B737 Hydraulic LOW PRESSURE light

[barrichello72]

Hi folks
one question about the low pressure light for the hydraulic system of the 737ng.

from the QRH: Hydraulic Pump Switch - OFF

A hydraulic pump LOW PRESSURE light illuminated indicates related hydraulic pump output pressure is low.

Intermittent illumination of the hydraulic pump LOW PRESSURE light may be the result of single electric pump operation and a high demand on the hydraulic system.

What does that last statement mean? That if the light comes on the EDP is less than normal but more than 1300psi? The single bit throws me off.

Thanks for any input

[Capt Pit Bull]

Say Engine Driven Pump (EDP) output low or zero. Most obvious reason seized engine core, so you’ve done a severe damage shutdown.

now the hydraulic system pressure is being maintained by the electric pump only. It has about 1/4 the flow rate of the EDP. So if you make a lot of hydraulic demands you may exceed the pump output at which point system pressure decreases. The onside accumulator will help a bit but the pressure could drop enough to triger the low pressure light (and the master caution and HYD on the system annunciator panel.

But this will be transient as the pump will catch up.

e.g. Single engine ops with a seized engine. Cycling gear and flaps simultaneously can give this condition for a few seconds. By the time you move your eyes to the panel it has usually gone out again.

So don’t sweat intermittent HYD when cycliing hydraulic services on one engine.

a windmilling engine is still turning the EDP and so this condition is less likely.

hope that covers it.

[bArt2]

I had the case a few years ago. Shortly after arriving at top of climb we got a system-B engine driven pump low pressure light. A few minutes after finishing the checklist we got a system-B electric pump low pressure light. System-B quantity had decreased to zero. Decision was made to return and during descent we got a system-A engine driven pump low pressure light, with system-A quantity slowly decreasing. So the last part of the flight we only had the system-A electric pump and it seemed to do it’s job quite well, never noticed any flickering of the hydraulic pressure light.

[Capt Pit Bull]

I had the case a few years ago. Shortly after arriving at top of climb we got a system-B engine driven pump low pressure light. A few minutes after finishing the checklist we got a system-B electric pump low pressure light. System-B quantity had decreased to zero. Decision was made to return and during descent we got a system-A engine driven pump low pressure light, with system-A quantity slowly decreasing. So the last part of the flight we only had the system-A electric pump and it seemed to do it’s job quite well, never noticed any flickering of the hydraulic pressure light.

ouch! That is a different scenario, namely contents loss rather than EDP failure.

And it sounds pretty concerning. Fluid loss from BOTH systems? How did that happen?

[Capt Quentin McHale]

Capt Pit Bull,

Seized engine core seems a bit drastic, I would be looking at a sheared EDP drive shaft first (then again engine instrument parameters and aircraft handling will obviously alert you of a seized engine).

The only accumulator I can recall on the 737 is the Sys B BRAKE SYSTEM accumulator for Brake System ONLY.

No accumulators for either Hyd Sys A or B that I can recall. Please correct me if I am wrong.

Rgds McHale.

[bArt2]

ouch! That is a different scenario, namely contents loss rather than EDP failure.

And it sounds pretty concerning. Fluid loss from BOTH systems? How did that happen?

Pumps had a mechanical failure and disintegrated, therefore loss of hydraulic fluid. Never got an answer as to how it is possible to loose two pumps in two seperate system at roughly the same time.

[Capt Pit Bull]

Capt Pit Bull,

Seized engine core seems a bit drastic, I would be looking at a sheared EDP drive shaft first (then again engine instrument parameters and aircraft handling will obviously alert you of a seized engine).

The only accumulator I can recall on the 737 is the Sys B BRAKE SYSTEM accumulator for Brake System ONLY.

No accumulators for either Hyd Sys A or B that I can recall. Please correct me if I am wrong.

Rgds McHale.

Sure, that was just an example of one way you could be deprived of an EDP. Regarding accumulators I expect you are right (about 14 years since I last flew one and there are a lot of types rattling around inside the old noggin!)

The general principle holds true for the OP, compromised hydraulics plus using lots of services -> transient low pressure.

Cheers.