Cory6188
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What Does It Mean To Have An Inop APU?

Wed Mar 12, 2008 3:19 pm

On my CO flight yesterday from BOS-EWR on a 735, I noticed that the cabin was uncomfortably warm while we were still sitting at the gate, still boarding. Shortly after I got on, the pilot came on the PA to apologize for the warm cabin, explaining that the APU was inop on the aircraft and that they would get the number 1 engine started as soon as possible to help cool things down in the cabin. Then once we pushed back from the gate, we sat on the tarmac for about 5 minutes or so, which the pilot then explained was because they were using crossbleed air to start the number 2 engine. Once we arrived at the gate in EWR (after waiting in a holding pattern above New York for about a half hour, *sigh*), the pilot left the number 1 engine on for a good 5-7 minutes or so before finally turning it off. The catering truck actually tried to pull up to the rear door and then immediately stopped once the driver noticed that the engine was still running.

I know that the APU provides power and things like that, but what exactly does it mean in the grand scheme of a flight for the APU to be inop? And if the APU wasn't working, how did we have power while at the gate in BOS before starting the engine?
 
drewwright
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RE: What Does It Mean To Have An Inop APU?

Wed Mar 12, 2008 3:29 pm

You had power at the gate because the aircraft was connected to ground power from the terminal.
The APU is a gas turbine generator whose basic function is to provide power on the ground when it is not available from the terminal, and to provide air for air conditioning and engine start. It can also be used in the air in case of the in-flight failure of one or more engine-driven generators. An inop APU has little impact on the actual flight, which is why their maintenance can be deferred.
 
PanAm747
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RE: What Does It Mean To Have An Inop APU?

Wed Mar 12, 2008 3:32 pm

APU = Auxiliary Power Unit, for readers who might know the terminology.

The APU was designed for airports that did not, at the time of its initial development, have the modern "plug-in" facilities that virtually all of them have today.

Basically, the APU allows a plane to turn off both of its engines and run a smaller engine to power things such as the air conditioners. While today, many (if not most) airports have separate lines that hook up to a plane to do the jobs that an APU can do, the APU sometimes is needed at for example a "remote stand" parking location.

Think of it like the generator on a Recreational Vehicle - you don't need to run the engine that actually powers the vehicle down the road just to run the TV at night when you're parked!! If a campsite has hook-up facilities, then you don't need your back-up. But if those hook-ups aren't there - or aren't working - you can use the generator - your APU - instead.

I have flown on planes that had an inoperative APU, and that's okay - an America West 757 flight in September, 2003, was delayed slightly because the APU wouldn't work (and it was an ex-Eastern bird showing its age!!), but we did fly with out it.
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Cory6188
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RE: What Does It Mean To Have An Inop APU?

Wed Mar 12, 2008 3:34 pm

Cool -- thanks for the detailed explanations. I figured that it couldn't have been that big of a deal to not have it operational.
 
roseflyer
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RE: What Does It Mean To Have An Inop APU?

Wed Mar 12, 2008 4:01 pm



Quoting Cory6188 (Thread starter):
Then once we pushed back from the gate, we sat on the tarmac for about 5 minutes or so, which the pilot then explained was because they were using crossbleed air to start the number 2 engine.

The APU creates pressurized air to start the engines. Without it working, an external pressurized air source is need to engage the starter to start the engine.

A crossbleed start is often used to start the second engine. It takes pressurized air from the running engine to start the second engine. It does require the engine power to be increased though in order to get enough crossbleed air.
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flynavy
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RE: What Does It Mean To Have An Inop APU?

Wed Mar 12, 2008 4:01 pm



Quoting Cory6188 (Thread starter):

I know that the APU provides power and things like that, but what exactly does it mean in the grand scheme of a flight for the APU to be inop?

Well, for one, if your APU is inop, you wouldn't be able to use it in-flight for an engine start should you loose one or both engines.
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apodino
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RE: What Does It Mean To Have An Inop APU?

