Fuel Tank Rules

[Airbus_A320]

This is something I have been wondering about for a while.

What exactly is the airworthiness directive that was issued regarding fuel tank sparks/explosions. Was it something like keeping the tank filled to a certain amount all the time to prevent sparks from igniting the vapors? And if so, what happens if you need to use all the fuel in the tank to get the required range?

Just wondering, something I've wondered for a while, just always forgot to ask.

[ExpressJet145]

Most aircraft dont have AD's for low fuel as far as Im aware...fuel pumps is what can cause a spark, and that usually happens when the tank is empty and not sucking on anything. That is the reason why all 757s have to have 1000lbs in the center tank at all times on any flight per most airlines. That was the reasoning, or part of, of TWA 800. The fuel tank causing sparks.


Edit: Im sure you are aware no airliner would run the tanks dry, however each aircraft has a minimum allowable for fuel inflight. All of which vary depending on aircraft. After a certain amount, aircraft can start havin pnuematical and hyrdrolic failure, per a UAL Airbus Captain I heard once. Not sure if that is infact accurate .

[Airbus_A320]

Ah... that makes sense then. 1000lb isn't a whole lot, I thought it was a lot more than that. I guess the pilots would just turn off the pumps for the center tank once it reaches that level?

But I think I remember reading that the fuel level meter is what sparked in the TWA 800. And what happens when fuel level goes down too low to cause pnuematical and hyrdrolic failure?

[ExpressJet145]

Ah... that makes sense then. 1000lb isn't a whole lot, I thought it was a lot more than that. I guess the pilots would just turn off the pumps for the center tank once it reaches that level?

But I think I remember reading that the fuel level meter is what sparked in the TWA 800. And what happens when fuel level goes down too low to cause pnuematical and hyrdrolic failure?

To be honest im not sure, maybe AJ can spread some light on that issue.

It was the FUEL PUMP sucking dry air which caused a spark from what a 757 captain told me. So thats why they keep the tank went no less than 1000lbs at all times, or atleast try to. Its not required, although on some airlines it is. Its recommended.

[MCM]

Pneumatic and Hydraulic Failure?

The decrease of fuel isn't going to cause Pneumatic and Hydraulic Failure until the engines stop running because there isn't enough... and then, you've got a rather major problem, in the fact you have no engines running!

There have been various measures on different aeroplanes for Centre Wing Tank fuel levels. Because of minimum reserve fuel, the wing tanks aren't really a problem... if the fuel in the wing tanks is below the fuel pump level, you have bigger problems on your hands.

With the CWT, you do try and use all the fuel from it. So, certain aeroplanes had policies implemented that prevent the use of all the fuel, so the pumps remain covered by the fuel at all times. Newer aeroplanes are often made with different styles of pump where this isn't really an issue.

[ExpressJet145]

I'm just going by what every pilot I've ever met has told me. Once you reach low fuel levels the aircraft starts shutting down electronically and pneumatically. No idea how or why. But numerous UA and US captains have told me that. Can an airline pilot who knows shed light on this?

[MaxPower]

I was just watching the Avianca 707 Flight 52 crash on NG channel.

Why I mention this is because they ran out of fuel, seeing how many minutes they have left of fuel was staggering. Yet the ATC kept them in air, although the flight crew requested to be prioritized due to low fuel. I know it's a sad relation to this thread, but I just felt I should post it. When was the 1000lb (requirement at all times) fuel set up ? And how many minutes do the flightcrew have untill it switches over to the reserve fuel tanks, considering they're still maintaining the same or less speed.

Avianca Flight 52 had been in a holding pattern over New York for over one hour due to fog limiting arrivals and departures into John F. Kennedy International Airport. During this hold, the aircraft was exhausting its reserve fuel supply.

On Thursday, January 25, 1990, the aircraft performing this flight, a Boeing 707-321B registered as HK-2016, crashed into the town of Cove Neck, Long Island, New York after running out of fuel. 73 out of the 158 passengers and crew on board were killed.

[MCM]

MaxPower:
There were a lot of issues involved in the 707 crash.

They have nothing to do with the CWT fuel carried for the fuel pump issues though.

