ILS Outer marker check..What do you do?

[Orion1]

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:

[square leg]

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)

[machonepointone]

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.

[LEM]

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

[jtr]

Thanks LEM, you just changed my personal SOP s

[Tinstaafl]

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.

[LEM]

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:

[Dan Winterland]

Finish my coffee.

[W.SHIRRA]

Very nice reply! Some sugar, maybe?

[Dan Winterland]

Just one, thankyou.

[411A]

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.

[LEM]

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!
"£$%&^ !

[Traffic]

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.

[LEM]

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

[Shore Guy]

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

[Orion1]

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:

[steamchicken]

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).

[LEM]

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:

[HotDog]

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.

[LEM]

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


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

[HotDog]

Oh that sharp Gallic wit is ever so cutting!:rolleyes:

[Suffering Sucataash]

LEM,

What can I say, I don't agree with you.

Now I expect you will tell me of the thousand ways in which I could have killed my tens of thousands of passengers as I don't agree with you.

We'll I'm downloading the NZ report and will have a carefull look, but I'm sure I know my 3 times tables and would pick up the wrong lobe well before the OM check.

However if I am at "..."DME/OM and on the correct G/S and indicate say 200' above the check height and don't correct my minima, then I'll hear the other bloke say "DECIDE" and touch down, or worse, at the same time.



I'll have a read and get back to this topic.:suspect:

[Dan Winterland]

I was an instructor in the RAF when they decided to switch from using QFE as a landing datum to QNH overnight. I won't go into the numerous details of the arguement except that it was not very popular and was eventually changed back - except that now the radar circuit was flown on QNH, QFE being set by the pilot and setting confirmed by the controller on final approach.

Soon after, a mate was flying an instrument instructional sortie making an approach to a local airfield which was some 200' above our base and had cloud cover down to the deck. (Quite legal under the RAF rules at the time providing you planned to go around at DH). At about 300' on the Alt, my mate noticed the runway threshold just underneath the aircraft and made a hasty go around. He had not set QFE and the controller hadn't confirmed it.

In the subsequent incident report, he freely admitted that if he had done a check at the outer marker, the error would have been picked up.

So as well as finishing my coffee ;) I always check the height and make sure the GA alt is set in the alt window - as per the company SOPs.

[Chimbu chuckles]

Orion 1

While I agree that this subject is a big black hole it is really very simple.

Alt checks at the OM/DME distance occurr for the following reasons:

1/. Gross error check in case of incorrect subscale setting,
2/. To catch any instrument errors that have not been allowed for,
3/. Yes if you try and capture GS from above you may get a false GS...never seen it myself but there you go.

1/. is fairly rare in two crew ops as most have sufficient altimeter cross checks...but it can happen.
3/. is again very rare

The biggest uncorrected 'error' you will see in day to day ops is caused by ISA Deviation. The reason the old rule stated if the altimiter was undereading ignore it was because that is safe and essentially, in ISA+ Australia, going to be the case virtually every ILS you ever fly...with the possible exception of Melbourne/Hobart in winter.

Pressure sensitive altimiters are not corrected for air temperatures which deviate from ISA. This correction must be done manually and the correction applied to all IAL heights/altitudes including MSA/LSA but not to assigned altitudes, where ATC should make the allowance for you:hmm:

I'm not going to pull out the ISA correction tables in my Dassault Manuals tonight but the rule of thumb I use is

+/- 4' per degree of ISA Devn x (Alt/1000') and that agrees with the tables to within a few feet until the temperatures get antartic cold and the height above the datum gets very high indeed.

Alt = height above the temperature datum, i.e. the airfield.

On an ISA+20 day in Australia sliding down an ILS past a check height at say 5DME/1300' to a MSL runway you would get;

-4x20x(1.3)= -104'. All other things being equal you would expect to see 1200 odd feet a 5 DME if you were on GS. Of course altimeters aren't perfect so there may also be some position error, which you should know and therefore be able to allow for, and perhaps some minor/in tolerance mechanical errors too....essentially though if you were indicating 1200 odd feet at the OM you could confidantly say "Outer Marker check altitude OK" or whatever your SOPS call for.

