A nasty VOR failure mode
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A nasty VOR failure mode
If somebody tells you that GPS is dodgy but conventional navaids can be relied on, this is a nice lesson.... caused by a subtle failure in a KN72 signal converter.
The panel section is here.
The video is here (20MB mpeg).
The HSI and the CDI are both tuned to the same test source and should indicate identically at all times.
Explanation:
The aircraft has two separate VOR receivers, NAV1 and NAV2. These share a VHF antenna. The HSI is fed from NAV1, and the CDI is fed from NAV2. There is also an RMI (KI-229) whose VOR needle (the thin one) is switchable NAV1/NAV2 but in this example it is working from NAV2.
NAV1 is a KX155A radio, with a KN72 VOR/LOC converter. NAV2 is a KX165A radio and there is no external converter.
The KN72 converter processes the VHF VOR/LOC signal and generates the deviation of the HSI bar, the TO/FROM flag, and the INVALID flag.
In this installation, the KN72 converter failed, in a manner which produced invalid indications on the HSI bar, invalid indications on the TO/FROM flag, but a valid behaviour of the INVALID flag. This is obviously a dangerous failure mode because the main indication of the instrument being untrustworthy - the INVALID flag - is useless. In contrast, a GPS is extremely unlikely to produce an apparently dynamic (changing) instrument indication which is wrong.
The KN72 failure is not evident if the signal is strong. With a signal corresponding to flying within a few miles of a VOR, the KN72 outputs are fine. As the signal is weakened, say at 10D, the instrument indication starts to fall apart, in a gradual manner which would not be obvious to a pilot who is tracking the VOR in the conventionally trained manner and who does not have a separate source of situational awareness (e.g. a moving map GPS).
All 3 instruments were driven from the same ultimate signal source; a VOR/LOC avionics test set. The CDI and the RMI show correct indications at all times, being fed from a different system to the HSI.
The LOC behaviour of the faulty KN72 was not tested because immediately after the video was mode the KN72 was replaced. When the inability of NAV1 to track a VOR was originally suspected, airborne tests revealed a blind spot (on both NAV1 and NAV2) at 3 o'clock. This was caused by one of the two VOR/LOC antennae having a broken connection at its base; an extremely subtle fault which could remain undiscovered for years, revealing itself only when flying a VOR approach with a "right base" join, and flying it using the VOR (and not a GPS, in the OBS mode, which is how most people do it). Due to component lead times, it took some time to get the antenna assembly replaced. Because ILS approaches are operationally much more important than VOR approaches (which can be flown using a GPS) extensive ILS tests were done but they failed to show any LOC problems. After the antenna assembly was fixed, no further faults were suspected. Any normal avionics test would not have revealed the KN72 failure, and it may have been present for years.
The panel section is here.
The video is here (20MB mpeg).
The HSI and the CDI are both tuned to the same test source and should indicate identically at all times.
Explanation:
The aircraft has two separate VOR receivers, NAV1 and NAV2. These share a VHF antenna. The HSI is fed from NAV1, and the CDI is fed from NAV2. There is also an RMI (KI-229) whose VOR needle (the thin one) is switchable NAV1/NAV2 but in this example it is working from NAV2.
NAV1 is a KX155A radio, with a KN72 VOR/LOC converter. NAV2 is a KX165A radio and there is no external converter.
The KN72 converter processes the VHF VOR/LOC signal and generates the deviation of the HSI bar, the TO/FROM flag, and the INVALID flag.
In this installation, the KN72 converter failed, in a manner which produced invalid indications on the HSI bar, invalid indications on the TO/FROM flag, but a valid behaviour of the INVALID flag. This is obviously a dangerous failure mode because the main indication of the instrument being untrustworthy - the INVALID flag - is useless. In contrast, a GPS is extremely unlikely to produce an apparently dynamic (changing) instrument indication which is wrong.
The KN72 failure is not evident if the signal is strong. With a signal corresponding to flying within a few miles of a VOR, the KN72 outputs are fine. As the signal is weakened, say at 10D, the instrument indication starts to fall apart, in a gradual manner which would not be obvious to a pilot who is tracking the VOR in the conventionally trained manner and who does not have a separate source of situational awareness (e.g. a moving map GPS).
All 3 instruments were driven from the same ultimate signal source; a VOR/LOC avionics test set. The CDI and the RMI show correct indications at all times, being fed from a different system to the HSI.
