Loss of all engines due to fuel exhaustion
 

Loss of all engines due fuel exhaustion is rare although the Air Canada 767 `Gimli Glider` is the most publicised example. There have been several dead stick landings by jet transports. Some aircraft have RAT's that supply electrical and hydraulic power to vital services. Others do not have RAT's and the Boeing 737 series is included.

Although dead stick landings are rarely practiced in simulators the exercise is tricky enough especially if no previous simulator practice. The situation gets grim if the reason for a dead stick landing is fuel exhaustion since the APU becomes inoperative and in turn that may cause flight control difficulties because of no hydraulics.

Boeing 737 hydraulic systems have standpipes that prevent total fluid loss. I am sure other jet transports have similar design precautions.

I wonder if the same standpipe principle could be incorporated in aircraft fuel tanks that feed fuel to an APU. With total fuel exhaustion resulting in no APU operation the result could be disastrous. A standpipe installation in an appropriate fuel tank could be designed to maintain (say) two hours of APU fuel availability as a precaution against loss of all engines caused by fuel exhaustion. Any thoughts?

Not a design/certification requirement.

APU can have MEL applied so Murphy's Law dictates that your fuel exhaustion event will coincide with your APU being unserviceable.

With no noise from the main engines, time from max cruise ALT to granite is approx 20 minutes in a commercial airliner - why the need for 2 hours APU ops?
If you want an extra 2 hours for APU ops, then why not mandate taking an extra 2 hours fuel for all your main engines?

Standby power/batteries/RAT/static inverter is a well designed and certified system for the fuel exhaustion/loss of elec generators scenario.

- yup! I think most of us would prefer any remaing fuel to be available for the engines? You would have to feed the engines from the standpipe and the APU from the botom of the tank, so on that one buttock-clenching day at 3 miles finals the motors quit but you crash with an APU running beautifully!

Now, perhaps a switchable reserve tank....? Then it is your choice what to do with the gas.

Two hours? I doubt the plane will be up there for that long with the engines having no fuel left.

Its not like an automobile stalled along the side of the road. We can't send the first officer walking back with a gas can to fetch some more.

Or you could just be vigilant about fuel loads and gauging and land before it runs out.

- not sure that will catch on............

Or the leak will be in the standpipe.

737 does'nt need a RAT. It has a manual backup flight control system.(or non electric fly by wire:))

Fuel contamination is probably about as likely as simple exhaustion. In which case a segregated reserve of fuel for the APU doesn't help.

It's a cert requirement that the aircraft be controllable after the loss of all engines. The 737 can be flown without hydraulics. The 747 (prior to the -8) requires hydraulics, but the windmilling engines will provide sufficient hydraulics to control the airplane in all engine out condition (as been demonstrated during a couple volcanic ash encounters) - when we went to the 747-8, the new wing required more hydraulic power to control, while the new engines windmilled at lower speeds and provide less hydraulic power, so the -8 got a RAT. The 757/767 required more hydraulic power than the windmilling engines would provide and got a RAT from day one.
Also, there is a cert requirement that electrical power can be supplied to the essential systems for at least 30 minutes after the loss of all electrical generation, hence the need for a big battery. In addition the 777 has a generator on the RAT (I'm thinking the 787 does as well but I'm not positive about that).

The problem after an all engine power loss (regardless of cause) is not that the airplane won't fly acceptably, it's that if you don't have an airport within gliding distance your chances of a happy outcome are not very good. The Gimli glider, Sully's miracle on the Hudson, the TACA 737 that landed on the dike, and the A330 that did a dead stick in the Azores all demonstrate the problem is not lack of hydraulics or electrics to control the aircraft, the real problem will be lack of thrust to get to a suitable spot to land.

I think that rather than invent a way to keep the APU running, the emphasis should be on preventing all engine power loss events.

The APU is just a nice-to-have. The RAT and free-spinning engines will provide enough hydraulics and electrics to retain controllability.

US FAR 25
 

§25.1553 Fuel quantity indicator.
If the unusable fuel supply for any tank exceeds one gallon, or five percent of the tank capacity, whichever is greater, a red arc must be marked on its indicator extending from the calibrated zero reading to the lowest reading obtainable in level flight.

The stability required to achieve a successful outcome sans hydraulics in the B737 is reliant to a large degree on available engine thrust, which can assist with both pitch and yaw.

When the engines go quiet that assistance is lost, so unless you can provide the equivalent of 3000 PSI of pressure the outcome may not have a fairytale ending.

The last one I flew had 2 hydraulic systems and a manual backup. Actual wires to the control surfaces. Perhaps the new generation is different?

Prior to my retirement (in 2008) The B757/767 QRH had no provision for inability to restart either engine after both had failed. Is that still the case? Although not a requirement for rating renewal I would occasionally suggest to crews in the sim that they might like to practise dead stick landings just for the experience. I added a section in 'How To Do Well In The Sim' to give some guidance but this input was never 'official' and I'm not sure of its efficacy.