A nasty VOR failure mode

[peter-h]

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 was not evident with a strong signal. With a signal corresponding to flying within a few miles of a VOR, the KN72 outputs were 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 should have been suspect too but 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.

I posted all this 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. A reasonably solid 1970s design, with no less than eight trimpots (presumably due to the wide component tolerances esp. capacitors) and no obvious mechanical failures. I have the full maintenance manual with the schematics but it is hardly worth getting it repaired because it requires a bit of a bench setup.

 

[SCCutler]

Fascinating... good on you to note its odd behavior. Under this failure mode, you could have a number of successful VOR checks near the VOR station, or at a VOT site, and never observe a failure.

Guy I knew was flying an ILS, very pleased with his skills, when he looked at the number 2 NAV indicator (no. 1 being the HSI), saw he was three dots low (!). GS indication was on the fritz, but the indicator did not go to its doghouse as it was supposed to have done. Cross-check!

 

[gismo]

There are several possible failure modes that will provide a "valid" flag along with incorrect nav information, one of the most common is a perfectly centered needle regardless of tracking errors. All the lack of a flag indicates is that the nav converter thinks it's seeing enough signal to provide acceptable nav info if the circuitry is functioning per design. There's virtually nothing in the nav converter to generate a flag as a result of a malfunction within other than a total loss of signal through the input stage.

Also FWIW, while the VOR converter shares some circuitry with the LOC converter, there is a significant portion that is unique to each decoder. The shared portions include the input amp and the drivers for the needle and flag, pretty much everything else is separate. As a result, problems in one decoder rarely have symptoms in the other and the fact that one appears to function properly offers little assurance that the other is in the same condition.

Both decoders work with the "video" signal which consists entirely of signals in the audio range and the simplest DSP could easily handle the conversion entirely in software. I suspect (but do not know) that the more modern nav receivers (e.g. GNS430W, G1000, etc) decode the VHF nav signals that way. I don't know of any that process the VHF RF signal digitally, there would be little point as the tiny bit of circuitry required for the downconversion to IF is inexpensive, robust, and effective plus the necessary high speed A to D converter would add noticeable expense. It would be possible to process the IF (typically around 10 MHz and/or 455 Khz) digitally but AFaIK none of the current crop of GA avionics does that either.

 

[flyingcheesehead]

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.

The G1000's GIA 63 LRU's contain the nav radios, and they commuincate with the PFD via a purely digital Ethernet interface.

As for how things work internally to the GIA 63, I don't think that's information we'll get from anyone but Garmin.