Your Airliner Has A Hidden Engine!

Your Airliner Has a Hidden Engine - AeroSavvy


How many turbine engines are on an Airbus A320?
Two, right?   Would you believe three?

How about a Boeing 747?
Four engines? Wrong again.   A 747 has Five turbine engines!

Hiding inside the tail of most every airliner is an extra engine called an Auxiliary Power Unit or APU. Time to find out what’s hidden in the tail of your aircraft!


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Honeywell 331 APU
The Honeywell 331 Series APU.
Source: Honeywell

Airliners are loaded with redundant systems to help make them reliable. Years ago, aircraft manufacturers and operators figured out that installing a small turbine engine in an aircraft’s tail is a great way to provide an extra level of safety, convenience, and comfort.

The auxiliary power unit is an actual turbine engine, more appropriately called a turboshaft engine. Unlike the aircraft’s main engines, the APU outputs almost no thrust. Most of the power produced is used to run an electric generator and provide pneumatic (air) pressure.

Comfort and Convenience

The auxiliary power unit is usually run on the ground during passenger boarding and deplaning. The APU turns an electric generator that powers the electrical system on the aircraft when the main engines are off. It also provides pneumatic pressure for air conditioning and, more importantly, starting the main engines. This eliminates the need for using an external power source and a noisy air compressor cart to keep the cabin comfy while boarding.

Next time you hop on an airliner (even a small regional jet), listen carefully as you board. You’ll hear the familiar whine of a turbine engine, even though the main engines aren’t running. You’re hearing the APU doing its thing!


The most important job for the APU is to provide redundancy. The APU’s electric generator can be used during flight in case one or more of the aircraft’s main generators have a problem. Similarly, in the event of a pneumatic malfunction in the main engines, the APU can provide compressed air for cabin pressurization and airframe ice protection. The redundancy provided by the APU is one of the reasons twin engine jets are allowed to fly long distances over the ocean. It’s a pretty big job for a small turbine engine hiding in the tail.

Small turbine engine?

Garrett TPE331 engine on a Metroliner.

Auxiliary power units aren’t exactly small. Many APUs are variants of full size turbine engines used in other aircraft. Check out the classic Garrett (now Honeywell) TPE331 engine: For years it has powered large turboprops like the Jetstream 41, Metroliner, and Beechcraft King Air. A variant of this engine, the Honeywell 331, is used as an APU in many Boeing and Airbus wide body jets. Instead of a propeller, the APU has an accessory shaft that powers a generator.

Your Airliner Has a Hidden Engine - AeroSavvy
The Boeing 747-400 APU is a variant of a Pratt & Whitney turbofan engine used on Cessna Citation and Mitsubishi Diamond business jets. This is a very powerful engine. It takes a lot of horsepower to run the systems on the 747 when the main engines are shut down.

Firing up the little turbine engine

APU control panel on a Boeing 757/767

Operating the auxiliary power unit couldn’t be easier. The APU control panel on most airliners consists of a switch or button and a couple of lights – that’s it! Starting the unit is as easy as starting a car. APU manufacturers have done a great job designing these engines so they take care of themselves. If there is a problem, the system is smart enough to shut the APU down and let us know. It’s pilot-proof!

Crews normally run the APU before flight for electrical power, air conditioning and engine start. After landing, the APU is started so engines can be shut down as soon as the aircraft arrives at the gate. If the APU is needed during flight, it can be started in about a minute. It’s a wonderful thing!


Watch an APU Start!

The video shows the start sequence of a Boeing 767’s APU (a Honeywell GTCP 331-200). When the flight crew begins the start, the air intake door opens on the aircraft’s tail and an electric motor begins to spin the engine. Fuel is added and within seconds, the APU is up-to-speed and providing electric and pneumatic pressure for various aircraft systems.


Do all airliners have an APU?

Most every modern turbojet powered airliner (including the smaller regional jets) have an APU installed in the tail. Only a few large turboprop aircraft have them due to the weight restrictions of smaller aircraft.

Look for the tailpipe!

Next time you’re at the airport, take a look at aircraft tails. You’ll notice an exhaust outlet for the aircraft’s auxiliary power unit. The APU is hiding inside the tail, just in front of the exhaust.  So that’s what the little tail pipe is for!

747APU-exhaust    767APU-exhaust

Some APU exhausts are on the side of the fuselage, near the tail.  Photo Credit: Eric Salard – CC BY-SA 2.0

Where does the APU get its air?

