787 APU Autostart

[sgs233a]

Originally Posted by PaxLambda

I hope my question is not too much outside the topic. If all main-bus power was lost around 08:08:43, how is it possible there were FR24 ADS-B transmissions with time stamps until 08:08:51?

PL

Do we know how well synchronized all the timestamps that have been released are? Presumably most of these things (especially ADS-B) would be GPS time synced, but I'm not sure it was definitively stated in the interim report.

[PaxLambda]

Originally Posted by sgs233a

Do we know how well synchronized all the timestamps that have been released are? Presumably most of these things (especially ADS-B) would be GPS time synced, but I'm not sure it was definitively stated in the interim report.

Indeed, a 4-5 seconds shift in the ADS-B timestamp vs the preliminary report timing would explain several "discrepancies".
Thank you for this idea.
PL

[chrisrobsoar]

Originally Posted by PaxLambda

I hope my question is not too much outside the topic. If all main-bus power was lost around 08:08:43, how is it possible there were FR24 ADS-B transmissions with time stamps until 08:08:51?

PL

This strongly suggests that the transponder was powered, perhaps by the main battery.

[NSEU]

Originally Posted by chrisrobsoar

This strongly suggests that the transponder was powered, perhaps by the main battery.

It's generally not normal for the transponder to be powered by the main battery on Boeing aircraft (although British Airways did have one transponder powered by Standby Power on the 747-400).

Even if it was, it doesn't explain why it cut off when the RAT started powering the aircraft.

[Musician]

Originally Posted by NSEU

Even if it was, it doesn't explain why it cut off when the RAT started powering the aircraft.

The likely explanation is that the hobbyist ADS-B receiver collecting data for FR24 lost "sight" of the aircraft, much as it did on the Eastern end of the runway, while the transponder kept transmitting.

[WilliamWatson]

Originally Posted by tdracer

On the 787 (also 777 and 747-8), when the fuel control switch is cycled to CUTOFF, the electrical generation doesn't stop when the engine N2/N3 drops too low - it happens much sooner than that.
In order to facilitate a 'breakless' power transfer during an engine shutdown, the fuel control switch sends a signal to the electrical system, which reconfigures to drop the related generators from the system before they drop off line. This takes ~0.25 seconds, not the several seconds it takes for the N2/N3 to drop sub-idle.

Do you know if this is documented anywhere? This sounds very good logic and known to Boeing insiders but I'm not sure how to convince people without a source document?