"RST Engineering" wrote in
m:

.
.

It has been nearly fifty years since I went to Boeing school, so
please forgive the misrememberances. As I recall, once the engines
spool up and take the load, the APU is shut down and once the aircraft
has departed, the squat switches have to be enabled to restart the
APU. It cannot be started airborne.



At least that is my memory. The reason I say that is that I was
trying to do some autopilot work for which I needed electrics and
hydraulics and with the airplane on jacks, I couldn't get the APU to
fire up. Perhaps one of the sparkies pulled a breaker somewhere so
that I couldn't accidentally activate the gear via the
APU/electrics/hydraulics, but I also remember something in school
about it.

You remember correctly, but only for the transversely mounted APUs in the
707 and 727. All those tail mounted APUs can be run throughout the flight
and sometimes are if, for instance, an engine driven genny is U/S. They can
be started in flight, even, and for Cat 3 landings they usually ae, though
this is usually not a requiement to do the Cat 3


Bertie