At the minima...say 200' the error would be -4x20 x (.2) = -16'. Hardy worth worrying about in any real way given the other errors, including parallax, but in an otherwise perfect world at 200' indicated your radalt might say 216'...no biggy as I say.

It is all opposite in temperatures colder than ISA...in the above example using ISA-20 you would expect to see approx 1400' at the OM but you can also see the mistake in just 'adding the difference' as the old rule stipulated. At the DA 200' your radalt migt say 184' which is a bust....so for a checkride I'd be adding the 16'...pedantic I know but if you didn't at least discuss it I'd fail you!!

Where it gets real interesting is high LSA/MSAs...say for instance MSA 10000', airfield elevation 1000', ISA-20. +4x20x(9)=+720'...that's how much you altimeter will be in error...putting you possibly only a few hundred feet above the highest obstacle within that sector.

Not being able to intelligently discuss the above for an Instrument Rating Oral in places like Canada would end the Test.

All the above does NOT mean getting paranoid about 20' here or there...it means prior to TOPD you have a good idea of what altitude you expect to see at the OM/DME GS Check and if the altimeters are close then that's good enough...have a coffee and slide onto to the minima:D

Chuck

[Traffic]

Chuckles

As always, your one thousand words paint a very clear picture indeed.

I notice you slashed OM/DME together which is really the essence of this debate I would have thought.

The OM alt check should be merely the confirmation of the comfort zone you should have got yourself into a lot earlier, perhaps as the coffee was coming through the door.

All those mental arithmetic commonsense checks one learns along the way are the circuit breakers designed to save your bacon. Just because a screen tells you one thing, it does not mean you put your thumb in your bum and your brain in neutral the rest of the way down the GS.

[LEM]

The Air New Zealand incident, like many others, proves more than one point:

Despite our mental calculations- times 3 arithmetic, etc., people still bust the correct altitudes during the final approach, because they don't do a proper glide check.

Capturing a false glide slope, or experiencing wrong indications on our glideslope indicators, is a rare event (never had one, personally). But it's still possible, and it is the rarity of the event which makes it so dangerous.

Distance indications are not always available, so it's not always
that easy to make a X3 mental check.


Let's have a look at the Faleolo incident, then at the Alitalia crash in Zurich, 1990.
Both of them did not check the glide properly: the 767 crew went around at ~400ft, (!), after the whole approach screwed up!
Had they called, loudly ,"At 2500 we must be at 7.5 DME" and "At 2000 we must be at 6 DME" etc, they would have been totally safe, instead of waiting for the DME to check their altitude: they happened to be at 900ft. at 8miles, and at ~400ft. at 5.8 miles, very close to Apolina island!
I'm aware that such callouts seem so obvious, so basic, so natural... but if you make a habit of thinking that way, and declaring loudly "2500, 7,5 DME, Glide Checked ", then you are safe on every approach, where the numbers are not so straightforward.
In 1990 an Alitalia DC9 crashed short of rwy 14 in Zurich: they did not check the glide, and flew the approach 1400ft parallel and below the glide.
They were waiting for the outer marker to do the check ("Il marker é passato?"), and they didn't realise they were low.
It's easy to say, from behind a keyboard, they missed the obvious, that they should have made a constant X3 mental check and so on.
Truth is the conditions (from a human factor point of view) were bad, the elevation was high, and so on... and they died. In the cockpit, when things go to worms, our brain decreases its abilities to 80, 50, 30%..... even if we don't like to admit it.
So we must adopt a fool proof method, wich GUARANTEES no mistakes can be done, even by retarded people!
Only then we won't experience anymore accidents which seem so stupid, but unfortunately fill our statistics, like the Faleolo or Zurich or Guam, and the list is looong.

Please don't get me wrong when I say you must check the DME or marker at the altitide instead of the opposite.
It's just a way of thinking the check which saves your sweet a§§.
Actually, when you make sure your glide is correct, you simultaneously check the altitude accuracy.
And you can make or confirm all the corrections you like due to temperature and whatever.

