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APU Inlet

A346Dude From Canada, joined Nov 2004, 1198 posts, RR: 8
Posted (5 years 1 month 4 weeks 5 hours ago) and read 9842 times:

This may have an obvious answer, but I am wondering where the APU gets its air from? An APU is obviously not very large, but surely it must need a fair amount of air to run... is there a subtle inlet in the tail or upper fuselage of most airplanes?

Also, what size range are we looking at, say for an A320 or 737 sized APU. i.e. if it was an actual engine, roughly how much thrust would it produce?

You know the gear is up and locked when it takes full throttle to taxi to the terminal.

25 replies: All unread, showing first 25:

Pilotboi From United States of America, joined Sep 2007, 2366 posts, RR: 10
Reply 1, posted (5 years 1 month 4 weeks 5 hours ago) and read 9841 times:

Quoting A346Dude (Thread starter):
is there a subtle inlet in the tail or upper fuselage of most airplanes?

Yes, it's usually subtle, unless you know where to look. It's different for every aircraft.

757/767: (just between the horizontal and vertical stabilizers)

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Photo © Tim Samples

MyAviation.net photo:

Photo © Jose Muniz

A330:

MyAviation.net photo:

Photo © SPMP

MD88: (hard to see from this angle, but just forward of the stairs on the bottom side of the fuselage)

MyAviation.net photo:

Photo © Billy

CRJ: (bottom of the leading edge of the vertical stabilizer)

MyAviation.net photo:

Photo © Billy

MyAviation.net photo:

Photo © Billy

[Edited 2008-03-24 18:13:36]

ArmitageShanks From UK - England, joined Dec 2003, 3361 posts, RR: 16
Reply 2, posted (5 years 1 month 4 weeks 5 hours ago) and read 9835 times:

Most aircraft have an inlet near the bottom of the vertical stabilizer and an outlet on the rear of the tail. Some aircraft have inlets and outlets on the side of the tail. It varies with aircraft.

On the CRJ its located at the base of the vertical stabilizer and the outlet is in the rear of the tail.

There is an inlet door that controls the speed of the air entering the APU in flight to prevent surging. If the door is not working the cruise speed is limited to 300 mph. I would assume its similar for other aircraft.

Check your bags, check your brain.

AeroWeanie From United States of America, joined Dec 2004, 1600 posts, RR: 52
Reply 3, posted (5 years 1 month 4 weeks 4 hours ago) and read 9805 times:

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Most 707s didn't/don't have APUs. The E-8 J-STARS has an APU inlet aft of the wing on the right side of the fuselage. The exhaust is on the other side:

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Photo © Agustin Anaya

The 727 APU inlet is in the MLG wheel well and the exhaust is above the wing on the right side.

The 737 APU inlet is at the bottom of this picture, with the infamous "bicycle seat":

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Photo © 737doctor

Planenutok From United States of America, joined Mar 2008, 21 posts, RR: 0
Reply 4, posted (5 years 1 month 4 weeks ago) and read 9752 times:

Quoting ArmitageShanks (Reply 2):
On the CRJ its located at the base of the vertical stabilizer and the outlet is in the rear of the tail.

Not true. On the CRJ 700 that i work on the apu has a door on the right side below the vertical stab as shown in this photo. The inlet at the base of the vertical stab is the pack inlet.

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Photo © Andy Jung

[Edited 2008-03-24 22:19:40]

[Edited 2008-03-24 22:27:19]

Pilotboi From United States of America, joined Sep 2007, 2366 posts, RR: 10
Reply 5, posted (5 years 1 month 4 weeks ago) and read 9746 times:

Quoting Planenutok (Reply 4):
Not true. On the CRJ 700 that i work on the apu has a door on the right side below the vertical stab as shown in this photo. The inlet at the base of the vertical stab is the pack inlet.

I knew something didn't seem right with the photo I was using. That is correct.

Planenutok From United States of America, joined Mar 2008, 21 posts, RR: 0
Reply 6, posted (5 years 1 month 4 weeks ago) and read 9745 times:

Here is another photo that shows the door open.

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Photo © Joe Statz

HAWK21M From India, joined Jan 2001, 31201 posts, RR: 58
Reply 7, posted (5 years 1 month 3 weeks 6 days 22 hours ago) and read 9718 times:

The B757 Inlet Extends upwards between the Vertical & RH Horizontal Stablizer part of the Empennage.
The B737 has its Inlet on the RH Fwd & below of the RH Horizontal Stablizer & extends inwards.
The "Bicycle seat" was later installed as a Vortex generator after it was found that Airflow seperation existed from that area.The earlier Types did not have this VG.

regds
MEL

Think of the brighter side!