747APU-intakeTurbine engines need a lot of fresh air to run. So, where does the APU get its air? You have to look carefully to spot the air intake. The intakes are usually covered with motorized doors that open when the APU is started. The best time to look is when the airplane is at that gate. Once the aircraft is taxiing, the APU will likely be shut down and the door will be closed, hiding its location.

767APU-intake   737APU-intake

APU Trivia!

  • The first airliner to have a gas turbine APU was the Boeing 727 in 1963. The 727 was designed for smaller airports with little (or no) ground support equipment. The 727’s APU was located in the main landing gear bay. The exhaust was through louvers in the top of the right wing!
  • Aérospatiale-BAC Concorde didn’t have an APU. Because the aircraft was small and extremely weight sensitive, it was designed without an APU. Concorde only operated into large airports with plenty of ground support equipment.
  • Two other classic airliners, the Boeing 707 and Douglas DC-8, were also built without an APU. These airplanes were widely used well into the 1990’s and beyond. A few of these aircraft were modified to include an APU in a wheel well. The KC-135 tanker (a variant of the 707) was upgraded to include an APU in its tail.



  1. Thanks Ken, good article. Although I knew of the APU, its exhaust and some of its functions, there’s one thing I’ve always been curious about. Where does its air supply come from? Turbines gulp a good chunk of air and I don’t see obvious inlets to supply it. Thanks.

    • Hi Chris, thanks for the great question! It’s such a good question that I added an air intake section to the article. Every airliner is a little different so you need to look closely to spot the intakes!

      Thanks for reading,

  2. That’s really interesting that some APU units are Garret TPE331 engines. I always imagined them being much smaller ! Definitely learnt a few new thing !

    • Hi, Joe!

      That surprised me as well when I started flying the 757 and 767. When I was in training and discovered what type of APU was in the aircraft, I couldn’t help but laugh. I had logged thousands of hours with the 331’s on Metroliners and Jetstreams. Great engine!

      Thanks for reading!

    • Great comment – I didn’t know that! It makes sense because other popular airliners of that era, like the DC-8 and 707, lacked an APU as well.

      Thanks again for the comment, I added it to the article. 🙂

      Thanks for reading!

        • You’re exactly right. Weight was everything on Concorde. And the designers never planned for it to fly into smaller airports lacking ground support equipment.

        • Good discussion but let’s not overlook what else APU’s do. APU’s are relied on to provide emergency instant Main engines restart power in midair. In the NY emergency landing Pilot Sully was able to land on the Hudson River and communicate with the tower because the Honeywell APU was providing electrical and hydraulic power. One Alaska airlines plane whose engines had stalled due to injesting MT St Hellen’s Volcanic ash during flight over Montana was able to restart it’s engines mid flight utilizing it’s Hamilton Sundstrand APU. After the engines stalled that passenger plane had lost 13,000 ft of altitude. The black box in most APU’s start the APU automatically. On the F-16 that happens in 8 seconds. APU’s are lifesavers!

  3. Thanks for the great article! Really enjoyed reading it!

    Now I have a little question regarding the APU start-up: In case the main engines are running (e.g. just after landing) – providing there is adequate supply of bleed air, is there any way to start the APU using this bleed air? Or you have to use anyway its D/C (?) starter?

    Thanks in advance and keep up the good work!

    • Hi, Nick,

      One of the benefits of an APU is that it can be quickly started with an electric motor. No need for cross-bleed start capability. Cross bleed would add a lot of unnecessary weight, complexity and plumbing.

      Thanks for reading!

      • Realizing this is an old post but jut to keep things correct, the 331-500 APU installed in the Boeing 777 is capable of a cross bleed start. This APU and electric start and air turbine start 9ATS) capability. The ATS is mounted on the APU gearbox right next to the electric starter.

  4. I remember reading that Capt. Sullenburger switched on the APU just after hitting the geese over NYC. This action, not included in the engines out checklist, likely saved lives. Since the A320 still had power from the APU after the main engines flamed out the plane remained in normal law and the flight envelop protection stayed active. This allowed the pilots to just command full nose up on the side stick and the computers figured out the rest maximizing the plane’s glide distance to make it to the Hudson River.

    • Hi, Jon, great comment!

      Starting the APU isn’t one of our engine failure memory items, it’s on the full checklist that we run after the memory stuff is complete. But I’ll guarantee you we don’t wait that long. The APU is one of the FIRST things we reach for in certain emergencies.

      Thanks for reading!