The LOC behaviour of the faulty KN72 was not tested because immediately after the video was mode the KN72 was replaced. When the inability of NAV1 to track a VOR was originally suspected, airborne tests revealed a blind spot (on both NAV1 and NAV2) at 3 o'clock. This was caused by one of the two VOR/LOC antennae having a broken connection at its base; an extremely subtle fault which could remain undiscovered for years, revealing itself only when flying a VOR approach with a "right base" join, and flying it using the VOR (and not a GPS, in the OBS mode, which is how most people do it). Due to component lead times, it took some time to get the antenna assembly replaced. Because ILS approaches are operationally much more important than VOR approaches (which can be flown using a GPS) extensive ILS tests were done but they failed to show any LOC problems. After the antenna assembly was fixed, no further faults were suspected. Any normal avionics test would not have revealed the KN72 failure, and it may have been present for years.
Last edited by IO540; 19th Oct 2010 at 21:11.
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Thanks for that. An excellent piece of useful info! 
I suggest notifying the relevant regulatory bodies if not already done so they can issue the appropriate safety advisories. Or it could be stuck on this forum and forgotten about over time.
I suggest notifying the relevant regulatory bodies if not already done so they can issue the appropriate safety advisories. Or it could be stuck on this forum and forgotten about over time.
I had a King KI209 (the VOR/LOC/GS head with built in converter) fail in flight with no flag. We were IMC on vectors to a short gate ILS at an airport with no DME. The student was late starting the descent so I was not surprised to see the GS full down but it remained full down when we intercepted the loc. I finally realised what was going on when we were only 100 ft above the FAF GS check altitude still with a hefty descent rate and the needle had not budged. I called missed approach and we went home flying the lOC/DME to minimums to get back in. The instrument never showed a flag and intially passed its bench test. After cooking for awhile it finally replicated the fault. Bottom line any piece of electronic magic can fail, sometimes in subtle ways, so keep your situational awareness and when that little voice in your head is saying "hey that ain't right"....listen !
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I suggest notifying the relevant regulatory bodies if not already done so they can issue the appropriate safety advisories
As BPF shows above, all kinds of failures are possible and some of these could kill you.
I just posted my illustration for those who have been taught that VORs are somehow superior to GPS
The way in which the three components of a VOR (CDI) receiver are generated (the deviation bar, the to/from flags, and the Invalid flag) is a disaster absolutely begging for a place to happen. They are generated by separate circuits, which can fail individually.
In contrast, a GPS usually has a single processor which either runs or doesn't. The LCD will have its own "processor" and this is how the LCD can freeze on a given indication when the main processor crashes, but it will not show anything meaningfully changing. The GPS satellite receiver is usually a separate module, with its own little processor, and if that fails, the main processor will put up a big "no satellite reception etc" message across the screen.
Whereas the VOR (and LOC/GS) system uses separate circuits which can fail individually. There lies the danger.
I don't know how e.g. a G1000 system does this. Does it use analog separation of these signals and then an A-D converter to get the LCD representations, or does it use a DSP to do a direct digital conversion receiver? The latter would be unlikely, on VHF.
I opened up the duff KN72 (there is no exchange option on it so it is basically worthless) and here is the inside.
Last edited by IO540; 20th Oct 2010 at 07:53.
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Hidden Nav failure
The un-suspected fault with a broken connection to one leg of the Nav antenna would normally show up when testing the system as insensitive operation. Typically you're looking for the fail flag not to appear until you attenuate the test set by at least 55dB, and with a leg disconnected, it normally fails at 30-35dB down. On an EASA aircraft maintained under CAA LAMP, this test is done on every annual, but on most N reg, there is no requirement in the maintenance schedule
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The un-suspected fault with a broken connection to one leg of the Nav antenna would normally show up when testing the system as insensitive operation. Typically you're looking for the fail flag not to appear until you attenuate the test set by at least 55dB, and with a leg disconnected, it normally fails at 30-35dB down.
It will still pass if the tester just happens to be standing on one side (the "right" side) of the aircraft, because on the TB20 the vertical stabiliser masks the other antenna pretty well - so well in fact that a few miles from a VOR you cannot hear the ident at all if the VOR is exactly in the 3 o'clock blind spot.
On an EASA aircraft maintained under CAA LAMP, this test is done on every annual,
but on most N reg, there is no requirement in the maintenance schedule
If you fly say 20nm away from the VOR, and then do a reasonably tight (but with not much of a bank angle) 360 orbit while listening to the ident, the duff antenna is quickly revealed. That is how it was finally detected. This is a potential killer - even without the KN72's contribution...
