On Sept. 21, 2005, a scheduled airline Airbus 320 took off from Bob Hope Airport (BUR) in Burbank, Calif., bound for John F. Kennedy International Airport (JFK), carrying 140 passengers and six crewmembers. While on climb-out, the copilot attempted to retract the landing gear, but the nose gear would not successfully return to the stowed position even after several attempts. The crew requested permission to make a flyby of their maintenance base at nearby Long Beach Municipal Airport (LGB), to try to get a visual indication of the problem. During the low pass, ground crew noticed that the nose landing gear had rotated 90 degrees off axis, perpendicular to the airframe.

After consulting their operations headquarters, it was decided to plan an emergency landing at Los Angeles International Airport (LAX), which had a much longer runway and modern emergency equipment. To reduce the possibility of fire, the aircraft orbited nearby for two hours to burn fuel. During this time, local news agencies got wind of the story, creating a media sensation complete with passengers aboard the plane watching their own drama via satellite TV, until the captain wisely shut off the service.

With the entire world watching, the pilots executed a gentle landing, keeping the nosewheel off the runway until the last possible moment. When the tires made contact, they quickly shredded and streams of sparks erupted as metal met concrete. The aircraft slowly came to a stop without fire, explosion or emergency evacuation slides; much to the disappointment of TV news directors everywhere. After a quick check by the airport fire chief, the air-stair truck pulled up to the aircraft and the passengers casually exited as news helicopters provided live feeds via long range camera.

Investigators determined that a faulty nose gear steering mechanism caused the wheels to pivot past their normal centered position and would not allow the gear to fully retract. Even though this incident highlighted a potentially disastrous system failure, the remarkable fact was that the landing gear successfully supported the weight of the aircraft with no rolling tire contact, and should be viewed a remarkable feat of engineering design and certification.

From the time of the Wright brothers, landing gear have evolved from simple skids to highly complex systems that are capable of sustaining tremendous landing loads, and support complex functions such as electronic nosewheel steering and anti-skid braking. This complexity and strength requires proper maintenance in order to ensure that the landing gear will perform as designed, especially in an emergency. To help maintenance managers and technicians keep their gear in top shape, we collected advice and maintenance tips to make every landing one that you can walk away from.

Even with the smoothest landing, the sudden shock of the aircraft contacting the ground produces a tremendous amount of force. Add just a few more feet per second sink rate, and the forces increase rapidly. As aircraft grow larger, designers strive for more strength, and light weight with exceptional reliability. Stronger materials and improved hydraulic metering in the struts have aided designers in creating capable landing gear, but this capability comes with a cost - landing gear are both expensive to make and maintain.

The basic retractable landing gear looks simple enough from the outside, but inside the shock strut is a robust metering system that transfers the landing loads through the strut fluid and compressed gas. Many landing gear now have crushable cores as an added safety feature in case of a worst-case emergency landing. Truck or bogie systems have positioning systems to allow angled contact with the runway and help tuck the gear into the wheel well. Retracting mechanisms need to lock and unlock precisely at the right time to allow the gear to extend and retract in the correct sequence.

Many large business aircraft have sophisticated nosewheel steering and braking systems, along with weight on wheels switches that are part of the flight control system to transition the aircraft from flight to ground modes. All of these components combine to make up a complex system that requires close attention to detail during periodic and scheduled maintenance events. During the daily or preflight inspection, landing gear need to be checked for leaks, play or wear in actuator linkages and a thorough inspection for cracks. Loose linkages or bearings can affect the gear's ability to extend and retract properly, and sloppy nosewheel steering linkages can make for awkward taxiing.

Most landing gear are painted white to aid in the daily visual check, but it is important to keep the components clean or you may miss the beginnings of a serious failure. You should never use a pressure washer to clean landing gear components unless you have specific instructions from the manufacturer. High pressure can ruin seals and push debris into the strut. Whenever you replace a tire or service the brakes, visually check the axle and axle attachment area for damage or cracks. Some aircraft have high-tech coatings on the axle shaft that protect against wear, but are easily damaged during tire installation. Always follow the maintenance manual for selecting the correct grease and hydraulic fluids, as some may have additives that can attack seals or trap moisture, which can lead to corrosion pitting.

