OFF RADAR. A tale in the tail

While waiting inside the aircraft, have you ever been caught off guard by the brief flicker of cabin lights just before the plane pushes back for departure? Oh, is there any engine problem, you would have wondered. Don’t worry, the flicker of lights is because the pilot has switched off the electricity supply coming from the airport, and switched on the APU, or the auxiliary power unit.

The APU is the aircraft’s extra engine, which not just helps start the main engines, but also powers the on-board electric systems including the cabin lights, air conditioning and the cockpit systems.

And now here is another interesting detail. The APU is located right at the end of the aircraft’s tail. If one has noticed the hole at the end of the tail, well, that is the exhaust outlet for the APU.

The APU might be the smallest engine in a plane, but it is no less important. In fact, pilot and author Mark Vanhoenacker says that many consider it the most important. It’s not surprising.

In an article he wrote for an American publication, Vanhoenacker explains that the APU gives the power to start the main engines. So now we know why the pilot switches off the power supply from the airport and puts on the APU. And an APU is needed because the plane’s main engines are simply too massive to be switched on otherwise. The air from the Unit helps the engine blades get going.

During this time, the air conditioning for the cabin comes from ‘bleed air’ off the engines. Once the engines come on stream and don’t need the air, the air conditioning goes back to its normal mode. In older planes, the air conditioning had to be manually shut for the engines to be started. In the new ones, the whole process is automated.

The APU is small enough to be switched on using aircraft’s batteries. After the main engines warm up, the APU is turned off. But pilots have the option to bring back the APU in service in mid-air if there is a need for a power back-up. Back on the ground, pilots, especially those manning large aircraft, are careful to use the APU efficiently. That is because a large plane’s APU can be almost as big as the engine of a smaller airline. So if power supply from the airport is available, then pilots prefer to switch off the APU to save fuel, says Vanhoenacker.

By the way, the APU is not situated at the end of the tail in all the aircraft. In some, including in Boeing 727, the Unit is located in one of the wheel wells, the compartment for landing gear). And yes, the APU is a smart character. In case of a problem it shuts down automatically and alerts the pilot.

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