Sign in to follow this  
VOJu24

Fuel Cutoff in flight: EDP still working.

Recommended Posts

Hi all,

 I just played around with the 747-400 to understand the implications of an engine shutdown. For that I cutoff fuel to one engine in flight. I would have expected that when the engine has stopped, the corresponding engine driven pump would not provide pressure anymore. However that doesn't seem to be the case in flight. On the ground it does stop working.

 

So in the manual it says 'The pumps provide pressure to the hydraulic system when the engine is rotating'. So is the explanation for this behavior, that the air blowing through the engine provides enough rotation to keep the pump working?

 

To me it just seems like that just the airflow shouldn't be enough energy for those powerful hydraulic pumps, but maybe I'm wrong.

(Note that I'm only asking about the engine driven pump. The behavior of the demand pumps is logical so far.)

 

Thanks a lot 

 

David Clapper

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

Okay, Thank you :)

This is the reason the 744 doesn't need a ram air turbine. You can lose all four engines and windmilling HP rpm will maintain sufficient hydraulic pressure to power the flight controls. If you watch the overhead during engine start you will notice that the EDP low press light extinguish at quite a low N2.
  • Upvote 1

Share this post


Link to post
Share on other sites

This is the reason the 744 doesn't need a ram air turbine. You can lose all four engines and windmilling HP rpm will maintain sufficient hydraulic pressure to power the flight controls. If you watch the overhead during engine start you will notice that the EDP low press light extinguish at quite a low N2.

 

That is actually very impressive. And how could I miss it doesn't have a RAT. Thanks a lot for the info

Share this post


Link to post
Share on other sites

This is pretty common for most hydraulic systems. I work on a variety of business jets powered by Honeywell TFE-731 engines - much smaller than those on a 747-400 of course, but the principle is the same.

 

During engine start, the hydraulic pressure will almost immediately come up to 3000 p.s.i., just from the energy of the electric starter motor, even before the engine lights off at 12-15 percent N2. Hydraulic pumps used on aircraft are extremely efficient.

  • Upvote 1

Share this post


Link to post
Share on other sites

This is the reason the 744 doesn't need a ram air turbine. You can lose all four engines and windmilling HP rpm will maintain sufficient hydraulic pressure to power the flight controls. If you watch the overhead during engine start you will notice that the EDP low press light extinguish at quite a low N2.

Did not know that, well thought off design.

 

Thanks for the constructive contribution btw....

  • Upvote 1

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this