ILS Outer marker check..What do you do?
 

I am hoping I can get some help on this topic. Recently the Aussie AIP changed the requirement to check the a/c's altimeter at the outer marker. It used to be that if your altimeter read less than the outer marker check height, you were clear to desecnd to DA, and if the altimeter was high, you added the difference to the DA.
This guidance has dissapeared.
I understand that Australia is/was the only country to do this check. I have heard it was carried out for a few reasons, some being: to check the altimeters accuracy, to check the G/S has not been 'tampered' with and to confirm the receiver is getting the correct lobe.
Our flying school is trying to write some guidance on what to do at the OM check and I would like to find out what the rest of the World is doing.
Can anyone give me the background behind this check, and also tell me what they do at the OM?

Thanks is advance:confused:

We do the following:

a) Check the OM alt vs. the approach plate alt (+/- altitude correction which depends on the given temperature on that day, this having been done during the briefing)

b) Check/confirm the MDA (CAT I) or DH (CAT II/III)

c) Check that the correct Missed Approach Altitude is set in the Altitude window of the altitude alerting device at hand.

d) be configured properly for the landing (pitch/power/speed/config etc...)

Oh yes, the altitude difference (temperature above/below ISA) one might have at say the FAP/OM becomes less and less as one approaches the MDA. How much less depends on the actual difference and is not that easy to quantify me thinks. DH is a different matter as one is using Rad Alt. If one has RA then use it during the approach to compare the DME with it. Obviously the DME is not always at the threshold but it gives one an idea of whether the baro altimeter is functioning properly or not. (+/- 300 ft per DME)

Where I teach we do not have radar altimeters and all the goodies some other aircraft have. What we teach is to crosscheck the indicated altitude against that published on the approach plate and to start timing the approach. The reasons are as follows:

The height check is to confirm that the correct QNH or QFE is set. It would be a bit eye-watering to attempt to descend to DA with, for example, 1013 set instead 0f 1003.

The approach is timed as a precaution in case the glideslope is lost. The approach can then be continued as a localiser only procedure. On the majority of such approaches which do not have DME the MAP is defined as a time from the OM, hence the timing.

I'm not at all sure about the comment by Square Leg in the previous reply about MDA for a Cat 1 approach. I thought MDA was for a non-precision approach (VOR, NDB). Any full ILS is a precision approach which includes glidepath guidance and therefore is flown down to the Decision Altitude/Height.

This is a vital question, but incredibly there's a big black hole in the aviation culture on this subject.
I have personally noticed that the majority of the colleagues I have flown with didn't know how to check the glide slope correctly.
Even some directives from the manufacturer or other authorities lead in the wrong direction.

The problem is: how to check I'm on a valid glideslope.
To my knowledge, nobody has an official rule to refer to for doing an altimeter accuracy test at the outer marker in order to make a correction at Decision Altitude.
The tolerances and variables in real life are too many to specify exactly what is the maximum acceptable altimeter reading error.
Everybody uses good common sense for accepting/refusing the altitude value they read.
If it's colder than standard, you can expect to read higher than published, for example.
Anyway, altimeter accuracy is not the point during the approach (you have checked it before departure, you've completed a whole flight without complaints from ATC, etc...).

The point is checking the glideslope.
You can leave your safety altitude only if you make sure the lobe is correct.
Various fatal accidents have happened because people failed to do this check correctly .

What most people think is the correct method is: "When I overfly the outer marker, I must be at 1300 ft."
BANG! DEAD!
The truth is exactly the opposite!
"When I descend through 1300ft, I must check the outer marker is there".

A simple point like that seems to be so hard to understand, sometimes!
If you are on a wrong low lobe, or if your instrument gets stuck, with the first incorrect method you are gonna wait for the outer marker to do the altimeter check.
You you are gonna find your a§§ on the ground before you ever reach the marker.
Instead, if you do it properly, at 1300ft you expect to hear the marker: if not, at 1200ft you are gonna start worry seriously, at 1100 even more, at 1000ft you get the hell out of there!

It's amazing to see people calculating exactly, with their own formula, hom many feet deviation they will get with ISA minus 7°C, waiting for the outer marker in order to do a meticolous check, instead of waiting for the altitude to do it!!

The above is true, of course, also in case the official glide check is by the DME instead of the outer marker.
"At 1300ft I must read 5 DME" and not "At 5 DME I must read 1300ft".

Got it? I hope so! :ok: :ok: :ok:

LEM

Thanks LEM, you just changed my personal SOP s

Oz didn't always require the altitude difference at the OM to be added. That was brought in ~8-10 years ago. As has been said, the OM check is to confirm that you're on the correct lobe: The GS should agree with the dist/alt or OM/alt. If not then you have to consider you're on the wrong lobe.

Just to express myself more correctly, in case the values disagree by a large amount, make sure your altimeters are correctly set of course, but this is common to both methods (the right one and the killer one).
LEM :ok:

Finish my coffee.

Very nice reply! Some sugar, maybe?

Just one, thankyou.

Call the tower...and hope they reply.
Oh yes, and wake the First Officer up....;)

One particular airline that I recall, used to insist that at BOM, the OM was crossed at the charted altitude, irrespective of anything else....temp, for example.

Sooo, the guys did this unfailingly, until one poor chap landed long (poor speed technique, steep glidepath) and ran off the end.

Henceforth, this policy was abandoned.

Good common sense should prevail, many times it does not, especially with some instructions from the head shed.

Dan & 411, forgive me for not being on your frequency, but not everybody in the world belongs to your Saxon culture.
I really can't see your point (coffee, calling tower etc...)
Would you please use plain language so everybody can understand your point of view, if you really have something to add?
Muchas gracias
Spaciba bolchoi
Grazie mille!
"£$%&^ !

LEM

It is less Anglo Saxon culture than a sense of humour.

Let me translate what they are trying to say...albeit too subtley for the humourless. Any approach requires a liberal application of commonsense.

Ah, I see the light, now!
Thankyou :}

Our outfit does it with a simple "Outer Marker - Altitude Checks" callout. PNF makes the call and both are to check actual vs. planned/charted altitude.

This report will make the hairs on your neck stand up and the reason for the importance of this check (Big file).

http://www.caa.govt.nz/Occurrences/00-2518.htm

Thank you all for your replies. LEM you have given me food for thought, and might I add that is the best explanation regarding the ILS OM I have heard. Thanks again:ok:

LEM - an interesting point. Thinking about it, it fits with the maxim about "aviate, navigate, communicate" - the altitude, then the distance, then anything else. The logic is good too - your way means starting with the facts, then checking the plan rather than checking the plan and then checking if reality fits it (which can turn into making reality fit the plan).

Re : the CAA report
What did I tell you?
Why in the hell so many people don't know how to check the glide?
What do we need to change this philosophy in our textbooks?
Other crashes?

It took only 15 pages out of the 200+ to find what I was looking for:

" ... If terrain had been a factor during this event and a marker type ILS was in use, the first available checkpopint may have been too late to prevent a CFIT event. "

OF COURSE! IF PEOPLE WAIT FOR THE MARKER TO CHECK THEIR ALTITUDE, THEY ARE PLAYING THE RUSSIAN ROULETTE!

:yuk: :yuk: :yuk:

LEM, your philosophy is correct but you are not by any means, the only one putting it into practice (hopefully). Have you ever made an IGS approach to RW13 at Kai Tak? My logbook is full of them and we managed never to fly into the checkerboard before the move to Chek lap Kok.

Oh gosh! I thought I was the only one in the world to be on the right side! :{


Hod Dog has arrived!!!!!!!!!!!!!!!!!!!!!!! :rolleyes: