Aviation Mythology and Misconceptions

[silverstrata]

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I thought I would start a thread on the erronious ideas that some professional pilots have. See how it runs, it might be interesting.

I will start it off with the argument from someone on this forum that you can do an ADF approach without an ADF. In short, he had misread the FCTM for the Boeing 737. But he may be a trainer - who knows. My answer was as follows:


Sorry, but you cannot do an ADF approach without an operating NDB and ADF - period !

If you do not have a ground station and the appropriate cockpit instrument, you are doing an RNAV FMC approach, not an ADF approach. And we have no RNAV approach plates in our Euro-Jepps, and so I'm presuming that RNAV (FMC/GPS) approaches are not yet certified to any of the airfields we visit in Europe.

Your reference in the 737 FCTM is referring to whether "raw data" or "map display" is selected - not whether the ADF is working or not. In other words, you can use map display mode, if you have overlaid VOR or ADF pointers for cross-checking (which many systems can do) or you flip between raw data and map display to check that the map is in the right position.

Note the note at the bottom of this section: - "Compare VOR and ADF systems to detect possible map shifts". You cannot do a map cross-check, if the VOR or ADF is u/s or not fitted !!


The 1996 Croatia USAF CT-43 crash is what happens, when people use a (map-shifted) RNAV FMC approach, instead of the raw NDB/ADF.


Ditto the B-Med Airbus going into Addis Abeba. The FMC plot and terrain display were off by some 3 miles, leading to a go-around just 60 ft over the high terrain. Now while the VOR would give occasional signals of its unreliability (the VOR was the primary fault), the FMC gave no indication that it had a map-shift. This was the biggest complaint in the report, that the FMC knew it was getting bum VOR information, and therefore may have a map-shift, but did not bother to tell the pilots. And the pilots could not check for a map-shift, because they were comparing bum data with bum data.


The map-plot indicated that the crew were highly influenced by the map and terrain display (rather than the VOR raw data), which showed them passing nicely through the valley. However, the terrain display was likewise mapshifted, and they were 3 miles north of the true inbound. If they had been following an erroneous and displaced VOR radial, it would have eventually taken them to the VOR, whereas this flight paralleled the VOR inbound, which is what would happen if you followed an erroneous FMC position on the map display. Which is why we do no do FMC approaches, especially when we do not have GPS input.

And I am not even sure that GPS-RNAV is entirely a solution at present (as was recommended in the B-Med report). It works well with high accuracy and reliability, but Europe is still concerned that the US may degrade GPS signals or switch the system off during periods of international tension. This is the primary reason for Europe developing and launching the Galileo GPS system. When that is up and running, and there is full confidence that erroneous signals can be detected and warnings given, then Europe may proceed to RNAV approaches.