Canoecarrier From United States of America, joined Feb 2004, 2844 posts, RR: 13 Posted (10 years 10 months 2 weeks 1 day 2 hours ago) and read 3812 times:
First post in the Tech Ops. Kind of like a non-swimmer in the deep end, so please be easy on me.
I worked a TW flight a few years ago at COU east of STL. I remember it was really late and we were called by TW to see if we would stay around to fuel the plane when it came in. It missed approach the first run, and when it did finally make it in, they taxied to a remote stand and shut down.
Long story short, we didn't have a GPU powerful enough for startup. The crew told me that low fuel procedure was to dump the fuselage tank into the wings, basically exhausting the only tank usable for the APU. So, when the plane shut down, and we didn't have a GPU, they couldn't start up.
The fix was to gravity feed fuel (2 hours later after lots of calls to STL MX). I turned a valve under the #2 leading edge slot which allowed fuel to drain into the center fuselage tank. After enough fuel was in they were able to start the APU off the battery.
I guess what I'm asking is, did this happen a lot? And if so, wouldn't this happen frequently?
As a side note, I was a junior in College, and the plane had just flown the U of Illinois football team to play U of Iowa. It was full of catering. The FA's were so happy to get back to STL they gave me and the crew I was over all the beer, banana's, and subs on the plane. My roommates thought I was a hero for a week.
SlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 67
Reply 1, posted (10 years 10 months 2 weeks 1 day ago) and read 3755 times:
Canoecarrier 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-8D 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.
B747skipper From , joined Dec 1969, posts, RR:
Reply 3, posted (10 years 10 months 1 week 6 days 5 hours ago) and read 3627 times:
Dear Canoecarrier -
The fuel system of 727 is very simple...
3 tanks... Left wing is nbr. 1 tank, center wing is nbr. 2 tank, right wing is nbr. 3 tank.
Fuel feed - tank to engine for takeoff...
After climb established, flaps up, feed the 3 engines from nbr. 2 tank (if it has more fuel)...
When fuel amounts equal again in the 3 tanks, back to tank to engine.
Landing with LOW FUEL - tank to engine, all crossfeeds OPEN.
You CANNOT transfer tank-to-tank in a 727 (nor in a 707)...
Honest, your desciption of nbr. 2 tank empty. Must have fed 3 engines from nbr. 2 tank until dry.
Stupid and dangerous thing to do...
I have not flown a 727 since the Jurassic era, but hell, so simple, that I can even remember.
Canoecarrier From United States of America, joined Feb 2004, 2844 posts, RR: 13
Reply 4, posted (10 years 10 months 1 week 4 days 4 hours ago) and read 3547 times:
All I can say is I remember the TW crew specifically saying that they had "dumped" the fuel remaining in the center tank #2 into the wings. My recollection is that we were their alternate airport and they had missed twice into STL, so the fuel being quite low in the #2 tank is believable.
Again, we didn't have the 115V GPU or a Huffer, so the fix i.e. to get fuel in the #2 so the battery could start the APU, makes me believe they had nearly no fuel left on board. Whether or not they were actually able to crossfeed into the wing tanks, skipper seems to seem that is improbable. Thanks for your comments.