B 757 Centre Hyraulic Pumps Unserviceable

[machcruiser]

In the B 757 with both centre hydraulics pumps unserviceable, the QRH says to put off C1 & C2 electric pumps switches to off.

The inoperative items are the centre autopilot and the Left auto stabilizer trim.

My question is why not deploy the RAT manually and get the centre hydraulic system back on line and have all system functioning normally?

[expedite_climb]

Because if the RAT deploys you are going to have to divert - fuel burn etc will increase.

Why do you need the centre system back ? All the controls are otherwise available via left and right systems?

[Golden Rivet]

The MEL states that you must have at least one centre pump serviceable, so for the purposes of this conversation I assume they are both lost during flight.

The arming requirements for RAT deployment are air mode, aircraft above 80kts and both engines out.

As stated by Expedite climb you can fly quite merrily on the L/H and R/H Hyd systems.

I can only assume that along with the extra fuel burn, the RAT is not continously rated above the the aircrafts ETOPS certification.

If the L/H and R/H systems were to fail together your only means of having a 'good day' is the Centre system RAT and you would not want to jeopardise its integrity by using it when technically not required.

Regards

[machcruiser]

My scenario is as Golden Rivet pointed out that that one pump is under MEL and now if the the other pump fails along with the either the left or right hydraulic then why not use the RAT. At least you would have the centre system to help you.

[shopsywhopsy]

I agree with Golden Rivet, save it for one of those days, when things go a bit pear shape. <img src="eek.gif" border="0">

[E. MORSE]

Machcruiser, you are missing the point.

In this scenario there is simply no need for the quote- centre system to help you -unquote.

Both L and R are doing their job.. .You have been redundant.. .And you still are redundant.. .there's no need to divert as well.

Cheers

[VH_KAM]

Whats the point? In the 767 (and I assume the 757 is the same) doesnt the RAT supply hydraulic power to the flight controls only ?

[Golden Rivet]

Vh-Kam

You are correct in your observation, the Rat does only feed flight controls/autopilot, but this is no different to the centre system normal operation.

You will find the only difference being the normal centre system feeds off a stack pipe in the resevoir, where-as the Rat feeds off the bottom of the tank.

[whats_it_doing_now?]

The answer to this is far less technical than you all imply...

Surely the real reason is that its incredibly embarrassing to taxy round an airport with your RAT out!

[pterodactyl]

Certainly not necessary to deploy the RAT on the 767. The 757 should be the same I thought.

It is important to recognise the difference between hydraulic pump failures and Hydraulic system failure. Hyd System failure would normally be the result of fluid loss.

Best to follow the manufactirers recommended non normal procedure. If we are dealing with C1 and C2 electrical pump failure it says turn the affected pump(s)with pressure light(s) illuminated off and then if the demand pump. .pressure light is on put it to ON.

Normally the Air Demand Pump (Centre Hyd Demand Pump) will operate automatically with only C1 & C2 pumps off so the Centre system is still operative anyway.