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ILS Outer Marker Check heights

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Old 20th Aug 2002, 13:44
  #21 (permalink)  
 
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Creeeeeaak. (sound of can of worms opening!)

Chimbu. Not commenting on RAAF policy at all, just attempting to define the un-definable -- 'significantly off glidepath'.

Besides, it's a well known fact that we have larger than average needles at P-3s.

ruprecht.
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Old 20th Aug 2002, 14:13
  #22 (permalink)  

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I think too much is being read into the OM altimeter check...sure if you forget to do it all then your in a bind....but on a typical day ISA is just a theory and therefore the altimeter will never say the exact check altitude.

In 9000 hours of IFR RPT & Charter I've never seen it.....usually in Northern climes you're most likely to see it underread by 70 to 100' at the OM...so you're a little high on GS at that instant...perhaps even 1 dot (not me you understand )...just get back on slope and get on with it.

I suppose down around East Sale it's probably ISA a lot more often...but still common sense must prevail.

For my money if the candidate makes mention of the approximate temperature deviation he expects to see, PCE if required on that aircraft, flies a nice controlled ILS within tolerance and completes the OM check (with any sensible correction if altimeter is indicating higher (very rare in Oz) then he just earned 9.8 out of 10. Few of these mental gymnastics happen while flying the approach...they were taken into account prior to TOPD.

As for syn angles...give me a break.

Lets face it 99.9999% of the time the only way you'll see a significantly 'overreading' altimeter at the OM is if you forgot to reset it... and the temperature correction required reduces to a value so small as to be almost unreadable on the altimeter unless it's arctic outside.

The aim of the game is to get to the published DH safely and land...not dick around in IMC.

IMO.

Chuckles.

Last edited by Chimbu chuckles; 20th Aug 2002 at 14:32.
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Old 20th Aug 2002, 16:34
  #23 (permalink)  
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There is an important reason for doing the altimeter check - The altimeter may be in error!

Of course, you checked the altimeter(s) when you taxied out and they were within the 60-75 ft limits set by AIP ENR 1.7 p1.2

But, what has happened to one of the altimeters during the flight? Does it have the same error as you saw on the ground? Which altimeter may have the "increased error" anyway - the pilot's or the copilot's?

You cannot tell what is going on and which altimeter seems to have the "increased error" until you do the altimeter check during the ILS approach.

The AIP states that you MUST conduct the check.

Therefore, a common sense interpretation of the requirements is that, if NOT on glideslope at the check point, you must proceed with the LLZ approach, or, conduct the missed approach and have another go.
 
Old 20th Aug 2002, 20:39
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OK, so let me get this right. Although a limit of 1 dot is quoted as being within limits for an ILS approach ( I guess someone really smart thought of this because they thought pilots were human and not triple autopilots handeling the fly-by-wire computers), the new kids on the block are now saying that if you are outside one needel width!!!! you have to fly to the LLZ MDA????? Is the one dot limit going to be amended?
I sure hope the you never fly to anywhere near CAT 1 min, as you will be shooting aproaches all day. Best you allow NO.1 to be loittered for ILS approaches to save gas as you will need it.

It is true that an altimeter may work on the ground but may fail in flight. But I guess it can also be argued that although the gear retracts OK, it might not come down again, but do we fly with them down all the time?

As said before, there can be lotsa things to influence the ALT reading at the OM check: QNH, Temp are the main ones I guess (not withstanding mech failure, rat infestation etc). If you fly somewhere really cold, you have to make the PEC corrections (Is that the same for the OM and DA?), if the temps are above ISA temp error is not a problem. A QNH error should be seen when comparing ALT Vs DME (hitting the glide at 3000' and ten miles). The OM check will check both. If you are slightly above or below the glide (ie within limits and correcting!!), and the crossing ALT is sensible with relation to the check height (IE: in the correct sense, and not too excessive), I would suggest you could continue to the ILS min.

