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Am I correct to assume that every electrical system on an aircraft can be manually switched OFF/ON by the Pilot OR is there indeed any system, on which the Pilot does not have the ability to switch OFF/ON ?

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everything has a fuse that can be pulled –  ratchet freak Jul 21 at 15:15
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@ratchetfreak - not so, if the engine is running, it's computer can not be shut dow by either switch or fuse/breaker. –  Steve H Jul 21 at 16:31
    
@SteveH you can still shut down the engine it is connected to –  ratchet freak Jul 21 at 16:41
    
@ratchetfreak - not in flight, shuting the engine off simply send an electrical command to the computer to shut fuel flow off, but it does not shut the computer off. the engine will windmill and the computer will remain powered after shut down. –  Steve H Jul 21 at 17:01
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In the event of complete aircraft wide electrical failure (or some crazy pilot pulling every breaker in the aircraft), it should be comforting to know that the FADEC will continue to keep the engine running. for that it needs a power source independent of the aircraft power. The designers felt that there is no reason the crew needs to have control over this power source. Indeed, they probably have other things on their minds at the moment like the boss complaining that the internet is down ;-) –  Steve H Jul 21 at 18:19

3 Answers 3

up vote 9 down vote accepted

One example of a system that cannot be switched off by the pilot that is probably more common to all other airframes than just the one airframe I'm indicating.

Whenever electrical power is applied to a Gulfstream G550, whether it is powered by battery, generators, an apu, or some external power source, each engine's FADEC Electronic Engine Control (EEC) computer will be powered and the pilot can not switch them off.

The pilot could only indirectly turns them off by switching off all power sources.

However, when the engines are running, he can't even do that since there is an engine-driven generator dedicated to powering the EECs in the event of a total power loss.

In fact, while the engine is running or even windmilling above 35% rpm, the EEC's dedicated generator is supplying all power to the EEC and there is no switch or circuit protection that can be manipulated by the pilot to interrupt it.

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The pilot is generally given ultimate control over everything in the plane, when it comes down to it. Just like your car, they have to be able to shut everything down at the end of a flight. Take a look at this 737 classic overhead panel:

737 upper panel electrical controls
Original: http://www.angelfire.com/il2/aphs/73oh2.jpg

You will notice all important switches have an "OFF" position. The black and red switch covers protect the main power connection switches, which you generally want to be "ON" during the flight. The BAT switch at the top connects the battery to the DC electrical bus. The switches below that control the switching of standby power, and the connection to the engine generators. At the bottom are the controls to transfer between different power sources.

Of course there are many other switches for different systems. Which systems have dedicated switches will depend on the make and model of the aircraft. As others have commented, some low level systems like engine control may be absolutely necessary for some things to function, and will not be easy to switch off.

As ratchet freak commented, they also have access to the circuit breakers. Here is another picture from the 737, behind the first officer's seat. The pilot's handbook has instructions for what breakers should be switched off in what circumstances. Outside of those instructions, the effect on other systems of pulling one breaker could have unexpected consequences. The circuit breakers can be used to switch off equipment as a last resort.

737 Circuit breaker panel

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In the name of safety, I don't agree completely with the idea of circuit protection being a responsible part of this answer. The pilot should not feel free to arbitrarilily make the decision to pull a CB. He has a reference handbook that tells him what CBs to pull in what situations and to go outside of that can make a dangerous situation. But I conceed perhaps a dire situation may warrant it. Anyways, because of this, there are many systems that can not be directly switched off and have no guidence for crew to deactivate by circuit protection. –  Steve H Jul 21 at 16:54
    
@SteveH I agree, thanks for your comment. I will try to edit the answer to be more clear about such points. –  fooot Jul 21 at 16:59
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The black and red switch covers protect the main power connection switches, which you generally want to be "ON" during the flight It not only protects them, but also makes them quick and easy to find; I don't know much about airliners, but in smaller aircraft, the first instinctive reaction to smoke in the cockpit would be to go for the electrical master switch. –  falstro Jul 22 at 6:37
    
This answer sort of depends on which system and which airplane. There are many circuit breakers outside of the cockpit that the crew cannot access. –  Sports Racer Jul 22 at 13:11

On an R22 helicopter (I quote from the Pilot Operating Handbook):

Each tach, the governor, and the low rotor RPM warning horn are on separate circuits. Either the battery or the alternator can independently provide power to the tachs. A special circuit allows the battery to supply power to the tachs even if the master battery switch is off.

It is clear that the designers intend the the tachometers should work at all times, and there is no "off switch".

To Falstro's point, the procedure for an electrical fire in flight includes turning off the alternator and battery master, and this is the precise moment when I am pleased to find that the tachometers are still working, since the governor and RPM warning horn are not.

It is true that there is also a circuit breaker for the tachometers, but there is no procedure, emergency or otherwise, which demands for these to be pulled. The only time I have ever known these to be used is by instructors to simulate emergencies for training purposes.

The R22 is a pretty simple system and I don't know much about airliners but I imagine that it's impossible to access the flight data recorders in flight and I'm sure the designers try quite hard to make sure that they have power under all circumstances including catastrophic aircraft failure and hijack.

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welcome to Aviation.SE! You have made a very nice first answer, to which I have made a few simple spelling edits and put your quoted paragraph into a quote block. –  CGCampbell Jul 22 at 12:24

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