Hi folks, 

 

I am just performing LFPG-AFR54-KIAD (with 77W instead of 388...) and I encounterned the HYD QTY LOW L and HYD PRESS PRI L failures. 

According to the non-normalchecklist switched off the PRI L pump. 

 

My question: 

Despite the fact (?) that the hyd fluid quantity indication reads zero, the pressure still states 3000.

Why? No fluid, no pressure...

 

Thanx in advance. 

 

 

 

Why? No fluid, no pressure...

 

You are assuming the problem is low fluid level when in fact it may be a failed level gauge. 

Hi Dan, 

 

thank you for the hint. 

After my save landing on KIAD's RWY 01R, I cheat a bit check the aircraft's system faults: 

1. EDP 1 HYD FAIL

2. L SYS QTY REFILL REQ

 

Well, seems to be independent, but accidentally on components of the same HYD system (L). 

Do I interpret it right that truly the ENG L primary pump is defect, 

and the HYD L gauge presented erroneously an empty quantity level L? 

 

This would explain the full pressure L... Ok, learnt something! Thanx. 

This is correct behavior. All the hydraulic reservoirs have standpipes that serve as the main feed for each system. For those who aren’t sure what a standpipe is think of the water reservoir in a toilet. The drain that prevents an overflow if the float doesn’t shutoff the water is a standpipe.

 

In your case the left engine driven pump is supplied from this standpipe. The left demand pump which serves as the backup pump feeds from the bottom of the reservoir. All hydraulic quantity indicators will read 0.00 when the fluid level is at the top of the standpipe. This leaves enough fluid in the reservoir to supply the demand pump, even when quantity reads zero. The only minor issue I see is the demand pumps should be around 2850 PSI, not 3000.

 

Brian

 

 

In your case the left engine driven pump is supplied from this standpipe. The left demand pump which serves as the backup pump feeds from the bottom of the reservoir. All hydraulic quantity indicators will read 0.00 when the fluid level is at the top of the standpipe. This leaves enough fluid in the reservoir to supply the demand pump, even when quantity reads zero. The only minor issue I see is the demand pumps should be around 2850 PSI, not 3000.

 

I assumed the gauging would be in the standpipe, which is the case for metering vessels in many industries.  I hoped to find some diagrammatic information in the FCOM but missed it or it is not there.  Know of any on line source?

This guy gives a pretty good description of how things work:

http://okigihan.blogspot.com/p/filter-units-filter-modules-are.html

 

He also makes mention of the 0.00 standpipe relationship when discussing the CHIS.

Thanks Brian, that is an interesting source.  Do you know the gauging method used in the pressurized reservoirs?  Measuring levels in a pressurized vessel subject to movement and g-forces is tricky.  My guess would be a capacitance-based measurement.

The quantity indicating system on the 3/4/5's uses a float to find the fluid level in the reservoir. The float mechanically moves the indicator that is on the transmitter. The float also moves a synchro which gives a signal equal to the fluid level in the reservoir.

 

Not sure what the NG's use.

My guess would be a capacitance-based measurement.

Good guess, that's exactly what's used.

EDIT: Useless Post - 777 forum not 737.