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FlyFlyFly
Topic Author
Posts: 6
Joined: Fri Jul 18, 2014 6:26 pm

787 systems

Thu Mar 19, 2020 3:15 pm

I have a couple of different questions regarding B787 systems.

“Air conditioning”: The CAC deflector door Prevent debris from entering the CAC inlet during ground operations. Buy why the deflector doors may retract on the ground during high (>35C) and low (<2C) temperatures?


“Passenger oxygen system”: Pax masks automatically deploy when cabin altitude reaches the greater of:

14500, or depature airfield +1500 ft or, arrival airfield +1500 ft.
My question is as I understand in case the airfield +1500 is greater than 14,500 ft the masks will not deploy until cabin altitude until reaches that airfield +1500. I can not understand what is the point behind that. Why boeing did not just only put 14,500 as a limit?


Thanks
 
unimproved
Posts: 190
Joined: Sun Jun 22, 2014 7:14 pm

Re: 787 systems

Thu Mar 19, 2020 3:35 pm

The deflector doors also block airflow which you need in extreme temperatures.

There are some airports around the 14k mark, so making it a hard limit could lead to unnecessary deployments.
 
FlyFlyFly
Topic Author
Posts: 6
Joined: Fri Jul 18, 2014 6:26 pm

Re: 787 systems

Thu Mar 19, 2020 3:53 pm

[quote="unimproved"]The deflector doors also block airflow which you need in extreme temperatures.

Why CAC loves extreme weather (hot and cold)?
 
kalvado
Posts: 2733
Joined: Wed Mar 01, 2006 4:29 am

Re: 787 systems

Thu Mar 19, 2020 5:44 pm

FlyFlyFly wrote:
“Passenger oxygen system”: Pax masks automatically deploy when cabin altitude reaches the greater of:

14500, or depature airfield +1500 ft or, arrival airfield +1500 ft.
My question is as I understand in case the airfield +1500 is greater than 14,500 ft the masks will not deploy until cabin altitude until reaches that airfield +1500. I can not understand what is the point behind that. Why boeing did not just only put 14,500 as a limit?


Thanks

There are a few very high airports, where 14500 may be too low. Look at this list: https://en.wikipedia.org/wiki/List_of_highest_airports
I remember a story of AA flight arriving to one of those high ones (LPB?) and masks deploying once outflow valve opened.
 
thepinkmachine
Posts: 413
Joined: Tue Apr 28, 2015 4:43 pm

Re: 787 systems

Thu Mar 19, 2020 9:32 pm

FlyFlyFly wrote:
unimproved wrote:
The deflector doors also block airflow which you need in extreme temperatures.

Why CAC loves extreme weather (hot and cold)?


When it’s very hot or very cold outside, the CAC’s and Packs need to work harder to maintain desired temperature inside the airplane. The deflector doors retract to provide higher airflow into the system.

Ability to provide adequate air conditioning takes priority over risk of FOD.
"Tell my wife I am trawling Atlantis - and I still have my hands on the wheel…"
 
Lpbri
Posts: 219
Joined: Tue Sep 27, 2016 7:18 pm

Re: 787 systems

Sun Mar 22, 2020 12:33 am

On the ground, the CAC deflector door is deployed to protect the CACs from FOD and nose gear spray. The door retracts when the OAT is 95 F or higher. They deploy to the 50% position when the OAT is 95 F or higher and engine speed rises above 50% N2. They retract when both N2s go above 90%.

Cabin O2 masks drop when cabin altitude is more than 15000 ft. Airfield elevation is irrelevant as automatic deployment is inhibited when on the ground or airspeed is less than 80 knots. The system gets its data from the Common Core System.

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