Starlionblue From Hong Kong, joined Feb 2004, 15870 posts, RR: 66
Reply 8, posted (5 years 1 month 3 weeks 6 days 21 hours ago) and read 9712 times:

Quoting A346Dude (Thread starter):
Also, what size range are we looking at, say for an A320 or 737 sized APU. i.e. if it was an actual engine, roughly how much thrust would it produce?

IIRC, I once figured out that the 747 APU produces more power than both engines on a Twin Otter combined.

"There are no stupid questions, but there are a lot of inquisitive idiots." - from Citadel by John Ringo

DALMD88 From United States of America, joined Jul 2000, 2365 posts, RR: 15
Reply 9, posted (5 years 1 month 3 weeks 6 days 7 hours ago) and read 9616 times:

The 767/757 uses a Garrett(now Honeywell) 331 model. This same core engine was fitted with a reduction gearbox and flown as the TPE-331 that was used on Metro commuter aircraft. It was also used on the Mu-2. I think the top HP produced was 1000/1100 shp.

A346Dude From Canada, joined Nov 2004, 1198 posts, RR: 8
Reply 10, posted (5 years 1 month 3 weeks 6 days 3 hours ago) and read 9577 times:

Quoting Starlionblue (Reply 8):
IIRC, I once figured out that the 747 APU produces more power than both engines on a Twin Otter combined.

Interesting. Thanks for the replies.

You know the gear is up and locked when it takes full throttle to taxi to the terminal.

Buzz From United States of America, joined Nov 1999, 697 posts, RR: 24
Reply 11, posted (5 years 1 month 3 weeks 4 days 23 hours ago) and read 9479 times:

Hi DALMD88, Buzz here. On the 737-300's we have the Sundstrand APU is rated at 90 hp to drive the generator, plus enough bleed air to pump up the cabin. Pardon me that I don't know the whole output.
Figure that 220-350 (757 size) pounds per hour of fuel used has to make some kind of power.

Humor: with the major push to "Reduce APU Use!" (it costs money to run that thing!) I recall a similar effort about 10 years ago. We had a DC-10 parked a mile or two from the terminal, we'd taxi it up every morning to save time.
So we'd climb in (in the dark), start the APU and get #2 engine started then shut down the APU and wait for the gate to clear. We kept the APU use down, and would shake our heads wondering what the bean-counters were thinking.
20 minutes of APU time at 350 lbs/hr or 20 minutes of CF-6 time at 1100 lbs/hr.

g'day

Askr From Poland, joined Mar 2008, 45 posts, RR: 0
Reply 12, posted (5 years 1 month 3 weeks 4 days 21 hours ago) and read 9470 times:

Was'nt Boeing promoting an APU-thruster idea? A larger APU that would double as a third engine on twins?
AFIK nothing came out of this idea, as the fuel consumption was way to high when using it as an APU, while the thrust output way to low to make it something usefull.

ATC-PL Wanabe :) - 2nd application is in... 11 July...

Starlionblue From Hong Kong, joined Feb 2004, 15870 posts, RR: 66
Reply 13, posted (5 years 1 month 3 weeks 4 days 20 hours ago) and read 9467 times:

Quoting Askr (Reply 12):
as'nt Boeing promoting an APU-thruster idea? A larger APU that would double as a third engine on twins?
AFIK nothing came out of this idea, as the fuel consumption was way to high when using it as an APU, while the thrust output way to low to make it something usefull.

Yes I heard about this too. Sort of "son of Trident"-ish.

"There are no stupid questions, but there are a lot of inquisitive idiots." - from Citadel by John Ringo

David L From United Kingdom, joined May 1999, 9212 posts, RR: 42
Reply 14, posted (5 years 1 month 3 weeks 4 days 19 hours ago) and read 9460 times:

Quoting Starlionblue (Reply 13):
Sort of "son of Trident"-ish.

Although, as I recall, the 3B had a "booster" engine plus an APU.

Askr From Poland, joined Mar 2008, 45 posts, RR: 0
Reply 15, posted (5 years 1 month 3 weeks 4 days 18 hours ago) and read 9450 times:

777 LR had something like this considered:
777LR With Additional APU Thrust?!? (by AlexG Mar 18 2003 in Tech Ops)

ATC-PL Wanabe :) - 2nd application is in... 11 July...

Starlionblue From Hong Kong, joined Feb 2004, 15870 posts, RR: 66
Reply 16, posted (5 years 1 month 3 weeks 4 days 16 hours ago) and read 9439 times:

Quoting David L (Reply 14):

Quoting Starlionblue (Reply 13):
Sort of "son of Trident"-ish.

Although, as I recall, the 3B had a "booster" engine plus an APU.

