View Full Version : Interesting ILS failure mode
http://youtube.com/watch?v=GelRBhJ4gmI
This shows how you can get a "valid" glideslope almost no matter where you are.
hugh flung_dung
2nd Feb 2007, 11:11
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
2nd Feb 2007, 11:32
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
2nd Feb 2007, 11:47
IO540 - good post. Very interesting.
Jamongris
2nd Feb 2007, 12:20
That is very informative, thanks for the post. Not just about the errors, but how ILS works generally.
Brooklands
2nd Feb 2007, 12:26
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
2nd Feb 2007, 12:55
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.
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
3rd Feb 2007, 11:48
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
3rd Feb 2007, 20:40
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
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
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.
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
4th Feb 2007, 10:35
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?
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
4th Feb 2007, 18:20
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
4th Feb 2007, 19:03
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
4th Feb 2007, 19:53
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.
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
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.
His reply is correct according to the rules, of course
Not quite.
Given conflicting information, the correct answer is to disregard both, get the aircraft to a safe level and position and then unless one is able to resolve the discrepency make an approach using a different aid either at the same aerodrome or at the alternate.
The the case being discussed that is exactly what the pilots did. They did not jump onto the VOR procedure and make an attempt to save the approach, they went round again, took time to think about the situation and then made a safe approach using a different aid.
In the private world of flying, pilots and owners are very slow to report radio faults because it is goping to cost money. Hence you hear "watch the ADF it is a bit weak or is a bit off". In the professional world that is unacceptable.
Regards,
DFC
Fuji Abound
5th Feb 2007, 10:10
I think in an interesting debate such as this it detracts from the quality of the discussion if remarks are made presumably purely with the intention of antagonising or provoking a response - as much as some might say they add to the pithiness of the debate.
:)
High Wing Drifter
5th Feb 2007, 10:20
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?The obvious answer is go-around, but what if the GPS says you are fine and the ADF says you are going to die? Why not use the method on the plate instead and avoid all confusion and don't die for either reason.
That has little relationship to the specific scenario in the film, in which case the answer would be to correlate position against the DME and height as defined on the approach plate. If you used the GPS distance in such a scenario then you might conceivably be just as dead. Of course you can always recalculate an unofficial crosscheck, but who and why!?!
Obviously the thread has got a bit sidetracked. What suprised me is how apparently easily, in the modern age and flying a modern airliner, the pilots allowed a gross positional error to remain undetected until somebody saw something strange out of the window. Presumably in IMC the result would have been different.
I think the sensible answer is that one should go-around if there is any discrepancy in instrument indications. This unfortunately is not very helpful in the "ADF" scenario where a huge error is pretty common.
Fuji Abound
5th Feb 2007, 11:24
Having watched the video and read the report I agree with the comment made in the video - before this incident some “pilots” assumed the ILS was always accurate unless otherwise flagged. Therefore, despite the NOTAM, this crew assumed that because it appeared the glide slope had been captured, it had been captured. The doubts came from the unusual pitch change and the external visual clues, not the DME, but were in them selves not sufficient to provoke the crew to react immediately to the lack of correlation in the information available to them because they were convinced they had captured the glide slope, which, in their experience was “infallible”.
DFC suggests that what is to be learnt is the importance of cross checks - never relying on a single instrument. He is clearly correct.
I0540 suggests a moving map GPS may have helped. In this instance the primary cross check had to be of range. Whilst this information may have been provided by almost any GPS, in many cases would the information be presented clearly, and notwithstanding the integrity of the GPS, would the range be the required range? For example, the range might be that to the center of the airport not the threshold. In other words if the pilot were cross checking with his GPS in the “amateur” world of GA would he necessarily know precisely what the GPS was telling him? The aircrafts position on the moving map would have been of little help.
In this particular case the correlation between the glide slope and the DME provides the best instrument clues that something is wrong and in themselves are sufficient to provoke the procedure to be aborted.
It is of course suggested that the localiser can never be susceptible to this type of error. Does that mean the localiser can be relied upon without any other cross checks? Is there any scenario in which the localiser might provide incorrect information without the crew being given adequate warning? In this instance a cross check with a moving map would provoke similiar concern that all was not well. If it did, and the moving map GPS happened to be one of DFCs three and six boxes on which would you rely?
In light aircraft I am becoming a great devotee of the G1000 displays. However, even with these displays this emphasises the importance of understanding the information with which you are provided, and the source from which the systems are deriving the information.
LD Max
6th Feb 2007, 18:47
Fascinating video. I note the YouTube version is edited to fit the permissable download time, and the person who posted it admits to editing out a lot of the technical lessons learned in terms of the technology.
So if this has been suggested before, I apologise.
It occurs to me, that a modification of the intercept procedure would eliminate the possibility of this particular error. Before anyone jumps down my throat, I am NOT advocating this should be done outside SOPs, but only that we consider this as a rhetorical suggestion.
A normal GS provides a beam width of 1.4 degrees. 0.7 degrees above and below the GS, i.e. Full Scale Deflection of the GS indicator represents 0.7 degrees. Most HSIs show 2 dots above and 2 dots below the GS. Each dot represents 0.35 degrees and 1/2 a dot would represent 0.17 degrees. At a range of 1 mile this equates to around 17 feet and at 1/2 a mile, approximately 8 feet.
It seems to me, therefore, that pilots would be advised to fly the entire procedure with a 1/2 dot fly down indication. (Or recalibrate the display so that glideslope intercept occurs 0.17 degrees ABOVE the actual glideslope.)
This would mean that (especially) the Autopilot would continue to fly straight and level on the localiser until a 1/2 dot fly down indication is achieved. As we have seen this will have an insignificant effect on the profile or performance, but it WOULD provide an airborne failure indication of the GS lobe transmitter.
If no fly down indication is forthcoming, the aircraft would continue to fly straight and level and the autopilot would not commence the GS descent profile. It would then quickly become obvious that the aircraft had flown through the glideslope without descending - upon which a missed approach procedure can be commenced safely and without any particular hurry.
Just a thought.
Regards,
LD Max
I must be going thick, LD-Max, but I don't follow the meaning of the half-dot. Maybe I missed something in that movie.
Normally, one intercepts the localiser first, and one is in altitude hold (whether flying manually or on the AP).
Slightly before the GS intercept, the GS flag goes away and the GS indication is UP. At the GS intercept, the GS indication passes through the middle. At this point, one tracks the GS (whether manually or coupled).
If one is going to be receiving a false GS of any sort, I don't see how the half-dot would help, because the false GS could still look "right".
dublinpilot
6th Feb 2007, 21:54
I think his point is that if the g/s indication shows something other than perfectly on the g/s then you know it hasn't failed as per the video. Because if it had failed as per the video, then the g/s would be showing perfectly on the g/s. Hence his suggestion to fly slightly off the g/s.
Interesting video....thanks for posting.
dp
OK, I see it now. That wouldn't help. As a designer of electronic gear for some 30 years, I can say the system could fail with a half-dot indication almost as easily as with a perfectly zeroed indication. Especially if the half-dot offset was designed-in as the default indication when captured.
There is no way to solve this problem comprehensively, other than some kind of digital verification (with appropriate security) transmitted by a separate monitoring unit.
Do airline autopilots fly with the indicator (the HSI, or the flight director) perfectly centred? GA autopilots certainly don't; well not unless it is dead calm.
englishal
7th Feb 2007, 08:47
The thing with MM GPS is that it gives you an instant picture of where you are, where you should be, when the Loc / GS is going to come in - and if it doesn't you know instantly there is a problem. All this without twisting , tuning and trying to fly the aeroplane. Had this crew had MM GPS they would have known BEFORE they intercepted the loc that there was a problem.
Seems almost criminal to not include a few grand piece of kit in an aeroplane costing 10's of millions which could save a couple of hundred lives. There have been numerous serious accidents which WOULD have been avoided if this kind of kit was onboard. In fact a friend of mine killed himself, his co-pilot, his god children and their mother by flying into a mountain on an IAP. Had they had GPS it is 99.9% certain they would be here today.
In fact a friend of mine killed himself, his co-pilot, his god children and their mother by flying into a mountain on an IAP. Had they had GPS it is 99.9% certain they would be here today.
Two points regarding such statements.
1. They were not on the IAP if they hit a mountain. Perhaps one should ask why they were not on the IAP. Or if they were, why did the IAP fly them into the mountain.
2. Unless the mountain is marked on the GPS and flagged then there is little to be gained.
If extra or new equipment could have been used to solve the problems that this incident highlighted I would expect that it would have been highlighted by the investigators or even the people who made the video.
Never assume that adding more information into the situation will be the answer. It can be simply another distraction or the final bit of information that leads to overload.
Moving map displays for Large commercial aircraft are basic in terms of the information they provide and very well filtered to remove all unnecessary information. They are also very expensive.....never mind Garmin430 that would be peanuts in cost terms.
The larger type manufacturers are moving to the areas of terrain awareness and warning systems as well as HUDs and synthetic vision to improve safety.