Wed Mar 12, 2008 4:54 pm

The thing is, on the CRJ, losing an APU can be a big headache. The ground air connection and the power connection that is usually found in the Jetway is not compatable with the CRJ, so stations need to keep Air Start Carts and GPU's just for the CRJ, and many stations do not. In fact, at one airline, an inop APU will instantly prevent the plane from flying to several cities, including YYZ believe it or not, just because the station refuses to stock the necessary equipment to handle such a plane.

The other problem with the CRJ, is the APU heats helps heat the plane in the winter time, and cools it in the summer time. If you lose it, and have to wait for a Huffer cart, the plane will take forever to cool or heat up, and you take delays. And the APU on the CRJ is very unreliable as well, as its one of the most common mel items that you see on the CRJ.

Quoting Flynavy (Reply 5):


Quoting Cory6188 (Thread starter):

I know that the APU provides power and things like that, but what exactly does it mean in the grand scheme of a flight for the APU to be inop?

Well, for one, if your APU is inop, you wouldn't be able to use it in-flight for an engine start should you loose one or both engines.

Not to mention that the APU is a backup generator so if you lose one of your engine driven generators in flight, or have one on MEL, you still have the APU generator as a second generator.

One more thing. This to me seems like a thread that belongs in the tech/ops forum, not the GA forum.
 
fr8mech
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RE: What Does It Mean To Have An Inop APU?

Wed Mar 12, 2008 9:42 pm

APU's have become very important in the grand scheme of things for ETOPS operators. The ETOPS program relies on the APU as a back-up source of electrical power should one of the engines or IDG's fail in flight.

Quoting Flynavy (Reply 5):
you wouldn't be able to use it in-flight for an engine start

As I recall, the APU is not used for inflight starts. Ram air through the engine is sufficient to allow a relight. The APU is started immediately after an IFSD in order to provide a redundant electrical source for the aircraft. I'm going to guess it depends on the aircraft and the operator as to whether you establish power from the APU once it's running or if it sits as a spare source.
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HAWK21M
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RE: What Does It Mean To Have An Inop APU?

Wed Mar 12, 2008 9:59 pm

APU Supplies Pneumatics & Electricals.When the APU is INOP or Inoperative,the Engines use pneumatic starters that need a pneumatic source,this can be achieved through a Ground cart or from a already running Engine.
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Alias1024
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RE: What Does It Mean To Have An Inop APU?

Wed Mar 12, 2008 10:42 pm



Quoting Fr8Mech (Reply 7):
As I recall, the APU is not used for inflight starts.

It can be used for in-flight engine starts on the CRJ. Windmilling starts are possible as well.
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PhilSquares
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RE: What Does It Mean To Have An Inop APU?

Wed Mar 12, 2008 11:01 pm



Quoting Flynavy (Reply 5):
Well, for one, if your APU is inop, you wouldn't be able to use it in-flight for an engine start should you loose one or both engines

On the 744, there is no APU start in flight.

Quoting Fr8Mech (Reply 7):
As I recall, the APU is not used for inflight starts

Depends on the airspeed. If you have a certain N1/N2 indication then you don't need it. Otherwise, you do need the APU to crank.

Quoting Drewwright (Reply 1):
An inop APU has little impact on the actual flight, which is why their maintenance can be deferred.

Not if it's going on an ETOPS flight. Normally, the APU must be operative for dispatch under ETOPS rules.
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Cory6188
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RE: What Does It Mean To Have An Inop APU?

Wed Mar 12, 2008 11:14 pm



Quoting Flynavy (Reply 5):
Well, for one, if your APU is inop, you wouldn't be able to use it in-flight for an engine start should you loose one or both engines.

I guess I should be glad that neither of the engines stopped working from BOS to EWR, then...
 
pwm2txlhopper
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RE: What Does It Mean To Have An Inop APU?

Thu Mar 13, 2008 12:58 am



Quoting Cory6188 (Reply 11):
I guess I should be glad that neither of the engines stopped working from BOS to EWR, then...

