Boeing 757 hydraulic cooling

[Bus429]

The 757 Maintenance Manual specifies a minimum of 600 USG/1800 kgs fuel in wing tanks to cool hydraulic fluid. This figure is quoted for maintenance checks of, or involving, the hydraulic system(s).

Is there a lower minimum operational quantity? I have often seen 757s arrive with a lot less than 1.8 tonnes in each wing.

[expedite_climb]

Do you have a MM referance ?

[idg]

Could it be that this limit is for ground testing only?

[Bus429]

Expedite,
PMA AMM Task ref is 29-11-00-862-019.

IDG,
That is my thinking. However, this question is prompted by a similar query from a pilot last week. He was presented with a fuel leak during a sim session. As a consequence of design or a sadistic instructor, the fuel leak came with a hydraulic overheat.

[mono]

Can't be much less than 1800 per wing. What with 2 G/A and a divert to think of for reserve fuel. Our co min fuel is 3 tonnes which is only a few kgs less. Land with less than this and an ASR has to be filed.

I think you will find that when flying it will be less due to the cold soak of a flight reducing the fuel temp so allowing more efficient cooling. Remember the MM figure has to allow for a vast range of ground temperatures (tropics to the poles). Only a guess though.

[Cornish Jack]

Had a bit of a head scratch about this on the 744 - similar system but four tanks with different levels for the i/bs and o/bs. The interest was prompted by the mention of a fairly standard arrival fuel of 12 tonnes across the tanks which brought the i/b tanks down close to the MM stipulated minimum. The limit WAS intended for maintenance purposes and pre-supposed potentially long-ish running with no aerodynamic or other cooling.
Thanks to the efforts of one of our F/Os a series of temp checks were taken during approach and landing at fuel levels which were the norm. The results were (of necessity) limited but when plotted gave an interesting divergence between the i/bs and o/bs. It's not an area in which extrapolation could be usefully applied due to the small sample structure but, certainly, the temps were heading upwards.

[Dan Winterland]

From my experience of the 747, it's the 1 and 4 systems which heat up. Not just because they are the gear and flaps systems, but because the cooling pipes in the tanks have to stretch up the tanks due to the geometry of the wing and get uncovered by fuel sooner.


Hi CJ, are you back working for the wooly pully?

[t-bag]

Did this question come from the Alteon sim in Manch? It is rather embarassing sitting in the back when the guys are low fuel and then have a multiple hyd failure-"it wasnt me" doesnt cover it some how!
I know of 757s landing with 1000KGS in each main and all hyds still working,but the engs at alteon are convinced that that wouldnt happen in real life.

[Lynx]

Bus 429,

You are correct in that the MM gives a fuel figure for Hyd pump cooling but this is for extended operations on the ground.

As far as I am aware there is no operational min fuel state associated with Hyd cooling in the air. It is assumed by Boeing that the air flow over the wing would provide any cooling required.

[Dogma]

I get very concerned about landing with less than 4 tons in the 757.:ugh: