787 altimeter inaccuracy

[Fursty Ferret]

Just been made aware of (yet another) BAB for the 787 where the altimeter measurement differs between Capt and FOs PFDs because of an FCM fault.

Apparently this can cause altitude excursions of up to 1000ft (great in RVSM airspace...) and we have a new unannunciated checklist to play with.

The thing is, all the training material for the 787 says that air data displayed on the PFDs is voted single-source, so how can one side provide different data to the other?

Is this another "the pilots don't need to know how this actually works" situation?

[Sidestick_n_Rudder]

I’ve been told on numerous occasions by Boeing and airline instructors that “altimeters will always show the same, no need to compare them”.

Yet, I’ve seen myself a difference of ~100ft between Capt. and F/O altimeters in cruise, which resolved itself after a few minutes...

My guess is that the system uses voted data from 3(?) independent sources. However, if one of these sources fail, then I suspect each side uses one of the two remaining independent sources.

As you already mentioned, Boeing didn’t bother to provide an explanation to us, pilots.

[Yaw String]

Yep,me too.
Have more than once pushed the levers forward assuming Windshear..