View Full Version : 737 Flashing fuel low pressure lights on T/O

29th Nov 2009, 06:54
As we have steadily been reducing our take off fuel due to various fuel saving measures, on short flights I now frequently observed the fwd fuel pump low pressure lights to be flickering on and off just after lift off.
Boeing does say that this can happen, and the action to take is to reduce the pitch until the lights extinguish.
OK, but this reduces the climb rate during the initial climb out, and you have to reduce pitch quite a lot to extinguish the lights.

Is there any published minimum/recommended minimum take off fuel amount anywhere for the 737-3-800's?
I'm just not comfortable in this situation, and what if one (or both) aft pumps should fail in this situation?

Any thoughts or suggestions (not involving 2 tons of extra fuel)?

29th Nov 2009, 08:30
Not sure about NG's and I'm sure that in the present climate most companies would choose to have you quietly 'ignore' the event, but when I started (on the 200) there was an 'urban (father to son type) legend' that Boeing said 4000 or 4200 (cannot recall which) as a minimum for departure. Certainly on the NG, 4200 will occasion only the briefest flicker on a fully 'reduced' departure - the problem comes when 'rigid' SOPs with some companies stop you reducing in some situations eg de-iced etc (which Boeing did approve on the Classic if asked).

29th Nov 2009, 08:49
Yes, the problem gets worse in a high thrust, low fuel (less than 4000kgs) situation. It happens frequently, but most pilots seem to take a; Oh well, that's the way it is, approach to the problem.
That works as long as all goes well, but with an additional problem it could get ugly.

29th Nov 2009, 09:28
I would be concerned how far from the fule uptakes are the fule sensors ie will the fuel uptake take a gulp of air and leave me with a flamed out engine?

29th Nov 2009, 09:43
The engines are feeding from the aft fuel pumps. This problem affects only the fwd pumps, but a 50% reduction of pump capacity immediately after lift off is not exactly an ideal situation.
I'm not sure how the system works, but I think a fuel low pressure light means precisely that, low fuel pressure, ie air in the pumps/lines.

29th Nov 2009, 11:24
Boeing assure us that the engines will run at that altitude without pump pressure. It is not a problem of fuel supply to the engines - the issue lies in the fact that we do not really like to see pumps running dry and there is, of course, a QRH procedure to follow if we do. 'Flickering' or short duration lights are not considered to be a 'problem' but I can fully understand the concerns that prompted the question.

3rd Dec 2009, 21:20
Apart from the Boost pumps,The fuel can flow to the engines even if the Wing boost pumps are INOP.Not so for the Center tank though.

Old Fella
6th Dec 2009, 04:58
As a retired Flight Engineer I find it inconceivable that departure fuel loadings have been reduced to a point where the forward fuel pumps are being allowed to cavitate on take-off as a regular occurrence. Just how short are the sectors being flown with such low fuel state at take-off? I would be confident there is a minimum fuel for take-off stated somewhere. Also there would be a maximum pitch angle limitation for low fuel states. If not, why not?

6th Dec 2009, 06:32
Even on our shortest legs (30 Minutes) I have never observed the Fuel Pump Low Pressure lights illuminate. The average fuel load with a close in alternate would be between 9 and 10,000 lbs.

Maybe I am just not looking at the overhead panel on take-off and a single pump low pressure in each tank would not trigger a Master Caution.

6th Dec 2009, 07:31
After nearly 10 years of shorthaul on the 737 (sectors with 35 to 45 minutes flighttime) i have seen it quite often. More often on the old classics than now on the NG though. Might have to do with the usual pitch value after take off, on the classics it was usually between 17 and 22 nose up, on the NG with all performance tricks used it is between 8 and 15 nose up. Main difference that we didn't use derates on the classics but do use derate in combination with assumed temperature and improved climb speeds on the NG which leads to very low power take offs on short sectors.

6th Dec 2009, 11:50
Look at what the MEL says about a pump inop.

From memory, engines tend to suck fuel ok, so even with no boost pumps working, fuel should get sucked ok, but also thought sucking is done by a low press sucking pump on the engine, on some engines, these can fail, engine will still work ok if a boost pump is working and feeds the hi press pumpn on the engine.

Look at what the MM says about running boost pumps with the low px lite on !!!

All will be ok until lady luck looks the other way one day, then things will be changed.

6th Dec 2009, 15:12
b737NGyyc agrees with post #2 (4000kg) - however does not actually know if his lights do flicker:confused:. They are actually in your peripheral vision, especially at night unless you look at your feet during t/off:)

:ok: on the pump warnings tech stuff however.

8th Dec 2009, 15:43
Don't know if still exist but center tank fuel pumps should be off for t/o if fuel in tank less than 2300kg. Select on when aoa reduces after acceleration.

9th Dec 2009, 19:45
Had all low pressure lights illuminate only a couple of weeks ago. First time I've ever seen it. By the time we'd looked at each other and said WIHIH they'd all gone out again. This was literally seconds after airborne. Engineers were baffled. Just under 5000kg in tanks. 300 series by the way.

9th Dec 2009, 21:58
Used to occur on the B744 too, short sector low fuel load (19Tonne), Solution.....increase timing in software before warning illuminates.....
Went up like a home sick angel though!!!

11th Dec 2009, 00:51
Do not rotate more than needed as MR. Boing says, problem solved.

11th Dec 2009, 06:18
Don't know if still exist but center tank fuel pumps should be off for t/o if fuel in tank less than 2300kg. Select on when aoa reduces after acceleration.

I have never seen that in any aircraft manuals that I have used, 200 300 or NG

11th Dec 2009, 07:31
If it's a fuel pick up issue isn't it irrelevant that the engine can run without the pump? I mean air is air and not fuel regardless of the pressure or am I missing something?

14th Dec 2009, 18:58
Thankfully ive never seen the main tank lights flicker ,irrespective of fuel load, (NG) but have seen the centre tank ones flicker a few times. Book says less than 453 kgs leave the pumps off, personally I leave them off if theres less than a 1000kgs and this usually keeps the low pressure lights out