An A330 aircraft was taxiing towards its departure runway. After being authorized to line up, the flight crew noticed that they needed to apply some engine thrust to align the aircraft on the runway, but they did not regard it as significant, because the runway was slightly uphill. The flight crew applied takeoff power, and the aircraft started to accelerate.

At around 70 kt, the flight crew heard a loud bang, followed by a BRAKES RELEASED ECAM alert. They performed a rejected takeoff at approximately 80 kt. The braking was not as efficient as expected. They selected the thrust reversers, but only the left reverser deployed. The aircraft safely decelerated, exited the runway, and stopped on a taxiway near the runway. The flight crew called for the airport emergency services.

The firefighter arrived at the aircraft and informed the flight crew that the forward right wheel of the Right Hand (RH) main landing gear was missing and that debris from the incident was spread across the whole runway.

The passengers were safely disembarked via stairs.

Investigation showed that the cause of the wheel separation was the failure of the wheel bearing. When the bearing failed, the wheel lost its location. It damaged the brakes and some hydraulic lines before being ejected.

Although the exact cause remains unconfirmed, the wheel bearing failure was most likely a result of improper maintenance.

The bearing of an aircraft landing gear wheel ensures the following functions:

• It maintains the wheel in the correct position
• It transfers static loads from the landing gear axle to the wheel when the aircraft is parked (aircraft weight).
• It ensures low friction between the wheel and the axle, minimizing wear and heat generation during taxi, takeoff, and landing.
• It transfers dynamic loads during taxi, takeoff, and landing:
  • vertical loads due to the touchdown or runway irregularities
  • longitudinal loads due to braking
  • lateral loads during turns or lateral deviations

The loads listed above apply both radially and axially on the wheel bearing. That is why tapered roller bearings are used for aircraft wheels, because they can sustain both high radial and axial loads.

A tapered roller bearing is a separable component, and it can be divided into two primary parts: the cone assembly and the cup. This enhances disassembly and assembly maintenance tasks.

The cone assembly is a complete, non-separable unit. It consists of the inner race, the tapered rollers, and a cage. The purpose of the cage is to hold the rollers in place and maintain their correct spacing.

The cup, the other separable part, functions as the outer race of the bearing.

Each aircraft wheel is equipped with two tapered roller bearings. The axle nut maintains the wheel assembly on the axle by seating on the inner cone of the outer bearing. It provides a preload that ensures correct positioning and optimum performance of the bearings.

On each bearing, two seals are used to contain the bearing grease and prevent contamination. A circlip maintains the seal in position.

(fig.3) Example of a nose landing gear wheel with its two tapered roller bearings

Due to the key role of the landing gear wheel bearing and the significant loads they are exposed to, the consequences of a bearing failure can be significant. They may include:

• Sudden seizure of the bearing
• Damage to the landing gear axle and wheel
• Damage to the brakes
• Emission of sparks and heat that may lead to a fire
• Parts Departing from the Aircraft (PDA), including the loss of the wheel and tire assembly.

The primary causes of wheel bearing failures are related to incorrect maintenance practices. It is essential to strictly follow the procedures of the Aircraft Maintenance Manual (AMM) or the Component Maintenance Manual (CMM) to ensure proper accomplishment of the maintenance tasks and reduce risks of error.

Safe operation of a wheel bearing is only possible if proper shop maintenance is carried out to prepare the wheel and its bearings, and if the wheel is properly installed on the aircraft.

Each time a wheel is removed from an aircraft in an unserviceable condition, it must be sent to a certified MRO for shop maintenance. The wheel bearings must be dismantled, cleaned, inspected, lubricated, and reinstalled according to the wheel CMM procedures.

When a wheel and tire assembly is sent to the wheel shop, the level of inspection of the wheel differs if the removal was due to tire wear (standard overhaul) or following an abnormal event, such as a hard landing or a brake overheat.

When removing a bearing from the wheel, the circlip and seals should be cleaned and their condition checked. If any damage is found, the damaged component must be discarded and replaced by a new one during reinstallation.

After the bearing is removed from the wheel, its general condition must be checked. The radial clearance of the cage must not allow contact of the cage with the cup when the bearing is rotated.

Removing the grease from the bearing cup and cone assembly is necessary to perform their detailed visual inspection.

The bearing cage must be free of any deformation or damage and rotate without any friction.

The rollers must be free of any sign of corrosion, stain, marks, or the presence of spalled areas. Particular attention should be paid to the large end of the rollers for scoring damage.

The physical roughness of the rib of the inner ring that is in contact with each roller end must also be checked using a probe or a ball pen.

The cup must be free of scratches, or spalled area. If stains are found on the cup raceway, they should be cleaned with the appropriate cleaning agent listed in the wheel CMM.

A probe or a ball pen can be used for roughness checks to detect any surface defect on the cup raceway: if a defect is felt while moving the probe/pen on the surface, the cup must be discarded.

Insufficient greasing is detrimental to the performance and reliability of the bearing. It can lead to premature bearing wear and corrosion.

Excessive greasing can increase the rotational resistance (torque) of the bearing and, consequently, generate excess heat, degrading the grease quality and affecting bearing life. In addition, excess grease may leak from the bearing onto hot adjacent components, resulting in smoke and potentially fire.

Mixing a cone assembly with a cup coming from another bearing should be avoided. Keeping the cone assembly and the cup of a bearing together ensures optimum fitting and performance.

After the bearing is reinstalled and the wheel is overhauled, the CMM instructions for bearing protection and wheel storage must be respected to prevent contamination and degradation of the bearing pending the next reinstallation of the wheel on an aircraft.

Proper wheel bearing maintenance is essential for safe bearing operations, however, it is not sufficient. The correct installation of the wheel on the landing gear axle is also necessary.

As for any maintenance procedure, the appropriate tools listed in the AMM must be used to perform the maintenance. They include:

• an axle thread protector
• an axle nut adaptor
• a torque wrench with the appropriate torque range.

The protection fitted on the bearing of the new wheel to be installed should only be removed shortly before installation to prevent the risk of contamination of the bearing by external materials.

The use of the cone-shaped protector during both removal and installation of the wheel prevents damage to the axle thread.

When the wheel is positioned on the axle, an initial seating torque must be applied on the axle nut using the axle nut adaptor fitted on a torque wrench.

The initial seating torque ensures proper alignment of the wheel and bearings as well as the correct seating position of the rollers on the inner ring and cup raceway.

As the wheel turns, the tapered rollers move along the inner race (the cone) and the outer race (the cup). Because of their conical shape, the rollers are guided into their correct operational position, aligning themselves precisely against the cone rib. It may take several rotations to ensure all rollers in both the inner and outer bearings are correctly seated.

Incorrect seating of the rollers may result in damage and premature failure of the bearing.

The initial seating torque being too high for the normal wheel operation, the axle nut must be 'backed off' (loosened), in order to prepare for the final torque application.

The wheel rotation during the back-off eases the loosening of the axle nut. It also ensures that the rollers remain correctly seated.

A final torque must then be applied to the axle nut. This crucial step sets the correct 'preload' (the optimal amount of internal load) for the bearing to operate efficiently.

Overtorquing the axle nut may result in increased bearing friction, leading to premature wear and failure.

Undertorquing the axle nut may result in incorrect roller seating, leading to potential damage and premature failure.

When the final torque is applied, the two cross bolts must be installed and locked to prevent the axle nut from moving.