fflood
Posts: 38
Joined: Mon Jan 08, 2001 11:11 pm

APU's In General

Sat Feb 07, 2004 5:16 am

I know that the APU assists in aircraft starts, maintains the a/c while on the ground and the engines are off, etc... but how long does it stay on after start up? Does it stay on all the time (unlikely, I think) or does the pilot shut it down in flight or before t/o ? Does it use up much fuel? After all, it is a jet engine of sorts. What are its other functions, if any?
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broke
Posts: 1299
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RE: APU's In General

Sat Feb 07, 2004 5:35 am

Normally, the APU is used on the ground to provide electrical power and air for heating, cooling, and engine starts.
The early jet airliners did not have APU's. The 727 is the first jet airliner that I know of that had an APU, but it was an afterthought on that airplane. So it is installed between the main landing gear wheel well and its shroud extends into each wheel well. It definitely had to be turned off before take-off because if you didn't, you lost all indication of whether it was running or not at liftoff and you first clue would be a wheel well fire warning alarm. The only way to turn it off in flight was to pull the fire handle for the APU, guessing of course that the APU was the source of your problem. Not a comfortable feeling.
There were attempts to add APU's to both 707's and DC-8's, but none of these worked very well.
On the twin jets of today, the APU is required equipment for ETOPS operations and it must be flight rated. It is used to provide a backup source of electrical power. Also, the APU must be operational with its generator on standby for Category III approaches. The idea being if you lost a engine driven generator while on the approach, the APU driven generator would come on line and not leave you in a lurch, low, slow, missing nav aids, in bad weather. So if your APU is inoperative on a twin, you cannot make a Category III approach.
Since the DC-9/BAC-111, the APU's have been flight rated on every airplane, twin engined or not, and it is almost always mounted in the rear of the fuselage. You can often see the APU exhaust extending from the airplane tail cone. On the trimotors, it may be located below the #2 engine, ala L-1011, DC-10, MD-11.
At major airports, electrical power and airconditioning is now provided by a central unit that is more fuel efficient and cheaper to maintain than an APU and supplies; these services will handle several airplanes at a time.
At smaller airports, the APU is still used on the ground.
 
Mr Spaceman
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RE: APU's In General

Sat Feb 07, 2004 8:37 am

Hi guys.

On some aircraft such as the Canadair Regional Jet (CRJ), the APU is also used for the main source of bleed air for pressurizing the cabin during takeoff & landing.

By using the APU for cabin pressurization during takeoff and initial climb, no engine intake air is used, thus allowing for higher power settings and more available thrust. Once the CRJ reaches a certain altitude (somewhere below 10,000 feet I believe), the bleed air for the cabin is switched back the engines and the APU is shut down.

The reverse procedure is done during final approach and landing. At some stage of the descent, the CRJ's APU is started up. Then the engines's bleed air for cabin pressure is switched over to the APU. This alows the engines to produce more thrust if it's needed in the event of an aborted landing and go around.

This info is from an article that was in Flying magazine several months ago called Flying The CRJ. I can't remember the exact details about the order in which the APU is used for cabin pressure with regars to altitudes, etc, but I'll read it a post any corrections tommorow. Big grin


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CX Flyboy
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RE: APU's In General

Sat Feb 07, 2004 11:36 am

On the 777 we turn it off immediately after the engines have started. Upon landing we start it up on the way to the gate and try to start it as late as possible so that it is up and running just as we are pulling into the gate. once running, it burns around 200-300kgs an hour.
 
XFSUgimpLB41X
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RE: APU's In General

Sat Feb 07, 2004 12:13 pm

Mr. Spaceman- eh.. not really. My airline has a very strict APU usage policy.

We rarely takeoff or land with the APU. The only time we take off or land with it powering the bleeds is when cowl and/or wing anti-ice is required. Then as soon as climb thrust is set, the bleeds are transferred to the engines and it is turned off. If required for landing (i.e. anti-ice being used for the approach/landing).. it is turned on at the last minute just before we start getting ready to configure the flaps so Canuck Bob doesnt start yelling at me about "CONFIGURE BLEEDS!! CONFIGURE BLEEDS!!"
We dont even turn it on when we land at a place where air and GPU is available at the jetway. At out-stations the thing will run the entire time if power/air is not available and will be turned on during my after landing flow and shut off during my before takeoff flow.

One drawback of not having it on at the gate is i had a guy accidentally pull the external power the other day (the plug fell out) after i had shut down the operating engines. We were deplaning and of course the whole airplane goes dark.. whoops!

It burns around 100-150 pounds an hour. It is a Garrett GTCP-36-150RJ...from what i hear is the same basic engine as what power the Jetstream. In contrast- the 747-400 has a JT8 if i remember correctly. That badboy sends out enough bleed air to crank 2 engines at a time (like the 777 does).


