Interesting ILS failure mode

[IO540]

http://youtube.com/watch?v=GelRBhJ4gmI

This shows how you can get a "valid" glideslope almost no matter where you are.

[hugh flung_dung]

Fascinating! Thanks for highlighting this, it deserves a much wider audience.
We teach cross-checking of altitude against DME during the approach and this is a good story to convince people to do it after they've finished training.

HFD

[763 jock]

NZ distributed a CD of this incident to the airlines. I seem to remember that there was also a problem with the control tower monitoring system due to a maintenance error. If it can happen, it will!

[tiggermoth]

IO540 - good post. Very interesting.

[Jamongris]

That is very informative, thanks for the post. Not just about the errors, but how ILS works generally.

[Brooklands]

IO,

Very interesting. It does raise the question of what do you do if there is no DME associated with the ILS like at Cranfield.

Brooklands

[englishal]

Brilliant! You learn something every day....Thanks.

This is why having a GPS as a backup for situational awareness could save your life.

I experienced loc / glide slope distortion once, I was flying an AP coupled ILS when a heavy jet taxied across the end of the runway. This led to a very marked sudden pitch up of about ~20° and a ~20° roll to the left and corresponding drop in airspeed.

[IO540]

No credit to me, I picked it up elsewhere.

I recall that the ILS was notamed "unmonitored". This begs the question: why did they fly it? They briefed for the VOR approach in this knowledge but they still flew the ILS. There is no way to check that an ILS is working, other than a check against unconnected navaids, or a GPS.

I suppose the answer is in their company procedures manual but I would not have flown a known-suspect IAP without secondary position monitoring. They did the secondary monitoring (DME) and went missed immediately upon realising it didn't make sense. But they did leave it quite late, IMHO.

It's easy to ask these questions, for us "GA pilots" flying with single autopilots, single ILS receivers and with no redundancy in anything. We know that relying on a single source for anything important (like navigation, and thus terrain clearance) is stupid. When we fly with an autopilot we watch it because we know the thing could pack up at any time and do so without any indication of fault (and they do; don't I know it). Flying an ILS, we probably keep half an eye on a GPS, just to make sure we are heading for the right airport In fact many of us fly nonprecision approaches on the GPS overlay, and use the navaid receiver as a monitor rather than primary (the only safe way to fly an NDB one, IMHO).

It's easy to forget that airliners often don't have a moving map GPS so the crew has no independent and clear confirmation of where they are. They fly blind, relying on the FMS. The FMS is very good and has multiple redundant inputs, but you can't beat a moving map. I haven't got a clue what % of airliners has no GPS but I bet it is the great majority of the world's fleet. Most of the time, in the civilised world anyway, it doesn't matter because anytime they are near terrain they are under radar control.

I've read quite a number of books on airliner accidents and it's obvious that if the crew had something like a Garmin 296 velcroed to the dash, and glanced at it every once in a while, the accident would not have happened.

[aluminium persuader]

One thing to bear in mind -

When you fly a procedure or are vectored for the ILS the plan is always to intercept the glidepath from below and to allow a period of level flight between establishing on the localiser & descending with the glidepath.

Therefore, as you begin to intercept the localiser beam you should always be receiving a "fly up" signal (below-glidepath indication) from the glidepath.

If at this point you are not getting a below-glidepath indication check your position & range from touchdown.

ap

[Piltdown Man]

Brooklands: I think the point of the video was that proper cross-checks of the glideslope vs. distance should be performed (especially in the third world where signal integrity systems may not be installed or ignored if they are). Whilst Cranfield doesn't have a DME, when performing an on ILS RWY 21 you should be at 1,560' when passing the (L)OM and 640' at the MM. These two checks will give an indepenent check on your G/S performance.

PM

[DFC]

It's easy to forget that airliners often don't have a moving map GPS so the crew has no independent and clear confirmation of where they are. They fly blind, relying on the FMS

IO540,

Please do not try to drag professional pilots down to your own level.

