B757-200 Brakes/RAT

[Winston]

On the B757-200 if the normal brakes fail and the alternate brakes work correctly what indications does the pilot get?

Is it a status message and if so when does it come up (immediately or after landing) and what does it say? ]

Second questions is a bit long winded (so stick with me) If both engines fail the rat deploys to power the centre system flight controls-why? The left and right systems both have electric pumps and both the centre pumps are electric so if the APU is running why can't they power the flight controls.
I appreciate that there is a degree of load shedding when engines fail but in that case if the left and right hydraulic systems fail and then either engine fails the RAT would be required.

[Old Aero Guy]

To reply to your second question, you are correct that if the APU were running, then the electric motor pumps would pressurize the left and right hydraulic systems if both engines failed. However, if the APU were running, it would mean fuel was on the airplane and the engines had stopped for some other reason, such as dual mechanical failure. Dual mechanical failure is an extremely rare event. Running out of fuel is a much more likely reason for dual engine failure (along with volcanic ash or massive rain ingestion). No fuel means no APU, so you can't count on it run the left and right hydraulic systems and power the flight controls. Therefore you need the RAT to power the center system to saisfy the regulatory agencies (and most pilots) that the airplane can be controlled after a dual engine failure.

In contrast, there is no RAT any 707, 727, 737, DC-9 or MD-80/90 because all depend on manual reversion if all engines fail.

[EPRman]

Winston,

The normal brakes are powered by the right hydraulic system, the alternate brakes from the left. The transfer of braking from the right hydraulic system to the left hydraulic system is automatic with the loss of right system pressure. There is no indication apart from the R HYD SYS PRESS EICAS message and the SYS PRESS light which will give you a clue. However, if the brakes are on the alternate system you can't arm the autobrakes as there is no autobrake with the alternate system.

In normal operation the APU is shutdown in flight. Therefore the aircraft has 2 electrical supplies ie. the engine driven generators. In the case of a double engine failure you will lose electrical power to the electric hydraulic pumps. The Ram Air Turbine will deploy and supply hydraulic power to the centre system and therefore flying controls until either you restart one or both engines or the APU comes on line. Incidentally, it provides adequate pressure to about 130kts.

Hope this helps.

[Winston]

Thanks a lot guys that answers both questions, if I get anymore questions I know where to come!!! Cheers

[Blacksheep]

...and on the 757-200ER (Extended Range) model, if all electrical power is lost and the RAT is deployed, the centre hydraulic system powers a Hydraulic Motor Generator that provides emergency electrics without running the batteries down...

Clever little b*ggers, these windmill thingymabobs, ain't they? ;)

**************************
Through difficulties to the cinema

[PAXboy]

If I may, two other RATty questions:

1) Presuming that, upon double engine failure, deployment of the RAT is automatic: From failure - how long before it pops out of it's hole and provides juice?

2) Is there a max operating speed for the RAT, whereby it will not deploy until IAS has dropped? Come to think of it, if the systems have all failed it might mean that the ability to relate IAS to deplayoment is not available. :rolleyes: Any more dumb questions like that and I can see me being sent to the back row of Y, next to the galley.