Something to note- the APU is only available for bleed air extraction at 15,000 feet and below. Above that its only use is as a generator (up to its service ceiling of 37,000). I think thats enough random rambling for now.  Big grin
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L-188
Posts: 29881
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RE: APU's In General

Sat Feb 07, 2004 1:16 pm

The early jet airliners did not have APU's

APU's on jet aircraft provide electrical and pneumatic power.

Now that being said, most people don't realize that a lot of the piston powered airliners had an apu also.

Have seen two examples of these up close and personal. One was installed on a B-17 and the other one was from a B-29

The B-17 was a single cylinder Briggs and Stratton motor driving a generator, the same thing that you get when you buy a portable home generator today. The B-29 had a bigger two cylinder one, but the same basic idea. A small motor driving a dedicated generator.
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air2gxs
Posts: 1443
Joined: Mon Jun 18, 2001 1:29 pm

RE: APU's In General

Sat Feb 07, 2004 10:23 pm

The B747-classic does not have a JT8 "apu". I doubt the -400 has one either. The Classic uses a GTCP-331. The exact consumption escapes me, but I think its around 1100#/hr under load.

Again, I have no -400 experience, but a JT8 in the tail just doesn't seem right, unless you're a B727.
 
N243NW
Posts: 1597
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RE: APU's In General

Sun Feb 08, 2004 4:36 am

The 727 is the first jet airliner that I know of that had an APU, but it was an afterthought on that airplane. So it is installed between the main landing gear wheel well and its shroud extends into each wheel well. It definitely had to be turned off before take-off because if you didn't, you lost all indication of whether it was running or not at liftoff and you first clue would be a wheel well fire warning alarm. The only way to turn it off in flight was to pull the fire handle for the APU, guessing of course that the APU was the source of your problem. Not a comfortable feeling.

Don't forget another reason why APUs aren't mounted in the wheel wells anymore. On startup and shutdown, the APU would sometimes shoot flames out the exhaust. Either the ground crew happening to be in that area might well become toast, or the flames shooting out of the aircraft (keep in mind that it was very close to the fuel tanks) could scare the crap out of anyone in the immediate vicinity of the aircraft.

Cheers!
-N243NW Big grin
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Mr Spaceman
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RE: APU's In General

Sun Feb 08, 2004 6:13 am

Hi guys.

>> XFSUgimpLB41X, Thanks for your reply. I knew ther was a CRJ pilot out there, but I couldn't remember who!  Laugh out loud

I was simply trying to reply to Fflood's question in the original post ......"What are it's other functions, if any?" by posting info from an article I read a while ago. As it turned out, my memory was good, except for the title! Big grin

In the May 2003 issue of Flying magazine is an article on page 68 called "Take the right seat of an RJ" The following are excerpts from that article which is what I based my info on. It's writen in the context where the Captain of the CRJ is talking to the reader /co-pilot.

This parts is during takeoff & climb .......

"You call a positive rate, and I respond with a "gear up" call. Tower wants you to contact departure. There we go through 1,000 feet agl, and our minimum maneuvering speed. Flaps up, please, set the climb thrust and complete the climb check. And don't forget to call departure.

The flow is easy from here: flaps up to zero, set the climb thrust using your target N1 setting, thrust reversers off, transfer the bleed air system from the APU to the engines and shut down the APU."


This part is during the descent ......

"The reference speed for the approach is 141 knots at 47,000 pounds, the pressurization system is set for the field elevation and the fuel is balanced between the two main tanks. We just got cleared to 5,000 feet, so let's keep on going -- landing lights are on, the seat belt sign is turned on. Behind us, we can hear the flight attendant putting the cart away. Contact Knoxville Approach. You're busy over there.

Let's crank the APU now and use it to keep the temperature in the cabin comfortable. We'll also transfer the engine bleeds to the APU so we don't lose any performance if we need to go around."


As you can see, there's no mention of the APU being use only because it was needed for cowl and/or wing anti-ice. (which doesn't mean that wasn't part of the reason & just not explained) Only engine performance was mentioned in the article.


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AJ
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RE: APU's In General

Sun Feb 08, 2004 7:14 am

On the Boeing 747-400 the APU was changed from the Garrett (Alliedsignal) of the Classic to Pratt & Whitney. It is not a JT8, more like a PT6 core. It can start two engines at once, although it sometimes struggles to start two RB211s!

The -400 can also have an APU Bleed to Pack takeoff to improve engine efficiency, however I've never seen it used (read about British Airways doing it out of Jo'burg).
 
XFSUgimpLB41X
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RE: APU's In General

Mon Feb 16, 2004 4:10 am

Mr Spaceman: I was just trying to clarify what we do  Smile..I do believe other companies burn their APU more than we do. The engines don't really provide too much bleed air (hell..the C-130 has the same bypass ratio as we do), so if its going to be a long sit on the ground we'll keep it running to keep the temp comfortable. I believe other carriers are starting to go more toward this...it defintely saves gas.


I remember my dad has done a few packs off takeoffs on the 400 when there are problems with runway length.




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