Instruemnt rated pilots have been flying for far longer than GPS or even FMS has been available.

To say that someone who does not have a GPS will not know exactly where they are all of the time shows a total lack of understanding with regard to simple basic radio navigation.

--------

To rely on a unmonitored, unapproved navigation aid which the manufacturer and the authorities restricts to VFR flight only to monitor another aid which while being approved is also unmonitored is no better than relying solely on the unmonitored one (either one) in the first place because when they tell you different info.......which do you use?

Every approach aid has a height crosscheck at some point. There is no such thing as an approach without one.

Regards,

DFC

[IO540]

To say that someone who does not have a GPS will not know exactly where they are all of the time shows a total lack of understanding with regard to simple basic radio navigation.

Evidently, DFC, you are incorrect in some cases. Did you watch the video?

Piltdown Man - I agree 100%; I always thought that a distance check (to something, usually a DME but could be a VOR radial) at the actual or expected GS intercept was a standard procedure with any ILS.

[DFC]

Evidently, DFC, you are incorrect in some cases. Did you watch the video?

Yes, it reguluarly comes up in training.

However, even the U-Tube version frequently flashed to the DME readout.
Perhaps the IO540 version would have the camera flash frequently to the GPS readout showing x miles to go which could also being ignored.

Remember that the film highlighted the fact that even after they had outside visual references which should not have appeared as they did, it took some time for them to wake up and for the brain to resolve that actually being at xxx ft close to the town was not a good place to be.

Ask an instructor about tunnel vision.

Higher than normal rate of descent, earlier than expected descent point, less power than normal, towns appearing in unusual places..........what good is an unapproved GPS that no professional would risk their aircraft on anyway be when one ignores all that?

I can just picture the meeting with the chief pilot!
So you went round (or did not go round) because your £400 unapproved handheld GPS told you to despite the copilot, the observer, the ILS, the DME, the outside visual references all saying something that your toy did not?
Boot, Door, Ass skid to the dole office for a long time!


Why is it that people who have only been flying sime GPS was established think that thre is simply no other way to fly?

Perhaps you should look up that BA incident some time ago where map shift was a factor.

In the professional forum, this is being highlighted as a failure in corsschecking and situational awareness. Not as a "they should have had GPS" debate.

Situational awareness failure - the brain knows where it is but the mind does not recognise the problem with being there at that time.

Typical GA version is being IMC at a level below the surrounding terrain - they know where they are in a vertical and horizontal sense but for some reason the brain does not shout "idiot" loud enough.

Don't confuse situational awareness failure with being lost!

Regards,

DFC

[pumper_bob]

Surely this should be a debate of the pilots refusal to see an abnormality in the approach, the blind belief in his instruments that worked well enough in the earlier flights? GPS is a brilliant tool, it can guide a tomahawk (not a PA38, the missile version ) into your front room via the toilet window if they should so wish. Now if the Mil place enough trust in the system to that degree, then i think you should accept it will get you to within a few hundred meters of a touchdown zone in pretty much the right place. The good old fashion WWII stuff does indeed work. But you have to scoot around the sky to follow the routes they prescribe and the kit in the plane needs regular health checks to make sure it tells the truth!!!!! I have seen many differences in various vor readings in different planes, as much as 10deg! Without in In/Opp sticker in sight. Use ALL tools available to you, why would you do otherwise? But as i said earlier, surely this is more a question of why the pilots took so long to realise things weren't right? CRM, Get homeitis, etc etc, all of these and probably more factors were the cause, not the lack of GPS or the old wartime stuff! It sounds simmilar to something we teach in the Advanced Motoring Course, most accidents happen within a mile or so of the begining or end of your journey. This is because you know the area intimately and your mind wonders to events that are likely to happen once you have parked up, and you take your eye off the all important ball. And when starting your journey your mind is filled with things like nav, purpose of journey and you are monitoring the vehicle to make sure all is well.
Think of a Ryanair pilot who has done 3 sectors and is on his fourth into his home base, not a hitch all day and he is faced with the scenario in the video, i think the same situation could be very likely?
However it was a very helpfull video that explained something i have often pondered. Does anyone know if the localiser works in the same way only in the horizontal plane?