And now we have come to the second interesting point: do we correct or not for the temperature?
As C. Chukle has so rightly pointed out, if we fly in extremely cold conditions, and at altitude, like Calgary, then we must correct our minima.
The 4XaltXISA deviation rule of thumb is well known and still valid if we don't have tables or more accurate means of correction.
In Calgary the error could be very significant.
Here are a few interesting articles on the subject:
http://www.bluecoat.org/reports/Long_98_Cold.pdf
http://www.fmcguide.com/media/calgary.pdf
Academically, we should always correct, even at sea level, but there don't seem to happen many crashes because we decide at 184 feet instead of 200.
Anyway, this is really a big black hole in our culture, as it seems only Canadian ATCO make allowances for very cold temperature when radar vectoring, and even the FAA is dragging behind trying to establish new safer rules in North America.
Also, if we consider that at despatch an altimeter is considered acceptable with an error as big as 75ft (if my textbooks are still correct), and on a B733 the max allowable difference between altimeters at sea level is 50ft, then we realise the Gods really love us!

Is the average pilot really safe, or just a bit safe and a lot lucky?
I suggest a new rule for establishing when to decide at the minima: At the corrected barometric decision altitude/height, or at 200 ft radioaltimeter, whichever comes first.

[safety_worker]

Whilst on the subject of temperature corrections.

Consider minimum altitudes in 'cold' regions.:( (basically at altitudes)

Does anyone out there have a table for regions / climatology (coldest temp for a time of year) / correction chart?

Do companies prescribe minimum flight altitudes for certain areas at certain 'cold' times?

What are your policies on minimum altitudes?

safe wishes.:ok:

[LEM]

I'd like to pull Boeing's ears on this:

Landing procedure
[...]
At final approach fix/OM, verify crossing altitude.
(B733 NP.20.31)

My congratulations to Boeing for upholding this killer mental approach.


LEM

[big pistons forever]

As a newly minted IFR pilot my first approach in IMC weather was to CYXX. I was in a PA34 and was vectored to final between the IF and the FAF ( outer marker ) for the ILS. The approach has no DME so I did not know exactly how far back I was but rapidly became suspicious because I was quickly approaching the glide path check altitude with the glide slope needle still full down. I went around at the check altitude with the needle still full down, diverted to better weather and did a LOC only approach. The radio shop subequently confirmed the glide slope had failed with a permanent full down indication and no flags. ( and yes I checked it on the ground before takeoff ). THAT needless to say made me a big believer in Glide path check altitudes:ok:

On a seperate topic I will not allow an approach to be changed after established on the LOC/in bound track ( in IMC ) on any airplane I am in, Therefore the practice of timing the FAF to MAP segment is a waste of time. If the glide path fails I/we would immediately start a missed appoach, make sure nothing else is wrong and then rebrief the approach we are now going to fly. Changing the approach type from the one briefed and the one you are mentally up to speed inside the outer marker strikes me as introducing too much risk for the small time saving gained.

[Sick Squid]

Safety-Worker,

My company prescribes a formula of 4% errror per 10 degree drop below ISA of the mean temperature layer below the aircraft (in reality we would all use ISA at altitude for that....) Therefore for 10,000 feet indicated with a temperature of ISA minus 30, you would find your altimeter overread by 1200 feet on that formula.

In addition, where the wind at the relevant MSA is forecast to be 50 knots or more, we increase the MSA by 2000 feet. In mountain wave conditions, then a vertical clearance over the highest ridge at least equal to the height of the ridge above the surrounding terrain should be selected. Memories of the Mount Fuji 707 I think...

On my type (320) inthe sphere of operations we use then these are generally of academic interest... however, I still mention the drift-down criteria as a routine in my emergency brief because the one day we will be caught out is over the alps, in a chilly northerly airmass with very strong winds, and only just make the criteria. Also, it is a self-discipline thing not to forget it, having flown big twins (and big 4-engined as well) over mountainous terrain with MSA's well above the drift-down alt regardless of temperature corrections.

Reverse will also apply.. I well remember first time flying into the Gulf and noting the point at which the rad-alt came active being well different to what it would be in Europe, all down to density altitude.

Squid

[Aslan]

There are often approaches around where the OM check requires the use a DME distance and altitude instead of the beacon.

Following the logic of LEM, is there a rule for @ OM altitude, e.g. if DME dist > then given OM dist then .....,
if DME dist < then given OM dist then ....? I can't figure one out.