      • Ken i suppose that these engines do not have the same specifications as when they are mounted on a plane as the main engine right? The thrust coming out from plane’s exhaust is not comparable. I think it’s much less right?

        • Great question!
          The APU engines have the same core as their main engine cousins, but produce very little thrust. The Honeywell 331 APU is essentially a turboprop engine without the prop. The Pratt and Whitney APU used in the 747-400 has the same core as the business jet engine, but it lacks an N1 – the big fan in the front that provides most of the thrust.

          Thanks for reading!

  5. Hi

    Just curious, i used to be a flight dispatcher at GLA. Certain aircraft had to get a “air start” where ramp based equipment was used to start the engines with big air tubes.. Is this because they dont have an APU? or cause the aircraft has been powered down overnight? or something else?!?!


    • Hi John,

      Some older aircraft, like the Boeing 707 and Douglas DC-8 didn’t have an APU. They relied on an external source of electricity and compressed air to get the engines started. On newer aircraft, if the APU is not working, a start cart will be used to start the first engine.

      For readers that may not know, “Air Start” carts are large gasoline or turbine powered air compressors that can be towed into position next to the nose of an aircraft. A large hose from the cart is attached to an air duct under the nose. The compressed air then powers the engine starter motor to start the engine. Once one engine is started, compressed air from that engine can be used to start other engines.

      Great question. Thanks for reading!

      • Cheers for the answer Ken.. They aircraft i used to dispatch were nearly all 757 or 767’s.. As far as i can remember it normally happened in the morning, hence why i thought about the power!!

        Totally enjoyed reading through this site!


  6. Would the Apu be able to keep an twin jet airliner pressurized in the unlikely event of a dual engine failure?

    • Most airliner APU’s will provide enough air for some pressurization. More importantly, in the extremely rare and unlikely event of a dual engine failure, the APU will keep the electrical systems running so the crew can concentrate on restarting the engines.

      Thanks for reading!

  7. Hi Ken, Like many others I have been wondering about the reasons for the Egyption Airlines plane”Crash” and I would prefer to think that it was not the result of an act of sabotage. One of the reports said that the plane went into a flat spin of 360 degrees followed by a sudden dive of many thousands of feet. While I am not an expert in these things, it occurred to me that such behaviour can be the result of a failure in the tail plane area and I wondered I there is anything in the APU area that could have caused a structural failure in that area?

    • Hello Ian,

      It’s way to early to discuss a cause of the EgyptAir crash.

      As for APU failures and risks, I haven’t heard of an accident caused by an APU malfunction. It’s certainly possible, but really, really unlikely. APUs and their enclosures are designed withstand a catastrophic failure. They are one of the safest, most reliable systems on the aircraft.

      Thanks for reading!

      • APU’s are designed to be the last power source running in the event of Main Aircraft Engine catastrophic failure. In some Aircraft like the F-22 fire retardant paint was used on the APU gearbox providing an additional level of last minute power safety.
        Most APU’s have a safety containment ring/band around the APU spinning turbine/compressor section. ( It spins at about 60,000 RPM in some APU’s) If the Turbine rotor bursts, this metal band is designed to prevent any Turbine wheel shrapnel from damaging the outer structure of the aircraft. It is quite an ingenuous admirable design.

  8. G’day Ken, thank you for that, I agree that it is too early to come to any definate conclusion, but I try to look at things that will discount all of the stupid conspiracies that are suggested whenever a plane crashes or goes missing for a couple of hours. Now it seems that many of the “Experts” are saying that the “evidence” points to there being a bomb in the Avionics bay. That is before the black boxes have been recovered and decoded by the BEA or the NTSB. When I read of the 360 flat spin, I was wondering what could have caused it and thought that the loss of the tail plane was possibility and then wondered what might or could have caused it. I am probably wrong but it is a better idea than suggesting that it had been hi-jacked and flown to Diego Garcia as has been said about MH370.

    • Hi Ian,

      Don’t put too much stock in reports of a “360 spin.” I think that info is from radar data which can’t determine if an aircraft is in an aerodynamic spin. I understand that it’s an interesting exercise to try to figure out a cause, but there is precious little data available at this time – much of which may be inaccurate.

      TV “experts” are often more interested in their own exposure than informing the public with useful information.

      I always recommend that people take a deep breath and let the investigators do their (painfully slow and tedious) job.

      Thanks for the comments!

    • Hello Ian,

      I don’t know where the 360 spin theory comes from but surely not from somebody who knows a minimum about aviation. How such an idea could be determined when the wreckage and the way it’s spread over the sea is not even clear yet?

