captjetblast
Posts: 286
Joined: Thu Aug 30, 2001 5:59 am

Auxiliary Ram Air Turbine

Mon Feb 18, 2002 11:27 pm

I know that a ram air turbine automatically deploys from the belly of the jet incase of an "all engines stop".

How does it work exactly, and, is it there and pict of a deployed auxiliary turbine?

Thanks
 
A/c train
Posts: 674
Joined: Sun Jun 10, 2001 7:57 am

RE: Auxiliary Ram Air Turbine

Tue Feb 19, 2002 1:31 am

Not sure of any pics but here goes,
If all other methods of Hyd system supply fails, RAT or HYDRAT on some a/c, is deployed into the airstream to drive a HYD pump, RAT is only generally used in emerg situ. The RAT drops down out of its box (RATBOX), begins to windmill and turn a HYD pump to pressurise one of the systs.
On a 757 which im most familiar with, the RATBOX is stowed behind the STBD trailing edge, in the wings to body fairings and it is in quite literally a box, the whole thing has to come out on skin changes in the area.
The assy is usually on two bearings which incorporate swivel couplings. The couplings from the suction and pressure connections for the HYD pump mounted in the assy casing.
When stowed the assy(assembly) is held by a lock mechanism and is enclosed by doors, in the area of the RAT dropping for common sense reasons there are usually RAT caution signs incase of inadvertent op.( WHICH HAS HAPPEND), on the u/side of the fuselage. Spring loaded guides ensure that the prop lies fore and aft when in the stowed position and on initial lowering into the airflow, to prevent contact with adjacent structure.
The RAT is a CSU incorporating two variable pitch prop blades controlled by a mechanical centrifugal governor mechanism. Any tendancy to accelerate or decelerate is corrected by the Centrifugal force acting on the governor weights. Speed increase, blades coarsen off, Decrease, blades finer pitch.
Heating elements fitted to spinner and LE of each blade for de-icing reasons, power provided by Alternator in Turbine unit.
Oil filler+Drain plugs provided.
On modern a/c the RAT may be deployed AUTO , or by op of latch/handle in ckpt, extends in approx 5 secs EST.
As the RAT spins up on deployment, a volumetric fuse cartridge allows the Turbine to spin up before the HYD syst load is applied to the pump, the Cartridge bypasses fluid, gradually applying load to pump, full PRESSURE can be devloped within 4-5 secs, and is indicated by green light in RAT control switch.
The RAT is stowed and checked on the GRND by using HYD rig connected to the a/c hyd syst. The hyd syst is used to back drive the RAT as a motor . This drives the RAT in the normal direction and the governed RPM can be checked- 4500 rpm approx, when the hyd supply is off the RAT runs on as a pump and as the pressure rises the RAT pressure green light illuminates on the RAT switch.
RAT can be used for elec and HYD backup. Some a/c have a HYDRAT or both. There is still more to be said if you want but I think you're bored by now!!!,
regards,
a/c
 
A/c train
Posts: 674
Joined: Sun Jun 10, 2001 7:57 am

RE: Auxiliary Ram Air Turbine

Tue Feb 19, 2002 1:35 am

About the RAT inadvertent operation, it has happend, with all the sense lines and all that bananas on todays modern a/c i.e 777, the a/c senses weight off wheels and no HYD press or no Electrical power and inadvertently drops the RAT , hence caution signs in area of RAT operation, Tuffty works on the 777, he could tell you a bit about it.
regards,
a/c
 
captjetblast
Posts: 286
Joined: Thu Aug 30, 2001 5:59 am

RE: Auxiliary Ram Air Turbine

Thu Feb 21, 2002 3:40 am

Thanks!
One more question: which is its size, diameter once deployed?
Regards.

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