747-400 demand pump selection
Thread Starter
747-400 demand pump selection
My aging and rapidly decaying brain cells can't cope with an apparent anomaly in the 400 hydraulic QRH drills.
QRH 1 - "HYD OVHT SYS ( )"
EDP - OFF
Demand pump - AUTO
QRH 2 - "HYD PRESS SYS ( )"
EDP - OFF
Demand pump - ON
Obviously the above are extracts and not the complete drills but the post drill conditions are the same - EDPs switched OFF and, therefore, the aircraft is reliant on the demand pumps to provide hydraulic 'muscle'. Sooooooo... the question is:-
Why does one case have the demand pumps in AUTO and the other (essentially same conditions) have the demand pumps at ON.
The HYDIM logic triggers would provide continuous running for either case, since the demand pump AUTO logic is looking for > 1600 psi from the EDP to switch off the demand pump. So WHY !!!
This is with Boeing at the moment and am awaiting a reply (and have been for several weeks) but would anyone like to hazard an educated guess ? PLEASE!!
QRH 1 - "HYD OVHT SYS ( )"
EDP - OFF
Demand pump - AUTO
QRH 2 - "HYD PRESS SYS ( )"
EDP - OFF
Demand pump - ON
Obviously the above are extracts and not the complete drills but the post drill conditions are the same - EDPs switched OFF and, therefore, the aircraft is reliant on the demand pumps to provide hydraulic 'muscle'. Sooooooo... the question is:-
Why does one case have the demand pumps in AUTO and the other (essentially same conditions) have the demand pumps at ON.
The HYDIM logic triggers would provide continuous running for either case, since the demand pump AUTO logic is looking for > 1600 psi from the EDP to switch off the demand pump. So WHY !!!
This is with Boeing at the moment and am awaiting a reply (and have been for several weeks) but would anyone like to hazard an educated guess ? PLEASE!!
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Posts: n/a
Thanks for the challenge, CJ... I think I have an answer for you, but I have to honest and tell you that I had to get the wiring diagrams out to refresh my decaying brain cells 
I think it is important to recognize that in the second case (QRH2) the demand pump has failed to _automatically_ supplement the SYSTEM (total) pressure when the engine pump pressure (we assume) has dropped below 1400psi (GE engines logic value only).... hence the "HYD PRESS _SYS_" msg. In the QRH procedure, we are bypassing the automatics by selecting the demand pump to ON. Note: The SYSTEM pressure sensor is different from the EDP pressure sensor and is the one which puts up the EICAS msg (as well as illuminates the overhead SYS fault light).
In the first case (QRH 1), there is nothing wrong with the automatics.... You "just" have an overheat problem, which, I was told recently, will be generated by a faulty engine pump in most cases.
Hope this makes sense??? (It's a lot easier to understand this stuff with pictures in front of you)
Cheers.
Q.
I think it is important to recognize that in the second case (QRH2) the demand pump has failed to _automatically_ supplement the SYSTEM (total) pressure when the engine pump pressure (we assume) has dropped below 1400psi (GE engines logic value only).... hence the "HYD PRESS _SYS_" msg. In the QRH procedure, we are bypassing the automatics by selecting the demand pump to ON. Note: The SYSTEM pressure sensor is different from the EDP pressure sensor and is the one which puts up the EICAS msg (as well as illuminates the overhead SYS fault light).
In the first case (QRH 1), there is nothing wrong with the automatics.... You "just" have an overheat problem, which, I was told recently, will be generated by a faulty engine pump in most cases.
Hope this makes sense??? (It's a lot easier to understand this stuff with pictures in front of you)
Cheers.
Q.
Thread Starter
Many thanks Q
That will do nicely!
I'm afraid that I was approaching it from the other viewpoint - why shouldn't the selection be to ON for the OVHT. (I could, in fact, see why it wasn't NECESSARY, because of the output pressure monitoring) Once I'd started looking at it from that standpoint I was trying to argue the logic the wrong way round.
Obviously this living upside down that you chaps do down there in Oz is good for the brains. Perhaps I should come down and try re-charging the grey matter
B rgds
That will do nicely!
I'm afraid that I was approaching it from the other viewpoint - why shouldn't the selection be to ON for the OVHT. (I could, in fact, see why it wasn't NECESSARY, because of the output pressure monitoring) Once I'd started looking at it from that standpoint I was trying to argue the logic the wrong way round.
Obviously this living upside down that you chaps do down there in Oz is good for the brains. Perhaps I should come down and try re-charging the grey matter
B rgds
