Canada A333 at Montreal on Dec 25th 2021, gear damage on landing

Last Update: October 4, 2023 / 16:06:58 GMT/Zulu time

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Incident Facts

Date of incident
Dec 25, 2021


Air Canada

Flight number

Aircraft Registration

Aircraft Type
Airbus A330-300

ICAO Type Designator

An Air Canada Airbus A330-300, registration C-GFAF performing flight AC-901 from Fort Lauderdale,FL (USA) to Montreal,QC (USA), landed on Montreal's runway 06L when the crew received indications of a malfunction of the right main gear. The aircraft became immobilized on the runway. Emergency services responded and found the two forward tyres on the right main landing gear blown in addition to substantial damage to the gear bogie, which made even towing impossible.

The Canadian TSB reported the aircraft sustained substantial damage, rated the occurrence an accident and opened an investigation.

O=n Oct 4th 2023 the Canadian TSB released their final report concluding the probable cause of the accident were:

Findings as to causes and contributing factors

- On 17 December 2021, while the aircraft was taxiing for takeoff, for an undetermined reason, one of the bearings on the No. 4 wheel seized and caused the protective sleeve to rub against the bogie beam bushing, causing localized overheating of the bogie beam base metal.

- Given that the overheat indicator strip closest to the beam at the No. 4 wheel on the axle was its original colour, applicable procedures allowed for the replacement of the axle and the bogie beam bushing without a thorough damage assessment.

- Given that the component maintenance manual does not provide any specific repair for a bushing replacement, the Disassembly and Assembly sections were used as references. Consequently, the inspection criteria during bushing replacement focused on ensuring the correct dimensions rather than detecting damage, which eliminated the requirement for non-destructive tests.

- Given the intact indicator strip, the clear transition between the blackened area and the adjacent protective layer, and the intact paint on the beam around the bushing, the technicians who performed the visual inspection during the replacement of the damaged bushing concluded that the beam was in good condition, even though the bogie beam base metal had overheated.

- Two cracks emanated from a previously undetected area of overheating under the bogie beam bushing. One of the 2 cracks spread and caused a fracture of the No. 4 wheel bore.

- Upon landing on 25 December 2021, the fracture emanating from the No. 4 wheel bore spread rapidly and resulted in the bogie beam breaking into several pieces. No longer supported by the wheels, the shock strut scraped the runway until the aircraft came to rest.

Findings as to risk

- If the brake temperature sensor leads are reversed on the landing gear bogie, the brake overheating indication in the cockpit will be associated with the wrong wheel. Maintenance personnel might then inspect the wrong wheel, and damage could be missed.

- If maintenance actions that can be carried out by a number of technicians at different times are combined into a single operation on a process router, an action could be omitted or not checked, creating a risk that the maintenance performed will not be safe.

- If scoring and other damage to the inner surface of a bogie beam are not detected during visual inspections, there is a risk that cracks will develop.

- If spots of missing corrosion-inhibiting protective layer inside a bogie beam are not identified during visual inspections, there is an increased risk of corrosion.

The TSB analysed:

Bogie beam failure

First visual examination of the fracture surfaces

When the aircraft is in flight and the landing gear is down, the bogies are inclined such that when the aircraft lands, the rear tires are the first to touch down on the runway. Therefore, forces are transferred gradually when the wheels touch down.

An analysis of the vertical acceleration forces at touchdown indicates that the landing occurred without excessive force and that the maximum vertical acceleration was recorded when the main landing gear’s front wheel tandem touched the runway. Slight variations in force occurred in the next few seconds, but no noticeable increase was recorded, indicating that the beam quite likely failed when the front wheel tandem touched down.

The damage marks left on the runway led to the same conclusion, which was that the beam very likely fractured on touchdown and the landing gear collapsed immediately. These observations indicate that there was a pre-existing condition and that the failure was not the consequence of an impact.

The fractographic analysis of the beam’s fracture surfaces, conducted by Safran Landing Systems’ (Safran) metallurgical laboratory under TSB supervision, determined that the surfaces showed chevron-shaped markings, which are typically associated with the rapid spread of a fracture. Furthermore, an examination of these chevrons helped to determine the direction in which the fracture spread, as well as the starting point of the fracture. All of the markings observed pointed to the No. 4 wheel bore, where the fracture surfaces had a different granular structure over approximately 1 cm2.

Laboratory examination of the bogie beam fragments

An examination of the fracture surfaces under a scanning electron microscope revealed the presence of cadmium coming from the corrosion-inhibiting protective layer within the base metal granular structure, near where the fracture started. The presence of cadmium indicated that it had reached its melting point and, therefore, overheating had occurred.

After the Barkhausen noise analysis, which revealed that the metal had been altered, a nital etch test determined that the surface temperature of the beam’s base metal (300M steel) had exceeded 850 °C in some places around the bushing shoulder. The weakened steel then led to the formation of 2 cracks.

The investigation determined that the area of overheating was not a result of the occurrence flight. A review of past occurrences was then carried out and it was discovered that brake overheating and a bearing failure had occurred on 17 December on the same bogie. It was then deemed necessary to further study this overheating.

Brake overheating and bearing failures before this occurrence

Cockpit indication

On 17 December 2021, while the occurrence aircraft was taxiing into position for takeoff, the electronic centralized aircraft monitor (ECAM) generated a BRAKES HOT message for the No. 3 wheel. The crew taxied the aircraft back to the terminal to have the brake inspected by maintenance personnel. Given that the brake temperature sensor leads for the No. 3 and No. 4 wheel brakes had been reversed during a previous maintenance activity, no deficiencies were found with the No. 3 wheel. Because the No. 3 and No. 4 wheels were on the same axle, this reversed overheating indication had no impact on the operational procedures that the pilots had to follow.

