Mr Spaceman From Canada, joined Mar 2001, 2780 posts, RR: 15 Posted (9 years 2 months 4 weeks 1 day 3 hours ago) and read 6180 times:
Hi guys.
If you enlarge the first photo below you can clearly see a small door/scoop that's opened up on the upper surface of the Boeing 777's tailcone.
What is this object's purpose?
From what I can tell from a photo search, this small door is never open while the 777 is in the air. It's only seen open when the 777 is parked at a gate or sometimes while taxiing. This makes me think that it's the cabin pressure control valve's exhaust port.
However, because this door opens upward with the opening aimed forward, it appears to be an air intake scoop of some sort. So, what is this thing?
Embqa From United States of America, joined Oct 2003, 9286 posts, RR: 13 Reply 1, posted (9 years 2 months 4 weeks 1 day 3 hours ago) and read 6133 times:
It's the APU intake.
"It's not the size of the dog in the fight, but the size of the fight in the dog"
Mr Spaceman From Canada, joined Mar 2001, 2780 posts, RR: 15 Reply 2, posted (9 years 2 months 4 weeks 1 day 2 hours ago) and read 6120 times:
Hello Embqa.
Thanks for your quick reply.
OK, so it's the intake for the APU. Does the 777 never takeoff or land with the APU running, or does the airliner switch to a different air intake source during high speed flight to prevent that little door on the tail from being ripped off?
LMP737 From , joined Dec 1969, posts, RR: Reply 3, posted (9 years 2 months 4 weeks 1 day 2 hours ago) and read 6120 times:
The APU inlet door can be opened at anytime in flight, airpseed does not matter. There is only one source of air to the APU, that being of course the APU inlet.
Mr Spaceman From Canada, joined Mar 2001, 2780 posts, RR: 15 Reply 4, posted (9 years 2 months 4 weeks 1 day 1 hour ago) and read 6076 times:
Hello LMP737.
Thanks for your reply.
OK, so now I know the moveable scoop on the 777's tailcone is the only source of air for the APU's intake and that it can be opened anytime during flight.
With this info and the fact that I couldn't find any photos of a 777 with it's APU intake opened during flight, I guess it's safe to say that a 777's flight crew normally shuts down the APU before takeoff, and it stays off unless it's needed.
Dc10hound From United States of America, joined Dec 2000, 463 posts, RR: 6 Reply 6, posted (9 years 2 months 4 weeks 9 hours ago) and read 5919 times:
I guess it's safe to say that a 777's flight crew normally shuts down the APU before takeoff, and it stays off unless it's needed.
Would this be true?
Yes Chris, it would be true. If one of the engine generators is inoperative:
B777 MEL
""24-3 Engine Driven
Generator Systems
(IDG and GCB)
(Continued)
-Dispatcher ’s minimum flight plan fuel
requirement is increased an additional 200 lbs.
per flight hour and 540 lbs.per hour of APU
ground operation,and
(M)
-All generator control units,including APU
GCU,are verified operative
and
-Affected IDG is disconnected
and
-Opposite engine-driven generator operational
history is reviewed and APU and IDG oil is
serviced ,and
(O) -APU-driven generator is used to supply buses
of the inoperative generator throughout the
flight,and
-Prior to each departure after both engines are
started,verify that ELEC BACKUP SYS or
ELEC BACKUP GEN L and R status
messages do not appear within 30 seconds.
Some airplanes require that the APU generator be available for CAT III landings. I don't know about the triple 7...
"Eagles soar. But weasels never get sucked into jet intakes.."
Mr Spaceman From Canada, joined Mar 2001, 2780 posts, RR: 15 Reply 7, posted (9 years 2 months 4 weeks 6 hours ago) and read 5865 times:
Hello Si02y & Dc10hound.
Thanks for your answers.
OK, so the 777 only uses it's APU while on the ramp for electrical power and engine start-up, however, if an engine generator fails inflight, it would be started up and used.
The APU is also on the 777's MEL list and must be operative prior to departure.
I guess the APU is normally used for only ground operations on most airliners. I'm aware however, that the CRJ regional jet sometimes uses the APU's bleed air during takeoff & landing at certain airlines.
I suspect that the retractable intake for the APU on the 777 is a unique design for that aircraft because I've never seen it on other types.
-Backup electrical power system (MEL item 24-
7)is operative,and
-At least one external power system (MEL item
24-15b)is operative,and
-Procedures do not require use of APU
generator,and
-Auxiliary power breaker (APB)remains open,
and
-Both engine-driven generator ’s operational
history is reviewed and APU and IDG oil is
serviced per MPM 24-05,and
-Backup AC power system is verified operative
prior to each departure as follows:
•Confirm that ELEC BACKUP SYS or ELEC
BACKUP GEN L and R status and advisory
messages are not displayed 30 seconds,
after both engines have been started.
NOTE:
APU may be used for pneumatic source.
And the APU itself being u/s:
B777 MEL
49-1 Auxiliary Power Unit
(MMEL 49-11-1)
May be inoperative provided:
-Flight remains within 180 minutes of landing at
a suitable airport,and
-Airplane is not dispatched on routes between
the U.S.mainland and Asia,and
-At least one external power system (MEL item
24-15b)is operative,and
(M)
-APU driven generator is placarded inoperative
in accordance with MEL item 24-5,and
-APU bleed air system is placarded inoperative
in accordance with MEL item 36-10.
NOTES:
1.This MEL item is for use when the APU itself
is unusable.
•If APU is serviceable as a pneumatic
source,but cannot be used to provide
electrical power,placard per MEL item 24-5.
•If APU is serviceable as an electrical power
source,but cannot be used for pneumatics,
placard per MEL item 36-10.
.
"Eagles soar. But weasels never get sucked into jet intakes.."
Mr Spaceman From Canada, joined Mar 2001, 2780 posts, RR: 15 Reply 10, posted (9 years 2 months 3 weeks 6 days 4 hours ago) and read 5674 times:
Hello Dc10hound.
Thanks for your additional info.
I understood the MEL list for an inoperatine engine driven generator that you provided, but, I failed to respond correctly. My fingers didn't type out what my mind was thinking!
I should have typed ............
The APU is also on the 777's MEL list and must be operative prior to departiureif one of the engine generators in inoperative
Also, Thanks for letting me know that the 767 & 757 have similar APU air ilet doors in approximately the same location.
Here's a nice view of an Air canada 767-3 being pushed back from Terminal 1 at Toronto Intl (YYZ). You can see the APU inlet door is still open. Now I know that if I'm watching a 777, 767 or 757 and that little inlet door is closed, then the APU isn't running.
I've enjoyed viewing airliners from this spot on Terminal 1's parking garage at YYZ for over 20 years. Sadly though, Terminal 1 is about to be demolished!
MD-90 From United States of America, joined Jan 2000, 8417 posts, RR: 13 Reply 12, posted (9 years 2 months 3 weeks 19 hours ago) and read 5087 times:
This may not make much sense, but I suspect that given how the inlet is located slightly to one side of the tail, the air flow back there is fairly low pressure (relatively). Which is why it should be operable at any phase of flight without an undue drag penalty.
FSPilot747 From United States of America, joined Oct 1999, 3599 posts, RR: 14 Reply 13, posted (9 years 2 months 3 weeks 18 hours ago) and read 5068 times:
Slightly off topic, but I heard that a large part of the noise from a jet taking off is from the APU and not the engines. I wouldn't be surprised, but is it true?
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