Down and Locked
Inspection and lubrication tips for landing gear systems
By Joe Escobar
Although every part and component of the aircraft is important, the landing gear is an especially critical system. It is the only means by which the aircraft is able to safely transition from flight to ground. Failure of the gear prior to or during landing can be catastrophic.
Recent NTSB reports show that there has been an increase in accidents and incidents attributed to landing gear problems. Numerous causes are listed ranging from part failures to improper maintenance. When you throw in the additional factor of severe operating conditions such as lack of pilot training and agricultural operations, the possibility of defects being present on the landing gear increases. The landing gear functions in some of the harshest operating and environmental conditions on the aircraft. Whether the landing gear is fixed or retractable, thorough inspections and lubrication are essential.
The first step of performing an effective inspection is to ensure that the gear is clean. Excessive dirt, grease, and other contaminants can hide otherwise easily visible flaws. Dirt and debris on the gear can cause binding in moving components such as actuators, chains, limit/indication switches and hinge points. A clean landing gear system ensures freedom of movement and ease of inspection.
A thorough visual inspection begins with a good light source. A good 10X magnifying glass also comes in handy at times.
Rich Mileham, Safety Manager for the Great Lakes Region FSDO, notes that cracks are the foremost area of concern when it comes to the landing gear. He adds that they are very difficult to detect. Some indicators of cracks include chipped paint, soot residue, or corrosion. Be sure to spend a little extra time on any areas of the system that have had a past trend of cracking. However, do not assume that just because you have never found a defect on any particular assembly that you can become complacent and not worry about looking at it very closely. All areas of the landing gear system deserve the utmost of attention when it comes to inspection. Be sure to follow the requirements of the maintenance manual and any applicable AD’s or Service Bulletins when performing the inspection.
Sometimes, it cannot be verified by a visual inspection whether or not an indication noted is a crack. This is where a NDT test comes in. Some AD’s require recurrent dye penetrant inspections on certain components. When performing a dye penetrant inspections, the issue of cleanliness is even more important. The greases and lubricants used for the landing gear system can set up in cracks. If not completely removed, these lubricants can prevent the penetrant from flowing into the discontinuity, causing false results in the test.
Some areas of the landing gear are particularly susceptible to corrosion. Areas where water can be trapped are especially susceptible. Any sitting water should be removed and the area inspected thoroughly. The ledges in wheel wells are particularly vulnerable to sitting water. Ensure that all drain holes are clear.
The area inside the axles is another corrosion prone area. Its relative inaccessibility makes corrosion removal difficult. The key is to find any corrosion early and remove it before it becomes a serious problem.
Some other areas to watch out for when it comes to corrosion are at dissimilar metal contact areas, under ferrules and identification tapes, and on wheel bearing races. Landing gear hardware is particularly prone to corrosion. All exposed hardware should be treated with corrosion preventive compound (CPC) as required.
Following recommended lubrication schedules can help prevent corrosion and wear in internal surfaces. Lubrication serves two purposes. First of all, it provides a film that allows two metal components that are in contact with each other to move freely. Second, it provides protection from corrosion by preventing water and debris from penetrating the fittings. It is important to always ensure the correct grease or oil is being used. Mileham notes that use of the wrong type of lubricant can often be as bad or worse than using none at all. Failing to lubricate even one component can lead to an incident. When applying grease, make sure that plenty of new grease comes out of the fitting. This will ensure that any moisture or contaminants are flushed out.
Falcon Crest Accessories, Inc. in Houston, TX is an FAA certified repair station that performs overhauls on landing gear.
"Preventive maintenance matters!" says Mark Berkenmeier, Falcon Crest’s Director of Operations, stressing the importance of proper lubrication. "You can definitely tell whether or not the customer performed routine lubrication as required." He adds that problems such as worn bushings and corrosion on hardware are more prevalent in poorly maintained gear assemblies.
Eagle Aircraft Services Inc. in Ft. Lauderdale, FL is another repair station that performs landing gear overhauls. Nick Kyriakopoulos, Director of Maintenance at the facility, states "It is usually the simple stuff like improper lubrication that leads to corrosion problems encountered during overhaul."
Watch those switches
Areas of the gear system that need particular attention are the switches. These include limit switches, safety (squat) switches, and associated components. These switches are small in comparison to the mechanical components of the gear system, and are easily prone to break in the event of seizure. If the switches are protected by rubber boots, ensure the boots are not deteriorated or cut. If they are open switches, they need to be kept clean and lubricated as required by the maintenance manual.
The landing gear mount bolts and surrounding areas require particular attention during inspection. The bolts are under severe shear stresses. They can become worn and bent or even develop cracks. Any time the bolts are removed, they need to be carefully inspected. The bolt holes should also be checked for wear. A common method to do this is to place the gear in a broken over condition. The assembly can then be grasped and moved from side to side to check for excessive play.
Hard landings can wreak havoc on the landing gear and associated structure. Forces from a hard landing can be transmitted along structural members to remote areas of the aircraft. Subsequent normal loads can cause failure at a later time. Any time an aircraft experiences a hard landing, it should be carefully inspected in accordance with the applicable maintenance manual.
Careful with that tug
Poor towing techniques can also damage landing gear. Sudden stops and starts while towing can cause damage at the tow pin area. Oversteering due to not following towing limits can also cause significant damage. On the low end of damage possibility, passing towing limits can cause bent or broken towing pins or bent towing stops. On the extreme side, serious damage can be sustained by the landing gear or wheel well structure. Mr. Kyriakopoulos with Eagle Aircraft Services says that they often reject otherwise excellent condition King Air landing gear for cracked towing limit stop support blocks.
Check that rigging
The landing gear system should be checked regularly for proper rigging. The minimum checks necessary are usually established in the aircraft maintenance manual or continuous inspection program. During these operational checks, all associated systems should be examined such as unsafe indication, safety switches, and gear warning. The emergency system should be exercised periodically to ensure proper operation. Inactivity, dirt, and corrosion can render the system inoperative when it is needed. All components of the emergency system including cranks, levers, and placards should be checked.
Last, but not least, tires and brakes need to be part of the landing gear inspection. The tires should be kept clean at all times. Any solvents, fuels, or other fluids spilled on them should be cleaned up immediately. The tires should be inspected for cuts, worn spots, bulges on the sidewalls, foreign objects embedded in the tread, and tread condition. Brakes should also be inspected for security, wear, and leaks.
When inspecting or lubricating any landing gear system, always refer to the manufacturer’s maintenance manual for specific requirements. With careful inspection and lubrication, the number of major incidents or accidents attributable to landing gear failures can be reduced.