737-800: Change in hydraulic reservoir quantity on landing gear retraction
 

Retraction of the landing gear causes roughly 20% drop in indicated system A reservoir quantity in 737-800. And stays so throughout the flight even after putting the landing gear lever to OFF position. It returns only after extending the gear again. Where does this fluid quantity get trapped? And why does it not return through the return lines?

The fluid is trapped inside the retraction jacks. It stays there until you lower the undercarriage again

By retraction jacks, do you mean uplock actuators?

Inside hydraulic cylinders...

Assuming the hydraulic actuators (Jacks) are double acting cylinders, fluid is displaced from the return side of the actuators by the internal pistons as the actuators perform their stroke.

The hydraulic fluid displacements are not equivalent between the up and down stroke of the actuators since the actuation shaft displaces fluid from one side of the piston and reduces available piston surface. Normally you would require more force (thus more hydraulic fluid) to retract the landing gear than to extend it, so the non-shaft side of the piston is generally the one pressurized for retraction.

This applies to hydraulic systems in general-yours may be different. I am not at all familiar with the details of the 737 landing gear.

Gear retraction is achieved using fluid pumped into hydraulic cylinders from the reservoir, which pushes the landing gear up into the wheel well. The fluid is trapped when landing gear lever if selected OFF. The fluid is then released for returning to reservoir when wheels are extended.

Skyjob,
Are you saying you always gravity drop your landing gear on your aircraft? How does your landing gear lock down?

Just asking the question out of curiosity. All the hydraulic retractable gear aircraft I've flown move landing gear under hydraulic pressure in both directions.

https://www.cpi-nj.com/wp-content/up...or_650x387.jpg

The volume of the piston rod is the clue to this puzzle.

Thanks Machinbird for the explanation. 20% of reservoir A quantity is about 4.3 litres which can easily be displaced by the piston rod in all three hydraulic actuators (uplock, downlock and MLG actuator) in landing gears, considering that all three are pressurized from the non-shaft side for retraction.

You are welcome, and thank you Jhieminga for the nice generic graphic of a hydraulic cylinder.

First off the assumption of the hydraulic retract actuator being dual action is most likely wrong, most gear on large jet aircraft only pressurize for retraction. Gear down is usually part free fall with the final execution of a relatively small actuator that pushes the side brace etc over center.

Not correct
 

The above graphic of a hydraulic jack would increase the the fluid in the Resivoir on retraction.

It also applies less force during retraction that extension ........ Nice graphic ..... Just 100% wrong !

A and C,
You have apparently made the assumption that the words 'extend' and 'retract' on the graphic refer to landing gear extension and retraction.

You are looking at a generic hydraulic cylinder graphic, and 'extend' refers to extension of the piston rod with 'retract' referring to retraction of the piston rod. Nothing more. The cylinder could just as well be mounted on an excavator or on the rudder of a ship.

Machinbird, here is the deal. Most jet aircraft simply have a high hydraulic demand to retract a landing gear, it is a demanding task. Extension is simple, open the door, release and secure. Simple engineering allows for gravity, often springs and a small hydraulic demand from another actuator to assist the gear into extended and locked position. Once this is done simple mechanics of an over center link generally are sufficient to hold it in it's down position yet are generally reinforced by a small hydraulic actuator. The retract actuator has one goal and requires a high demand of force.

Sorry, but it is a double acting cylinder on the 737. The 737 gear is hydraulically operated in both retract and extend although it can be dropped by gravity and air loads if necessary.
Reference: https://www.google.com/url?sa=t&rct=...z1mHvw&cad=rja

The 737NG MLG actuator is located horizontally outboard of the MLG trunnions. See the PDF located here: http://virtualairlines.hu/737NG_Vide...EAR_BRAKES.pdf
Look eleven PDF squares down on the RH side for the MLG drawing and blow up as required.

To retract the gear, from that drawing, it appears that the piston rod end of the hydraulic cylinder pushes on the landing gear strut below the trunnions while a walking beam assembly attached to the head end of the hydraulic cylinder pulls on a point at the top of the strut above the trunnions thus rotating the strut at the trunnion pivot points to retract the gear. A walking beam hanger prevents the hydraulic cylinder and walking beam from moving out of the plane of the wing.

50 yrs ago, we had fairly complex specialty training devices so that pilots could see and understand the mechanisms they were operating. At some point, it became impractical to provide this level of training and pilots are now just taught what they need to know to operate the aircraft. I seem to remember that this change in training philosophy occurred with the introduction of the 747 and subsequent aircraft.

Machinbird
 

How utterly stupid of me to assume that the graphic of hydraulics cylinder marked extend and retract in a thread about the displacement of fluid in a landing gear system was in fact a hydraulic cylinder from an excavator !

I will crawl back under my rock and defer to your sky god like status.

A and C
Sorry. Didn't mean to rub you the wrong way. I see from your background that you likely already understood why a typical hydraulic cylinder would have unequal displacement to extend and to retract the piston rod.

Grounded 27 - The gear is pressurised in both retraction and extension mode - the selector interchanges the pressure and return systems.

from 737NG-FTD-29-01003

"Retraction of the landing gear causes a noticeable drop in system A hydraulic reservoir level. Starting with the system at 100% before flight, retraction of the landing gear causes roughly 20% drop in indicated system A reservoir quantity.

Thermal contraction during cruise causes an additional drop in reservoir level. Operators have reported 5 to 10% further drop in system A reservoir level after some time spent in cruise, and Boeing's calculations indicate that thermal contraction after prolonged cruise at low temperatures could cause as much as 20% drop.

Similar changes of system B reservoir level can occur. Boeing's calculations indicate that thermal contraction could cause as much as 20% level drop. Leading edge extension just prior to landing removes an additional 20% from the reservoir."

Given that Boeing recommend max fill levels of 90%, its not unusual to see A system down to 70% in flight.

Grounded27- I agree that extension can be done by gravity alone as well and is done during non-normal conditions.
But that doesnt give us any control over the movement of the gear and might lead to its free fall, suddenly increasing drag and even damaging the mechanical structure. To avoid this risk every time we extend the gear, the actuators we use are dual acting hydraulic actuators, which move the gear by hydraulic pressure during both, extention and retraction.
This is evident from the drawings and schematics provided in AMM. I would have inserted the picture here but I am not authorised to do that yet. In the version that I have, it is in chapter 32-32. Might be someplace else in your AMM. But please do have a look yourself.