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On September 21, 2005, at 1818 Pacific daylight time, Jet Blue Airways flight 292, an Airbus A320, N536JB, landed at Los Angeles International Airport, Los Angeles, California, with the nose wheels cocked 90 degrees. Jet Blue Airways, Inc., was operating the airplane as a scheduled domestic passenger flight under the provisions of 14 Code of Federal Regulations (CFR) Part 121. The airline transport pilot licensed captain, first officer, 4 flight attendants, and 141 passengers were not injured. The flight departed Burbank, California, at 1531 as a non-stop to JFK Airport, New York, New York.
When the landing gear handle was positioned to the up position in the initial climb, the ECAM system listed a fault (L/G SHOCK ABSORBER FAULT) message for the nose landing gear (NLG) shock absorber. The gear handle was then moved to the down position and the crew received an error message of a fault for the nose wheel steering (WHEEL N/W STRG FAULT). After determining that the nose landing gear was cocked 90 degrees, the crew landed at an alternate airport, and the NLG tires and both wheels were worn down into the axle.
The first officer was the pilot flying. He noted no problems during the initial departure and observed a positive rate of climb.
Immediately after takeoff, the cabin crew detected a problem. The first three rows of seats were emptied and the baggage was moved as far as possible. Able-bodied persons were placed in the exit rows and they were instructed on how to operate the doors. The attendants made sure that each person was aware of the emergency procedures that were going to take place.
After detecting the fault in the landing gear the FO decided to divert the flight to Long Beach, California. The captain performed a flyby of the tower for verification on the gear status. The tower responded that the nose gear was canted 90 degrees to the left. After an advice from the company representatives, he decided to divert to LAX due to optimal field conditions, runway length and a better emergency/abnormal support services. The flight stayed in flight for several hours to burn fuel so that the flight can land at a lighter weight.
The captain noted the fuel burn to make sure the center of gravity stayed within limits. The plane touched down at 120 knots and normal braking was applied at 90 knots. The nose gears were held off of the ground for the longest time possible. At 60 knots, the engines were shut down. No Ground Spoilers, reverse thrust or auto braking was used. The air traffic control tower confirmed that there was no fire.
At touchdown, the NLG tires deflated and tore apart and both of the wheels were worn into the axle. The airplanes’ trajectory was not affected by the abnormal NLG configuration or the tire damage that followed and the plane stayed on the runway centerline.
The airplane was an Airbus A320, serial number 1784. The operator reported that the airplane had a total airframe time of 14,227 flight hours and 5,098 landing cycles. It was on a continuous airworthiness inspection program. Maintenance records indicated that Jet Blue maintenance technicians replaced a proximity sensor on the nose wheel prior to the previous flight’s departure from New York earlier that day.
The landing gear normal extension and retraction system are electrically controlled and hydraulically operated. The electrical system consists of landing gear control lever, two Landing Gear, and Control Interface Units (LGCIU), a gear-hydraulic selector valve, a door electro-hydraulic selector valve, 32 proximity sensors and their related targets, and a set of indicator lights. The ECS has two subsystems. Each is controlled by a separate LGCIU. At any given time, one LGCIU is in CONTROL and the other is in MONITORING mode. They use the date from the proximity sensors.
The hydraulics components include three gear actuating cylinders, three-door actuating cylinders, three gear uplocks, door uplocks and door by-pass valves each an NLG downlock release actuator and two MLG lock stay actuating cylinders as well. This particular airplane model has a green hydraulic system which provides hydraulic power to operate the landing gear.
There is clearance around the wheels when the NLG is in the retracted position. In the event of a failure in the mechanical centering of the wheels, the wheels are allowed to rotate a certain amount until contact with the NLG bay roof. Several Airbus tests have shown that this amount of rotation does not cause the NLG to jam in the bay and achieve free fall. On takeoff after retraction, if the nose wheels deviate from their mechanically centered position while in the landing gear bay, an L/G SHOCK ABSORBER FAULT caution light illuminates.
A set of proximity sensors and targets on the nose landing gear detect if the gear is extended or compressed. The sensors detect whether the wheels are aligned or not. In case they are not aligned, the gear does not retract. However, a failure condition can exist that results in the NLG system sensing “ground/compressed” when the gear is extended and a mechanical failure allows the NLG wheel to rotate to a position greater than 6 degrees.
The Airbus model A320 has an electrically controlled NLG steering system using the BSCU and it is controlled hydraulically by the steering control module and two steering actuators. The BSCU computes and sends steering commands electrically to a servo valve in the steering control valve. It is used to hydraulically position the nose wheel assembly to a given position. The BSCU is fed back position signals from sensors on the NLG and from a sensor that gives the position of the steering control module servo valve. Hydraulic pressure to extend the main landing gear is only applied if the respective doors are closed. The A320 has two types of BSCU standards, the conventional and the common (EMM).
A series of five steering tests are performed once the NLG is down and locked. It first performs a brake test. After the hydraulic power is available to the steering servo valve the BSCU performs the steering test. The complete cycles take 5.0 seconds to completes and continuously keeps performing it until touchdown of the main gear assembly.
In case of the flight in question, the failure of anti-rotation lugs was caused due to the repeated cyclic pre-landing tests allowing the nosewheels to deviate from the 0-degree position on the landing gear retraction. The design of the BSCU system logic also contributed to the fault. It prevented the nosewheels from centering. No procedure was present to reset the BSCU under these conditions.
Bench binocular microscope examination revealed that ratchet marks were present on the lugs which indicates a fatigue crack. It originated within the radius between the slot side of the lug and the lower surface of the upper support. The fracture had propagated through 95 percent of the fracture face.
After the incident, Airbus issued an Operations Engineering Bulletin (OEB) which described the method for resetting the BSCU in flight. It discusses the steps to be taken if the L/G SHOCK ABSORBER FAULT ECAM message and the WHEEL N/W STRG FAULT ECAM caution light was on. FAA issued methods to perform NLG shock absorber charge pressure check and repetitive borescope inspection of NLG upper support/cylinder lugs to mitigate the fatigue cracks that were induced by the BSCU. It introduced a modified and more robust upper support approach in A320-32-1310.
New software standards by Airbus were also introduced which reduced the number of pre-landing test cycles to 8 per flight. This reduces the likelihood of fatigue. The design of the upper support assembly was updated and specific inspection requirements were also provided. (National Transportation Safety Board Accident Database System (ADMS2000), 2010)
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