Regards,
DFC
PS: Here is something to do - Put your handheld GPS in simulator mode and start flying round an airport a long way from base that you never visited. Eastern Europe or anywhere that you have absolutely no idea of the terrain or airspace. Then ask yourself if it improves your situational awareness or if in fact it takes so much playing with that you have no spare time to do anything else!
englishal
7th Feb 2007, 16:45
Clearly everything is black and white to you DFC.....The trouble is in the real world there are a million shades between black and white.
1. They were not on the IAP if they hit a mountain. Perhaps one should ask why they were not on the IAP. Or if they were, why did the IAP fly them into the mountain.
2. Unless the mountain is marked on the GPS and flagged then there is little to be gained.
They failed to go missed at the MAP. It was a VOR DME approach and although we don't know what happened for whatever reason they never went missed. Which meant they hit the mountain 15' below the summit. HAD they had a GPS, it would have been painfully obvious that they had passed the MAP, and if it was more spohisticated with TAWS (which many / most have these days) they would have seen their course ending in a big red splat. Surely even for the most stupidest pilot out there that would start getting the alarm bells ringing.
Oh and for the record, this wasn't some silly little PPL CFIT we're talking about, this person was a JAA ATPL, FAA ATP, CFI, CFII and MEI. "Co-pilot" was ATP.
I have been in a plane (the back of) attempting to intercept an ILS....except there was a nearby VOR dialed in (very similar frequency, 112.4 instead of 114.2 or something like that). HAD we continued and flown a LOC approach (because GS wasn't coming in) we'd have hit the hill the VOR was on. The GPS showed us flying towards the VOR and 2&2 clicked and the situation was "saved".......
Fuji Abound
7th Feb 2007, 20:50
Firstly, this is a GA forum - that is why it is called private flying.
The discussion arose out of an incident in a commercial aircraft, with three crew. That environment is totally different from the average SPPPLIR ops.
Clearly in a commercial aircraft there is no possibility of operating with a three and six unapproved GPS unit - end of discussion.
In GA however, a three and six unapproved unit is possible, as is a panel mounted Skyforce, a panel mounted G1000 or an Avidyne unit.
If you have used a G1000 you would be totally convinced that the situational awareness this gives, coupled with the seamless integration with the automatics, procedures, weather and terrain information and avionics is a huge leap forward in the improvements it brings to situational awareness, cockpit management and workload. Could a professional pilot perform as well in SPIR ops with “traditional” equipment in all circumstances - of course not. However, it is not that simple. There is a great deal of information available, and the smooth operation of the kit in a potentially very high work load environment demands familiarity. Without that familiarity it is another distraction, the consequences are potentially disastrous. It is not a panacea.
At the other extreme are the typical moving map non approved units used by many.
They were not on the IAP if they hit a mountain. Perhaps one should ask why they were not on the IAP.
No - that is not the question to ask. If every pilot was always on the IAP this type of accident would never occur. Yet the accident record is “littered” with pilots of considerable experience who find themselves in just such situations. In short pilots are trained with all the expertise and experience accumulated over the years by instructors not to let this happen, but it still does. Would a moving map of any type provide a secondary reference that the aircraft was not correctly established on the IAP or might it add to additional confusion, where none had existed before?
In my opinion IF the pilot is familiar with the unit so that it does not become another distraction then it can only aid him in situational awareness. If only once it leads him to doubt that he is correctly established on the IAP and enables him to reposition it has proved its worth.
Unless the mountain is marked on the GPS and flagged then there is little to be gained.
I don’t follow your argument. Many incidents of this type are caused by the pilot missing a clue that they are at some other point in the procedure. In Englishal’s case we can all conject why the pilots did not go missed, however in addition to the information they should have had from their DME or from the markers they would have seen on a moving map that they had passed the threshold.
In short I am not suggesting relying on a moving map, particularly in an unapproved fit, but it aint half a bad backup which will give you a clue that what the primary instruments are telling you might just not add up!
Englishal - sorry to hear about your freind.
LD Max
14th Feb 2007, 18:03
OK, I see it now. That wouldn't help. As a designer of electronic gear for some 30 years, I can say the system could fail with a half-dot indication almost as easily as with a perfectly zeroed indication. Especially if the half-dot offset was designed-in as the default indication when captured.
There is no way to solve this problem comprehensively, other than some kind of digital verification (with appropriate security) transmitted by a separate monitoring unit.
Do airline autopilots fly with the indicator (the HSI, or the flight director) perfectly centred? GA autopilots certainly don't; well not unless it is dead calm.
