Hydraulic Pressure

[sirtopper]

Well its been a few weeks now and this is still my only aircraft outside its hanger, the rest are tucked away. Love it lots but still unable to complete a flight with VNAV. Not too worried as I know its being looked at. Besides makes the descent a lot more fun.

My question is regarding the engine driven hydraulic pumps. After coming to stop at the gate and shutting down the engines the hydraulic pressure remains at 3000 psi right up until the engine N2 speed is around 5%  then it suddenly drops, no slow decay. I would have thought the engine driven hydraulic pumps would requires a lot more engine speed to sustain that pressure?

Many Thanks IXEG for your continued development.

[poodster]

What is your VNAV doing that's causing you to not complete a flight? I've completed quite a few with VNAV engaged

[Litjan]

Hi sirtopper,

the engine driven pumps are variable-dispacement piston type axial pumps and they can output 3000 psi at very low rpms. If you would run a lot of consumers (i.e. flaps, gear) at this low rpm you could probably see pressure drop a bit sooner.

Theoretically (no leakage) you could even maintain pressure in a closed system without any type of pump action (think of the brake pressure accumulator), but I think we timed the pressure drop to the rpm after the real aircraft.

Jan

 

[sirtopper]

Thanks Jan, I tested the pressure drop with flap and brake pumping load. Although the pressure drops while pumping the brakes it is still able to replenish to 3000psi until around 5% N2.

 

1 hour ago, poodster said:

What is your VNAV doing that's causing you to not complete a flight? I've completed quite a few with VNAV engaged

Hi poodster, I dont use VNAV on departure as I never select runway until in the air. Normally for me VNAV shows a delayed TOD which means it never is able to get on the right profile. Or while descending on what seems to be the right profile the speed bug it follows seems to go above the bar pole (maximum speed ) and I have to intervene. It has nearly worked but always seem to be something messing it up so I don't tend to bother with it much.

Tony

 

[poodster]

Just now, sirtopper said:

Thanks Jan, I tested the pressure drop with flap and brake pumping load. Although the pressure drops while pumping the brakes it is still able to replenish to 3000psi until around 5% N2.

 

Hi poodster, I dont use VNAV on departure as I never select runway until in the air. Normally for me VNAV shows a delayed TOD which means it never is able to get on the right profile. Or while descending on what seems to be the right profile the speed bug it follows seems to go above the bar pole (maximum speed ) and I have to intervene. It has nearly worked but always seem to be something messing it up so I don't tend to bother with it much.

Tony

 

Ohhh ok. Yea the VNAV right now doesn't like those changes during flight. I wonder if things would be better if you chose a runway on departure, just to get an end waypoint, and then changed it in mid air. 

Maybe it's worth a try?

[sirtopper]

6 minutes ago, poodster said:

Ohhh ok. Yea the VNAV right now doesn't like those changes during flight. I wonder if things would be better if you chose a runway on departure, just to get an end waypoint, and then changed it in mid air. 

Maybe it's worth a try?

I will try a flight with a full VNAV before take off and see how she performs

Cheers Tony

[poodster]

13 minutes ago, sirtopper said:

I will try a flight with a full VNAV before take off and see how she performs

Cheers Tony

Good luck! Hope it goes well.

[sirtopper]

52 minutes ago, poodster said:

Good luck! Hope it goes well.

Well very nearly again.. short hop from my local EGNJ to EGNT filled out the FMC all set and did not edit it in the air. All went well from 400' selected VNAV and LNAV and then CMD at 1500'. All went well. before TOD reset Altitude to ILS FAF approach altitude 2200' just before reaching FAF the descent stopped and held at 2800' checked all FMC restraints 2200' and MCP window 2200 set. QHN correct. So nearly made it on VNAV  

Cheers Tony

[chadryan]

I have also been sticking to using level change instead of VNAV as I've found the VNAV TOD is way too late in most cases

[Tim013]

Curious as to why people are so fascinated by VNAV.  Seems like real 737 pilots rarely use it.  Seems they either hand fly, or use the MCP.  LNAV already takes away the fun of navigating, why have VNAV suck out the fun of planning descents? 

Tim