I really don't have time to list them all, but there are some interesting CRM studies done on the crash... highlighting the fact that the Autopilot was inoperative... the captain had hand flown the aircraft all night, the captain's English was poor, and the F/O was the only one who could communicate with ATC, so the captains urgency on the situation never translated well through the F/O to ATC, and they never used the magic emergency words that would have got them priority, there were ATC handover issues whereby they didn't pass on the information that they had low fuel due to the horrendous weather conditions in the area and the large amount of traffic, the fuel guages on the 707 weren't particulally accurate, so you didn't know exactly how much fuel was on board, amongst other issues.

As to "Reserve tanks", for example the 747 has reserve tanks, but they are not the last tanks to be drained of fuel... the fuel from these tanks is transferred into the inboard main tanks after they reach a certain level. (quite high, you still have over 60t at this stage). The last fuel you use comes from the main tanks (on the 747 anyway).

The CWT fuel issue is PURELY to keep the pumps covered... its not got anything to do with minimum reserves of fuel. It just means, that when they calculate minimum fuel, they have to consider the fuel in the CWT as UNUSABLE, so all fuel calculations are done on not requiring it. You pretend its not there.

Just for general info, a 767 I'm guessing would normally have roughly 4400lbs of fuel remaining after touchdown as an absolute minimum legal fuel... although rarely would anyone land with the absolute minimum. If you are going to land below this figure for some reason (maybe a leak for example) then you would have to declare an emergency (the proper way) and not be asking... be telling ATC what you are doing.

ExpressJet145.

So you know how the systems work... The Hydraulic, Electric and Bleed Air (pneumatic) systems work from the engines. As the engine runs, air is taken from a compressor stage in the engine (so pressurised) that is used in the pneumatic system. Electrics are also provided by a generator (IDG) that is attached to the engine to generate the electricity for the aircraft. Hydraulic pumps can be driven by either the electric or pneumatic part.

These systems are independant of the fuel state, until there isn't enough to run the engine, in which case the generator/pump/bleed air system won't work. Its the same as if you shut down the engine for other reasons... all these generators stop.

Of course, there is more than one source for this power... each engine usually has one system, and in the case of a twin, there is normally another source as well.

In addition, the APU is usually able to power an electric bus, and the bleed air system. An air driven hydraulic generator will provide hydraulic support.

In addition, Twins (and I'm told the A380) have a RAT - Ram Air Turbine. This will provide limited electrics and hydraulics in the case of failure of both engines.

Of course, the specifics change on aircraft type.

So, I can assure you, being low on fuel will have no affect on the pneumatic, electric or hydraulic systems of the aircraft, until the point that the engine can no longer run, or starts surging due to the inadequate supply of fuel.

Although I think you wanted an airline pilots perspective... AJ seems to be the resident expert: care to support me AJ, or have I got it backwards ...

[ASpilot2be]

Hey MCM, you seem to be quite knowledgeable about all this. I am curious as to what your profession is?

[MaxPower]

MCM, Great post !

Thanks for enlighten us about the reserve fuel "transfer" to the main tanks in general.





MP

[474218]

To be honest im not sure, maybe AJ can spread some light on that issue.

It was the FUEL PUMP sucking dry air which caused a spark from what a 757 captain told me. So thats why they keep the tank went no less than 1000lbs at all times, or atleast try to. Its not required, although on some airlines it is. Its recommended.

On TWA 800 the center wing tank pump was "off" and the tank was "empty", with only residual fuel (fuel that can not be used) left in the tank. The source of the fumes. Even though the pump was off there is still electrical power in the cable (wire) leading to the pump. A spark from the electrical wire is what caused the explosion. But keeping a mininum of 1000 pounds of fuel in the center tank, the pump and electrical wires are covered with fuel and no sparks can occur.

[LRJet Guy]

I believe it was discovered that the aircraft involved with TWA 800 had numerous write ups with cabin lighting flickering, not working, interphone problems, and some other things. The common denominator in all of them was that wiring for those services was routed in and near the center tank. It is entirely possible that it was not only wiring to the pumps that caused the spark.

Side note. One of our captains saw it happen in its entirety. He was actually looking at the aircraft right before it exploded. He and his FO were taken offline for a while and deposed by the Feds. Not the Feds that are "here to help" either.