While my logic may be fallible, I seem to find that if a DME dist > given value at the spec OM alt., it can be either

1) flying down the lower lobe of the GS (definitely not safe) or
2) flying with OAT > ISA (which is OK)

Can somebody check my logic here.
thanks

[gliderboy]

LEM

In China many OM's are at 800' (PEK is an example). It may be a bit late to verify if u r on the correct lobe.

I think the old dme/alt gross error check (say at 10 dme ) is the ball park check for lobe (or say 6 dme and 1800') and the OM verifies the G/S more accurately wherever the OM may be.

I think a bit of common sense needs to be applied here.

(prepares to get shot down.......)

gliderboy

[FlexibleResponse]

The point is checking the glideslope.

LEM is correct.

Consider the case of a near CFIT caused by flying a G/S that was left in the “test position” in the edited quotes in the following report (link to full report supplied by Shore Guy above).

CAA NZ

AIRCRAFT SERIOUS INCIDENT REPORT
OCCURRENCE NUMBER 00/2518
B767-319ER
ZK-NCJ

Synopsis
Early on the morning of Sunday 30th July 2000 the Air New Zealand Duty Line Manager was notified of a suspected false glideslope capture experienced by Air New Zealand flight NZ 60during approach to Faleolo International Airport, Apia, Western Samoa.
NZ 60 had been cleared to Faleolo via a FALE arrival for an ILS runway 08. The approach was planned to be an auto coupled ILS, using a low drag approach profile. During descent the aircraft was established on the 15 nm arc as per the STAR procedure.
Approaching the localizer course at 2800 ft LOC was armed, and the auto flight system subsequently captured the localizer inbound course. During the turn on to the localizer the aircraft was decelerated and configured to Flap 1. APP was armed after localizer capture and the auto flight system captured the glideslope shortly after. The crew reported a rapid energy increase, with speed increasing to near the flap 5 limit speed. To assist with energy control, while continuing to configure the aircraft for landing, the crew used speedbrakes and landing gear. The flight instrumentation glideslope deviation indicators displayed ‘on glideslope’ throughout the approach. Shortly after landing flap selection the PF (Pilot Flying) noted an anomaly in DME versus altitude. Around the same time the PNF (Pilot Not Flying), while trying to establish visual contact with the airfield and runway, became aware that visual cues did not correspond with what was expected. The SP (Supplementary Pilot) also became aware of an anomaly in aircraft position at approximately the same time as the two other crew members.
A go-around was commanded, initially climbing straight ahead followed by a climbing left turn, to pick up the 340o radial FA VOR to rejoin the 12 nm arc for a subsequent approach.
This second approach was flown with careful attention to distance and altitude, using the
published DME recommended altitudes as per the LOC (GS out) table on the approach plate for glide path management. The glideslope deviation indicator also indicated on glideslope throughout the second approach. The glideslope indications were ignored and the approach continued to a successful landing.

Subsequent analysis of the FDR information established that the aircraft had descended on a glide path of approximately 3.5° to a point approximately 5½ miles short of the runway with ‘normal’ localizer and glideslope indications displayed on the flight instrumentation.
It was later established that the ILS glideslope transmitter had inadvertently been left in
control (monitor) bypass mode, with the unserviceable transmitter selected. In the bypass
mode, the glide path transmitter executive monitor was unable to shut down the faulty
transmitter or to transfer to the serviceable transmitter. The result was the radiation of invalid glideslope information consisting solely of the carrier plus side bands (CSB) signal component. The side bands only (SBO) signal component was missing from the glideslope transmission.

The distinction between a ‘false’ glideslope and an ‘erroneous’ glideslope must be
appreciated:
__ A false glideslope is a recognised phenomenon and is a normal by-product of the ILS
transmission. A false glideslope provides a distinct, but incorrect, path to the origin of
the glideslope.
__ An erroneous glideslope, however, does not provide a defined path. Whilst continuously indicating to the crew that the aircraft is on slope irrespective of its position in space, with no warning flags visible, very little or no guidance is being given to the aircraft. A crew using an erroneous glideslope is utilizing a system that has an error present, for example; a glideslope transmission that has a component of the signal missing or the components being radiated in the incorrect phase relationship. An
erroneous transmission may occur intentionally during maintenance or testing, or inadvertently due to maintenance error.