Thanks for the reply. Apologies for not getting back sooner... been busy.
I think what you are saying is that if the lobe transmitter was re-calibrated as I suggested then a failure would still show half a dot fly down. Actually that's not what I meant.
I merely suggest that if you intercept the G/S as normal, but delay the descent until you get a 1/2 dot fly down indication then this would provide a failure indication of the G/S. The reason being that if the lobe transmitter had failed, you would always get a centered needle on the G/S indication even after you had flown right through the glideslope.
My suggestion to re-calibrate the display is simply to do with the aircraft. If the display were recalibrated so that a centred indication was given when the aircraft was, in fact, flying 0.17 degrees above the glideslope, then pilots (and autopilots) would not have to modify their normal procedures. In this case if the lobe transmitter failed, then the pilot would see a permanent 1/2 dot FLY UP indication.
Either way, no descent could occur because the aircraft would continue to fly straight and level trying to intercept, until someone twigged the DME was indicating they had flown through the glideslope. Much safer, I think, than starting a descent before the DME starts screaming at you.
In terms of failure indication, you are quite right that separate monitoring is the only sure way to know. But in the case of this incident, the ILS was notamed as "unmonitored" - meaning that the normal fail-safes were out of the loop. Since the pilot has no failure indication other than the flags, this particular mode of failure was impossible to identify in the cockpit.
My suggestion to fly the procedure with a 1/2 dot fly down indication would provide an additional means of identifying such a failure.
You're also right of course that it would take a very skillful pilot to fly a perfect profile and perhaps the "perfection" of a "perfectly centred" G/S should be treated with suspicion... However this particular crew were coupled to autopilot and would be used to flying a very accurate profile, which in itself did not indicate anything unusual.
Regards
S-Works
14th Feb 2007, 18:22
I had an interesting experiance flying the ILS into Cambridge yesterday. GS was indicating normal and then about 2 miles from the MAP it started fluctuating up and down at half scale in either direction. Now I have been known to have bad days but not a half scale in both directions.
It turned out that an aicraft had joined a tight base leg and flew accross the GS. Had I not been monitoring the DME/Height profile it might have ended in tears!
LD Max
14th Feb 2007, 18:59
I have been in a plane (the back of) attempting to intercept an ILS....except there was a nearby VOR dialed in (very similar frequency, 112.4 instead of 114.2 or something like that). HAD we continued and flown a LOC approach (because GS wasn't coming in) we'd have hit the hill the VOR was on. The GPS showed us flying towards the VOR and 2&2 clicked and the situation was "saved".......
Sorry to hear about your friend too. Any accident is tragic and it's all very well to treat such events as lessons to the rest of us, but I do think it important to learn the RIGHT lessons.
The right lesson to be learned from your own anecdote is the importance of "identifying" the radio aid.
Firstly, note that neither of the two frequencies you mention (and I realise they are arbitary examples), are ILS frequencies. The LOC transmitter operates in the “normal” airband VHF frequency range between 108.1MHz and 111.95MHz. Note the frequencies are all assigned odd-tenth decimals.
VORs on the other hand operate between 108.0MHz to 117.95MHz. The band between 108.0 and 112.0 is in even-tenth decimals to preclude any conflict with localiser frequency assignment.
It is therefore possible to mis-dial something like 110.5MHz (ILS) and dial in 115.0MHz (VOR). If you know the rules, this should become obvious when cross-checking the radios during approach checks.
Secondly as you know, all LOC transmitters identify themselves by Morse code. Localisers always start with the letter "I" (dit dit). I believe there are no VOR idents which start with the letter I, (bit more research needed to be sure). In any case, a navaid needs to be positively identified before use - but the lack of a letter "I" when identifying an ILS, should be enough to alert you to listen more carefully.
Finally, I cannot agree with your point about GPS being used as a "backup". It may have saved your skin this time but just a week ago, a buddy of mine was flying with a GARMIN GPS 430 in a twin, just south of Ocala in Florida. Just for fun, he wrote down the GPS coordinates of where the GARMIN unit said he was. Later, on the ground, he looked up the coordinates on a map. To his astonishment, the coordinates placed him a few mile South East of Kissimmee - about 100 miles away from where he actually was.
We could put this down to a "glitch", "finger trouble" or simply picking the wrong numbers from the display. Either way GPS is probably one of the best ways of being wrong about your position, without any clear indication of the error. There is a saying that GPS can be very precisely inaccurate - and that is the danger.
At the end of the day IMHO, there is no substitute for good IFR procedures.
Regards,
LD Max