you have a tendency to push a bad situation.So give it a break Mark and do the math. and Sling get some experiance
j
[This message has been edited by before landing check list (edited 11 May 2001).]
[This message has been edited by before landing check list (edited 11 May 2001).]
I know that flt time a ATP and CFII does not mean all but I do have a litle bit of experiance and the insurance companies rated a BH206 the worlds safest aircraft. I think it has only one engine. Also more fatal accidents in airplanes occur in twins even though more singles fly. That is because a failure in a single makes up yuour mind, in a twin you have a tendency to push a bad situation.So give it a break Mark and do the math. and Sling get some experiance
j
[This message has been edited by before landing check list (edited 11 May 2001).]
[This message has been edited by before landing check list (edited 11 May 2001).]
Guest
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BLCL, I've been at this a few years as you obviously have, so also speak from experience. I'm not arguing with your math-I agree with you that 2 engines gives you twice the chance of ONE engine failing but that is not a valid argument against having 2 engines. My point was that you wouldn't use that "double the chance of a failure" reason as a justification for not having 2 generators, artificial horizons, etc, so why use it for engines? I agree with your point about the potential to mis-handle an engine failure in a twin, and to possibly be worse off than if the failure had occurred in a single. All things being equal however, a well trained, competent pilot suffering an engine failure in a twin should come off better than the same pilot having an engine failure in a single.
Guest
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Just a thought...the original question refers to 'singles vs twins', which has broadened out to ask about twin gens/hyds etc. as well as the assumed engines.
Any thoughts about numbers of crew at the sharp end? FWIW we always seemed to get more snags (as in whinges, not proper defects) from a twin pilot S61 than we did from a single pilot S76...is this because the S76 is more reliable/less complex? (don't think so!)
Would a flight with 4 pilots be safer than 2 or 1?
Me?, I just fix 'em
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Ground tested, no fault found
Any thoughts about numbers of crew at the sharp end? FWIW we always seemed to get more snags (as in whinges, not proper defects) from a twin pilot S61 than we did from a single pilot S76...is this because the S76 is more reliable/less complex? (don't think so!)
Would a flight with 4 pilots be safer than 2 or 1?
Me?, I just fix 'em
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Ground tested, no fault found
Guest
Posts: n/a
I have 6000 helo hours with about 4000 on twins. I fly for a police agency over extensive built-up areas with overhead tram lines in the CBD areas. We operate 2 dauphines and have had engine shutdowns in-flight. All I can say is that unlike LAPD who have lost aircrew and civilians on the ground due to engine out in singles we have never had an auto into the suburbs. Thank goodness we have twins otherwise we too would have lost souls. (Try auto into 50,000Volts).
Guest
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OK, let me start again. I am not saying one is more safer then the other. No way. It is too broad a statement. All I am saying is there are those times when you are using 100% of 100% available and you cannot afford to have A engine (on a multi engine)cough let slone quit. I was just saying the blanket statement about multi are safer then the single is a under educated statement. The correct answer is it depends. And nasus, I have 8 1/2 years flying over cities doing police work and most of my time was above ETL and did everyhing I could to accomplish the mission by staying out of the wrong side of the height/as envelope. Of course you cannot do it all the time. You just have to believe in god and Turbomecca, Alison, GE, P&W, Lycoming etc. I was doing alot of it in a single and wish I were in a twin, especially in a UNHOSPITABLE enviroment such as a city at night.
j
j
Guest
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G'day Before Landing Check List.
I too have a few years of flying over cities (16 yrs) on Police ops day & night and all I can say is that sure single turbines are safe and the chance of having a failure is remote But it sure makes for a less stressful flight when you don't have to keep looking over your shoulder for a place to auto into or worry about height velocity curve etc. We use a helo that can fly away OEI from a 400' hover over the city no problems and that makes it easier to concentrate on the mission then having to worry about a forced landing area. OK it's more expensive but there's no way I'm going back to flying singles over cities again...I'd rather quit!
I too have a few years of flying over cities (16 yrs) on Police ops day & night and all I can say is that sure single turbines are safe and the chance of having a failure is remote But it sure makes for a less stressful flight when you don't have to keep looking over your shoulder for a place to auto into or worry about height velocity curve etc. We use a helo that can fly away OEI from a 400' hover over the city no problems and that makes it easier to concentrate on the mission then having to worry about a forced landing area. OK it's more expensive but there's no way I'm going back to flying singles over cities again...I'd rather quit!
Guest
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Although a sometimes busy management excercise, the Renton twins were better together than either of them alone. Only my perspective. Having spent many hours in mountainous environments, I've always liked knowing that I would have a little more thinking and planning time before landing after an engine failed. Very often it was only a 1/2 reduction in the rate of descent. Never the less it was more time.
[This message has been edited by VLift (edited 16 May 2001).]
[This message has been edited by VLift (edited 16 May 2001).]
Guest
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Gentlemen,
Having flown single engine over Very deep water I agree with Deek01 when you only have one maybe french is best. Neverinbalance quotes the gazelle with it's Astazou donk and I agree again that it is a damn good engine WHEN you only have one. But, marksix quotes training as a means of eliminating a single engine mind set when the emergency comes to visit. - again I agree Two engines can give you the time to sort out your **** - even if you are limited to a landing. Hoverman, Nasus and Scattercat generally give the same message - if you are over a confined conurbation area low level the only way to ensure the safety of the people on the surface is with perf A criteria and TWO engines. I have had the pleasure of single engine failures on differing types, but without the other donk it would be into the - very lucky to get away with it group, and I for one do not wish to join. fly safe - use two!!!
Having flown single engine over Very deep water I agree with Deek01 when you only have one maybe french is best. Neverinbalance quotes the gazelle with it's Astazou donk and I agree again that it is a damn good engine WHEN you only have one. But, marksix quotes training as a means of eliminating a single engine mind set when the emergency comes to visit. - again I agree Two engines can give you the time to sort out your **** - even if you are limited to a landing. Hoverman, Nasus and Scattercat generally give the same message - if you are over a confined conurbation area low level the only way to ensure the safety of the people on the surface is with perf A criteria and TWO engines. I have had the pleasure of single engine failures on differing types, but without the other donk it would be into the - very lucky to get away with it group, and I for one do not wish to join. fly safe - use two!!!
Guest
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The CAA did a survey on the twin/single flight safety argument for light fixed wing a few years ago. They concluded that low hours/unpracticed pilots were more likely to have a serious accident after a single engine failure in a twin than their single equivelants due to the added complexity of fault diagnosis and the larger choice of flying options. However, high hours/well practiced twin pilots were less likely to have a serious accident. I have to agree with MightyGem, crew training in twins is vital.
neverinbalance: Quite right, the Chinook does have the most technical failures of any RAF helicopter, although since the Mull crash it certainly doesn't have the worst safety record. The technical failures are, however, rarely engine related. In four years on type I have had one precautionary shut down, and heard of a few others, but can't remember one failing.
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Through S**t to the Bar
neverinbalance: Quite right, the Chinook does have the most technical failures of any RAF helicopter, although since the Mull crash it certainly doesn't have the worst safety record. The technical failures are, however, rarely engine related. In four years on type I have had one precautionary shut down, and heard of a few others, but can't remember one failing.
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Through S**t to the Bar