Findings

Aircraft Equipment

3.2.8 The absence of the SBO signal from the transmitted glideslope signal leaves a balanced CSB [only] signal that may not be immediately identified by the crew as an invalid “on slope” signal. The CSB [only] signal is interpreted by the airborne ILS receiver as a glideslope with infinitely wide limits. The result
may be the course deviation indicators remaining centered on the display with no warning apparent to the crew. Shifting the phase relationship between the CSB and SBO signals may have a similar effect.


Ground Equipment

3.4.4 Pilots unknowingly using an ILS transmitter, while the transmitter is on maintenance or test, may use erroneous information because there may be no flight deck warning.


5. Safety actions taken or agreed

5.1 Air New Zealand

As per the provisions of NZCAR 12.59 (Investigation and Reporting), NZCAR 119.79 (Internal Quality Assurance) and AC 12-2 (Incident Investigation), Air New Zealand has raised corrective and preventive actions to address the issues
raised by this report. In addition to actions aimed specifically at system improvement, the relevant activity is as follows:

__ Increasing flight crew knowledge and awareness regarding glideslope altimeter checks.

ICAO State Letter AN 7/5-01/52 and Attachments


1. I have the honour to draw your attention to a number of incidents which have occurred in recent years resulting from the operational use of instrument landing system (ILS) signals being radiated during ILS testing and maintenance procedures, specifically for phasing and modulation balance testing. Such signals may be perceived onboard aircraft as “on-course” and/or “on-glide-path” indications regardless of the actual position of an aircraft within the ILS coverage and with no flag or alarm indication in the cockpit. The use of ILS localizer and/or glide path signals for approach guidance during these testing and maintenance procedures can therefore result in false indications to the flight crew and has the potential to cause a controlled flight into terrain (CFIT) accident.


US Department of Transportation
Federal Aviation Administration
National Airspace System Operations Program

Evaluation of Glideslope Operations and Maintenance
With Respect to Incidents of
Near-Controlled Flight into Terrain

Area Investigated
Description of Area
The group evaluated reports of two incidents of near controlled flight into terrain. Part of this evaluation included flight data recordings that were conducted by an aircraft using a GS facility that was not radiating sideband energy. This resulted in a
constant on-glide path indication to the cockpit, though it did not exactly duplicate the radiated signal of a GS facility that is radiating a signal in quadrature as is the case during phasing. The group also evaluated applicable publications that govern
facility maintenance and aircraft operations.

Conclusions
Certain changes can be implemented that will enhance the integrity of terminal flight operations. Though there have been no reported incidents of CFIT in the NAS, there are unanswered questions about the specific events, which led to the international
incidences of near-CFIT. The possibility of a CFIT incident occurring in the future is a concern and the probability of such an event is uncertain. Given the two events to date, and the fact that calibration of the system produces a hazardously misleading
GS signal, it appears improvement in GS system integrity is necessary.

Recommendations
Actions taken
Operations Manual. The Boeing Company initiated a change to their operations manual. This change: requires aircrews to cross-check their altitude at the final approach fix (FAF). Though they consider this to be basic airmanship, they felt the added emphasis would enhance aircrew situation awareness. Refer to Appendix 4, Option I.

To those of us who are a little older, it is rather chilling in the light of the above report to reflect on those previously “unexplainable” CFIT events during ILS approaches that have happened over the years (and possibly LOC-only approaches when a “good” G/S was radiating in the test mode).

[Sven Sixtoo]

To pick up on a point raised by Big Pistons Forever:

My SOPs state that it is not permitted to join the glideslope from above (presumably to avoid situations of the kind BPF describes). Is there any one who would accept an approach descending onto the g/s then continuing down the slope? If so, what procedures are in place to prevent excessive RoD & inadequate terrain clearance?

I fly an ac with no ILS coupling and no GPWS of any sort.

Sven

[alf5071h]

My understanding of an ILS is that the only significant (powerful) false gideslopes occur above the normal beam at multiples of the glideslope angle (theta) e.g. 2 x theta, 3 x theta, which are 6 and 9 deg for a standard 3 deg slope. Also that the sense of the beam reverses at each multiple, thus only a ‘9 deg slope’ can be flown in the normal sense. Thus the hazard of a false glideslope is often overrated; the more significant hazard is not checking or incorrectly checking the altitude and range. LEM covered this point previously.

