Originally Posted by grounded27
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.
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.
Landing Gear Extension
When the LANDING GEAR lever is moved to DN, hydraulic system A pressure is used to release the uplocks. The Landing Gear extends by hydraulics, gravity, and air loads. Overcenter mechanical and hydraulic locks hold the gear at full extension. The nose wheel doors remain open when the gear is down.
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 hangar 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.