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If a aircraft landing gear fails to drop due to hydraulic failure, is it possible to drop the landing gears manually?

Yes. In almost all the aircraft with a retractable landing gear system, once the gear is up, they are held up by mechanical uplocks. Each wheel or gear has an uplock roller which is mechanically attached to an uplock hook in retracted position. So, to drop the gear, all that has to be done is to release the rollers from their respective uplock hooks. The rest of the work is done by something called the Gravity as the gear no longer has any system holding it in place.

In some aircraft, the procedure to lower the gear in the situation the main system fails is a bit more complex than others. In the Dash 8 for example, the main landing gear has to be pumped down. For that, there is a small independent hydraulic reservoir located in the nose of the aircraft. Initially, you have to pull two cables located in two compartments, one in the ceiling towards the First officer’s seat and one on the floor behind the center pedestal. By pulling the cable overhead, you can release the main landing gear doors and the main gear wheels from their uplocks. The nose gear can then be lowered by pulling the cable in the floor compartment. Because the main landing gear is heavier and as it falls against the direction of flight, just by releasing the uplocks it only extends partially. To extend the gear fully, you have to pump it down. For that we have socket in the lower gear extension compartment to which you can insert a metal handle which fastened to the cockpit wall behind the seat of the First officer.

Fun fact: For the first flight of the day as a new crew we used always pull these two cables to open the gear doors so that we can inspect the gear wells during flight.

The lower alternate gear extension compartment of a Dash 8 Q300. The cylinder you see is where the handle is inserted to pump the main gear out.

The things are easier in some aircraft. Like in the Airbus A330, all you have to do to gravity extend the gear is pull a cockpit handle to down position, pretty much like how you would pull the normal gear lever down. Here, the uplocks are electrically unlocked and the gear falls down. No pumping or pulling of lever required.

The A330 gravity gear extension handle. To activate it, the pilot have to simply pull it to down position.

Source: Quora

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