The concern related to the operator’s removal of a limit on tyre wear which was imposed by the aircraft manufacturer.
The reporter expressed a safety concern related to the allowable wear on the tyres for the [Operator] Boeing [type] fleet.
The reporter advised that they have observed wear similar to that shown in the photographs in the ATSB investigation into a tyre failure.
Last year, the Boeing aircraft maintenance manual (AMM) reduced the allowable limits for shoulder wear on the tyres with the removal of the allowance for canvas to be showing. [Operator] has re-instated this to allow canvas to show, however a limit for removing the tyre when the shoulder groove is missing, was removed. This again has since been re-instated.
The reporter advised that flight crews are conducting the aircraft inspections during maintenance on demand (MOD) transits, with aircraft mechanical engineers only required to inspect the tyres every check 2 or if a defect is raised by flight crew. However, most flight crews are not aware of the tyre check requirements and this will often go unreported. This means the tyre may exhibit severe shoulder wear before the check 2 inspection.
The reporter also advised that there has been an increase of severe shoulder wear since [Operator] operations management made a directive to fight crew to use less flap and idle thrust reverser while using high auto brake settings to reduce speed on the runway as a fuel saving measure. The reporter advised that since the change in flying methods they have detected carbon brakes glowing red hot during turn-around inspections.
[Operator] is aware of a tyre failure that is the subject of the ATSB investigation. [Operator] performs inspections of tyres at intervals recommended by the manufacturer. Tyres are replaced when wear or damage beyond allowable limits is found. The recommended inspection intervals take into account the allowable damage limits and the wear behaviour of the tyres. We are unable to comment on the similarity of any wear pattern between the [type 1] and the [type 2] in the absence of further details on the [type 1] findings.
The allowable limits [Operator] has in its documentation was discussed with the aircraft OEM (Boeing) and the tyre manufacture (Goodrich). Boeing has agreed that limits recommended by the tyre manufacturer may be used, and Goodrich has provided a No Technical Objection for the allowable limits that are in use today.
The aircraft manufacturer recommends tyre inspections at least every 48 hours (ref Boeing Maintenance data task 32-360-00). The wear limits provided by the manufacturer take this into account. Furthermore, flight crew perform visual inspections prior to every flight during their pre-flight walk-around inspection. This is conducted in accordance with the flight crew operating manual procedures which state that the crew must ‘check that the tyres are not too worn, not damaged, and there is no tread separation’. The flight crew will refer any unusual wear to maintenance staff prior to flight. If the pre-flight inspection is conducted at a non-maintenance port the flight crew can refer to the tyre condition chart available on their iPads, to help determine the condition of the tyres. [Maintenance] are available by phone 24 hours a day to provide additional assistance to the crew at non-maintenance ports.
[Operator] has noted an increase in the rate of wheel changes in early 2017, however this is generally attributed to the fact that there had been an increased population of new tyres in the fleet and the new tyres (R0) average life is approximately 120 landings while re-treads (R1-R4) average life is approximately 170 landings. Worn shoulders are not an uncommon reason for tyre removal. Most of the time, the worn shoulder is at the inboard shoulder of the inboard tyre location on a main landing gear. Currently, the tyre replacement rate has reduced to be in line with historical averages.
In any case, tyres are inspected regularly (every flight by flight crew and at least every 48 hours by maintenance staff), so wear is being picked up in a timely manner.
CASA offers the following response to the REPCON:
- An analysis of the Service Difficulty Reports (SDR) required to be submitted to CASA did not show any adverse or unusual tyre defect trends or patterns. It is further noted that the operator has stated that its [aircraft type] tyre replacement rate is in line with historical averages.
- The actions taken by the operator have been in accordance with the requirements and instructions of the original equipment manufacturers (OEM):
- Boeing (the aircraft manufacturer) has agreed that the limits recommended by the tyre manufacturer may be used (by the operator); and
- Goodrich (as the tyre manufacturer) has provided a No Technical Objection (NTO) to the allowable limits which have been set and are being applied by the operator.
- The response by the operator to each of the concerns raised in the REPCON is appropriate, these being that:
- The operator performs inspection of tyres at intervals recommended by the manufacturer
- The recommended inspection intervals take into account the allowable damage limits and the wear behaviour of the tyres
- Tyres are replaced when wear or damage beyond allowable limits is found
- Where licenced aircraft maintenance engineers (LAMEs) are not available to conduct tyre inspections, the responsibility to inspect the tyres falls to the flight crew who are trained to conduct the inspection and are provided with tools and engineering support to enable them to determine whether tyre wear is within acceptable limits
CASA notes that if flight crew cannot make a determination as to the acceptability of tyre wear using the resources at their disposal, they have the authority and obligation to raise a defect in the maintenance log - an entry which can only be cleared by an appropriately licenced aircraft maintenance engineer before further flight.
Nevertheless, CASA is still reviewing this matter with [Operator] and is awaiting further information.