I do recall seeing odd effects, over the years, on NAV1, on the 30-day VOR check, but they could never be reproduced, and anyway I almost never use NAV1 for VOR. The fault must have been there for years and most likely while it was G-reg on the PT CofA.
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The KN72 converter which does the stuff in this case is a 1970s design so has been around a while...
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The risk is with any conventional VOR receiver system, not just one with a KN72 inside.
The point is that the circuits driving the bar, the t/f flags, and the Invalid flags, are all substantially separate, and can thus fail separately. This is how it has been since day 1.
If this is a certification issue then VORs and NDBs and ILSs will be shut down too
I wrote it as a heads-up (as the Americans call it) because the flight training system maintains that if the Invalid flag is not showing, the instrument can be trusted, which is bollox.
The point is that the circuits driving the bar, the t/f flags, and the Invalid flags, are all substantially separate, and can thus fail separately. This is how it has been since day 1.
If this is a certification issue then VORs and NDBs and ILSs will be shut down too
I wrote it as a heads-up (as the Americans call it) because the flight training system maintains that if the Invalid flag is not showing, the instrument can be trusted, which is bollox.
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Have instrument failures killed people?
Who knows? I bet they have.
It probably depends on how near the borderline you are when it happens. If flying a VOR approach with a cloudbase of say 1500ft (which is what I had when I first realised something was really wrong with the VOR receiver) and no terrain around, it doesn't really matter - apart from ATC wondering what the hell you are doing.
But elsewhere it might matter.
Obviously the important thing is to have a separate situational awareness source. A moving map GPS is the best by far. If flying an ILS I would tune in the runway extended centreline as an OBS on the GPS, and the LOC intercept should take place exactly on that line.
This would have been a much bigger issue in the past because even airliners did not have GPS until very recently. But then airliners don't fly much to places like Kathmandu And when they do, they don't need a duff instrument to hit the terrain...
Now, in addition to the legally required 30-day VOR check (which is a formality because I usually fly with some VOR tuned-in, anyway) I get the avionics tested a few times a year (unofficially). I would like to be able to do it myself but while the testers (example) do come up on US Ebay they are still 4 figures, and I can get the test done for about 1% of that.
There have been some interesting cases in the USA where a number of pilots flying with a specific MFD, which has incorrectly colour-coded terrain depiction, have hit the rocks, at altitudes corresponding close to the colouring errors. Admittedly this was a stupid pilot error (what is "MEA" for, etc?) but the manufacturer was completely uninterested in fixing the colour coding. Presumably they did not want to fix it because doing so would be an admission of liability.
It probably depends on how near the borderline you are when it happens. If flying a VOR approach with a cloudbase of say 1500ft (which is what I had when I first realised something was really wrong with the VOR receiver) and no terrain around, it doesn't really matter - apart from ATC wondering what the hell you are doing.
But elsewhere it might matter.
Obviously the important thing is to have a separate situational awareness source. A moving map GPS is the best by far. If flying an ILS I would tune in the runway extended centreline as an OBS on the GPS, and the LOC intercept should take place exactly on that line.
This would have been a much bigger issue in the past because even airliners did not have GPS until very recently. But then airliners don't fly much to places like Kathmandu
And when they do, they don't need a duff instrument to hit the terrain...Now, in addition to the legally required 30-day VOR check (which is a formality because I usually fly with some VOR tuned-in, anyway) I get the avionics tested a few times a year (unofficially). I would like to be able to do it myself but while the testers (example) do come up on US Ebay they are still 4 figures, and I can get the test done for about 1% of that.
There have been some interesting cases in the USA where a number of pilots flying with a specific MFD, which has incorrectly colour-coded terrain depiction, have hit the rocks, at altitudes corresponding close to the colouring errors. Admittedly this was a stupid pilot error (what is "MEA" for, etc?) but the manufacturer was completely uninterested in fixing the colour coding. Presumably they did not want to fix it because doing so would be an admission of liability.
I think that there is a common misconception that "no flags" equals a servicable unit. When the flag disappears all it is telling you is that the unit is powered and is recieving a nav signal. That nav signal could be totally wrong but the flag will still disappear.....
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Here's an ATC recording of an engine failure in IMC, in icing conditions, where the 3D Synthetic Vision in the GPS played a role in getting the pilot down safely. Very professional and calm pilot considering the circumstances.
YouTube - Aircraft Engine Failure IMC
Let's say he'd been over higher terrain, no GPS and the VOR's would have failed in IO's manner, then this guy would not have been alive today.
