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.


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

  1. Xnuiem says:

    You dont see them anymore, but back in the day of classic DC-9 and before, there were starter carts. They were basically a small APU sized engine, usually with a diesel engine in there for power, that would give the plane electric power, but also had an air hose to force in the bleed air that is used to start the engines and power systems.

    DC-6s needed them as well. A huge leap forward (at the time) was that the Lockheed Electra actually had an onboard APU back near the tail.

    • Dan says:

      You actually still see air start carts. If the MEL permits the APU to be a DMI, you’ll have to have the cart to get the engines going.

    • HunnerWoof says:

      Oh, you still see them from time to time. They’re still used when APUs are INOP, particularly in outstations for regional aircraft.

  2. Sirwired says:

    Maybe you could ask Honeywell why they can’t put a *bleep*-ing muffler on the things! I get half-deafened when approaching a tarmac-loaded aircraft when the APU is running.

  3. Richard says:

    I imagine the cowlings on the “real” engines are for aerodynamics, not aesthetics. Otherwise you can bet Michael O’Leary would have his planes looking like flying plumbing supply houses in no time…

  4. Miles says:

    Even that little 100 horsepower APU puts out a lot of power: Each HP can become roughly 650 watts of electricity (after losses), so 100 HP becomes ~65,000 watts.

  5. thor says:

    The Honeywell “100” APU has been installed in nearly two dozen different applications including: Falcon 50, Gulfstream (GI, GII, GIII, GIV), BAe HS-125, Bae 146, Jetstream II, Canadair Challenger, Ground Carts and Military applications.

  6. Ron says:

    OK, the APU is an engine, but is it a jet engine? If not, then the 737 is still not a trijet…

    • Zoomtide says:

      Yes the APU is a jet engine. It gets it’s jet fuel from one (or both) wing tanks typically.

  7. Erik B says:

    Cool. Up until now I’ve only really know the APU as the culprit in my flight delays.

  8. A says:

    Those APU’s burn a ton of fuel, so while necessary it’s not ideal to leave them running while the planes are sitting on the tarmac. Back in the good old days I recall seeing the exhaust heat signature from nearly every plane sitting at a gate. Today, hardly ever.

    • Jimbo says:

      FYI, on an MD80, the APU burns about 250 pounds (about 37 gallons) of jet fuel per hour. By comparison, the average hourly burn for each JT8D-219 engine is about 3000-3300 pounds (about 450 gallons) an hour…

  9. Stu says:

    I missed a bar trivia question on this once. “How many engines does a Boeing 747 have?” Easy – four! I mean, duh. You can only imagine how frustrating it was when the moderator said “Five”. Cost us a win :(

  10. SingBlue says:

    This little story links back in to the “annoying neighbour” thread a few days back. I was once returning from Milan to UK on an LCC, when the pilot announced there’d be a slight delay as the APU was out, and therefore they needed to start the engines up to get the systems working before taxi/take-off.

    I don’t normally engage with neighbours on planes, but the middle aged lady next to me decided to go on a bit of a rant and finished with “Why can’t we just take off and let the systems power up as we fly?”

    Anyway, I’d studied a bit of accident investigation as part of my first degree, and quite calmly explained to her that without these systems, the temperature inside the aircraft would be close to that outside, and at cruising altitude that would mean we would be at a chilly -58C. Or dead, to use the technical term.

    I’ve no idea if any of what I said was remotely accurate, but the look on her face was priceless, and a member of the cabin crew overheard the exchange and comped me a couple of beers on the way back for brightening her day. (It was an LCC).

  11. matt weber says:

    Because the APU spend so little of its life actually running, the weight of the APU is much more important than the fuel consumption. Consequently the turbo-shaft engines in the APU don’t win any awards for efficiency. They were designed for minimum weight.. Let me put that another way. At typical rates a 500hp electric motor consumes about $28 worth of electricity per hour. That same 500hp APU consumes about $120 worth of Jet-A each hour. So it should come as no surprise that ground power is preferred whenever it can be used.

