Identification of issue
The main wheel for a large transport aircraft meets the minimum performance standard prescribed by ETSO-C135 except for the requirement hereunder.
ETSO-C135 § 3.2.2.2 Combined Yield Load (wheels tests) requires: "There must be no interference in any critical areas between the wheel and brake assembly, or between the most critical deflected tyre and brake (with fittings) up to limit load conditions..."
During ETSO-C135 qualification testing, interference occurred between brake and tyre before the combined limit loads were reached. Indeed the tyre selected by the aircraft manufacturer came in contact with the brake structure (Brake Actuator carrier torque take-out) at a combined load value of 72% of limit radial load and 90% of limit side load.
The wheel manufacturer is submitting to EASA this deviation request against ETSOC135 for the main wheel for this aircraft type.
Arguments supporting the deviation request
The wheel manufacturer claims that deviations concerning similar cases has been substantiated and approved previously and proposes EASA accept the relevant deviation as is, according to the following arguments.
Tests results have been submitted by the wheel manufacturer to the aircraft manufacturer. According to the aircraft manufacturer a side load is not predicted to exceed 90% of limit load for this condition more often than once in 10,000 flights.
The aircraft manufacturer stated that in case of interference only 2 positions (8 wheels per aircraft) would be affected, and that in case of a 2 tyres failure, this failure would be considered as minor, for airplane safety. Moreover, when conditions for this aircraft are compared with a similar aircraft model from the same manufacturer (same wheel rim diameter), similar combined load vectors and lateral tyre stiffness are found. Also, the lateral distance (clearance) between tyre and brake are similar for this aircraft and a similar model from the same manufacturer. With the fleet of this similar model accumulating more than 38,000 landings to date, no such interference has been identified.
Annex 1 includes a 7-page document issued by the aircraft manufacturer to substantiate more precisely those statements.
The wheel manufacturer recalls that a similar request for deviation has already been granted by a subcontracted National Aviation Authority on behalf of EASA for another of its main wheels.
Taking into account the facts presented above, the wheel manufacturer considers that this main wheel complies with the ETSO-C135 requirements, providing an equivalent level of safety.
The wheel manufacturer agrees that the information contained in this document is not proprietary.
EASA Position
EASA agrees that all aspects of the ETSO minimum performance standard have been met by this wheel, with the exception of paragraph 3.2.2.2, which requires that there shall be no interference between wheel brake and tyre at combined limit radial and side load conditions.
The wheel manufacturer requests a deviation to ETSO-C135 for paragraph 3.2.2.2, which states,
“There must be no interference in any critical areas between the wheel and brake assembly, or between the most critical deflected tyre and brake (with fittings) up to limit load conditions, taking into account the axle flexibility. Lack of interference can be established by analyses and/or tests.”
The justification of this deviation request is made with the following arguments,
a) Acceptable aircraft level risk
The aircraft manufacturer has analysed the aircraft level hazards associated with the potential tyre failure which could be caused by this interference. It has been determined that the likely failure mode, the loss of up to two tyres on a single gear, has consequences no worse than major for the aircraft. This classification has been accepted by the aircraft certifying authority.
b) Unlikely to reach this loading condition in service
The aircraft manufacturer quotes a statistical examination of side load factors collected from large transport aircraft in service. This shows that the case giving rise to this loading condition is conservative for commercial airliners and becomes more conservative with increasing aircraft size. EASA only partially accept this report in support of this deviation, since it is understood that the data in the report quoted is "clipped" data - i.e. the data was edited at source before use by the authors of the report. The report is useful, however, for showing typical, rather than extreme, operational loads and for the showing of trends. The report does show, however, that larger aircraft tend to experience lower lateral loads than smaller aircraft. The report also shows that high lateral loads are not typical of service experience which does contribute to this particular case.
c) Comparison to existing approved design in service
The aircraft manufacturer compares the tyre clearances for the subject wheel and brake assembly with those of a similar large transport aircraft already in service. They claim that the dimensions are similar as are the loads and the tyre stiffnesses, and so the service record (of no identified interferences) for the comparison aircraft can be used as supporting evidence that tyre/brake interferences are not likely. EASA consider this comparison valid, but it is made against a single type with some, but not extensive, service history and also having a different configuration of landing gear.
EASA has reviewed this requested deviation and agrees that the above arguments are acceptable compensating factors providing an equivalent level of safety for the intent of ETSO C135 requirements. Therefore, EASA envisages granting the requested deviation to ETSO with limitations stated on the ETSO Authorisation.