Servicing the struts has always been one of least glamorous but necessary maintenance tasks. Although the basic servicing principles of fluid quantity and gas pressure are the same, each manufacturer may have slight variations on the process, so be sure to follow the maintenance manual. "I have found that the best way to properly service a landing gear for the aircraft that I maintain is to do this with the aircraft on jacks," said Stan Brewer, aircraft maintenance manager at Atlanta-based Printpack Inc. Though some maintenance manuals allow for servicing with weight on wheels, the variations in aircraft weight, ambient temperature, plus temperature of the gear and fluid provide variables that can lead to less than desired results. "It is also best to service the struts at the same time so they match. I sort of learned this the hard way by servicing the L/H strut in the morning and the R/H in the afternoon. When the aircraft came in the next morning one strut was noticeably higher than the other," Brewer added.

All landing gear either have Life Limited Parts (LLPs), mandatory inspection/overhaul periods, or both. Keeping accurate records of landings and life remaining are critical to maintaining your aircraft safely. Landing gear components are typically tracked by the number of landing gear cycles. "At the mention of LLPs, most people think of engine parts. For aircraft, the most LLPs are indeed contained in the engines, but are closely followed by LLPs on the landing gear," said Roy Resto, vice president of technical operations at Milwaukee-based Tracer Corp. "For any LLP, you must keep impeccable records of the status of such parts [see FAR Part 91.417]. For large aircraft, your long-term maintenance planning must include careful monitoring of landing gear overhaul due dates. The most expensive overhauls are for aircraft, engines and landing gear, in that order. Because of this, it is a common practice to allocate financial reserves that accrue ahead of the landing gear overhaul," Resto added.

When it comes to landing gear overhauls, you have a number of different options. For a fast turn-around, you can exchange the complete gear assembly. Often the cost is fixed and you can plan accordingly. The problem with exchanges is that you may receive a gear that is freshly overhauled, but the lives remaining are much less than the gear you turned in. "With gear overhauls I've never gone the exchange route - I want to keep my landing gear so I know exactly what I have and what the history is," said Steve Rahn, chief of aircraft maintenance for Greensboro, N.C.-based VF Corp. "I'll put on a ship set of rentals and wait for mine to come back from the overhaul shop. It's more work for the techs having to jack up the aircraft again and do another removal and replacement, with all the measurements and ops checks involved, but I believe it's worth it in the long run," Rahn added. Be sure to carefully review your overhaul cost estimates and plan for a worst-case scenario if there are components that need repair or replacement.

Many landing gear components require repetitive Non-Destructive Testing (NDT) to check for cracks. Often these procedures take place during an overhaul, but many procedures can be accomplished in your hangar or on aircraft. When choosing a shop to perform these inspections, the widely recognized and accepted standard for NDT technicians is National Aerospace Standard 410, Certification & Qualification of Nondestructive Test Personnel. Based on military standard MIL-STD-410, the techniques, training and experience levels, and privileges are spelled out in a rigid methodical manner. The American Society of Non-Destructive Testing (see sidebar) also provides guidance and training standards such as the ASNT Standard Practice SNT-TC-1A. However, since the NDT technicians in and of themselves cannot return the aircraft to service, you will need to have either an appropriately rated A&P/IA or Part 145 Repair station sign off the work, so it makes sense to hire a shop that can both perform the inspection and return the aircraft to service.

Although we tend to think of landing gear as a simple mechanical device, it is actually a sophisticated system that needs to function correctly every time. Even though modern landing gear are designed for strength and reliability, proper maintenance is essential. From daily servicing to inspection and overhaul, you need to know and follow the manufacturer's recommendations, and plan for the long-term maintenance/overhaul costs. Take the time to learn as much about your landing gear as you can and ensure your technicians are properly trained to make sure every landing is one your passengers and crews can walk away from.