[B777] Why can't APU support hydraulics but RAT can?
Thread Starter
Join Date: Mar 2016
Location: somewhere
Posts: 16
Likes: 0
Received 0 Likes
on
0 Posts
[B777] Why can't APU support hydraulics but RAT can?
Just out of curiousity..
Why RAT, which is a small turbine, can provides hydraulics redundancy...
whereas APU, which is considered as the third engine in 777, cannot perform the same task?
Why RAT, which is a small turbine, can provides hydraulics redundancy...
whereas APU, which is considered as the third engine in 777, cannot perform the same task?
The APU doesn't drive a hydraulic pump directly but I think that your assumption that the APU doesn't provide hydraulic redundancy is incorrect.
When running the APU is pushing out lots of electrics and lots of bleed air that feed various systems, including the three hydraulic systems that have a combination of either electric or bleed air driven demand or primary pumps. If you run the APU in flight, say in the event of a single engine failure, I'd say the APU is providing hydraulic system redundancy, if needed, via the air driven and/or electric driven pumps.... ( you don't deploy the RAT just for one engine out).
As evidence of the APU s capabilities it is worth bearing in mind that it is also certainly powering the hydraulics, again indirectly via electrics/bleed air, during the initial stages of pushback, before any engines are running.
In most circumstances the RAT will provide essential but limited hydraulic power but it is really a fourth in line backup for hydraulics, after the two engines and the APU.
As an aside whilst the 777 APU is fairly powerful I've never heard it ever described or considered as a third engine.
When running the APU is pushing out lots of electrics and lots of bleed air that feed various systems, including the three hydraulic systems that have a combination of either electric or bleed air driven demand or primary pumps. If you run the APU in flight, say in the event of a single engine failure, I'd say the APU is providing hydraulic system redundancy, if needed, via the air driven and/or electric driven pumps.... ( you don't deploy the RAT just for one engine out).
As evidence of the APU s capabilities it is worth bearing in mind that it is also certainly powering the hydraulics, again indirectly via electrics/bleed air, during the initial stages of pushback, before any engines are running.
In most circumstances the RAT will provide essential but limited hydraulic power but it is really a fourth in line backup for hydraulics, after the two engines and the APU.
As an aside whilst the 777 APU is fairly powerful I've never heard it ever described or considered as a third engine.
Last edited by wiggy; 10th Apr 2017 at 09:11.
Thread Starter
Join Date: Mar 2016
Location: somewhere
Posts: 16
Likes: 0
Received 0 Likes
on
0 Posts
Thanks for the quick reply wiggy!
Well thats what i wondered as if APU could provide redundancy to the hydraulics.
However , as in "flying the big jets" and from "http://ads-b.ca/777/B777-Electrical.pdf"
they both didnt mention APU as a backup source for the hydraulics while did mention that for the RAT...
Therefore its hoped some experts here can shed some light on the issue.
Well thats what i wondered as if APU could provide redundancy to the hydraulics.
However , as in "flying the big jets" and from "http://ads-b.ca/777/B777-Electrical.pdf"
they both didnt mention APU as a backup source for the hydraulics while did mention that for the RAT...
Therefore its hoped some experts here can shed some light on the issue.
No probs, I'll have a look at the PDF later but I think the issue can be that the APU doesn't provide a direct hard backup (e.g An attached pump.), so on a simple diagram there's perhaps no obvious link....
Join Date: Mar 2014
Location: New Delhi, India
Age: 65
Posts: 9
Likes: 0
Received 0 Likes
on
0 Posts
Flying the big jets is very much about the 744. Or at least my 3rd edition is.
I don’t know about the 777, but on the 744 I don’t think you can start the APU in flight?
Also, a 744 doesn’t have a RAT, engines windmilling seems to be sufficient.
I don’t know about the 777, but on the 744 I don’t think you can start the APU in flight?
Also, a 744 doesn’t have a RAT, engines windmilling seems to be sufficient.
Hydraulic systems have a 'primary' pump and at least one 'backup' pump - the backup pumps are generally electric or air driven pumps (sometimes both). In the case of the 777, the primary pump source for the left and right is the engine driven pumps (EDP) while the center uses an electric pump.
As wiggy notes, the APU can't drive the EDPs directly but can and does provide the electric and/or pneumatic source for the other hydraulic pumps.
JeroenD, unlike the 747, the 777 APU is in-flight operable throughout the flight envelope (although it can't be used as a pneumatic source above some altitude). You are correct, the 747-400 (and earlier models) don't need a RAT, the windmilling engines will provide adequate hydraulics for basic aircraft control in an all-engine out condition. However the 747-8/8F does need a RAT - the combination of higher bypass engines (lower windmilling speeds) and higher hydraulic loads due to the new wing made the RAT mandatory.
As wiggy notes, the APU can't drive the EDPs directly but can and does provide the electric and/or pneumatic source for the other hydraulic pumps.
JeroenD, unlike the 747, the 777 APU is in-flight operable throughout the flight envelope (although it can't be used as a pneumatic source above some altitude). You are correct, the 747-400 (and earlier models) don't need a RAT, the windmilling engines will provide adequate hydraulics for basic aircraft control in an all-engine out condition. However the 747-8/8F does need a RAT - the combination of higher bypass engines (lower windmilling speeds) and higher hydraulic loads due to the new wing made the RAT mandatory.
Join Date: Jan 2002
Location: West Country
Posts: 1,271
Likes: 0
Received 0 Likes
on
0 Posts
Just a point of interest - the APU doesn't supply pneumatics above 22,000 ft so Hyd ADP will be unavailable and even below that altitude the mass flow from the APU isnt really enough to drive the ADP's at the correct speed.
As other posters have mentioned the APU does provide the electrical power to drive the elec Hyd pumps so indirectly it does provide hyd redundancy.
As other posters have mentioned the APU does provide the electrical power to drive the elec Hyd pumps so indirectly it does provide hyd redundancy.