    I have some interesting experiences with Start Carts and 747-400ER’s. Unless the cart is in ‘tip top’ shape, they often are unable to start the big CF6’s. In fact that is the only time I have known a jet engine not to start. The CF6 startup is fully automatic, and if the cart cannot provide enough pneumatic power to spin up the turbine to the required speed, no attempt will actually be made to light the engine. We went through 3 different carts before they finally found one that would successfully start the engine.

    In general the APU is on the MEL (minimum equipment list) only for ETOPS aircraft. On an ETOPS aircraft you have to be able to start and run the APU anywhere in the flight envelope. So it isn’t all that unusual for it to be US on domestic narrow body aircraft, or widebodies with more than 2 engines.

    Not mentioned is there actually two manufacturers of APU’s. At one time the business was ‘owned’ by Garret Airesearch, which through a series of acquisitions is now Honeywell. The 787 APU is made by Hamilton Sundstrand, and is rated 1100hp. It is pretty much an electricity generator only APU, since just about everything on the 787 is electric, including the engine start.

  12. tharanga says:

    Does the ram air turbine count as yet another engine, of a sort?

    • Jamie Mackay says:

      Not really as a RAT is exactly that, a turbine. Its simply a very small wind generator on the bottom of the plane.

  13. Geof OConnor says:

    As soon as I saw the title of this I knew exactly what it was! APU – yes. One of my more memorable delays was hearing the captain say there’s maintenance issue and our aircraft needs a new APU. It was a long wait, but the captain explained what it was and now that I know how large they can be, I’m surprised our wait wasn’t longer. It’s the jet engine sound we hear when we board the aircraft. I also learned without it, the pilots could not start the jet engines.

  14. Jeff Mills says:

    I remember my days at Frontier Airlines (the orig, one) and APU’s.When we had a RON that had to be Cabin cleaned and our Hobart wasn’t working we would have to start up the APU.Gave us ground folks a just to play Pilot a little! HA. Then when the planes APU was out we would get our jet Start ( air bottle) to get those Fans going! Great memories of the past!

  15. Henry says:

    I was flying on Jetblue from Bradley to FLL this winter and the APU broke while we were waiting at the gate. They did not cancel the flight, but rather (from what I understand), they toed a portable generator owned by the airport next to the plane. That provided the power for us to start our engines. However, they obviously detached the plane from that before we departed. I questioned at the time what would happen if the engines were to go out mid-flight. But, I assumed that since we did fly that night without fixing the part, the APU wouldn’t ever be necessary mid-flight. Is this common practice? Were we ever in any real danger?

    • Rob says:

      I can answer your question. I am an A320 Captain for jetBlue. No there was no danger with an inop APU. The APU is usually only used when we are at the gate just prior to push back for airconditioning and electric and then after we push off the gate to provide bleed air to start the engines. After that we don’t need it and it is shut down for the rest of the flight. However APUs are very reliable, so if it does break it usually is not a simple fix. So to try and keep the A/C on time it is very common to MEL (a list that of what we can fly without) the APU until the plane get to a maintaince base for an overnight where the mechanic’s have time to work on it.

      • Chris says:

        Thanks for the thorough explanation. In a situation when the Ram Air Turbine deploys, is it not necessary for the APU to be functional? Are the systems not dependent at all?

        • Jimbo says:

          Ram Air Turbines (RATs) are deployed only after complete loss of electrical power, and provide very basic emergency electrical and/or hydraulic power, depending on the airplane. A RAT is intended to allow minimal systems functionality to land the airplane soon as practicable.

  16. Miles says:

    The experts here can correct me if needed, but I don’t think the APU has any function during normal flight. The big engines make all the electricity needed, and if they flame out the plane’s velocity will push enough air through them so they can be re-lit.

    Evidence of the non-critical nature of the APU is that there is only one on the plane.

    • Jimbo says:

      An APU generator may be used in place of an inoperative engine driven generator if permitted by the approved Minimum Equipment List (MEL), which specifies the conditions that must be met if an engine driven generator is inoperative.

