B787 Question

[KelvinD]

Having taken quite a few photos of B787s at Heathrow & Manchester, I have noticed that virtually all departing aircraft have a little flap, directly behind the tail, which appears to open into the wind. None of the arriving aircraft have this flap open. Can anybody tell me what this is for?
Thanks

[slam525i]

If it's open at the gate/push-back/taxi, and closed on arrival, I'd guess it's an APU intake door, but a picture would help.

[KelvinD]

Here's a couple of them:
Ship Photos, Container ships, tankers, cruise ships, bulkers, tugs etc (http://www.kelvindavies.co.uk/kelvin///details.php?image_id=15885)
Ship Photos, Container ships, tankers, cruise ships, bulkers, tugs etc (http://www.kelvindavies.co.uk/kelvin///details.php?image_id=13930)
Ship Photos, Container ships, tankers, cruise ships, bulkers, tugs etc (http://www.kelvindavies.co.uk/kelvin///details.php?image_id=14003)
As I say, they are all departing. I can't find a single shot of a landing B787 showing this flap.

[p1andy]

It's the APU inlet

[slam525i]

What Exactly Is This? — Tech Ops Forum | Airliners.net (http://www.airliners.net/aviation-forums/tech_ops/read.main/115078/)

Same location as the 7(5/6/7)7 APU intake, so I'd guess APU, but I'm not sure. Someone else here will know for sure.

Question to people who know: Is it normal ops to have the APU running after starting both on the 787?

[ajd1]

In the Boeing Procedure turning the APU off is the 1st item after
engine start. It then goes into cooldown mode. I'll check how long
that lasts, but not long. I think.

[KelvinD]

Thanks very much troops.
Obviously some close it earlier than others. This pic was taken from the same location as the others yet it is closed already:
Ship Photos, Container ships, tankers, cruise ships, bulkers, tugs etc (http://www.kelvindavies.co.uk/kelvin/details.php?image_id=15909)
Thanks again all.

[PAXboy]

KelvinDObviously some close it earlier than others
If the APU has a 'cool-down' phase, then the closing of the inlet door will be automatic. Thus, dependent on local air temperature and how long the unit was running.

[Lord Spandex Masher]

As opposed to closing the door manually...?

[Tinwacker]

From the B777 AMM which will be similar.

The air inlet door opens to let air into the engine and load
compressor plenum.
The air inlet door is on the right, upper surface of the tail cone
immediately aft of the rudder trailing edge. This position
decreases the possibility of water or slush ingestion during
ground and flight operations.

As has been mentioned most APU's now have a cool down mode prior to full shut down, this maybe approx 30 - 60 seconds dependant upon type.

Unusual to see the APU door open in flight but can be allowed fror despatch to sumplement the electrical system if a fault on an engine IDG for example.

The B787 may have extra electrical requirements that necessitate the APU being run longer than most, then being shutdown during the flight phase.

B787 course not on the cards yet.....

[KelvinD]

Again, many thanks to all who replied.
Having a bit of time on my hands this morning, I had a dig around and found that someone raised this question on another site back in 2010, having seen a 787 demo flight at Farnborough. And some of the responses were downright fanciful!
However, Aviation Week had an article on the APU in 2013, reporting that the operation of the APU has its own problems. It seems that if the door is closed after shut down, heat continues to build up in the APU, causing the rotor shaft to bow after 20 minutes. After 2 hours, the shaft will straighten itself out. Boeing issued a note advising the APU must be selected 'On' after landing, in order to keep the flap open to allow cooling. Unfortunately, this can be done only with ground power connected as leaving the APU 'On' will drain the lithium-ion batteries within 15 minutes.
The full article is here:
Boeing Tackles 787 APU Heating Issue (http://www.aviationweek.com/Article.aspx?id=/article-xml/awx_05_22_2013_p0-581314.xml)
Incidentally, watching 6 of these leaving Heathrow yesterday, 4 had the flap open and 2 (both BA) had it closed.
Thanks again to all.

[ajd1]

KD is on the right track.
There is a Bulletin in effect to alleviate shaft bowing; after the cool down cycle (approx 3 mins) the APU switch is turned to the ON position (not START) to open the APU inlet door. After 40 mins the switch should be turned to OFF to close the door. If takeoff occurs during the 40 min wait, the APU switch can be turned off at a convenient time after takeoff.