With highly technical investigations, the ATSB considers it appropriate to provide an explanatory statement that describes the nature of the technical investigations in a manner which is more easily understood by readers of this report.
ATR42 and ATR72 aircraft are twin-turboprop, short-haul, regional airliner and cargo aircraft built in France and Italy by ATR, and have been in service around the world since the mid-1980s. They seat between 48 and 78 passengers. The entire fleet of the two aircraft types have operated in excess of 20 million flight hours.
There are currently a small number of both aircraft types operating in Australia.
On 20 February 2014, a Virgin Australia Regional Airlines ATR 72 aircraft, registered VH‑FVR, operating on a scheduled passenger flight from Canberra, Australian Capital Territory to Sydney, New South Wales sustained a pitch disconnect while on descent into Sydney. The pitch disconnect occurred when the crew applied opposite inputs to the elevator controls while attempting to prevent the airspeed from exceeding the maximum permitted airspeed for the aircraft type. The pitch uncoupling mechanism activated and the elevators deflected in opposite directions, resulting in aerodynamic loads above the design strength of the tail structure, causing significant damage.
The “pitch uncoupling mechanism” (PUM)
The elevator consists of a left and right control surface and is located at the top of the tail structure on ATR aircraft. It is operated by the pilots pushing or pulling on the control column in the cockpit, which raises or lowers the nose of the aircraft (known as pitch). During normal operation, the left and right elevator deflect in the same direction and in equal amounts.
There is also a mechanism called the pitch uncoupling mechanism (PUM), located between the right and left control surfaces in the tail of the aircraft. In the event of one of the elevators becoming jammed, the PUM can be triggered to have the elevators operate independently of each other. The PUM is activated by the pilot (operating the pitch control for the elevator not jammed) applying significant pressure to the control column in either direction. The pilot is then able to operate the elevator on the non-jammed side.
If significant pressure is applied in the opposite direction on each of the control columns, the PUM may also be triggered. An inadvertent activation of the PUM due to opposing control inputs is what occurred in 2014.
Flight crew procedure
Generally, flight crew procedures prohibit the simultaneous inputs by both pilots on the control column. Established handover/takeover communication protocols reduce the risk of any inadvertent dual input.
The Work of the ATSB
The ATSB has undertaken an extensive investigation into the safety factors and issues behind the incident. The investigation has considered design, engineering and certification aspects of the aircraft, as well as operational, maintenance, training and regulatory aspects.
On 15 June 2016 the ATSB released its first interim investigation report that identified a safety issue concerning the potential for PUM activation to exceed the strength of the aircraft.
During the continued investigation of the occurrence, the ATSB has obtained an increased understanding of the factors behind this previously identified safety issue. This has identified that there are transient elevator deflections during a pitch disconnect event that could lead to aerodynamic loads capable of exceeding the design strength of the aircraft structure. The ATSB also identified that these transient elevator deflections were not considered during the certification process. The ATSB considers that the potential consequences are sufficiently important to release a further interim report that expands on the identified safety issue, prior to completion of the final investigation report. Readers are cautioned that new evidence may become available as the investigation progresses that will enhance the ATSB’s understanding of the occurrence.
In order to ensure the veracity of the analysis of the evidence leading to the identified safety issue, the ATSB engaged the UK Air Accidents Investigation Branch (AAIB) to conduct a peer review. The AAIB conducted an analysis of the evidence relating to the safety issue and concluded that their findings were consistent with those provided by the ATSB.
Stakeholder engagement and cooperation
The role of the ATSB is to identify safety issues and communicate them to the relevant organisation(s) to address. Since 2014, ATSB has worked closely with ATR (the aircraft manufacturer), the European Aviation Safety Agency (EASA), the French Bureau d’Enquêtes et d’Analyses (BEA), the United Kingdom Aviation Accident Investigation Branch (AAIB), the Civil Aviation Safety Authority (CASA), Toll as the Australian operator of the Australian ATR42 aircraft, and Virgin Australia as the operator of the ATR72 aircraft. Each of these stakeholders has a specific role in relation to the continued safe operation of these aircraft and has taken the following action to address the safety issue:
ATR has continued to provide data to ATSB and cooperate more broadly to undertake further aeronautical engineering and assurance work to determine if the aircraft has sufficient strength to withstand the loads resulting from a pitch disconnect. Specifically, ATR has commenced a detailed engineering analysis of the transient elevator deflections that occur during a pitch disconnect. Indications from ATR is that this work will be completed in July 2017.