Also of significance are the approaches where markers have been replaced by DME, and where the procedure chart should provide an altitude / range table. How or when are these tables used, who has a SOP, how many times do you check altitude and range?

A good safety fall back for NPAs is to use the Rad Alt. NPAs are designed so that there will be terrain (obstacle) clearances of 1000ft, 500ft, and 250ft RA when outside the IAF, FAF, and MDA respectively; thus if any of these altitudes are seen within the range / fix positions then a go around should be flown.
Also, (and without RA) there are the very useful wind/GS based timing and VS calculations that aid situation awareness and provide a cross check, now how many of us use these?

[haughtney1]

As the NZCAA investigation stated
"__ Increasing flight crew knowledge and awareness regarding glideslope altimeter checks"

Quite simple really, and symptomatic of reliance on a normal indication..or situation..i.e. complacency.

Glideslope checks are not rocket science, they are merely part in parcel with good airmanship. As far as I can see no one has mentioned a personal check on glideslope, based on personal calculations that should match published figures..Jepps are not infallible.

More attention needs to be paid to this area.

:)

[FlexibleResponse]

My understanding of an ILS is that the only significant (powerful) false gideslopes occur above the normal beam at multiples of the glideslope angle (theta) e.g. 2 x theta, 3 x theta, which are 6 and 9 deg for a standard 3 deg slope.Dear alf5071h

Respectfully, I hope you are not confusing the difference between a false G/S and an erroneous G/S. Erroneous G/S can happen on any ILS installation at any time and are incredibly dangerous because the a/c equipment does not detect them as being erroneous.

The following rant is for the benefit of any pilot still out there who doesn’t understand why we carry the ILS Outer Marker Check (the topic of this thread):

All professional pilots should read and understand investigation reports such as why Air New Zealand NZ 60 on 30 July 2000 nearly crashed into the sea some 5 nm short of the runway, so that we may avoid been killed by a now “well known and understood” limitation of the standard ILS installation (see Shore Guy’s link above).

The problem with an erroneous G/S been radiated by an ILS installation which has been inadvertently being left in the maintenance/test mode (and is NOT DETECTABLE by the a/c instruments to warn the flight crew) identified by the report, HAS NOT BEEN FIXED.

The only way we pilots can verify a valid ILS glideslope before we trust our life and the lives of our pax using that G/S down to decision altitude/height (Minima), is to physically perform a check that the on-slope indication of the G/S intercepts a defined check altitude at a defined distance from touchdown.

The height check at the outer marker is shown on each ILS Jepp (approach) plate. At some installations the outer markers have been replaced by a defined DME distance instead. The procedure for the crew to carry out the check is defined in your company ops manual.

If the height does not check, the approach must be immediately discontinued.

The height check is made by reading the altitude on the barometric pressure altimeter. The altitude you read at the outer marker may be slightly different to the true height due to actual atmospheric differences to the standard (namely temperature).

As a guide to acceptable discrepancies for the check, in hot weather your indicated altitude may be up to about 100 feet lower than the height shown on the approach plate at the marker. In cold weather the reverse applies. This is normal and to be expected.

Lastly to address the topic of cold weather corrections that may be needed to be added to altimeter readings at Minima due to the extremes of airfield temperature from standard… This has nothing to do with the G/S validity check! Good airmanship dictates that any required changes to minima must be calculated and agreed upon in the cruise when destination airfield conditions are received, then briefed prior to the approach.

Corrections to minima should only be made in accordance with your company ops manual. However, for guidance, most companies will only require you to make a correction for very low destination airfield temperatures. In very low temperatures the height of your a/c above touchdown at Minima will be significantly lower (>10%) than your indicated barometric altitude and obviously this would be dangerous if you did not add a correction.

As a final note of interest, if your Minima is read from your Radar Altitude indicator (ILS CAT II/III), NO corrections are made or are necessary for any variations in temperature. Even in this case the outer marker check height is still made from reference to the barometric altimeter altitude!

[alf5071h]

FlexibleResponse, you are of course quite correct about erroneous G/S; my post focussed on false / multiple beams.