Seems it might be a safe bet to become a TACAN specalist Fishy.
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Old 20th Aug 2002, 22:33
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If this correction is so important, why isn't the rest of the world doing it?
Cant remember F.A.A. but JAR does a final fix check to determine, as far as I can tell, that you have the correct ILS tuned and have not captured a false G/S, ie; a gross error check. No adjustments to minima ever prescribed. If it doesnt look right you get out.
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Old 21st Aug 2002, 00:26
  #26 (permalink)  

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Perhaps all those AN pilots who've just gone back into the RAAF should be made IREs....seems the system may have become a little inbred!

L84wrk,

In the northern hemisphere where it can get really cold (i.e. JAR, FAR country) if it's significantly colder than ISA you use a table to correct all ' IF app' type altitudes, MSA/LSA/minimas etc. If it's not cold then you do nothing (just like in Australia).

It is just a gross error check as you say,however failing to make the correction for ISA devn (if required) is a fail on your IRT in places like Canada, for instance.

PEC is of course a different thing again and is an allowance for position error in the probes...the flat +50' only being required if the aircraft has not been tested and an actual number found...wasn't required at all on the Falcon I've been flying the last few years.

Donpizmeoff,

Your, presumably rhetorical, question about the ISA correction being the same at OM as DH is the point I was trying to make with my question about the 'just blindly add the difference at the OM'. If a chap doesn't understand what it is about he might see a difference of +50' at OM caused by bloody cold weather and just add it to the DH when the correct correction might only be 10'...and therefore hardly worth worrying about except on an IRT

You'd be amazed how many pilots, including Long haul wide body Captains, who have no idea about it, or insist that Boeing's ADCs allow for it automatically


Chuck.

Last edited by Chimbu chuckles; 21st Aug 2002 at 00:46.
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Old 21st Aug 2002, 00:37
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Jeez, Don! With all that sand over there, some of it must have got into your britches!

If you cast your mind way back, you'll remember that the glideslope needle in the old bomber is just under half a dot wide, so I think that this would count as being on glideslope at the OM. I'm guessing that your glideslope needle on the 777 may be a little smaller than that!

I'm not advocating being within this tolerance down to CAT 1 minima, just at the OM for the altimeter check.

It is a grey area. Do operators overseas use the OM check height at all?

r
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Old 21st Aug 2002, 02:49
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My two cents,

No not even in any way is it a gross error check.

You must cross the outer marker (preferably DME height check) on slope.

If say the check height was 1500' to a minima of 200' then you would then be descending a further 1300' from the OM/DME check to the 200' minima. If you crossed the check height at say 1600' and descended to 200' indicated, then you would have infact descended a further 1400', in reality leaving you only 100' AGL at minima. This is not the intent of the CAT 1 ILS. The 100' high should have been added to the minima making it 300' indicated. You would still descend 1300' from the check position and would be at the designed 200' minima above runway.

Temperature corrections etc should have been made to the minima prior to the start of the approach. To think you could reliably do this math during the approach is unrealistic particularly after a very long day. Hopefully though we can all still add the few feet that may be required.

If you are off slope at the marker/DME check, use the LLZ minima.

Not suggesting you shouldn't use the thick black pencil though, but that I believe is the rules.


Now who is up for a discussion on the application of catagory speeds?

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Old 21st Aug 2002, 03:02
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Mud Skipper thats how I remember it as well.

Also lets not forget the NZ 767 that had a false gligeslope capture somewhere in the Pacific - glideslope checks are very important.
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Old 21st Aug 2002, 11:03
  #30 (permalink)  
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Umm, I don't think so, Muddie. Using that same reasoning, the altimeter would be 100' out on landing, in which case it/they are WELL outside tolerance - unless you had inadvertantly set the wrong QNH.

The discrepancy at the OM is adjusted relative to actual airport elevation, meaning that a difference of (eg) 200' @ 1500' does NOT translate to 200' @ S.L. In fact it would be less than 20'.

So anyone who's interested in making these corrections should first check'
(a)the known altimeter error PRIOR to take off:;
(b)the PRECISE location of the OM vs DME;
(c)the DME caibrated error;
(dISA deviation at the OM;
(e)the EXACT QNH at time of passing the OM - and not the QNH at the landing airport;
(f)the parralex error due to seat position.

And just PRAY that the QNH is not even ONE mb/hp out!

Time to give it away. If there's a gross errror (spot that one?) then it's good to know WHY. But if you've done a thorough pre-flight, and followed up enroute as a PAID PROFESSIONAL will, you'll have earned your bread if you can pre-empt the OM crossing height and respond with a confident "Checked"!
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Old 21st Aug 2002, 11:28
  #31 (permalink)  
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rescue 1's comments lead to some not so well known information ..

Say what you will about ANZ but their safety culture is pretty much above board from what little I have observed. The Apia incident was thoroughly investigated .. not to cane the crew ... but to get at some of the underlying tech problems which helped the crew to set themselves up for a fright. One observation was that perhaps a number of accidents in the past might have involved similar sorts of tech problems which, while well understood by the elec/radio techs ... are generally not appreciated or known by the flight crews.

One of the main things to come out, as I understand, was a very definite tale that the ILS is a dinosaur bit of gear which works well most of the time but has a number of potential traps for young players .... it is not a bad idea to make sure that the aircraft is fed into the tube where it should be ... and that any available checks are kept an eye on down the ILS to check for any significant deviation from where you thought the aircraft might be ... markers, DME, whatever ....

I have seen a number of confused crews in sim exercises when the finger in bum, brain in neutral thought process couldn't resolve the lack of correlation between what was indicated and what was expected .....

Last edited by john_tullamarine; 21st Aug 2002 at 11:34.
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Old 21st Aug 2002, 11:38
  #32 (permalink)  
 
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Cougar:

I have heard mentioned that if you miss the Check you can go off the DME heights on the bottom of all RAAF Terminal plates but i have heard this one shot down by numerous people since and therefore do not use it myself.
Not sure on what basis people are shooting this down. For example, there are clearly airports where finals is flown over water, and it is quite impossible to install a marker beacon. In this case, there is no alternative but to use a DME distance vs height. Also, as some have already pointed out, this is a lot more accurate than using the beacon exclusively. Depending on the sensitivity of your marker receiver, your "overhead" of the beacon can be pretty nebulous.
All that is required (straight out of FIHA) is a "fix or facility that permits verification of the glidepath-altimeter relationship". Clearly, DME vs height is such a fix. How late you leave it is a point where there could be some debate. If you miss the OM at say, 6nm from the threshold, and don't get a check until 1nm, then you've perhaps left it a little late.
I think Chuck hit the nail on the head a few posts back - just carry out some sort of check, and make a commonsense correction if required. We are kind of "measuring with a micrometer and cutting with an axe" here, after all.

Have fun on your IRT Fishhead. Do they go for 12 hours too, just like real missions??

2 rules for IRT's:

1) 3 minutes of injudicious afterburner usage during an IRT means fuel for 1 less approach!

2)Always get the 30 day extension from your CO. That way, in a 12 year flying career, you'll do one less test than everyone else, and that's gotta be a good thing!

Cheers,

SW
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Old 21st Aug 2002, 14:08
  #33 (permalink)  
 
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Good stuff!

So if you are on slope and you check at the outer marker (albeit with the DME for accuracy) then you have checked the altimeter setting and made the correction, if any.

Why then do you need an addtional 50' PEC (assuming no AFM correction is available). I would think that you have checked the altimeter and the setting and have cross checked it with DME Marker etc.,

Doesn't that do away with the 50' add-on?

Remember the glide slope is monitored to .1 of a degree and if out of tolerance the stanby one activates automatically....?

Just thought I would ask?
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Old 21st Aug 2002, 15:55
  #34 (permalink)  
 
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Ruprecht,

Took your advice and removed the sand. Mate, the itch went as well!!! You are good. Did you fail young fishy?
Don
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Old 21st Aug 2002, 23:51
  #35 (permalink)  
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Wow.... I almost generated as much heated debate as the time I asked if anyone had seen the NAS proposal last year!