On 5 February 2016, as a result of this occurrence and a briefing from the ATSB, ATR released an All Operators Message. The message informed operators of ATR 42/72 aircraft of revised maintenance and operational documentation relating to the pitch control system and pitch disconnect occurrences.
EASA is the issuer of the aircraft’s type certificate and, as such, is the global regulator for the continued safe operation of the word-wide ATR fleet.
While acknowledging the safety issue identified by the ATSB, on 20 December 2016 (and updated on 21 February 2017), EASA issued a related safety information bulletin (SIB) that contained the following statement:
At this time, the safety concern described in this SIB is not considered to be an unsafe condition that would warrant Airworthiness Directive (AD) action ...
As the global regulator for the aircraft type, EASA is closely monitoring the detailed engineering analysis of the transient elevator deflections being carried out by ATR.
CASA has advised that they continue to work closely with both EASA and the Australian airline operators of the ATR aircraft, in relation to the continued safe operation of the aircraft type in Australia. ATSB has been advised that this has included both continual dialogue with and specific audits of the airline operators to assure that sufficient pilot training and operational procedures are in place to prevent the recurrence of this event.
Virgin Australia and Toll
Virgin Australia Airlines advised that, in response to this occurrence, they have taken action to reduce the potential for pitch disconnects and to manage the risk of adverse outcomes from such occurrences. These included:
- reviewing and revising (where necessary) policy and procedures associated with descent speeds, handover and takeover procedures, overspeed recovery and on ground pitch disconnects
- incorporation of a number of factors surrounding the event into training material and simulator checks
- improved pilot awareness through Flight Crew Operations Notices, manufacturer’s communications (All Operators Messages) and ongoing training and checking
- updated maintenance requirements following a pitch disconnect.
Toll Aviation and Toll Aviation Engineering advised that, as a result of this occurrence, they issued a safety alert to their flight crew and aviation maintenance engineers, which included copies of ATR and EASA service bulletins. This alert advised that, in the event of a pitch disconnect, the aircraft was to be grounded until the appropriate checks had been carried out.
Toll has also developed and delivered enhanced training to their flight crews focused on handover/takeover procedures, unexpected turbulence and high-speed scenarios, and the pitch uncoupling system. Toll have also conducted detailed inspections associated with major maintenance procedures.
While welcoming the safety action taken to this point, in particular ATR’s engineering analysis, the ATSB retains a level of concern as to whether the aircraft has sufficient strength to withstand the loads resulting from a pitch disconnect. As a result the ATSB recommends that:
- ATR complete the assessment of transient elevator deflections associated with a pitch disconnect as soon as possible to determine whether the aircraft can safely withstand the aerodynamic loads resulting from a pitch disconnect within the entire operational envelope. 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.
- EASA monitor and review ATR’s engineering assessment of transient elevator deflections associated with a pitch disconnect to determine whether the aircraft can safely withstand the aerodynamic loads resulting from a pitch disconnect within the entire operational envelope. 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.
- CASA review ATR’s engineering assessment of transient elevator deflections associated with a pitch disconnect, to determine whether the aircraft can safely withstand the aerodynamic loads resulting from a pitch disconnect within the entire operational envelope. In the event that the analysis identifies that the aircraft does not have sufficient strength, it is further recommended that CASA take immediate action to ensure the ongoing safe operation of Australian‑registered ATR42/72 aircraft.
Readers of this report are encouraged to read this report in conjunction with, the interim report released on 15 June 2016 report and an update on the ATSB website on 10 June 2014.
A final report is expected to be published in August 2017 and will include the results of the further testing and assurance activity being conducted by ATR, and will also include operational, maintenance, training and regulatory aspects associated with the incident.Last update 05 May 2017