I agree with the points that you make, they are always worth restating, either for those who did not know, who have forgotten, or just as a good reminder of an essential safety item.

I also agree about the NZ accident; there is a very good CD / video on the incident (from NZ CAA and/or FSF?); the FSF report can be downloaded here (www.flightsafety.org/members/serveme.cfm?path=fsd/fsd_jul02.pdf).

There was a similar event in Brazil involving a 777; a preceding aircraft checked the OM altitude and questioned the QNH – then adjusted the flight path; the 777 with all of its sophistication did not check the OM altitude, but was ‘saved’ from a terrain encounter by EGPWS. Although a technical fix for the particular problem is not available (ILS ground test mode), the industry has (or should have) put defences in place to prevent a repeat of this type of incident, but OM altitude checks are still required as these defences are only as good as the person implementing them.

[FlexibleResponse]

alf5071h,

I wasn't sure whether you had missed the significance of the erronoeus glideslope as opposed to the false G/S due to multipath reflections that we all learnt about in days of old.

I think most aviators including me were totally unaware of the possibility of the erroneous G/S been transmitted in the ground test mode until the initial reports of the NZ incident first surfaced in the aviation magazines around five years ago. The distinction between erroneous and false G/S was drawn in the investigation report.

Your posts reflect a huge depth of experience and are always very interesting and enjoyable to read.

Cheers!
FR

[alf5071h]

Thanks FR.
For those Ppruners who would like yet another viewpoint on the erroneous glideslope problem, see here (www.boeing.com/commercial/aeromagazine/aero_21/glidescope_story.html).
This link also confirms that the video on the NZ incident (“New Zealand 60 — A Free Lesson”) was distributed to all Boeing operators; additional copies are available from the FSF.

A briefing note related to altimetry issues is available from the Airbus safety web site here (www.airbus.com/en/corporate/ethics/safety_lib/index.html); or direct link to the specific item - Altimeter Setting
(www.airbus.com/store/mm_repository/safety_library_items/att00003777/FLT_OPS-SUPP_TECH-SEQ01.pdf)

[punkalouver]

"Whilst on the subject of temperature corrections.

Consider minimum altitudes in 'cold' regions. (basically at altitudes)

Does anyone out there have a table for regions / climatology (coldest temp for a time of year) / correction chart?

Do companies prescribe minimum flight altitudes for certain areas at certain 'cold' times?

What are your policies on minimum altitudes?

safe wishes."



I used to fly for a company whose routes were mostly in the Arctic , all aircraft have temperature correction charts and they are used. I don't worry too much until the temp goes below freezing and then I apply the correction to the altiitudes.

[Walter Sobchak]

I am also on the topic of temperature corrections.

How do you correct for it during a VNAV approach(B737NG)? According to Boeing you shouldn´t alter anything from FAF onwards because you are messing up the programmed approach logic, except as stated in FCTM 5.36 “... crews should make cold temperature altitude corrections by applying a correction from an approved table to the waypoint altitude constraints.”
:confused:
So what should we do?
Use the descent forcast page?
Change values from FAF onwards?

I am quite sure the isa dev on the descent forcast page allows only for a rather high temperature(can´t recall the value) and I anyway don´t know if that is only for descent planning anyway.

Anyone in the know? Meanwhile I am back to good old V/S.

Regards, the Walter

[wondering]

I reckon what alf5071h was trying to say is that a normally working glideslope has no lower lobe as someone else was suggesting. So catching a 6° slope as the next possibile slope should get everybodies attention fairly quickly. An erronoeus glideslope is, of course, a different beast.

[Cardinal]

Mathematically, I can accept that. But practical experience hasn't borne the same conclusion. One morning arriving at KDEN, dog-tired after a long night of flying, I settled into the routine of flying a visual approach, backed-up with the ILS, into our home airport. Captured the glideslope and decended on it, too tired to care about much of anything.

At about 10 DME things started to get my attention, as the farmfields I was idly gazing at were whizzing by rather close. A check of the radar altimeter showed less than 2000 feet. (1500? It's been awhile) My first officer had just noticed the same thing. We stopped the descent, recaptured the real G/S, and continued.

But this glideslope felt real. Solid as a rock, no flags, and produced the usual 750fpm rate of descent. It was just in the wrong place. Weird stuff happens.