Yes yes. I just like to mention the Trident 3B. Yet another funky design from Blighty. Big grin

"There are no stupid questions, but there are a lot of inquisitive idiots." - from Citadel by John Ringo

David L From United Kingdom, joined May 1999, 9212 posts, RR: 42
Reply 17, posted (5 years 1 month 3 weeks 4 days 16 hours ago) and read 9435 times:

Quoting Starlionblue (Reply 16):
Yes yes. I just like to mention the Trident 3B. Yet another funky design from Blighty.

Fair enough - same reason that I just had to chime in. Smile

DALMD88 From United States of America, joined Jul 2000, 2365 posts, RR: 15
Reply 18, posted (5 years 1 month 3 weeks 4 days 14 hours ago) and read 9422 times:

Quoting Buzz (Reply 11):
Humor: with the major push to "Reduce APU Use!" (it costs money to run that thing!) I recall a similar effort about 10 years ago. We had a DC-10 parked a mile or two from the terminal, we'd taxi it up every morning to save time.
So we'd climb in (in the dark), start the APU and get #2 engine started then shut down the APU and wait for the gate to clear. We kept the APU use down, and would shake our heads wondering what the bean-counters were thinking.
20 minutes of APU time at 350 lbs/hr or 20 minutes of CF-6 time at 1100 lbs/hr

I hear you. We have gotten the same speil. But yet we still taxi airplanes from remote parking in the AM because ramp doesn't have time to drag them. Also we have to use APU's at night to keep the planes from freezing. Seems not all of our new high tec gates have heat that works. Finding a remote heater is impossible, so fire up the APU for six hours.

HAWK21M From India, joined Jan 2001, 31201 posts, RR: 58
Reply 19, posted (5 years 2 weeks 6 days 11 hours ago) and read 9001 times:

The B757 APU can be replaced by the Fishpole hoist method or the APU cradle method.Which is more commonly used.I would like to think the former.
regds
MEL

Think of the brighter side!

Qantas744ER From United States of America, joined Jun 2005, 1251 posts, RR: 4
Reply 20, posted (5 years 2 weeks 6 days 10 hours ago) and read 8991 times:

Quoting Starlionblue (Reply 8):
IIRC, I once figured out that the 747 APU produces more power than both engines on a Twin Otter combined.

Correct the 744 APU provides 1450HP and one PT6A-20 used on the DHC6-300 Twin Otter produces 620HP both together making 1240HP.

So that is on hell of a APU the 744 has Wink

Leo

Happiness is V1 in Lagos

LMP737 From , joined Dec 1969, posts, RR:
Reply 21, posted (5 years 2 weeks 6 days 7 hours ago) and read 8950 times:

Quoting Pilotboi (Reply 1):
MD88: (hard to see from this angle, but just forward of the stairs on the bottom side of the fuselage)

A big problem with the location of the MD-80's APU inlet is skydrol ingestion . If there's a hydraulic leak in the wheel well or if the thrust reverser control valves are leaking the fluid will end up in the APU inlet. Which means an APU change and if it works it's way into the packs a pack burn along with changing the socks for the water separators. The best thing they did with the 717 was put the APU inlet on top of fuselage.

MQTmxguy From United States of America, joined Nov 2007, 196 posts, RR: 0
Reply 22, posted (5 years 2 weeks 5 days 17 hours ago) and read 8887 times:

ERJ APU intake is on the right side of the tailcone, right below the rudder 1 hinge with the stab

Well at least we can all take comfort in the fact that NW will never retire their DC-9s

MQTmxguy From United States of America, joined Nov 2007, 196 posts, RR: 0
Reply 23, posted (5 years 2 weeks 5 days 16 hours ago) and read 8880 times:

The red thing around the inlet the to keep rampers from sraying glycol into it.

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Well at least we can all take comfort in the fact that NW will never retire their DC-9s

Fr8Mech From United States of America, joined Sep 2005, 4242 posts, RR: 12
Reply 24, posted (5 years 2 weeks 4 days 18 hours ago) and read 8775 times:

Quoting MQTmxguy (Reply 23):
The red thing around the inlet the to keep rampers from sraying glycol into it.

It does no such thing. The only thing that can keep the rampers from spraying glycol in the inlet...is the ramper Big grin

. Some would see the red border as a target.

When seconds count...the police are minutes away!

MQTmxguy From United States of America, joined Nov 2007, 196 posts, RR: 0
Reply 25, posted (5 years 2 weeks 4 days 17 hours ago) and read 8768 times:

Quoting Fr8Mech (Reply 24):
It does no such thing. The only thing that can keep the rampers from spraying glycol in the inlet...is the ramper . Some would see the red border as a target.

Good Call, I tell ya rampers and pilots are always finding new and exciting ways to break things. Now mechanics never break things......(especially the ones from MQT and other heavy bases