Many thanks for everyone's input... I guess the one thing it really showed is that there does not seem to be a recognised 'standard' when it comes to this question - perhaps that's because it really is in the 'niff naff and trivia' section. But I believe that it's not, if for no other reason than for the reason of checking that the correct QNH is set. Let's face it, the Missed Approach point for an ILS is an altitude after all, not station passage, so it's pretty important to get it right.

Swingwing:
Have fun on your IRT Fishhead. Do they go for 12 hours too, just like real missions??
Nah, more like 6 hrs.... just enough time to qualify for a chocolate bar in the inflights!
1) 3 minutes of injudicious afterburner usage during an IRT means fuel for 1 less approach!
The idea never occured to me! Will give it a go........ it might scare the Flight Engineer though!
2)Always get the 30 day extension from your CO. That way, in a 12 year flying career, you'll do one less test than everyone else, and that's gotta be a good thing!
Don't forget, go see the OC and you can push that out to 60 days!

Don:
Ruprecht hasn't failed me yet.... it's next Tuesday for the test. He wouldn't have the guts to fail me anyhow....

All in all though, since it's Ruprecht that's my IRE, then I guess my answer to his quiz question will be "convert to a localizer approach, oh god-like IRE person"

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Old 22nd Aug 2002, 00:27
  #36 (permalink)  
 
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Fishy. Since it'll be my IRE renewal with CFS, I think I'll steer clear of such questions as CFS are bound to have their own interpretation.

How about: " so.... when can we descend below MDA on this circling approach?"

Just remember, if it comes down to my IRE or your IRT, your odds don't look good!

r
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Old 22nd Aug 2002, 10:57
  #37 (permalink)  
 
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AH
The old 1 in 4 chance of the candidate passing
I luv it
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Old 23rd Aug 2002, 07:20
  #38 (permalink)  
 
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Trying to figure out who Fishy and Rupy are. If Don has something to do with it you must be "Brand X".

If you are "brand X" then doesn't the new bomber have the more officer like equipment installed like glass, FD and a new AP that does it all for you.

Take my advice opt for another emergency rather than raw data, ask the IRE for a double assy app, that way you won't have to worry about being asked about OM check heights while you are helmet-firing with the double assy app !!

Hand flown raw data approaches are not authorised after any more than 6 hrs of flight time !
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Old 24th Aug 2002, 15:00
  #39 (permalink)  
 
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Folks,

While noting that this is predominantly a RAAF debate, it is a trifle concerning that the micrometer brigade have launched a take-over for the wood yard.

As an SIRE in a previous life, I used to worry about the brain power that went behind the "one foot below MDA and you fail" brigade, especially when applied to wonderful technologies like the digital readout in the F18 HUD where the actual level -off became a lottery simply as a consequence of display accuracy and system errors. Now I see it again in definitive "you must cross the OM on slope or modify/abandon the ILS" type statements.

Perhaps some of you should look upon IRTs being a check of the candidate's ability to recognise and compensate for his/her human failings while using a fairly average mechanical/electronic representation of the real world in a safe and consistent manner, rather than the alternative theory that an IRT is a legalised torture session designed to prove how clever an IRE you are and where the candidate is only there is confirm your self-assessment.

The rule is there to provide additional safety guidance, along with RADALT settings, SOPs, recency, etc. Half FSD is an IRT tolerance that reflects reality - as do correcting limits versus hard limits - and the obvious vagueness of marker beacon signals means that compensating your check altitude for temperature and actual GS indication is not inconsistent with the overall concept of a gross error check on the GS/altimeter relationship. In any event, PECs and QNH errors will ensure that the crossing altitude check will never be as precise as the charts indicate.

Standards are actually raised by encouragement and confidence building - not the opposite. TLAR (that looks about right!) is a sensible approach to OM check heights - if it doesn't look right, then protect yourself accordingly.
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Old 25th Aug 2002, 01:54
  #40 (permalink)  
 
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4dogs

The Wise Owl Award is bestowed.

As ever wise and directly to the point.

Well done guys, some excellent posts and some serious revision of the
'niff naff and trivia'
that I suspect a lot of us have either forgotten or ever really got quite right.

Good luck on your IREs.
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