      As Ken says experts need to do their job and determine what caused it. Now if you want to work with some facts here are some :

      – The A320 is among one of the safest most reliable aircrafts out there.

      – Besides some flap and landing gear issues which occurred very rarely personally i’ve never heard of an APU failure.

      – There were some automatic messages sent from the plane indicating smoke in lavatory and avionics of the plane. The pilot have made an emergency descent to depressurise the cabin and clear the smoke. What caused the smoke is to be determined.

      • Hi guys,

        These are all great comments. Unfortunately, I don’t believe this is the time or place to discuss this accident.

        In a few weeks time, I’m sure we’ll know more.

        Thanks again for the great comments!


  9. Hi Ken, of course you are right. I guess I have been goaded to an extent by a friend who believes every conspiracy theory that is ever mooted and tend to go on the defensive.
    Ehsan, thanks for your comments too. I can certainly relate to your comments about the A320s safety – I have flown a few thousand miles on them myself.
    I am sure that the experts will explain it all to us once they can access the information in the black boxes which, I hope, will be in the very near future.

  10. Hi Ken, am an aircraft technician.That’s a very resourceful and informative article on the APU, I definitely enjoyed it.

  11. Sorry to bring up this article from history past, but its so well written and your comments so helpful I had to ask one myself. When a jet is sitting on the tarmac due to a delay and they shut down the engines (and the APU is running), why does the air conditioning not work?

    • Hi Gordon,

      Good question. And I really don’t know the answer. Did this happen on one particular type of aircraft? It’s possible some aircraft can’t get conditioning air from an APU, or perhaps there was a problem with the system. Both the 757 and 767 can provide cool air to the cabin from the APU. I’m not familiar with other aircraft types so I can’t comment on those.


  12. Hi Ken,

    GREAT article!!! I heard about the APU from the movie SULLY (which, for those not familiar with the movie or the event, documented the emergency landing of an Airbus A320 on the Hudson River by Captain Chesley “Sully” Sullenberger). As a former mechanical engineer, I was interested in knowing exactly what the APU is and how it works. Your article answered ALL my questions. Fantastic! Many thanks!

  13. Cool site, although I wish it was a little more “professional” sounding. I don’t know if you are trying to appeal to a broad age range or what, but “jeepers!” just grates on me a bit. I also take a certain issue with you calling the APU a “jet” engine several times. This is a common mistake for turbines. Jet engines are typically turbines, but turbines are frequently not jets. “Jet” refers to the high-velocity reaction stream the provides the propulsion, and this includes rockets, pulsejets, ramjets and motorjets. A turbine that only or primarily provides mechanical power is not a “jet” at all, although it may be based on the exact same internals. d sounds the same. You see this all the time with turboshaft powered pulling tractors and cars that people call “jet powered” although they are turboshaft powered. Just a pet peeve of mine, and perhaps a bit misleading to some people.

    • Hi Kelly,

      Thanks for pointing out the “jet engine” reference; I’m familiar with the difference and you are correct. That was a remnant from a hastily written early draft and has been corrected.

      As for “professional sounding,” I won’t change a thing. 🙂

      Thanks for reading,

  14. A good article!And I have a question,some airplanes don’t have APU like DC8,if all the engines fail in air,how can pilots restart the engines

    • Great question!

      Even on aircraft with an APU, we don’t normally use APU pneumatic air for an inflight restart. If an engine can be restarted, it will be windmilling in the air stream. Inflight restart checklists specify an airspeed to maintain to assure the engine is windmilling fast enough for a successful restart. It’s like starting an old car by pushing it and releasing the clutch. Not starter required!

      Thanks for reading!

  15. Can a 747 APU be started in the air? I know it flamed out at 20,000 or before if it was left on (stories… ), but I never heard of anyone starting it in the air.

    • Hi Kathy,

      I’m not a 747 crew member. I checked our company’s 747-400 systems manual and it states that the APU can be started on the ground and left running for takeoff up to 20,000 feet. That seems to imply that it can’t (or shouldn’t) be started in flight. I don’t know if it can be started in flight for a non-normal situation.

      Thanks for reading!

  16. Dear Cap’n Ken,

    Excellent article! You have certainly aroused my curiosity in APU’s, I will be listening more carefully next time I’m on a flight- which happens to be on 6th Feb – Dreamliner 🙂

    God bless!

