View Full Version : Windmilling Hydraulic Pumps

ccy sam
14th Jun 2003, 15:08
Scenario: both engines on a 737 NG flame out due to fuel leak or whatever, the APU is not available and therefore the standby hyd. pump is not available because the transfer busess are not powered.
Question: will the hydraulic EDPs provide any pressure if the engines are windmilling? Is there enough pressure to operate the flaps or anything else?

Continuing with the same scenario, what speed at G/S intercept at say 3000agl would be required to make the rwy if you only had the gear available and no flaps?[glide approach]

14th Jun 2003, 23:50
ccy - remember, however, unlike on the ground, you will be using any pressure for the flight controls as well, so you may not get what you want! Obviously the faster the better for EDP pressure!

We do not work on a 10 mile 'speed' but glide clean until somewhere like 10 miles at twice the normal height, ie 6000' aal., then taking gear and flap as you can.

15th Jun 2003, 13:30
Consider this....
At approx 3% N2, there is 300PSI hydraulic pressure available!!

ccy sam
15th Jun 2003, 18:10
Thanks to those who replied. BOAC do you have any further details that would be helpful on a glide approach from say 25000ft. As far as I know Boeing don't give this kind of information. Things like how far could you glide from a given altitude etc. Thanks.

Dan Winterland
16th Jun 2003, 17:01
Boeing do provide info for some aircraft. According to the ACM The 747 will provide enough hyd power from windmilling engines above 160 knots.

17th Jun 2003, 07:47
ccy - work on 50nmls - ie twice your height. Don't know what EFIS you have on NG 737, but put chosen 'gate' height in MCP window (QFE/QNH?) and use the green arc as a guide as you glide. When you get there, put field elevation (or '0') in the window.

500ft for a 360 turn.

Flight Detent
17th Jun 2003, 18:54
Yes, Comxtec,
But there is no volume available, therefore, even though the gauge indicates 3000psi, as soon as you operate something, anything, that pressure almost disappears.
You need quite a bit more than 3% rotation to succussfully operate a service, that's why the B747 shows 160 kias to get anything useful from the EDP!


18th Jun 2003, 13:03
I agree, hence the different speeds of flap deployment when using elec pumps.
I was just throwing that out there because to this date, it amazes me that at that slow a speed 3000 psi can be put out.

18th Jun 2003, 15:18
We went playing once after school and what I recall:

735 @ 8 mile 3 glide 52k kg no relevant winds, powered controls, no LE/TE devices, manual gear, GRD spoilers, autobrakes, dry runway. Both engines f***ed out, of course. :rolleyes:

Started with about 220 kt at the top (210 best clean but 53070 is the limit you go to 220, so that was on the heavier side) and...

...off we went at the departure end of runway at about 80 kt. :\

The speed was decaying very slowly, something like 2 kt per NM.

We started with the gear quite late, 2 NM I'd say and once it was down, there wasn't a whole mile left on DME. I can be making this up, but I really believe we were doing something like 185 over THR. My guess is that 4NM G/down and you live happily ever after.

The NG may be quite another story after all, especially if yours is winglet equipped.

19th Jun 2003, 10:29
"it amazes me that at that slow a speed 3000 psi can be put out."

Did "comxtek" mean 3000psi... or 300psi (typo?).


20th Jun 2003, 03:26
No typo, 3000 (three thousand) psi!!!
But there isn't much volume. The EDPs put out approx 22GPM...but I'm sure that nunmber would be a bit less with only 3-4% N2. But yes, it doesn't take much to get 3000 psi.

Lu Zuckerman
20th Jun 2003, 09:37
Most hydraulic pumps used on modern aircraft are of the constant pressure variable delivery type. Although they are constant pressures this is somewhat of a misnomer. If the pump is in a 3000-PSI system it will put out 3000-PSI when there is no demand. When a demand is placed on the pump the pressure will drop to the level it takes to move the control surface or the landing gear. If the loads are light and it only takes 1100-PSI to effect the work then the system gage will show that level. When the surface or the gear reaches the desired position the demand will reduce to zero and the pressure will rise to 3000-PSI.

During the windmilling condition the pump will show a 3000-PSI level but the pump may not be able to react to a demand on the system (due to low flow). Some servo systems have internal bypass valves to allow manual reversion or, to reduce resistance to movement when the pressure in the system is less than a preset level (Approximately 700-1000-PSI. In this condition when there is minimal or no demand and the system pressure is at the 3000-PSI level the bypass valves will be closed. When a demand is placed on the system the pressure will drop and the bypass valves will open. To counter this designers incorporate redundant hydraulic systems ,power transfer systems , ram air turbines and electric pumps

If all hydraulic systems are lost then the pilots revert to thrust management for steering.