My B-727 manuals are three time zones away and I've been away from it for a long time, but as no one else has rung in on this one yet I will.
I don't recall the details of the B-727 fuel system but a few generalities: Most jet transports have the APU feed from one place, but if you have normal electric (115V 400Hz AC
) you can open valves and operate pumps to get fuel pressure where you need it.
Large jet engines (like the P&W JT
on the 727) require about 30-40PSI of air pressure (and a good volume of it) to crank the starter. Small jets may have electric starters.
The ignition system on the 727 does have a battery start mode, and in fact I think the high energy igniters (20 joule?) are off the battery bus.
The APU will take care of all these things, normal electric and pneumatic pressure, and start the engines.
If you had only 28VDC ground power you probably would not have been able to start them. If your air supply was not of sufficiently high output you probably could not help them start.
I don't have any 727 checklists handy so I can't help with what the crew might have done to get themselves into this pickle. If there had been just enough usable fuel left in the tank supplying the APU they might have been able to get it running just long enough to get its AC
generator online, which would have instantly given them all the AC
fuel pumps on the airplane.
Something like this happened back in the early 1980s with an MD
-80 over the Rockies somewhere. The crew ran the center tank dry and both engines flamed out - and their generators dropped offline, with just about full fuel in the wing tanks. The plane was above the altitude where gravity feed would deliver sufficient pressure to the fuel controls. The APU start circuit is wired so it can't be started in flight on battery power alone.
As I recall they happened upon just the right attitude in both pitch and roll to allow the unusable fuel in the center tank to cover the port supplying the DC start pump. As they had battery power, it delivered a little bit of fuel to the engines and it was just enough to start one engine briefly.
That was enough. When the engine came up into the speed range the AC
generator gave them back the AC
pumps in the wing tanks and they got both engines going again.
Very lucky crew.
I relate this because I believe the systems are somewhat similar.
Now maybe someone who really does know the B-727 will show up to set it straight.
Happiness is not seeing another trite Ste. Maarten photo all week long.