What Happens When Your Plane Flies With a Broken APU

by joeheg

During the boarding process of our American Airlines flight from Austin to Orlando, the Captain apologized to the passengers multiple times that the plane’s APU system was broken, so the plane was warmer than usual.

Listening to people walking down the single aisle of this AA 737, the main questions they had were “What’s an APU?” and “Is that OK?”

Since we write a travel blog, I know that APU stood for Auxiliary Power Unit. But even I didn’t know exactly what the APU actually did and how it would affect our flight. We’re now home safely, so I have plenty of time to check Google for what the APU does. More importantly, is it safe to fly if the APU is broken?

Please forgive me if I get any of the technical speak incorrectly, I’m just a passenger who tried to find information on the internet. If there are errors, I’ll gladly correct them.

What’s an APU?

From what I read, the APU is a smaller jet engine usually located near the tail of the plane. It runs while a plane is on the ground to provide power and air conditioning when the main engines are off.  In fact, the APU running the A/C is what can cause the mist to come from the interior vents.

Air is taken from an inlet (normally under the aircraft) and this is cooled and conditioned in the air conditioning system and then supplied to the cabin for passenger comfort. In warm moist air the cooling of the air can cause moisture present to condense and this can appear as fog/smoke coming from the air conditioning vents.

OK, the APU provides power and air conditioning. What else does it do?

As it turns out the APU is also how jets start their main engines. Those big turbines don’t spin on their own, so the bleed air from the APU is what helps start the engines.

Since my explanation is overly simplified, here’s a great breakdown of what the APU does in a modern airplane, both for the AC and starting the main engine.

Now that I have a better idea about what an APU does, is it safe to fly in a plane with one that’s broken?

Is it Safe to Fly With A Broken APU?

As it turns out, the APU is unlike the main engines in that it doesn’t have maintenance standards.

Unlike the aircraft’s main engines, which require regular tear-downs at very specific time points for maintenance, many APUs are treated more like pass/fail items, allowing operators to run them until something breaks, unless, of course, the unit is part of an ETOPS operation.

We’ll get to the ETOPS thing in a bit, but it helps to understand that the APU is not crucial to flying a plane (most of the time).

If the APU is broken, the plane can run on ground power and ground air conditioning while at the gate. While the A/C will not be as good as if it was supplied by the APU, it’s something. That’s why on our 100+ degree Austin afternoon, our Captain was apologizing for the broken APU. So if you can plug a plane into the airport’s power and A/C, how are we going to start the plane without an APU?

Airports have air start units, which are usually called “Huffer Carts.” These external engines provide compressed air to help start a jet engine when an APU isn’t working. Once one engine is started, the bleed air from that engine can be used to start the other engine.

When Can’t A Plane Fly With A Broken APU?

As you’d imagine, there are numerous rules when it comes to flying airplanes. If the Captain knows that the APU is broken before a flight, there’s a checklist to determine if the plane is still safe to fly to the next destination.

All aircraft have a MEL, or Minimum Equipment List, which is required before a flight. For a 757, here’s the MEL requirement for an APU:

Number installed: 1
Number required for dispatch: 0
Remarks: Except for ER operations, may be inoperative provided:
a) Both engine generators operate normally, and
b) Procedures do not require its use.

So the plane is allowed to operate without the APU, except for ER (Extended Range) operations, as long as both engine generators are operating normally and procedures do not require its use.

This gets us back to why planes operating ETOPS, or flights scheduled to operate more than 60 minutes from the nearest airport, have different FAA requirements for their APUs than other flights. While some flights, like our trip from Austin to Orlando, can operate without an APU as long as both engine generators are operating normally, it wouldn’t be the same for an ETOPS flight. 

Did it make a difference?

While the flight crew constantly apologized for the inoperable APU while we were boarding, it wasn’t that noticeable. Sure, the plane was warmer than usual, but that’s not uncommon for Texas flights in the summer. The one noticeable difference was that the onboard electronics kept resetting, causing the fasten seatbelt alert to sound each time the lights flashed on and off.

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This post first appeared on Your Mileage May Vary


CHRIS August 22, 2022 - 2:51 pm

Care to guess how many planes right now have their APU MEL’d……probably every single one of ZW and OO’s planes without a doubt!

SMR August 23, 2022 - 6:30 am

During summer these APUs go down like flies as they are over-used on the ground on very hot days. It’s not really airline – specific but regional jets suffer from APU problems the most. This summer I have been lucky and have had only 1 aircraft with a broken APU. Another thing airline have to consider when dispatching with a broken APU is where the airplane is going. Many islands that I fly to do not necessarily have ground air or a working huffer cart.


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