      The APU may be considered a “convenience” item, as they are not required to operate the airplane (the Boeing 727 was originally designed without one). In addition to providing backup electrical and pneumatic power, the APU allows an airplane to be self sufficient at smaller airports that do not have ground support equipment for large jets, and to reduce the amount of ground equipment required at larger terminals.

      In the event of an engine failure, the engine may be restarted if it did not sustain damage. If it is damaged, the airplane will fly on the remaining engine(s) to a landing at the nearest airport.

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  18. Wes says:

    I read somewhere that the 737-800 has to keep its APU running during flight for ETOPS routes (the statement specifically was addressing Alaska’s service to Hawaii). Anyone know if there’s any truth to that? If so, it seems like Alaska is up against a lot of factors to be profitable on its Hawaii routes, as I understand that it also must at least occasionally fly these routes at less-than-capacity due to weight restrictions.

    • Jimbo says:

      A very experienced 737 Captain advised me that Alaska began their 737 ETOPS operations by starting the APU at the ETOPS entry point, and running it until 30 minutes past ETOPS exit until they proved the APU would start reliably, then were approved to fly ETOPS with an APU available but not running.

  19. Jimbo says:

    IIRC, the 737 does not have a Ram Air Turbine (RAT) for emergency electrical power, so the APU is required to be run for the entire trip for electrical backup power.

    • Zack says:

      Nope, the 737 absolutely has a RAT, as do all airliners. If you were paying attention you would know it is a vital piece of safety gear.

      As for APUs not running much, or air-start carts (GPUs – Ground Power Unit) being “here or there”, anyone working a ramp job knows both of these things are widespread and often used. Look at the front #2 side of any UPS MD-11, etc. That black soot mark is from constantly used GPUs. APUs will run for hours and hours at a time, and with the high utilization of modern jets, they get a ton of usage per day.

      The original post is flawed, in that if you are counting any power-generating device as an “engine”, then you need to also include such things as the AC packs, which are jet-fuel powered turbines just like the APU. Typically there will be 2 of them in the “belly” (wing-fuselage fairing, forward of the main gear). They are also wicked noisy/hot like APUs and frequently run both in flight and at the gate.

      Also unmentioned is *how* the APU starts the engines, which is via bleed air. Tubes connect all three power-sources and the air spins-up the compressor blades to a point where fuel can be added and ignited. The new 787 does this all with electricity. Ever wonder why the cabin A/C stops functioning when engines start? Or why electrical power goes on/off at the gate multiple times? Switching power sources and diverting bleed air is why.

      • Jimbo says:

        @ Zack – Unsure the source of your information, but:

        1) I did a word search in the Boeing Flight Crew Operating and Systems Manuals for the 737-300, 400, 500, 700 and 800 – no hits for ‘ram air turbine’ or ‘RAT’…. A good friend of mine with thousands of hours 737 time advises the 737 has NO RAT. He advises that Alaska 737s flew ETOPS initially by starting the APU for the ETOPS phase of flight until they proved the APU would reliably start, then they were approved for ETOPS with an APU available but not running.

        2) I have been flying, instructing and checking pilots on the DC9/MD80 family for 18 years, and I have never heard of or seen a DC-9 nor MD82, 83, 87, or 88 with a RAT installed. The Douglas/McD/Boeing FCOM and Systems manuals are similarly silent on the topic… Their airworthiness certificates show them as being certified in the Transport category. NO RAT.

        3) Air conditioning packs are absolutely NOT jet fuel powered. They are “air cycle machines” that are “powered” by hot high pressure air compressed either by the engines, the APU, or a ground air cart. The hot, high pressure air enters the pack, is expanded and cooled, and mixed with hot air as necessary to provide the desired cabin temperature. There is absolutely NO jet fuel anywhere in an air cycle machine pack (“PACK”, I have heard, stands for Pneumatic Air Conditioning Kit, but I cannot find a source for that nomenclature).

        4) Also, in airplanes that use compressed air for starting, the start switches in the cockpit open a start valve to direct the compressed air to spin a air starter turbine on the engine’s accessory section. The air starter turbine is mechanically connected via gearing and a shaft to the compressor shaft of the engine. Starting air does NOT enter the compressor section.

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