View Full Version : 1 EO on B737 Classic - degraded AF Ops

6th Nov 2008, 11:09

Can somebody explain why I loose the FD on APP when one engine is out with the APU inop?


Capt Claret
6th Nov 2008, 12:04
Don't know about the 737 as I haven't flown one but if memory serves, on the venerable BAe146, when a generator dropped off line the electrical power interruption would cause the flight director to bias out of view. Cycling the Flt Dir switch returned it to service (Flt Dir not gen!).

6th Nov 2008, 12:46
Only one gen on line equals only one flight director (on the same side), due to the bus tie breaker opening on glideslope capture.

6th Nov 2008, 13:13
Hello Starlet

I cannot guarantee that this also goes for the classics, but this is the 737-800:
The ILS receivers are coupled to DC bus 1- and 2.
If you look at the electrical schematics you can see that the DC Bus 1- and 2 are separated by a cross bus tie relay. When you arm 'appr' and capture the glideslope this cbtr opens to ensure that failure of one electrical source doesn't affect both ILS receivers.
With one engine out and APU inop, the ILS receiver on the side of the failed engine receives power due to the closed cbtr.
Upon glideslope capture this cbtr opens and isolates the two ILS receivers, leaving one without power.

Port Strobe
6th Nov 2008, 15:56
Again this is an NG thing however I don't believe the electrical system is too far removed from the classics, may also be true. In the scenario described above, according to the simulator at least, if the PNF selects their flight director to off, the PF can get their flight director back. Not exactly critical but might help a bit if in IMC.

6th Nov 2008, 16:51
correctomundo! Same on the classic