BEG2IAH
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What Is Hole In The Tail For?

Fri Apr 23, 2004 3:33 pm

Hello guys!

I have one probably simple question, but it keeps bothering me for a while. What is this big hole below the tail? It looks like mouth or whatever. Big grin


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When engines are turned off some kind of exhaust keeps getting out of it with a nice smell of kerosene. Is it some sort of power generator's exhaust?

Thanks,
Ivan
 
sq452
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What Is Hole In The Tail For?

Fri Apr 23, 2004 3:34 pm

I think thats all its for is just exhaust, though not 100% sure on this. I thought when i was young it was a 3rd engine but i think its just exhaust,

someone out there know 100% what it is????
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Starlionblue
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What Is Hole In The Tail For?

Fri Apr 23, 2004 3:35 pm

That's the APU (Auxiliary Power Unit) exhaust. It's a turbine engine used to provide power on the ground or in emergencies. It also provides bleed air for engine start.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
je89_w
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What Is Hole In The Tail For?

Fri Apr 23, 2004 3:39 pm

As Starlionblue said, the APU provides power for the aircraft. It's what makes the noise when planes are parked at the terminal.
 
planespotting
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What Is Hole In The Tail For?

Fri Apr 23, 2004 3:40 pm

Yep i concur thats an APU alrite
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BEG2IAH
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What Is Hole In The Tail For?

Fri Apr 23, 2004 3:40 pm

Starlionblue, thanks for the reply.

Ivan
 
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Starlionblue
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What Is Hole In The Tail For?

Fri Apr 23, 2004 3:45 pm

No problem.

BTW here is an APU intake on the 757 (the hatch that has opened next to the fin)


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BEG2IAH
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What Is Hole In The Tail For?

Fri Apr 23, 2004 3:51 pm

Starlionblue,

I've never seen this. I guess you can learn something every day.

I started figuring out the plane from the back.  Smile Who knows how many other dumb questions I'll have once I get to the wings...

Thanks,
Ivan
 
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Starlionblue
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What Is Hole In The Tail For?

Fri Apr 23, 2004 4:43 pm

That's the great thing about this forum, all the stuff you learn!


Some more fun APU facts:
- The engine cores on the BAe-146/Avro-100 started life as APUs, for example on the 767. APUs are just jet engines although in the 40s-50s some planes had piston powered units.
- The APU for the Classic 747 has 1100 Shaft Horsepower. That's about the same as both engines on the Twin Otter combined.
- The 727 originally only had the APU as an option.
- The 727 APU could obviously not go in the tail so it is located in one of the main gear wells. It can only be used on the ground and is prone to set things on fire. Also, it often creates a tongue of flame snaking over the wing on startup. Pretty good if you want to scare the pax  Big grin
- I'm not entirely sure, but I think the 737NG APU intake is right under the exhaust:
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longhauler
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What Is Hole In The Tail For?

Fri Apr 23, 2004 7:59 pm

The B737 APU air intake is on the rear right side of the fuselage foreward of the horizontal stabilizer to the rear of the galley service door, even on the 737NG. You can see it in the picture above.

The vanes you see just outboard of the intake assist in high alititude starting by increasing airflow when airborne.
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gkirk
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What Is Hole In The Tail For?

Fri Apr 23, 2004 8:27 pm

Geez, Id hate to be behind that if it farted  Big grin
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nyskymasters
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What Is Hole In The Tail For?

Fri Apr 23, 2004 9:25 pm

Starlionblue,

With regard to the B737NG, there are what appears to be two exhaust outlets. Actually, the bigger "hole" is the APU exhaust outlet while the smaller one is, believe it or not, is a cooling inlet. Just another APU fun fact.
 
tristarenvy
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What Is Hole In The Tail For?

Fri Apr 23, 2004 9:37 pm

Speaking of APU's, there is a really interesting(and very sad) documentary that's on PBS every now and then, about a group of guys who find a lost B-29 that was ditched in a lake. The show documents the planes history, and the back story of the group.
The plane was put down in a fresh water lake, and the corrosion damage was minimal. They spend forever getting it airworthy. As they try and fly it out, the APU breaks loose in the plane, and catches fire. Everybody gets out, okay, but the plane is lost. They show the crew sitting in the snow, and one of them says, something like, "My tools were in there" in such a deadpan way. I swear I just about cried.
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What Is Hole In The Tail For?

Fri Apr 23, 2004 10:13 pm

Doesn't every plane have/need an APU?
 
lapper
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What Is Hole In The Tail For?

Fri Apr 23, 2004 10:36 pm

Some examples of APU inlets and exhausts for you, BEG2IAH:

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Photo © Harri Koskinen
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Photo © Freight-Dawg



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Photo © F van Moos
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Photo © 737doctor



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Photo © Jeff Miller
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Photo © Marc-André Veillard



Welcome to the boards!
 
JAL777
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What Is Hole In The Tail For?

Fri Apr 23, 2004 10:40 pm

That's where little airplanes come from.
 
starrion
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What Is Hole In The Tail For?

Sat Apr 24, 2004 2:06 am

"That's where little airplanes come from. "


Don't have a Cessna man!
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phollingsworth
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What Is Hole In The Tail For?

Sat Apr 24, 2004 2:52 am

Actually in this picture:


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The doors that are open are not the APU doors. They are actually open to allow access to the built-in work stand for the number 2 engine. The hatch on the bottom allows you to get to the stand from below, and the ones on the top allow you to work underneath the engine fan and core cowls. Even more fun are all the hinges to allow you to remove the #2 engine.

