Loss of Thrust on Both Engines

[B737-800W]

Hello,

Can a Loss of Thrust on Both Engines affects the pressurization system ie: Cabin Altitude Loss on B738?

Cheers

[Checkboard]

Of course, Yes. Air flow out of the cabin from various leaks, some designed (such as toilet vents) and some from gaps in seals and panels, as well as the cabin outflow valve. Once the engines stop pumping air in - the pressurised air in the cabin starts leaking out, the older the aircraft, the faster it goes.

[bucket_and_spade]

Thankfully the actual altitude of the aircraft will be going in the opposite direction to the cabin altitude in this scenario, helping things somewhat!

[B737-800W]

So, are you wearing your O2 masks during the drift down or are you waiting to hear the Cabin Alt Warning horn first?

[michaelknight]

With the loss of thrust on both engines you'd be initially on the STBY power system powered by the battery. Therefore you'll have a nice 'AUTO FAIL' light illuminated and you'll be on your own in MANUAL mode.

Best thing is to get the APU running if available and put it on line. Then the pressurization will be controlled automatically. But remember no engines, no duct pressure! So descent to 17,000 close the isolation valve and put the LEFT PACK on the APU.

If the cabin alt horn goes off :eek: MASK on then.

With both engines flamed out, you'll be busy initially trying to restart them. But remember, don't get sucked into trying to restart two dead motors, a la US Airways, if they don't relight, consider should be given to spending your limited time on preparing for a ditching....and there's a whole new checklist for that!

MK

[p51guy]

So you are going to skip the loss of both engines checklist when you lose both engines? It seems like a meaningless waste of checklist space if that is the case.

[Max Angle]

You are going to have around 20 minutes before you strike the ground if you are in the cruise at say 370 so plenty of time to set a good glide speed, point towards something you can land on, try and get the APU going, try to re-light and if all else fails get the ditching/crash landing checklist out.

[Ocampo]

"APU starts are not recommended above 25,000 feet", the air is really thin.

Or being some time into a long ETOPS flight it could also be soaked...even harder to start...:sad:

[Checkboard]

See the "cross-feed valve check" thread - APUs which are hard to start at altitude are started and run constantly on ETOPS legs.

The BA 747 which lost all four engines in volcanic ash eventually had the cabin altitude warning horn go off, and the cabin masks drop. As the FO's Oxy mask wasn't correctly fitted (it came to pieces as he grabbed it, as I recall), the Captain increased the glide speed to descend to 10,000 feet (left with a decision about whether he wanted a non-hypoxic FO, or more gliding time, he chose the FO! :} )

[mono]

Is loss of thrust the same as a flame out?

As an engineer I read the question as just that. A loss of power not a dual flame out/shut down.