Electric Aux pump B747-400
 

How long can operate HYD aux pump on the ground without any negative consequence?

I've never seen a limit for the Aux pump.

Maybe it is cooled by its own hydraulic fluid. Some aircraft have it in hydraulic system 1 as well and both are selected on for pushback or tow-in.

If there's no limit in the FCOM, run it all you want.

As said above Boeing use internal pump cooling, I believe Airbus don't in all cases so have pump limits where required.

No limits, however our book has us turning it OFF once aircraft is in final parking spot and brake has been set for 30 seconds or more. This is to reduce cycles on the pump for long term wear. So for MX, ON/OFF cycles seems to be of more harmful than time running.

Hi guys,


Just back from overnight trip and whilst away I posed torghabe's question to one of our line station engineers. His reply is straight from the B744 Maintenance Manual...


Intruder... yes there is a limit


JammedStab... yes the pump is cooled by its own hydraulic fluid


Depending on your companies choice, you will have either 1 or 2 HYD AUX pumps (sys1 and 4). These HYD AUX pumps are primarily for ground ops only for towing (sys1 for body gear steering and sys1 and 4 for brakes) (Note...with these systems pressurised they are also supplying pressure to sys1 and 4 dependent flt controls etc)


Each HYD sys (1,2,3,4) has its own heat exchanger mounted in the 4 main fuel tanks for HYD pump cooling eg...Sys1 in #1 main tank, Sys2 in #2 main tank etc. If you are going to run ANY hyd pump for more than 15mins and depending on which sys you run, you will need for hydraulic pump cooling purposes a MIN of 900kgs of fuel in #1 and 4 mains for Sys1 and 4 and a MIN of 2500kgs of fuel in #2 and 3 mains for Sys2 and 3.


The engineer and myself could only assume the reason that the limits are not in the FCOM is because if your fuel tank quantities are that low, you won't be worrying about Hyd pump limits.

It sounds like the same system as the B747 200/300 and the 15 minute limit was in our ops manual.

The temperature rise was very noticeable on that aircraft if the pump time approached the 15 minutes with low fuel quantities.

Wunwing,


You're on the money!! I recall my engineer friend mentioning the classic as well.

This question was born due to: in some cases our pilots forget set AUX pump to OFF position, after that pump can running 2-3 hours. Unfortunately this is not rare situation.

I may be wrong but I think that the Aux pumps can only be run on ground - the bus that powers them is not available in flight. The pump will also trip off if the system is pressurised by its ADP or EDP.