YouTube - Aircraft Engine Failure IMC
Let's say he'd been over higher terrain, no GPS and the VOR's would have failed in IO's manner, then this guy would not have been alive today.
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Receiver sensitivity
When I mentioned weak signal effects of a broken Nav antenna, it doesn't matter what relative position the test set is in - normally in the cockpit or sat on a wing. We're not looking for the just the bearing accuracy during a test, but also receiver sensitivity, and this will show up even with correct bearings, because the fail flag will drop into view far to early when the test set output is attenuated.
Incidentally, the fail flag does more than just indicate a satisfactory received signal and unit power. If the converter detects loss of either VOR 30Hz signal (reference phase or variable phase) it will also drop the flag. Similarly in LOC mode, if either the 90Hz or 150Hz signals of the ILS are missing, the flag should also appear, otherwise the CDI could indicate full deviation based on just one side of the ILS beam being received.
Incidentally, the fail flag does more than just indicate a satisfactory received signal and unit power. If the converter detects loss of either VOR 30Hz signal (reference phase or variable phase) it will also drop the flag. Similarly in LOC mode, if either the 90Hz or 150Hz signals of the ILS are missing, the flag should also appear, otherwise the CDI could indicate full deviation based on just one side of the ILS beam being received.
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In nearly all instances of flight one is flying a heading, and (if radio navigating) one uses navaid indications (including ones from GPS if applicable) to modify the heading. But one is still flying a heading.
With an instrument failure, the question becomes: how long it is before the duff instrument indication becomes apparent.
I suppose much depends on how "authentic" the failure is. On the dodgy VOR approach referred to, the failure was pretty damn authentic...
With an instrument failure, the question becomes: how long it is before the duff instrument indication becomes apparent.
I suppose much depends on how "authentic" the failure is. On the dodgy VOR approach referred to, the failure was pretty damn authentic...
Incidentally, the fail flag does more than just indicate a satisfactory received signal and unit power. If the converter detects loss of either VOR 30Hz signal (reference phase or variable phase) it will also drop the flag. Similarly in LOC mode, if either the 90Hz or 150Hz signals of the ILS are missing, the flag should also appear, otherwise the CDI could indicate full deviation based on just one side of the ILS beam being received.
. However there are still failure modes where the unit will provide erroneous nav information with no flag in view. My only point was to not blindly accept what you are seeing on the instrument just because no flags are in view.
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Very, very interesting story IO. Thanks for publishing it.
To me it's a mystery how electronics of that era ever worked at all, considering the pathetic tolerances of the individual components and the appalling reliability of same. If a modern PC were built with the same underlying material it would execute about 50 instructions before crashing. Indeed, in the 80's, Hawker Siddley believed exactly that and arranged for the CPU in a certain missile to be hard reset on every 500 rpm rotation of the vehicle!
Problem is, today we have unveriafiable software to fill the same evolutionary niche that once belonged to ghastly analogue and RTL electronics. Who can say that a GPS might not go off on a 'valid flag' adventure of it's own under the right circumstances? There's been no shortage of 'Innacurate GPS' anecdotes on here over the years and I bet none of those have been followed up with the agressive technical persistence that you've shown in this case. We'd need a technically literate CAA or AAIB for that.
I'm not quibbling with your underlying message, IO, that GPS (panel mount, IFR cert) is far, far more reliable that all this VOR/DME/ADF/ILS etc cluge up of antique electronics that should occupy airport dustbins. There it could be accompanied by the crushed and stripped remains of all the 30 year old spamcans that UK plc forces us to keep in the air, and which should be replaced with new aircraft and avionics, were it not for suffocating 'safety' bureaucracy and the lack of commercial finance for small businesses, which should allow owners to trade off finance costs against decent utilisation and the destruction of outlandish maintenance costs.
My own (strictly VFR) solution (in a 30 yr old spamcan equivalent of a Cortina mk1) is to monitor the 430 with the output of VOR, DME, even ADF and make sure it fits on the (printed) map. But if one day they don't agree, (It actually happened routing to one of those very narrow corridors at the Canyon), guess which one I'll believe! And because I believe the (cross checked) GPS, I'll live.
PS if you have a spare moment to have a look at some of the perplexing avionics issues in my aerial Cortina, I'd love the help. It's got my £40/Hr engineers beat and every time I thunp the table it's another £40 times xs hours to not fix the problems that were probably designed in by the original equipment manufacturers in the first place.
To me it's a mystery how electronics of that era ever worked at all, considering the pathetic tolerances of the individual components and the appalling reliability of same. If a modern PC were built with the same underlying material it would execute about 50 instructions before crashing. Indeed, in the 80's, Hawker Siddley believed exactly that and arranged for the CPU in a certain missile to be hard reset on every 500 rpm rotation of the vehicle!
Problem is, today we have unveriafiable software to fill the same evolutionary niche that once belonged to ghastly analogue and RTL electronics. Who can say that a GPS might not go off on a 'valid flag' adventure of it's own under the right circumstances? There's been no shortage of 'Innacurate GPS' anecdotes on here over the years and I bet none of those have been followed up with the agressive technical persistence that you've shown in this case. We'd need a technically literate CAA or AAIB for that.
I'm not quibbling with your underlying message, IO, that GPS (panel mount, IFR cert) is far, far more reliable that all this VOR/DME/ADF/ILS etc cluge up of antique electronics that should occupy airport dustbins. There it could be accompanied by the crushed and stripped remains of all the 30 year old spamcans that UK plc forces us to keep in the air, and which should be replaced with new aircraft and avionics, were it not for suffocating 'safety' bureaucracy and the lack of commercial finance for small businesses, which should allow owners to trade off finance costs against decent utilisation and the destruction of outlandish maintenance costs.
My own (strictly VFR) solution (in a 30 yr old spamcan equivalent of a Cortina mk1) is to monitor the 430 with the output of VOR, DME, even ADF and make sure it fits on the (printed) map. But if one day they don't agree, (It actually happened routing to one of those very narrow corridors at the Canyon), guess which one I'll believe! And because I believe the (cross checked) GPS, I'll live.
PS if you have a spare moment to have a look at some of the perplexing avionics issues in my aerial Cortina, I'd love the help. It's got my £40/Hr engineers beat and every time I thunp the table it's another £40 times xs hours to not fix the problems that were probably designed in by the original equipment manufacturers in the first place.
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To me it's a mystery how electronics of that era ever worked at all, considering the pathetic tolerances of the individual components
On top of that are the 3 installer-accessible ones. They are probably needed to take out the tolerance on the avionics being driven - another piece of stupidity. A huge amount of avionics adjustments are needed only because the inter-equipment voltages are sloppily specified. For example I have a KI-256 horizon whose pitch and roll outputs have something like a 10-20% tolerance (I have the maint. manual somewhere) and consequently when you change this instrument, the autopilot needs a re-cal which is a load of hassle (unless the 256 is a Mod 11 version
).If you look at a 1970s or 1980s state of the art piece of Tektronix or HP test equipment (I have a few next to me here) the thing is packed with trimmers. And it came with a massive maintenance manual full of instructions on how to set each one
In the age of 10% resistors, that's how it was done. All resistors are now 1% (0.1p) and I buy 0.1% ones for about 20p which is amazing; they were ~ £20 when I started.
and the appalling reliability of same. If a modern PC were built with the same underlying material it would execute about 50 instructions before crashing. Indeed, in the 80's, Hawker Siddley believed exactly that and arranged for the CPU in a certain missile to be hard reset on every 500 rpm rotation of the vehicle!
I was amazed to discover the KFC225 autopilot does not have a watchdog. If/when it crashes, it just crashes. No indication. Probably same for a Garmin x30 GPS.
Problem is, today we have unveriafiable software to fill the same evolutionary niche that once belonged to ghastly analogue and RTL electronics. Who can say that a GPS might not go off on a 'valid flag' adventure of it's own under the right circumstances?
There's been no shortage of 'Innacurate GPS' anecdotes on here over the years and I bet none of those have been followed up with the agressive technical persistence that you've shown in this case. We'd need a technically literate CAA or AAIB for that.
Having seen what trouble some people I know personally, face to face have had with "state of the art" avionics, I reckon the latest stuff is actually no more reliable than the "good quality" old 1980s/1990s stuff. A mech horizon might grind to a halt after 1000hrs but if an EFD1000 does 1000hrs (10 years?) you are doing extremely well. The big difference is that the individual instrument can be swapped locally; no need to visit an avionics shop in most cases.
PS if you have a spare moment to have a look at some of the perplexing avionics issues in my aerial Cortina, I'd love the help. It's got my £40/Hr engineers beat and every time I thunp the table it's another £40 times xs hours to not fix the problems that were probably designed in by the original equipment manufacturers in the first place.
) are just wiremen who follow wiring diagrams in the back of the installation manuals
Last edited by IO540; 21st Oct 2010 at 09:52.