All drop check operations should be performed as specified in the applicable maintenance instructions manual (MIM). These procedures should be thorough enough to ensure that the system is free of leaks and the operational integrity of the system has been restored following maintenance. Operational checks cover three distinct areas. They are the operation of the landing gear and doors, the operation of the landing gear position indicator and warning system, and the operation of the landing gear emergency system.

The first step in the drop check procedures is to place the aircraft on jacks. Refer to chapter 3 of this training manual.

Further preparation includes connection of a hydraulic test stand and external electrical power, removal of landing gear maintenance safety locks, and the proper placement of the landing gear control handle. As the operational procedure begins, check to make sure that the landing gear doors do not close in the path of the retracting main struts. This condition will be obvious (with hydraulic and electrical power on the aircraft) if the landing gear doors do not remain in the full open position when the landing gear control handle is placed in the UP position. Placing the landing gear control handle momentarily to the UP and DOWN positions several times will correct this condition by removing air from the wheel door cylinders.

Regulate the hydraulic test stand to operate at a flow of 4 gpm, and slowly increase hydraulic pressure. The landing gear down lockpins should start to retract. They should be fully retracted when the pressure reaches 1,800 psi, and then all gear assemblies should start to retract.

When the nose gear nears the up position, be sure the fairing doors are cammed to the closed position, and then check all gear doors to be sure they are closed and locked when the position indicator indicates the up-and-locked condition. Move the landing gear handle down and check to see that the wheel fairing doors open and gear assemblies extend. Visually check all gear assemblies to ensure they are down and locked. With the test stand regulated to 3 gpm at 3,000 psi, the gear should make a complete cycle (up and down) in 12 to 14 seconds. The maximum pressure required to retract and lock the gear is 1,800 psi at 4 gpm.

When you check the emergency extension of the gear, first retract the gear normally, secure external hydraulic pressure, place the landing gear handle in the down position, and then pull and hold the emergency extension handle fully aft. Visually check that all gear assemblies are down and locked by observing the landing gear position indicator in the cockpit, and then release the emergency extension handle. It may be neccssary to manually push the gear assemblies to the down-and-locked position. The force required to push the main gear to the locked position should not exceed 20 pounds applied to the axle hub. The force required to push the nose gear to the locked position should not exceed 10 pounds applied at the center line of the axle hub. Make at least one complete normal cycle of the landing gear, and then remove external power and aircraft from jacks.

NOTE: Some aircraft require resetting of the landing gear dump valves before recycling the landing gear. Refer to the applicable MIMs.

Troubleshooting of the landing gear system, like all hydraulic systems, requires that you understand the theory of operation of the particular system and the function and sequence of operation for each component. Troubleshooting steps provided in the MIM are normally aligned with the sequence of events or steps in the operational checkouts. They provide an efficient means of isolating the malfunction. The MIM requires that each step in the operational checkouts be performed in sequence. If trouble occurs during the procedure, it must be corrected before proceeding with the next step. These troubleshooting aids provide a logical cause for many anticipated landing gear malfunctions, including procedures for isolating and remedying the problem. Refer to the system schematic for the particular system and accompanying maintenance instructions, in addition to sound reasoning, to pinpoint the cause for a malfunction in an efficient manner.

Some landing gear malfunctions are related to improper maintenance practices, with the lack of proper lubrication being the predominant malpractice. A review of past discrepancies and previous corrective actions may also aid in analyzing malfunctions. Occasionally, discrepancies that are reported as a result of flight are difficult or even impossible to duplicate on the deck. However, too many discrepancies signed off with "Could not duplicatesystem checks 4.0," or similar corrective actions, show up as repeat malfunctions or as the cause of accidents. Every effort should be made to locate a sound logical cause for a reported malfunction by thoroughly checking the system, each component, linkages, clearance, and associated indicating systems. All phases of the operational checkouts must be verified by a quality assurance inspector.

Detecting internal leakage of components may require the use of special equipment, such as the ultrasonic leak detection translator or simple isolation of components by disconnecting lines, applying pressure, and measuring for allowable leakage limits. If troubleshooting time is considered significant, use a separate VIDS/MAF showing a common job control number and Action Taken code "Y." This form provides input data that accounts for the troubleshooting time separately from the actual repair time.