How long does an emergency battery on a jet has to power the systems legally?

[Diamondpilot]

I know that an emergency battery on a plane is required, by law. How long does an emergency battery in a plane like a B737-800 has to power the electrical systems? In europe.

I hope someone knows the answer :D

[Intruder]

For a US certified airplane operating in Europe, FAR Part 25 -- AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES -- would apply:

25.1309(a) The equipment, systems, and installations whose functioning is required by this subchapter, must be designed to ensure that they perform their intended functions under any foreseeable operating condition.
(b) The airplane systems and associated components, considered separately and in relation to other systems, must be designed so that—
(1) The occurrence of any failure condition which would prevent the continued safe flight and landing of the airplane is extremely improbable, and
(2) The occurrence of any other failure conditions which would reduce the capability of the airplane or the ability of the crew to cope with adverse operating conditions is improbable.
. . .
(e) Each installation whose functioning is required by this subchapter, and that requires a power supply, is an “essential load” on the power supply. The power sources and the system must be able to supply the following power loads in probable operating combinations and for probable durations:
(1) Loads connected to the system with the system functioning normally.
(2) Essential loads, after failure of any one prime mover, power converter, or energy storage device.
(3) Essential loads after failure of—
(i) Any one engine on two-engine airplanes; and
(ii) Any two engines on three-or-more-engine airplanes.

25.1331(a) For each instrument required by §25.1303(b) that uses a power supply, the following apply:
. . .
(2) Each instrument must, in the event of the failure of one power source, be supplied by another power source. This may be accomplished automatically or by manual means.

25.1351(d) Operation without normal electrical power. It must be shown by analysis, tests, or both, that the airplane can be operated safely in VFR conditions, for a period of not less than five minutes, with the normal electrical power (electrical power sources excluding the battery) inoperative, with critical type fuel (from the standpoint of flameout and restart capability), and with the airplane initially at the maximum certificated altitude.
The only hard time limit I can find is the 5-minute limit in 1351. Generally, 30 minutes battery power to essential equipment is available.

[Piltdown Man]

I think you need to poke about in EASA | European Aviation Safety Agency (http://www.easa.europa.eu) and go to regulations. Tell us when you know the answer but I'll tell you for sure, the number won't be what you are expecting - it will be considerably (frighteningly) less. Long live the RAT.

[EEngr]

I think 25.1351(d) pretty much defines the capacity. Back in my day at Boeing, we used 30 minutes (for the 767 and 747-400). But an argument can be made to extend this if the airplane can glide for a longer time from maximum certified altitude all the way down.

In other words, enough power for the essential systems including engine ignition in order to attempt re-lighting them.

[Diamondpilot]

Sorry for my late reply, I think it is 5 minutes, but Boeing and Airbus just use higher standards.

[JeroenC]

Dont know the regs but we have basic and auc batteries, giving 1 hour. Reassuring!