767 HDG

[NW3]

Hi All,

Might be something obvious that I'm missing, but does anyone know why the HDG on the 767 packs up on touchdown?

(e.g. scenario is L AC fail, R AC fail, APU gen fail, so you're operating on HDG alone)

Many thanks in advance,

NW3

[NSEU]

I can only assume that this design allows the aircraft to divert the Centre System hydraulic energy to devices more (potentially) useful than instrumentation during rollout.

Rgds
NSEU

[ajd1]

Well, it's not just instrumentation that the HDG runs.
In addition to the Capt's Flt Inst Bus, it also runs
L AC Tfr Bus
R AC Tfr Bus
Stby AC
Stby DC
Batt Bus
Hot Batt Bus

So if, as you say, it packs up on landing, you would be left with only the Hot Batt. Hardly ideal for taxying and unlike Boeing's usual logical approach.

I guess you've seen this in the Sim rather than the plane, so my guess would be that this is a Sim quirk; I'll check in the Sim next time I'm in.

[NW3]

AJD -

The battery will still power the full Standby system (HBB, BB, AC Sby, DC Sby), so it's no different to landing off a e.g. standby instruments approach in a non HDG aircraft... I just thought it was a bit strange that it shut down on touchdown!

Thanks for your reply though.

NW3

[Spooky 2]

Okay forgive me but what does packs up mean? If you think in retracts for reason, it does not, but I suspect I'm missing something in the translation.

[NW3]

Apologies - my fault for using some UK 'slang'!

In the simulator, the observed behaviour is for the HDG to stop generating power on touchdown. The L INSTR XFER bus, and I presume both transfer buses, lose their power, leaving only the standby power system.

NW3

[EEngr]

The HDP is shut down by air/ground logic to preserve center system hydraulics for the alternate brake system (and other stuff powered from the center system). The ADP capacity could be restricted due to reduced bleed air flow and it is better used for brakes and other controls once on the ground. There is more than enough battery/static inverter capacity for electrical loads still needed at this time.

At least, that's the way I remember it.

[Spooky 2]

Okay I get it now. Packs up is sorta like t-ts over here on the dark side. Thanks guys!

[Titobonifaz]

Just to confirm, in case of an all engine failure in flight in a B737-300 ER I Will not have HDG because they need bleed air from the center hyd.
ADP?