Damage assessment

During the inspection after the brake overheating indication, maintenance personnel noted that the No. 4 wheel was completely off-centre and the wheel hub was broken. The preliminary observations were the following:

- wheel bearings destroyed;
- damage to the axle sleeve;
- localized friction damage to the axle; and
- friction damage to the face of the beam bushing.

To assess the extent of the damage and the corrective action to be taken, the technicians used the information available and their observations. They consulted publications in effect for the aircraft to determine which existing procedures applied to the situation.

Brake overheating indication

As a result of the observed damage, the technicians determined that the unscheduled inspection after brake overheat procedure, described in the aircraft maintenance manual (AMM), needed to be carried out. One of the steps in this procedure requires the inspection of the axle overheat indicator strips.

The indicator strip closest to the beam at the No. 4 wheel on the damaged axle was still its original colour, which led the technicians to believe that the assembly had not been exposed to a temperature greater than 250 °C, at which point the strip paint colour begins to change from orange to brown.

Brake overheating generally produces heat that is transmitted to the axle through radiation over a large surface for a relatively long period. In the 17 December occurrence, the considerable mechanical energy produced by the sleeve rapidly rotating around the axle and rubbing against the beam bushing was converted into high localized thermal energy at the friction points. Although the temperature had exceeded 850 °C in some places, the significant metallic mass of the axle rapidly dissipated the heat, which explains why this heat had no effect on the indicator strip closest to the beam.

Damage to the beam axle bushing

The bogie beam axle bushing showed friction marks on its face, which is normally in static contact with the sleeve. The Safran laboratory report showed that seizure of the wheel bearing had caused the sleeve to rotate and that friction between the sleeve shoulder and the bushing had caused wear and melting damage to the bushing.

Bushing replacement

Given that bushings are an integral part of the beam assembly, the technician must consult the component maintenance manual (CMM) for their applicable limits, possible repairs, or replacement. Because bushing replacement is a specialized maintenance task, Air Canada technicians could not perform the task themselves. They had to use the services of the maintenance organization AAR Landing Gear Services (AAR), which was approved to perform this task.

Part replacement procedure

AAR technicians replaced the axle and bushings using the process router created by their engineering department and based on existing approved documentation, i.e., the CMM.

However, given that bushing replacement is not one of the repairs described in the CMM, AAR engineers used the Disassembly and Assembly sections of the CMM and combined them into one operation. As a result, this single operation included several actions that could be performed by a number of technicians at different times, but only required the signature of the technician to whom the operation had been assigned. This situation thereby undermined the inherently rigorous process routers and ran counter to the purpose of using them.

Visual inspection of surfaces

During the examination of the beam assembly at Safran’s laboratory, scoring damage was found on the protective layers of the beam’s inner surface. It is likely that this scoring damage was caused when the AAR technicians removed the bushing from the axle. The Disassembly section of the CMM does not provide a specific procedure or the tools required to remove bushings. AAR technicians generally use an adaptor plate placed against the bushing and a long aluminum punch that the technicians hit with a hammer. The punch’s length and weight make it difficult to manoeuvre within the limited space of the beam bore and increase the risk of damage to surrounding surfaces.

In addition to the scoring damage, the examination of the beam assembly conducted for the purposes of the investigation revealed that the Ardrox corrosion-inhibiting compound was missing in several spots in the area where the bushings were replaced.

Safran’s Service Bulletin A33/34-32-300 has a step that includes inspection and repair of the protective layers before installation of the axle. The beginning of the Assembly section in the CMM states that technicians must ensure that parts are clean and that the dimensions comply with those stated in the applicable section.

The Assembly section of the CMM does not provide a specific step for inspecting surfaces to identify any defects in the protective layers or the base metal. The Check section of the CMM provides criteria for inspecting parts, but this section was not used when the axle and bushings were replaced.

Although the scoring damage and missing Ardrox were not considered to be contributing factors to the failure, metal on untreated surfaces is susceptible to premature corrosion, which can reduce a part’s life limit.

Examination of the beam assembly revealed signs of localized overheating on the axle.

Despite the presence of these signs on the axle, the technicians who assessed the damage ruled out the possibility that the beam metal had overheated. They focused on the following points to arrive at this conclusion:

- The overheat indicator strip closest to the beam on the axle was intact.
- The blackened area showed a clear transition, contrary to what would be expected for the usual spread of heat in metal.
- The surface of the paint immediately around the bushing did not show any signs of excessive heat, such as blistering or gradual discoloration.

The documentation in effect allowed for the replacement of the damaged parts without a thorough assessment of the bogie beam damage. The technicians were focused on what they had to do, which was to replace the axle and bushings using the documentation available at the time to return the occurrence aircraft to service.

The report for the laboratory examination conducted by Safran determined that the structure of the 300M steel used to fabricate the beam had been altered, and 2 cracks were present when the axle and bushings were replaced. Some non-destructive tests, such as the Barkhausen noise analysis and the nital etch test, would have identified these deficiencies.
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Aircraft registration data reproduced and distributed with the permission of the Government of Canada.

Incident Facts

Date of incident
Dec 25, 2021


Air Canada

Flight number

Aircraft Registration

Aircraft Type
Airbus A330-300

ICAO Type Designator

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