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What If A Plane Loses Electrical Power?

Tue Aug 13, 2002 2:17 pm

Dunno if this should be in tech ops or general aviation. Move if needed!

Hypothetically, if a plane lost all electric power, is there a way for them to "flip a switch" and ATC knows the plane is in trouble? Do the radios have their own backup, or is it if a plane loses power, there is no way to communicate, other than to just find a place to land and just do it?

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RE: What If A Plane Loses Electrical Power?

Tue Aug 13, 2002 2:20 pm

Most planes have enough redundancy that at least the transponder will work. They can put in an emergency code.
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RE: What If A Plane Loses Electrical Power?

Tue Aug 13, 2002 3:55 pm

If a plane were to lose "all" electrical power, there is no "swich" to save the day.

But as Flyboy said, aircraft are redundant (as in generators, backup- generators, batteries, and back-up batteries) The odds of a total electrical failure are slim to none.

Break ground, and head into the wind. Don't break wind and head into the ground.

RE: What If A Plane Loses Electrical Power?

Tue Aug 13, 2002 4:58 pm

Dear Omegous -
Do not know if your question is for light airplanes or for airliners...
I have an answer for the two extremes, I fly a 747 commercially and own a little L-21 Super Cub for fun flying...
That little L-21 has a battery and a generator, but if the battery is dead and no jump cables available, I can start it with the propeller, the engine spark plugs operate with magnetos, which are completely independant from the aircraft's own electrical system...
In flight you may have a failure of the generator, meaning your battery will no longer get charged, same as in your car, you can fly the aircraft with the battery. I dont have much electrical items, only 1 VHF comm radio, transponder, position lights and landing lights... The battery would probably last for the rest of the flight... Bear in mind, with the VHF comm radio, that transmitting requires much more power than receiving only, you might inform the controller that you avoid transmitting to save your battery power...
Going to the other extreme, the 747 has 4 engine driven generators, 2 APU generators, and battery for last back-up... There is so much redundancy in the 747 electrical system, that it is legal to depart with one failed engine driven generator. The airplane electrical system would even work fine on just 2 generators, except that you might need to restrict the electrical draw on the galleys, warming up meals takes a lot of power...
Some of you will ask about the APU as source of emergency electrical power, unfortunately, though we can use the APU in the air, we cannot use the APU to provide electrical power, we can only use it to provide bleed air, for air conditioning and pressurization only... so forget the APU... a little note also, the APU generators and engine driven generators can be traded, they are the same... but surprisingly, we rather dispatch the aircraft with an engine generator inoperative, rather than one of the two APU generators, (ground handling reasons) should no ground power unit be available where we go, this especially for our cargo airplane (loading/unloading of pallets)...
Except for a few instances of the AC power of the engine generators being "knocked-off" by i.e. a lightning strike, which has happened a few times to 747s, power can generally be regained by going through a "loss of all generators" check-list... If not, the batteries will supply emergency power for some 30 minutes they say... I do not know of any such instances having been the case with a 747...
(s) Skipper
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RE: What If A Plane Loses Electrical Power?

Tue Aug 13, 2002 6:36 pm

Regarding our (SWA's) 737 fleet: In an emergency situation, the APU generator is capable of generating electrical power up to 35,000 ft, but you might have some trouble getting it started as it would be "cold-soaked" at that altitude. For continuous operation the altitude operating limits are as follows: APU electrical and pneumatic power is available up to 10,000 ft and pneumatic or electrical power (but not both) is available up to 17,000 ft.

Failing that, electrical power to run essential systems would be provided by the battery as follows: DC standby power would be available from the battery bus and AC standby power would be available from the battery bus through the static inverter. However, that AC power would be single-phase only and incapable of doing any real work (like running the electric motor-driven hydraulic pumps which require three-phase power for operation). But as long as your engine-driven pumps were functional, you would still have adequate hydraulic pressure.


Patrick Bateman is my hero.
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RE: What If A Plane Loses Electrical Power?

Tue Aug 13, 2002 7:12 pm


some A/C's, 767/757 have the turbine that can be deployed under the fuselage that produces electrical power. If my memory serves me correct, it's located aft the main fueltank.
Any corrections of my post is much appreciated.  Smile
When in doubt, flat out!
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RE: What If A Plane Loses Electrical Power?

Tue Aug 13, 2002 9:20 pm


Almost correct. On the 757/767 The Ram Air Turbine (RAT) is a small propeller which presents itself to the airstream whenever the airspeed is above 80 knots and both engines fail. It supplies power to the centre hydraulic system. Once extended it can only be re-stowed on the ground.

It is located in the body fairing aft of the right main gear.

Provided airspeed is above 130 knots, the RAT can provide adequate power for normal centre hydraulic system operation. There is a manual control for extending the RAT via a guarded switch on the overhead panel, in case it fails to deploy automatically.

A common misconception is that the RAT provides electrical power, it does not it only provides hydraulic power. Sufficient centre system hydraulic pressure provides power to the Hydraulic Driven Generator (HDG), which in turn provides electrical power as a fourth AC generating source, but the RAT only provides power to the flight controls portion of the centre hydraulic system.

The HDG kicks in to provide electrical power in case of loss of both AC busses.

Hope this all made sense?!
I used to love the smell of Jet-A in the morning...
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RE: What If A Plane Loses Electrical Power?

Tue Aug 13, 2002 9:48 pm

Thanks for the correction Rick.
Chould have known better then that, cause I got the RAT explained to me by a tech ones. Was some years ago, but anyway.
The word senil comes to mind.  Big grin
When in doubt, flat out!
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RE: What If A Plane Loses Electrical Power?

Tue Aug 13, 2002 11:48 pm

As for G/A aircraft, there are lots of variables to this situation and the main one is the single or multi issue. Even at that, some singles are equipped with dual alternators and some early twins had only one. Generally, since the engine ignition is magneto driven, there is no issue with the engine(s) conking out. If you take the example of a single with one alternator, the situation can quickly get dicey at night or in IMC. I had an alternator fail at night in the traffic pattern while shooting my "three to a full stop" and, even though I quickly shut everything down but the essentials and only turned on the landing lights over the fence, the battery was going fast. Don't take that 30-minute rule too seriously.

The worst situation in such an aircraft would be night IMC with electrically driven gyros and no vacuum redundancy, no flashlight and no handheld com. You are going to be quickly sitting in a totally dark cockpit trying to fly on basic instruments with no way to even say "goodbye, cruel world" to anybody. On the other hand, you are an idiot for being in that situation in the first place.

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