Have a fun day

Pete
 
BEG2IAH
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What Is Hole In The Tail For?

Sat Apr 24, 2004 3:51 am

Guys,

Thanks for the replies. These APU intake and exhaust holes make some of these planes look like real birds, kinda within the lines of what GKirk and JAL777 said.  Smile

Lapper, thanks for these pics. I'm really glad I joined A.net. I realized that I might have chosen a wrong profession. But now it's too late to change it.

Ivan
 
doug_or
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What Is Hole In The Tail For?

Sat Apr 24, 2004 6:37 am

Stefandotde- Aircraft only need APUs if there is no air on the ground for starting. It is nice for passengers and crew if there is no electricity, though its not a neccisity. IIRC the DC-8 had no APU, and some BAC 1-11 were also build without (though I couldn't find any pics in the database).
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IMissPiedmont
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What Is Hole In The Tail For?

Sat Apr 24, 2004 11:51 am

Phollingsworth, you just made me cringe remembering that. It's also the entry to where one goes to inspect the horizontal stabilzer. Instant crush if someone activates the hydraulic system. I hated that little door.

Fun though was the very rare hot start on a 727s APU. Flames leaping out of the right wing 6 feet from a window tended to get the passengers attention.
The day you stop learning is the day you should die.
 
musang
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What Is Hole In The Tail For?

Sat Apr 24, 2004 7:11 pm

Lapper - in the selection of pics of APU intakes, the 146 intake is on the side of the rear fuselage, behind the aft door. So its not visible in that pic. If the intake at the base of the fin caught your attention, thats the ram air intake to the cabin in case air con packs fail.

Tristarenvy - in that B-29 incident, it was an auxiliary petrol powered generator roped to the floor, that spilled fuel on itself and caught fire. I agree though, an immensely interesting documentary and very sad at the end.

Regards - Musang
 
citationjet
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RE: What Is Hole In The Tail For?

Sun Apr 25, 2004 1:05 am

On the 727 APU operation with flames, there are about half a dozen instances of passengers initiating evacuations on the 727 without the cockpit crew knowing it. When the flames shoot out the APU, the passengers yell fire and open the overwing exits and start climbing on the wing, all while the plane is still taxiing. Check the NTSB accident site.
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aloges
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RE: What Is Hole In The Tail For?

Sun Apr 25, 2004 2:06 am

"Check the NTSB accident site."

Done! Big grin Here's what I've found, it's indeed interesting:

http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001213X25234&key=1
http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001212X24351&key=1
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Mr Spaceman
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RE: What Is Hole In The Tail For?

Sun Apr 25, 2004 3:08 am

Hi guys.

Here's two photos to show the difference between the APU exhaust on a 737NG and an older 737-3 as mentioned by Nyskymasters. The 737-3 only has the one exhaust pipe, and the 737NG has both the exhaust pipe & cooling inlet.


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Photo © Patrick Lutz




Chris  Smile

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wing
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RE: What Is Hole In The Tail For?

Sun Apr 25, 2004 6:35 am

APU is a self contained gas turbine engine that supplies bleed air for engine starting or air conditioning.It also provides electrical power.
The upper hole is cooling air inlet,and the below is exhaust outlet in 737NG.
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Refueler1974
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RE: What Is Hole In The Tail For?

Sun Apr 25, 2004 9:01 am

Doesn't every plane have/need an APU?

The majority of your commuter aircraft do to some extend, but htere are still some, like the ATR-42 and SAAB 340, that do need external power while they are on the ground.
The only time you have too much fuel is when you're on fire.
 
JBirdAV8r
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RE: What Is Hole In The Tail For?

Sun Apr 25, 2004 10:21 am

Doesn't every plane have/need an APU?

Allow me to elaborate.

The majority of aircraft (mainly of the smaller variety) do not have, nor do they require APU's. Their power (when the engine is not running) is either provided by:

a) battery, and/or
b) a ground power unit

An APU has several purposes in most aircraft equipped with them: to provide bleed air for the air conditioning packs, electrical power, and in many instances, to start the main engines.

Many smaller jets have the ability to start their own engines under battery power using an electric starter motor. For larger jet engines, though, it is impractical for batteries to be used to start the engine (since it requires so much energy). Bleed air from the APU (or, in the case of aircraft that require pneumatic starts that don't have APU's, like the 707 and DC-8, and aircraft where the APU is INOP, a huffer cart) is used to start the engines spinning.

APU's are generally used on the ground, but some aircraft leave them on for awhile, generally through initial takeoff/climb and descent/landing or otherwise, depending on operating conditions, aircraft requirements, etc.
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musang
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RE: What Is Hole In The Tail For?

Sun Apr 25, 2004 6:02 pm

Refueller1974 - The ATRs, and I believe also the Saab 340, have no APU in the conventional sense, but can run the right engine with the prop hydraulically locked.

On the ATRs it was known as "Hotel Mode", and provided air + electricity. Generally the power lever was at idle, but could be increased slightly if necessary, for example on very cold mornings.

I've never had a definite explanation for the name, but I wonder if its a reference to ocean liners running the engines at low power in port to power the ship's systems, so they're in effect a floating hotel.

The problem with Hotel mode is the inconvenience to anyone working on the right side of the aircraft, due noise and fumes. It is warm though!

Regards - Musang
 
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Starlionblue
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RE: What Is Hole In The Tail For?

Sun Apr 25, 2004 10:49 pm

Thx to those who clarified the difference between the upper and lower holes on the 737NG.

Here is a 737 APU intake:
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