Quote:
Originally Posted by N121PP
I have been on several flights in the past where the pilot mentioned the APU was not working, but we would be able to fly in spite of that. I remember you saying previously ITT that was not a problem. However, doesn't the APU provide power in case of total engine failure (I know a rare thing)? Do all planes have a RAM air turbine? Is this all playing the odds and that's why you can fly with a non-functioning APU?
We're required to have two sources of electrical power at all times while in flight. Those two sources are normally engine-driven generators. The APU provides a backup so that, for example, if one engine-driven generator is deferred (i.e. currently not working and slated for later repair/replacement) we can still make the flight by simply running the APU for the entire flight.
Most of the time, we shut down the APU after starting one engine and we taxi out using just the one engine. Fuel savings is big these days and the APU will use about 400 lbs/hr iirc (that's about a gallon a minute). We hold off on starting the other engine for the same reason and, when we do start the 2nd engine, we use bleed air from the first engine, instead of the APU, to spin the starter.
With two engines running, each engine provides electrical power to a separate electrical bus. When only one engine is running, its generator automatically picks up the load and provides power to both buses. So even if the APU is running, it doesn't share the load. It's simply there as a backup if the single remaining engine-driven generator fails. In that case, the APU generator automatically picks up the load of the both buses (i.e. it can power everything).
This discussion is for twin engine planes. On planes with more engines (e.g. MD-11, B-747, Airbus 340), there is even more redundancy in electrical supply.
Every Part 121 plane (airliner) has some additional form of backup for electrical power. The minimum requirement is that the ship's battery can provide at least 30 minutes of power for essential items (i.e. the Captain's avionics and communications).
The CRJ has an ADG (Air Driven Generator), which automatically deploys from the right front of the plane if all electrical power is lost. It has a small prop which turns in the slipstream to power a small generator capable of powering essential items.
CRJ Air Driven Generator
The 757 has a Ram Air Turbine (RAT) which deploys automatically if both engines fail. It's purpose, however, it to power the center hydraulic system so that the flight controls still work. For many 757s, the only electrical backup is the battery (i.e. the 30 minutes mentioned above).
On some 757s and 767s (including all of the ERs), there is an HDG (Hydraulically Driven Generator). If all electrical power is lost, the center hydraulic pump turns a small electrical generator to power essential items. This is necessary for international operations, obviously, because the 30 minute backup provided by the battery might not be enough to get on the ground.