[IO540]

I agree, PB. I mentioned GPS only because nothing else makes a position error more obvious.

DFC and a couple of others are out to take the mick, and I should not have taken his bait, and won't again.

[mm_flynn]

i have often pondered. Does anyone know if the localiser works in the same way only in the horizontal plane?

Yes, it is basically the same concept. Key difference is that the ident is also transmitted in a localizer and this should not be transmitted if all components are not functioning - whereas the glideslope has no ident.

[High Wing Drifter]

PB,
My thoughts:

But as i said earlier, surely this is more a question of why the pilots took so long to realise things weren't right?

I pondered this because that impression is given in the program, it suggests they were pondering and musing on the problem. However that can't have been the case, they must of had significantly less than two minutes from capture and mere seconds from the DME crosscheck to a go-around decision, they must have acted pretty smartly to correlate the information and make the decision in a very high workload phase.

Use ALL tools available to you, why would you do otherwise?

Probably high workload is a reason why not. Introducing additional and unnecessary gadgets into the scan is probably going to slow the scan and thus hinder rather than help situational awareness. GPS distance will probably different from the DME distance and so will not provide a reliable glideslope check. The DME/glideslope crosscheck is on the Jepp briefing strip and is the position and height correlation to use.

[englishal]

I shouldn't worry about DFC IO540 (as you clearly don't )[ my mate who is a professional pilot, likes moving map GPS in the Biz Jets he flies.

[DFC]

my mate who is a professional pilot, likes moving map GPS in the Biz Jets he flies

Approved GPS units are used in most modern commercial transport aircraft and are often the primary reference by which the RNP is obtained.
No one and I in particular doubts the accuracy of GPS operating in an approved certified instalation. However, except in special circumstances, there is required to be a crosscheck available of the GPS computed position.

For many commercial aircraft there is two approved GPS, an automatic crosscheck using DME/DME (some aircraft have extra DMEs especially for this sole purpose i.e. readouts not selectable by the pilots) or VOR/DME or LOC/DME. Finally there is the two IRS. In total some 6 items are individually computing the position of the aircraft. Then add to that two pilots with their own raw data crosschecks which makes 7.5 independent systems checking the position of the aircraft.

Why 7.5?........the captain and copilot often have the same raw data sources selected so while they each check them the data could be corrupt for both pilots.

This is why DME stations are going to be retained long after VORs are history. DME DME is the most accurate crosscheck available.

Once again, the issue is not that they did not have the information available to deduce the actual position of the aircraft the issue is that they did not recognise the situation to be something that was not good for them at that time.

This was not a club puddle jumper with 1 vor 1 ils and 1 dme which have been out of calibration for years. This was a well equipped certified large commercial transport aircraft.

Why has the video been made?, why distributed to all professional training organisations? Because everyone recognises the lessons to be learnt from it in respect of situational awareness, crosscehcking and CRM.

I think tat you will also find that most companies prohibit the use of unapproved handheld electronic devices in the cockpit during flight. That includes the old handheld GPS toys as well as the DVD player

Regards,

DFC

[IO540]

I used to have an instructor for IMCR. An ATPL (or so he claimed).

One day I asked him: if you were flying an NDB IAP and the ADF was saying you are OK, but a handheld GPS was telling you that you are about to die, what would you do?

His reply: I would trust the ADF.

His reply is correct according to the rules, of course. But there is a 99.999% chance he will be dead.

Actually, his reply is a load of bo11ocks in any private flying context, because no law stipulates what equipment is to be used. Only equipment carriage is specified. In CAT operations this is different, but he would still have been just as dead.