Even if one had stopped you wouldn't have had too much to be concerned about, and would have landed safely. This is the kind of thing your flight crews train for.... And the chances you'd ever loose booth engines at the same time is almost non-existent.
 
EMBQA
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RE: What Does It Mean To Have An Inop APU?

Thu Mar 13, 2008 1:40 am



Quoting Flynavy (Reply 5):
Well, for one, if your APU is inop, you wouldn't be able to use it in-flight for an engine start should you loose one or both engines.

Why would you need too....?? All the crew would do is do a Cross Start,,, very-very standard procedure
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fr8mech
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RE: What Does It Mean To Have An Inop APU?

Thu Mar 13, 2008 2:51 am



Quoting EMBQA (Reply 13):
Why would you need too....?? All the crew would do is do a Cross Start,,, very-very standard procedure

Well, this should put many at ease, including the OP. There are many layers of redundancy built into aircraft. Some very obvious and some subtle.
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wilco737
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RE: What Does It Mean To Have An Inop APU?

Thu Mar 13, 2008 5:58 am



Quoting PhilSquares (Reply 10):
On the 744, there is no APU start in flight.



Quoting PhilSquares (Reply 10):
Depends on the airspeed. If you have a certain N1/N2 indication then you don't need it. Otherwise, you do need the APU to crank.

Same for the MD11F, if above FL250 you cannot start the APU!

And in a case of all generator off, Boeing and McDonnell Douglas DO NOT recommend to start the APU, it will reduce the battery life! So doesnt matter if the APU is working or inop, you either cannot start it or it is not recommended...

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jamotcx
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RE: What Does It Mean To Have An Inop APU?

Sat Mar 15, 2008 11:11 pm



Quoting WILCO737 (Reply 15):
And in a case of all generator off, Boeing and McDonnell Douglas DO NOT recommend to start the APU, it will reduce the battery life! So doesnt matter if the APU is working or inop, you either cannot start it or it is not recommended...

Strange how different companies think. We are told to start the APU (airbus) when we are below the max alt to start the apu from batteries in Emer Elec (running on battery only)

And can the thread starter confirm that engine 1 was the engine started first and left running after parking on stand?


Jamo
 
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RE: What Does It Mean To Have An Inop APU?

Sun Mar 16, 2008 7:50 pm



Quoting Jamotcx (Reply 16):
And can the thread starter confirm that engine 1 was the engine started first and left running after parking on stand?


On a 737, the ground air connector is located just at the front of R/H aircon bay, very close to #2 engine intake. Therefore you'll start #1 engine first on stand and only start #2 engine (via cross bleed) after ground air has been disconnected. Anything else would be too dangerous for the ground crew.

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Tristarsteve
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RE: What Does It Mean To Have An Inop APU?

Sun Mar 16, 2008 8:15 pm



Quoting PhilSquares (Reply 10):
Not if it's going on an ETOPS flight. Normally, the APU must be operative for dispatch under ETOPS rules.

Depends on the aircraft.
On B777 APU may be inop for ETOPS flight, there are two generators on each engine.
 
OPNLguy
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RE: What Does It Mean To Have An Inop APU?

Sun Mar 16, 2008 10:29 pm



Quoting Cory6188 (Reply 3):
I figured that it couldn't have been that big of a deal to not have it operational.



Quoting Apodino (Reply 6):
Not to mention that the APU is a backup generator so if you lose one of your engine driven generators in flight, or have one on MEL, you still have the APU generator as a second generator.

Looks like he beat me to it. Funny how dispatchers think alike...  Wink

Even though APUs can be deferred, we try to have them fixed ASAP, or if we can't, we try to keep such aircraft routed away from hot places like PHX and LAS, and "hot and high" airports like DEN and ABQ. In the winter months, we try and route them away from places where winter precip is falling, since an inop APU can complicate/lengthen the de-icing process than it would if the APU was operational If memory serves, that was one factor (of many) in the Air Ontario F-28 crash at Dryden back in 1989.
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