Landing gear hydraulic system maintenance is similar to the other types of hydraulic system maintenance. This system is inspected for internal and external leakage as well as proper operation during inspections. While performing operational checks, you must inspect the complete landing gear installation for adjustments, clearances, and sequence of operation. 

The adjustment of latches is one of your prime concerns. A latch is used in hydraulic systems as a device designed to hold a unit in a certain designated position after the unit has traveled through a part of its cycle. For example, when the landing gear is retracted in some landing gear systems, each gear is held in the up position by a latch. The same holds true when the landing gear is extended. Latches are also used to hold the landing gear doors in the open or closed positions. There are many variations in designs of latches. All latches are designed to accomplish the same thing, They must operate automatically, at the proper time, and hold the unit in the desired position. 

The main landing gear forward door is held closed by two door latches. As shown in figure 12-5, one latch is installed near the front of the door and the other near the rear of the door. To lock the door securely, both locks must grip and hold the door tightly against the aircraft structure. The principal components of each latch

Figure 12-6.Landing gear door latch installation.

mechanism, shown in figure 12-5, are a hydraulic latch cylinder, a latch hook, a spring-loaded linkage, and a sector. The latch cylinder is connected hydraulically with the landing gear control system and mechanically, through linkage, with the latch hook. When hydraulic pressure is applied, the cylinder operates the linkage to engage or disengage the hook with or from the latch roller on the door. In the gear down sequence, the hook is disengaged by the spring load on the linkage. In the gear up sequence, spring action is reversed when the closing door is in contact with the latch hook, and the cylinder operates the linkage to engage the hook with the latch roller. Cables on the landing gear emergency extension system are connected to the sector to permit emergency release of the latch rollers. An up lock switch is installed on, and actuated by, each latch to provide main-gear-up indication in the cockpit.

With the gear up and the door latched, inspect the latch roller for proper clearance. See view B of figure 12-6. On this installation, the required clearance is 1/8 inch 3/32 inch. If the roller is not within tolerance, it may be adjusted by loosening its mounting bolts and raising or lowering the latch roller support. This can be done because of the elongated holes and serrated locking surfaces of the latch roller support and serrated plate. See view A of figure 12-6.