U/s APUs

[dghob]

Waiting for departure on board a BA flight (747-400) from Singapore to London a few weeks ago. The cabin temp was reasonably high but not enough to worry about. Eventually one of the flightdeck crew came on the PA to say that due to the auxilliary power unit being inoperative the cabin temperature was in fact a bit high (accompanied by apology), and that the cabin lights would now be switched off while a ground unit got the first motor started. After what seemed to me a few attempts the motor was started and the lights came on, the other motors were started and off we went.

None of this worried me but it did raise a question, namely, how important is the apu once the aircraft's main motors are running? I suspect that it isn't important at all as long as the destination port has adequate ground facilities, but I recall an airline advertisement from many years ago that told us that if the red light on the coffee urn wasn't working, the flight wouldn't leave.

Any comments?

Cheers, dghob

[BlueEagle]

The APU being un-serviceable is an 'allowable' defect as it's function has no bearing on flight safety whatsoever and, on the B744, cannot be started in the air.

It would be unusual for any airline to allow such a defect to persist beyond a return to it's base, within the planned schedule, where the fault will be rectified. The B744 tends to operate to fairly well equipped airfields where ground power and air supply are available and at Singapore you can also get conditioning air for the cabin but it is not as effective as the APU.

The red light on the coffee maker is probably just an advertising 'blurb'!

[HZ123]

Once the engines are running the APU contributes nothing and is of minor importance. In the event of power failure while airborne there is an auxiliary aircraft power unit that could be deployed in the form of a large propeller that spins and generates power to re-ignite an engine.

That said the air con should have been run from a ground based unit, in this event, but as yoiu have stated the temp was bearable and to employ a mobile unit costs more. Thank you for using the right airline.

[yachtno1]

How intriguing HZ123. Which a/c type are you referring to ?

[Paul Wilson]

I'm no proffesional, but I was under the impression that a RAT (Ram Air Turbine I believe) was a bit like scraping the bottom of the barrel. Used when just about everything else has gone pear shaped, and only able to provide limited electrical and/or hydralic pressure. More of a device to enable you to choose where you are going to crash, and to do so in a bit more of a controlled manner. Although I believe I heard that the Air Transat Airbus deployed one after they ran out of fuel mid Atlantic.

I was also under the impression that on some types the use of an APU on takeoff allows a bit of extra performance from the engines, as bleed air from the engine can be selected off, which would give a bit more thrust.

[seat 0A]

Yes and in case of a loss of both engines, we unfold a large propellor and helo our way back to earth.
So nothing to worry about

[yachtno1]

I hate to admit it but I have actually deployed the RAT on a Bus ..I went for the wrong switch... well the cockpit was dark ..!

[FlyingForFun]

If the red light on the coffee urn wasn't working, the flight wouldn't leave

I was a pax on an A319 a few months ago, when we were a little late getting off the ground due to a coffee machine. Taxied to the holding point for the runway, and then turned right, despite the runway being on our left. I watched several other aircraft jumping ahead of us in the queue and getting airbourne, before the captain announced that one of the coffee machines was overheating, and he'd rather get the problem sorted out on the ground than in the air. This was followed by about 10 to 15 minutes more waiting, with cabin crew moving around with concerned looks on their faces. Eventually, the captain announced that they had been unable to fix the coffee machine, but they were going to switch it off as they were pretty confident the remaining coffee machines would be able to supply enough coffee for the flight.

Sorry to drag your thread off-topic!

FFF
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[sky330]

All depends of circonstances and type of aircraft.
As said on 747, the APU is unusable in flight so it really doesn't matter. The only question is do they have a ground cart to start the engines again at destination.
On A340, you loose part of your redundancy, but it is allowable 'defect' as with four generators and bleed system, safety is cerainly assured.

On twins, APU u/s is, as a rule, allowed with restrictions, for instance, it need to be operating for most ETOPS operation or long overwater flights.

Ram Air turbine on the bus, is an flow air driven hydraulic generator. It given you some hydraulic power in case of really dramatic scenario. Not to be confused with the 'emer gen', an emergency electrical generator using hydraulic power to produce electricity. Now, you can use the RAT to power the hydraulics that power the 'emer gen'. ... but it sure is not your day

Wasn't the DC-10/MD-11 also equipped with a similar system??

[Mr @ Spotty M]

The APU in the early days of ETOPS had to Serviceable for an ETOPS sector. That changed many years ago.
The only reason for the APU to be serviceable now is for turnrounds to save on GPU, Air Con cart and Airstart rig.
It is handy also for bleed off take-offs if you need the extra engine performance.
The main reason is if you loose one of the engine Generators and the APU is U/S, so is the aircraft and it Hotel time for the pax.