VNAV [NOTHING]

[emerydc8]

We recently installed the 747 MCP mod with altitude intervention in our 767s. We have two airplanes (767-300s) that are FADEC and we are having problems with the FMA on both. On a VNAV descent when capturing an MCP commanded altitude, instead of displaying VNAV ALT, it just says VNAV. The airplane wonders around +/- 100' like it's confused. Our maintenance is scrambling to figure out how to fix it as I'm told that going back to the old MCP requires about 80 wiring changes. Any maintenance gurus care to take a guess on what it could be? I'm thinking some pins are not configured but I guess it's possible that the FMC is not talking correctly to the MCP.

[Hardy Heinlin]

The word "VNAV" alone usually appears when a mode failure occurs. It's neither -PTH nor -SPD nor -ALT but just the word "VNAV" with a yellow line on it. Can your 767 FMA draw yellow lines over mode words?

The airplane wonders around +/- 100' like it's confused.

Maybe it tries to maintain a vertical speed of 0 fpm. -- If it couldn't maintain 0 fpm due to low speed, for example, the elevators would certainly try to maintain stick shaker speed (like the 744) to reduce the vertical speed as much as possible (during the mode failure).


Regards,

|-|ardy

[emerydc8]

It can draw amber lines through the FMAs, but in this case it didn't. It occurs when descending to an MCP altitude in IDLE/HOLD|VNAV PTH, so the correct FMA as it leveled off should have been SPD|VNAV ALT or SPD|VNAV PTH.

As far as holding altitude, it seemed to wonder around. When it descended more than 100' low, I decided to use FLCH. I think this STC came right from Boeing, so I guess they're going to have to figure it out. It seemed to work fine on the other aircraft, but this particular aircraft and its sister ship experienced this anomaly.

[Jeroen Hoppenbrouwers]

It seemed to work fine on the other aircraft, but this particular aircraft and its sister ship experienced this anomaly.

Argh, so not one single occurrence, either. Start comparing exact LRU hardware and software part numbers...

[emerydc8]

Thanks. Maintenance seems to think it's a software issue too. The mod comes from Boeing, so I guess they'll have to get involved.

[United744]

Boeing...LOL.

My guess is software - something isn't communicating. I wouldn't be happy with it just wandering, either. I have nightmares after the Airbus A330 that crashed during testing:

https://en.wikipedia.org/wiki/Airbus_Industrie_Flight_129

The autopilot took 3 attempts to engage

Airbus were strangely quiet on this point.

This was interesting, too. They think it was due to a particular RADAR site. After the incident, all air-traffic was quietly re-routed away from the area.

https://www.telegraph.co.uk/travel/news/kevin-sullivan-qantas-flight-72-computer-failutre/