Power in an airplane

Power in an aircraft

Tim Takeoff
01.03.2019
2 pictures
5 minutes

As a passenger in an aircraft you can switch on the light, drink coffee, charge your mobile phone and even watch television and the cockpit functions would also not work without a supply of power. But how do you power up such a large “bird” and what is so interesting about it?

Where does the power in an aircraft come from?

In general, a modern commercial aircraft uses an alternating current (AC) power supply with 115 volts at 400 Hz. This needs to be supplied and distributed in a variety of ways and only a few systems still need direct current (DC). Direct current? A normal battery would be very suitable for this, but the same problem would arise here as in a car: a battery to provide power for the entire flight would need to be unbelievably large. This doesn’t mean that there is no battery installed on board, but it is only needed to initially start up the systems or in the event of an emergency. You need a type of alternator, just like in a car, to ensure a permanent, stable power supply

The ground power

© Wikimedia Commons w:es:Usuario:Barcex

An all-rounder: the APU

If this is not available, the aircraft doesn’t need to remain without power. Ultimately, power is still needed for the lighting, for all systems in the cockpit and for lots of other equipment and pumps that are used to bring the aircraft to life. So if ground power is not available, the auxiliary power unit (APU) takes care of the power supply.

The APU works like a small, self-sufficient engine at the rear end of the jet and draws fuel from the normal tank containing aviation fuel. In addition to its function as a power generator, it can also supply the hydraulics and bleed air – a real all-rounder.

© Wikimedia Commons David Monniaux

Engine generators

The most important sources of energy during a flight are of course located on the engines. Here the generator is driven by an engine shaft, and because this always runs at different speeds, a “constant speed drive” (CSD) is used. This speed generates a constant voltage of 115 volts at 400 Hz AC power in a brushless generator, but because some systems still need direct current there are also still commutators on-board.

The power that is now generated is supplied to all consumers through a distribution (bus) system, guaranteeing an uninterrupted supply of power through a wide range of circuit breakers and relays.

Emergencies

All power generators can take on the function of the others in the event of an emergency. If there is insufficient power in the aircraft, certain consumers are switched off depending on the priority. For example, the galley is the first to be disconnected from the power supply when it gets very “tight”.

If a serious failure occurs, where neither the APU nor the engines can generate a supply of power, the on-board battery will initially be used. This can also provide alternating current through a power inverter in order to supply power to the most important systems.

The “RAT”

As the battery is often limited, a small type of wind turbine is immediately extended in the rear part of the aircraft in the event of a power failure. This is called the “ram air turbine”, the “R.A.T” or also fondly called the “rat” by pilots. The “rat” rotates in the wind and can generate power and hydraulic pressure for the most important systems in the event of an emergency.

What about lightning?

We often wonder whether an aircraft will be struck by lightning when flying in bad weather, even although aircraft normally fly at a distance of 50 to 100 kilometres around a storm. If lightning strikes unexpectedly, you often hear a muffled bang and see a bright light. Since an aircraft is designed in the same way as a Faraday cage (just like a car), nothing untoward can happen to the passengers. However, voltage peaks may lead to the failure of systems, but as all instruments are multiple-redundant devices, a complete failure is extremely unlikely.

It is possible to see at a later stage from a small hole on the exterior skin of the aircraft where the lightning strike entered and exited it again. A lightning strike is documented by the crew and communicated to the engineers – who then meticulously check the jet for damage so that the next flight can continue to fly safely.

If you would like to find out more about the important systems, we recommend that you read our blog article on hydraulics and engines!

by Tim Takeoff

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