  17. Hi! I have a question about whether or not the APU ever compressor stalls (and what you can do to fix it as a pilot?) and is there a inertial separator, Im just getting into turbine engines. So i apologize if its a silly question?
    Also if one of the engine driven bleed air intakes where to malfunction, does the APU need to be kicked in? or is there substantial pressure from the remaining engine (other engine) bleed air valves to run the systems appropriately..

    by the way, your articles are great! I for one really appreciate the effort you clearly put into them. Thanks

    • Hi Miller,

      The APU pretty much takes care of itself. The only direct control we have is to turn it on or off. If the system senses a problem, it shuts itself down and lets us know it sensed an error. Maintenance personnel can access the error codes and figure out what the problem was. The nice thing about centrifugal compressors (like the Garrett TPE331 derivative used as an APU on the 767) is that they are resistant to compressor stalls. I’ve never heard of it happening.

      There is probably some sort of inertial separator, but I really don’t know. We are given very little information about the actual engine in training.

      How the bleeds operate can vary considerably between aircraft. On the 767, we can dispatch the aircraft with one bleed inop and fly as high as 35,000 feet (that’s high compared to other aircraft). Our APU supplies pressurized air up to 17,000 feet. If we do have a deferred engine bleed, we run the APU as a back up during takeoff and landing.

      Thanks for reading!

  18. Hi Ken;

    Excellent informative article, another great example is to look at a Lockheed P-3 Orion APU, it show the top and bottom drawers open for intake and exhaust. And in helicopters such as the Boeing Vertol 107 (CH-46) we had hydraulic start APU’s.

  19. Dear Ken
    Thanks for all your articles. Makes very interested reading and of course knowledgeable.
    One small question. How does the fuel reach APU. It is a very long distance from wing tanks and has to be pumped.

    • You answered the question yourself! It’s a long distance from the fuel tanks to the tail. A fuel pump is used to pump the fuel through a fuel line to the APU.

  20. Great article and discussion. I have been a Honeywell engineer since 1977, working on mostly apus. I worked on the accessory gearbox design for the original 331-200 and -250 engines back then. I am currently the lead designer on the gearbox for the TPE1500, a replacement for the TPE331-10YGD engine on the Reaper drone aircraft; it will also be a new commercial offering with completely modern power section.

    I would like to point out that even as a propulsion engine, a turboprop engine produces nearly zero thrust, only lots of hot exhaust. Note the Pratt &Whitney PT6 exhaust.

    One thing about apu inlets—I could never figure out why Airbus insists on placing their inlet on the bottom of the fuselage. The open door gets really close to the tarmac on takeoff, working like a vacuum for runway debris.

    Also, I am no 747 expert by any means, but since it has four engines, the apu shouldn’t be required during flight. I also worked on the load compressor for the 331-500 on the 757 and 767. (That IGV mechanism is the most favorite thing I have ever designed. It has been copied several times over the years on other apus.). ETOPS and cold start capability was a great concern and design requirement for the twin engine aircraft.

    BTW, your lead photo is the Boeing version. The Airbus version has the inlet on the bottom.

  21. Is the APU supposed to be kept running when passengers are outside the plane?

    One time, when I got off a regional jet, and they were using stairs to board/unboard, they left the APU running.

    I’ve wondered if they normally do that. When you are outside the plane, it is extremely loud.

    • The APU supplies power and air conditioning to the aircraft when ground support equipment is not available. If ground power was not working or not available, it’s normal to run the APU. Yep, it’s noisy! Be sure to cover your ears when near it if possible.

  22. Nice article. I was wondering if you could tell me what the dB’s from the exhaust of the APU on a P-3c would be? I was in VP-1 as an AO and worked long hours in the bomb bay. I now have very bad tinnitus and I need to find out what the dB’s were. Thanks Ralph Turner

    • Hi Ralph, I’m sorry about your tinnitus. Far too many of us have suffered hearing damage in this occupation. Unfortunately, I don’t know the decibel levels in the aircraft you were on.

  23. I want to thank you for this article. I’m involved with a new system that will get its power from the APU, and can now point people to this article, instead of the sketchier explanation I’ve provided: The details of the exhaust and intake ports, and the history, were especially interesting to me.

    Did the KC-135s always have APUs? I flew on one as an observer in ’71, and I *thought* that it started-up off a power cart, the way that the USAF fighters did (but I may have misremembered).

  24. The APU-less B-707s sometimes had compressed air bottles in the wheel wells that would supply enough air to start one engine — whose bleed air could then be used to start the other engines.

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