The Australian Transport Safety Bureau (ATSB) has called on ATR to quickly complete an engineering assessment of the type's pitch control system, amid possible concerns that a serious design flaw may be present in the ATR 42 and 72 series.
The recommendation was made in the Bureau's second interim report into a 2014 incident in which the crew of a Virgin Australia ATR 72-600 (VH-FVR) made strong opposite inputs to the pitch controls, leading to a 'pitch disconnect', which uncoupled the left and right controls to the elevators.
This resulted in potentially catastrophic damage to the aircraft's horizontal stabiliser, while a cabin crew member was seriously injured by the aerodynamic loads generated during the event. However, the damage was not detected for five days, after which the aircraft was grounded for several months before returning to service.
In the earlier phases of its investigation, the ATSB identified that "transient elevator deflections during a pitch disconnect event that could lead to aerodynamic loads that could exceed the strength of the aircraft structure."
An interim report released by the Bureau in June 2016 identified that the opposite control inputs were "not part of normal procedures", but added that "the existing procedural risk controls alone may not be sufficient to prevent this type of occurrence".
The ATSB's second report follows greater analysis of ATR's design for the pitch control mechanisms and their performance during a pitch disconnect event.
It found that flexibility within the pitch control system cables led to a rebalancing of loads in the system following the activation of the pitch uncoupling mechanism, and "dynamic transient elevator deflections" shortly after the mechanism was activated.
It also found that there was "unavoidable movement of the control column(s)" after the mechanism had been activated.
"Each of these effects may contribute to elevator deflections greater than the aircraft manufacturer considered during the design and certification of the aircraft," the ATSB adds.
In December, ATR undertook an assessment of the short term risks with the system. This concluded that the system was safe as the ultimate loads could not be exceeded through the control column, and that the probability of a similar event happening was low.
Nonetheless, in January FlightGlobal reported that ATR planned to update the operating manuals of its aircraft to warn pilots of the risks involved in making large flight control inputs.
The manufacturer is also undertaking a detailed engineering analysis using an analytical model backed up by ground and flight testing. Further modelling of dual input scenarios and flight testing is being carried out, in association with the European Aviation Safety Agency (EASA), which is the certifying authority for the aircraft.
"In the event that the analysis identifies that the aircraft does not have sufficient strength, it is further recommended that ATR take immediate action to ensure the ongoing safe operation of ATR42/72 aircraft," the report says.
The ATSB has also called on EASA to monitor and review the engineering analysis to determine if the aircraft can withstand loads resulting from a pitch disconnect.
"In the event that the analysis identifies that the aircraft does not have sufficient strength, it is further recommended that EASA take immediate action to ensure the ongoing safe operation of ATR42/72 aircraft," it adds.