PFD: Amber QNH/STD mode alert

[Hardy Heinlin]

Good evening,

as we know, when STD is selected while the aircraft is below TA during climb or below TL during descent; or not selected while the aircraft is above TA during climb or above TL during descent, the respective QNH or STD setting on the PFD will be displayed in amber. The references for this are the FMC's active flight phase and the FMC's TA/TL entries.

Does anybody know the answer on the following question? It's not documented as far as I can see.

In case of a dual FMC failure, the PFD's amber QNH/STD mode alert will ...

(A) ... keep working based on last used TA/TL settings (stored in EIUs).
(B) ... keep working based on default values 18000/FL180.
(C) ... will no longer work, i.e. any selection appears in green.

Thanks!


Regards,

|-|ardy

[John H Watson]

From the beta forum....

I discovered today that if I pulled both FMC L & R CBs [on the ground], that an amber STD will turn green.

I believe the FMC was using the default TA of 18000' at the time.

Rgds
JHW

[Hardy Heinlin]

Ah, I overlooked that note. Thanks!

I vote for (C), that would agree with this observation as well.

Using 18000 on the whole planet as an alert reference, -- that would be a nuisance alert in most areas. So better make it all green, without any fixed reference.


Cheers,

|-|ardy


Edit: Actually, it proves that (C) is the only correct answer as the TA/TL values are irrelevant for the fact that your STD setting turns green on the ground; there is no TA/TL in the world below sea level.

[United744]

With the system operating normally, I have seen the alert for STD (turning orange during descent) when still above TL. i.e. the alert occurred before it was expected.

[Hardy Heinlin]

The reference for ACT DES is the TL entered on the DES FORECAST page.

[United744]

Ahhh! Thanks.