When aircraft are on the ground and need to be moved backwards, it is important to employ the use of aircraft towbar rather than use the engine’s reverse thrust systems. While most aircraft have a wide or full range of motion, towbars prevent damage to both the equipment involved and the equipment nearby. Instead of moving the aircraft from within, the towbar pushes from the outside using a system of a towbar mechanism attached to a carriage, such as a tractor or a tug. A driver on the ground operates the carriage and pushes the craft externally in a “pushback” motion without needing to run the jet engines. It is important to stay up to date with maintenance concerning the many parts of the towbar. This blog will explore the various aspects of the towbar system and how to properly prevent operational problems.
Main parts of the towbar system include the towbar body, the head mechanism, shear pins, tow rings, lift mechanism, carriage, wheels, and hubs. The body of the towbar is the tube that connects the wheels of the aircraft and the wheels of the carriage. It is imperative that the tube is inspected for straightness and never bows. If bowing, the body should be taken out of commission. Additionally, any cracks must be repaired by a welder upon discovery. The head mechanism attaches the body to the wheels in a moveable fashion. The head can also pivot and turn to withstand pushback force so it is important that all moveable parts are examined. Head mechanisms attach to the wheels through either a pin capture or a clamp style. To prolong efficiency, these parts, such as linkages, should be lubricated and checked for signs of wear. In addition, pivot bolts that some heads rely on should be fully removed for examination, and the actuator handle should also lock into position.
Shear pins are another part that should be removed entirely for inspection. They should be inspected for cracks or damages and be immediately replaced. The function of a shear pin is to carry the load up until they reach a predetermined stress (psi) at which point they shear. This is a safeguard that prevents damage to the aircraft caused by overexertion.
At the end opposite the head, the tow rings connect to the carriage and either allow for a swivel motion or hold the carriage in line with the towbar body. Welded tow rings are less complex because they do not account for movement, so maintenance mainly entails inspection for cracks in the ring. Swivel-type lunettes are tow rings that allow for movement, so they have many moveable parts that must be inspected. The threaded shaft should be checked for cracks, similar to the welded tow ring, but the inside threads must also be inspected as well for stripped, pulled, or missing threads. The lunnette’s ability to swivel should be maintained with lubricant, and a locking nut should be used to hold everything together.
A towbar might have a lift mechanism that allows it to reach the nose of the aircraft, so this function requires specific maintenance. They are often operated through a manual pump, so any hoses, seals, or pumps should be inspected for leaks or cracks. Additionally, fluid levels of the power system must be properly monitored and maintained.
Lastly, the carriage of any towbar system should be straight and tightly attached to any parts, such as the axle shaft, through secure hardware. All hardware should be inspected similarly to that of the head shaft with full part removal and a visual inspection for cracks. Additionally, wheel hubs and bearings should undergo full inspection every other year, including replacements wherever necessary.
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