What the F*&@ is an Auxiliary Power Unit?

If you ask someone at Honeywell how many engines are on any aircraft, you might quickly decide they’re all pretty stupid over there. Time after time during my visit to their operation in Phoenix recently [Disclosure: Honeywell paid for my trip to their media day], I had people tell me each airplane had one more engine than you’d expect. The 737? Three engines. The A320? Three engines. What the heck? Were these people insane? No. They’re just mighty proud of their auxiliary power units. For those not in the industry, you’ve probably never heard of such wizardry. So, what are these things?

Honeywell 737 APU

That twisted hulk of metal and wires above is called an auxiliary power unit, better known as an APU. Technically, an APU is a very small engine that’s found on pretty much every commercial aircraft. You just don’t see it hanging on the wings because it’s not meant to provide propulsion. And since it’s not publicly visible, it doesn’t have that pretty covering that you see on the regular engines.

An APU is primarily used to power aircraft systems when the engines aren’t turning. See, when the engines are running, they do more than push the airplane through the air. The power they generate is also used to run things like the aircraft’s electrical system. But when you’re at the gate, you need something else to keep the lights (and the air conditioning) on.

At some airports, there is ground power provided – they can basically plug the airplane in. But in most cases the APU is flipped on to keep things running at the gate. Oh, and then they use the APU to get those engines started when the time comes to get going. Then on nearly every flight, the APU just goes asleep and waits until it’s needed. If that sounds like a pretty easy job, it is, relatively. But APUs have to be over-designed in case they need to step in and help.

The biggest APU that Honeywell makes cranks out 1,700 shaft horsepower. That may sound like a lot of horsepower… for a car… but keep in mind that this APU is going to power all the systems on the A350. That’s one big bird with a lot of systems keeping hundreds of passengers comfortable. Smaller airplanes have smaller APUs. The smallest one Honeywell makes is only 100 shaft horsepower. I’m guessing that would be used to power a single electric lightbulb on a 1950 2-seater prop airplane.

Newer airplanes, however, need bigger APUs even if the airplanes themselves aren’t bigger because of the dramatically increasing power requirements. These things need a lot more juice because there’s a ton more automation onboard, and they keep being asked to do more. The latest thing Honeywell is pushing is the electric green taxi system, and that hasn’t even gone into commercial service yet.

You’ve probably seen airplanes taxi to the runway on one engine in order to save fuel, but Honeywell has put together this idea that you can use the APU to taxi the airplane. (There are other systems being worked on with the same goal of eliminating engine use during taxi by other companies.) Small electric motors are put on the main landing gear, and the APU can turn those wheels. That means you don’t need a tug to push the airplane back and you don’t have to turn the engines on until a few minutes before departure. It’s a lot more efficient than running the engines, though of course there’s extra weight on the airplane in the form of those motors. According to Honeywell, it’s a quick payback (of course they’ll say that), but it’s also still not in regular service yet.

Back to the point: it’s clear that the APU is getting more work. None, however, is more important than the work it almost never does.

Even though the APU tends to sleep during flight, it always has to be ready. If the engines quit, then the APU is needed to help power the systems onboard. It’s incredibly rare that it happens, but it means that the APU has to be built to be able to start in pretty much all conditions. (They were mighty proud that a Honeywell APU was instrumental in US Airways 1549’s ditching into the Hudson when both engines stopped.)

So the next time you hop on a 737, you can tell people that you love flying tri-jets. If you’re flying Southwest, that might be a great strategy to get an open middle seat next to you. It may sound crazy, but it’s also technically true.

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