Out of cuirosity, for how long could all 4 hydraulic systems operate solely with demand pumps? 

 

-Angelo Busato

Edited September 10, 20187 yr by killairbus

I don’t think they are time limited so they should run for as long as required. 

On 9/10/2018 at 4:26 AM, killairbus said:

Out of curiosity, for how long could all 4 hydraulic systems operate solely with demand pumps? 

 

-Angelo Busato

There are no limitations on how long the demand pumps can operate on their own, but they are normally operated in Auto when the engines are running which means that for most of the time they do not operate.

When they are in AUTO the demand pumps will only operate if the resepective hydraulic system pressure is low or the respective Fuel Control Switch is selected to CUTOFF.  However, Hydraulic Systems 1 and 4 Demand Pumps will run as required whenever the flaps are operated in order to maintain system pressure or when there is an unusually high demand from the respective hydraulically operated system (including a leak - but not for long if all the fluid is lost!). 

The early B744's had a single electric auxilliary pump fitted to the No.4 system, but modified and later B744 aircaft had a second electric aux pump fitted to the No.1 hydraulic system and these pumps are used for ground handling purposes.

I was waiting to hear an expert chime in before I added my thoughts.  Thanks Bertie.

As for hydraulic pump duty cycles...... in general the pumps are rated for continuous operation as long as there is a fluid to pump.  These pumps only run into trouble when there is a problem with the fluid as in low fluid or insufficient head where you can run into problems with heat or cavitation.  I spent my second career around petrochemical plants and those pumps tend to last as long as the operating unit.  Aircraft pumps as least as long.

I agree with you about the fluid loss, especially when the system is operating at 3,000psi.  With thanks for the complimet from one so called 'expert' - which incidentally I am not - to another! 

1 hour ago, downscc said:

I was waiting to hear an expert chime in before I added my thoughts.  Thanks Bertie.

As for hydraulic pump duty cycles...... in general the pumps are rated for continuous operation as long as there is a fluid to pump.  These pumps only run into trouble when there is a problem with the fluid as in low fluid or insufficient head where you can run into problems with heat or cavitation.  I spent my second career around petrochemical plants and those pumps tend to last as long as the operating unit.  Aircraft pumps as least as long.

Yes, most aux hydraulic pumps are quite rugged. As an example, The 3A electric pump on a CRJ-200/700/900 (which is the highest-capacity pump on the aircraft), is run continuously. It is switched on before pushback, and is not shut off again until after the aircraft is parked at the gate at the destination airport.

That pump does the bulk of its “heavy lifting” during landing gear retraction and extension, but it is always running in normal operations even when not called upon to do any work.

Generally, as long as the hydraulic fluid is being cooled with the heat exchangers in the wing fuel tanks, there shouldn't be a problem. There are time limitations on pump operation when fuel level is low on the ground.Last time I checked, it was 15 minutes.

In the air, there is air cooling.

Edited September 14, 20187 yr by Qavion2