An A321 was in cruise when the HYD G ENG 1 PUMP LO PR and HYD G RSVR LO LVL ECAM alerts were triggered. The flight crew applied the FCOM procedures and switched off the PTU and green hydraulic pump on engine 1.

During the approach, the flight crew switched the green hydraulic pump on again (not requested by the FCOM procedure). The landing gear was extended. Landing and rollout were uneventful and performed with the green hydraulic system selected ON.

The flight crew stopped the aircraft before commencing the taxi-in to switch the green hydraulic system to OFF once again. They resumed the taxi, and finally parked the aircraft at the gate. The engines were selected to OFF and the parking brake was set to ON. The yellow hydraulic pressure started to decrease toward 0 PSI, which was indicating an issue with the yellow hydraulic circuit.

In the meantime, the ground personnel observed that the right main landing gear wheel was on fire and extinguished it using a fire extinguisher at the gate.

The investigation showed that the head of one of the tie bolts of wheel n°3 had sheared off and progressively migrated toward the brake, causing damage to the brake assembly during the previous flights. This damage eventually caused a leak from the green hydraulic system, which triggered the ECAM messages that indicated low hydraulic system pressure and low fluid levels during the flight. The leak from the yellow hydraulic system happened when the parking brake was set to ON at the gate, causing hydraulic fluid to be in contact with the hot brakes and resulting in the wheel fire.

Aircraft landing gear wheels (nose, main, body or wing landing gear) have a similar basic structure, regardless of the wheel manufacturer. Each wheel assembly consists of a tire fitted over two half hubs, which are fastened together by tie bolts. (fig.1). For wheels fitted over brake assemblies, the inner hub of the wheel is designed to house the brake.

The tie bolts ensure the structural integrity of the wheel by sustaining the loads from the tire pressure, the aircraft weight, and the dynamic loads due to the impact of the landing gear during landing. Therefore, wheel tie bolts should be considered as one of the critical components of the landing gear assembly.

A tie bolt may be damaged and its screw or nut may migrate from its location, potentially causing damage and safety risks.

A reduced structural resistance of the wheel

Any missing tie bolt will result in an increase in the loads on the remaining tie bolts. This can adversely affect their condition and it may reduce the structural integrity of the wheel if it is not replaced (fig.2). Damage to the wheel and the aircraft can occur with the associated risk of injury to ground personnel during ground operations.

A risk of Parts Departing the Aircraft (PDA)

A damaged tie bolt that falls from a wheel can cause damage to the aircraft or injury to people on the ground. If the damaged tie bolt falls on the runway, there is a risk of damage to subsequent aircraft taking off or landing, and especially a risk of engine ingestion.

A risk of damage to the brakes and the aircraft

When a tie bolt is missing from a wheel that houses brakes, the tie bolt can migrate toward the brake, causing extensive damage and a risk of brake jamming. If the pistons of the brake are damaged, there is a risk of hydraulic fluid leak that may trigger a brake fire, as in the example described above.

Airbus identified a significant number of reported events related to tie bolt loss in the last five years, although only a few of these events resulted in significant damage. Prevention is key to reducing this trend by early detection of any missing tie bolts in line operations or even by anticipating a tie bolt failure through regular wheel tie bolt inspections that are requested by the maintenance planning.

There are numerous opportunities in the line of operations where maintenance personnel and flight crews can detect a missing or damaged tie bolt and prevent potential damage.

A missing or damaged tie bolt or may be detected during the daily check or preflight walkaround (fig.3). If the tie bolt damage is located on the inner hub side of a wheel equipped with brakes, or if the wheel has a brake cooling fan assembly fitted to it, it is more difficult to see if the tie bolts are damaged.

As an additional safety net, the flight crew should check for the presence of all the tie bolts when checking the condition of the wheels and tires during the exterior walkaround.

As per the AMM/AMP/MP procedure, if one of the tie bolts is found missing, the wheel must be removed and visually inspected for damage as well as the brake assembly.

Depending on the wheel type and manufacturer, the MEL may permit dispatch with one tie bolt missing on a wheel for a limited number of flights. In this case, the condition of the wheel and brake must also be checked before dispatch as per MEL maintenance procedure.

Many tie bolt ruptures and losses can be avoided if the damaged tie bolt is detected during planned shop maintenance inspections.

At each tire replacement, the tie bolts must be removed and visually inspected for any damage (refer to the CMM of the wheel manufacturer for the complete procedure).

Regular NDT inspections are planned to detect any structural damage on the bolt or any indications of fatigue or cracks and ensure that any damaged bolt is replaced before it fails. Inspection intervals vary for each wheel manufacturer and must be observed. These intervals may consider a maximum number of tire replacements on the wheel, or the number of flight cycles, or the maximum number of calendar days since the previous inspection. Often it is a combination of these and whichever occurs first. Refer to each wheel manufacturer CMM to determine the inspection intervals. In the case of an overload (e.g. Hard landing), some additional specific checks might be requested (refer to the CMM).

Traceability is key to ensure that the wheel tie bolts are inspected at the required intervals using NDT inspection methods.

Tie bolts do not have a serial number. Therefore, it is the wheel serial number that must be used to ensure there is a record of the number of tire replacements, flight cycles, and the last recorded NDT inspection date.

The good practices listed below should be followed to ensure that the bolts are inspected at the required intervals:

• After a tire replacement, the same set of tie bolts removed from a wheel assembly should be reinstalled on the same wheel to ease traceability, and the number of tire replacements since the last NDT inspection should be recorded
• When reinstalling the same set of wheel tie bolts onto a wheel assembly it is important to make sure that the tie bolts have not exceeded the number of allowable flight cycles, tire replacements, or calendar days passed since the last NDT inspection date, in accordance with the wheel assembly CMM
• If a wheel tie bolt is removed from a set, the replacement bolt must be a new bolt or a substitute bolt, but only if this bolt was inspected using NDT as per the CMM
• If a new set of tie bolts is installed on a wheel, the maintenance record of the wheel must be updated to indicate this accordingly
• Any tie bolt installed on the wheel must be the same part number specified by the wheel assembly CMM.

Correctly torquing and greasing the wheel tie bolts is important to ensure the overall structural integrity of the wheel assembly and to prevent damaging the bolts. The correct tightening torque value to apply for the wheel tie bolts is specified in the CMM and must be observed when assembling the wheel.

To improve the protection of wheels and brakes during parking and storage of aircraft, the recommendations provided in the AMM/MP/AMP procedure for parking/storage of aircraft should be applied.