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Consideration of transient elevator deflections from a pitch disconnect

Issue number: AO-2014-032-SI-02
Who it affects: All operators of ATR 42 and ATR 72 aircraft
Issue owner: ATR
Operation affected: Aviation: Air transport
Background: Investigation Report AO-2014-032
Date: 05 May 2017

Safety issue description

The aircraft manufacturer did not account for the transient elevator deflections that occur as a result of the system flexibility and control column input during a pitch disconnect event at all speeds within the flight envelope. As such, there is no assurance that the aircraft has sufficient strength to withstand the loads resulting from a pitch disconnect.

Application of the safety issue to both ATR 42 and 72 models
Although the flight control system in the ATR 72 has been assessed in this report, the ATR 72 is a longer version of the ATR 42 and the design of the flight control system is common to both models. The different length of the control runs is likely to have an effect on the flexibility, but the uncertainty that results from the lack of detailed engineering assessment means that the safety issue also applies to the ATR 42 model.

Initial safety action taken by the ATSB
On 11 November 2016, the ATSB notified ATR of the concerns identified in this report. The ATSB also notified the Australian operator of the aircraft, the Civil Aviation Safety Authority and the Federal Department of Infrastructure and Regional Development.

The issue was further discussed with ATR at meetings on 18 November 2016 and 1 December 2016. The European Aviation Safety Agency was also present during those meetings.

Recommendation

Action organisation: ATR
Action number: AO-2014-032-SR-014
Date: 05 May 2017
Action status: Monitor

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 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.

Recommendation

Action organisation: European Aviation Safety Agency
Action number: AO-2014-032-SR-015
Date: 05 May 2017
Action status: Monitor

The ATSB recommends that 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 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.

Recommendation

Action organisation: Civil Aviation Safety Authority
Action number: AO-2014-032-SR-016
Date: 05 May 2017
Action status: Monitor

The ATSB recommends that CASA review ATR’s engineering assessment of transient elevator deflections associated with a pitch disconnect, to determine whether the aircraft can safely withstand the 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.

Proactive Action

Action organisation: ATR
Date: 05 May 2017
Action status: Released

On 1 December 2016, in response to the identified safety issue, ATR advised the ATSB that they intended to:

  • perform a risk assessment to determine the short term risks associated with continued operation
  • conduct a detailed engineering analysis of the transient elevator loads during a pitch disconnect.

Short term risk assessment

On 15 December 2016, ATR provided the ATSB with the results of their assessment of the short term risks of continued operation awaiting the complete engineering work associated with the issue. Their assessment concluded that:

ATR considers that continued safe operation is ensured by considering

  • In the jamming situation, the ultimate loads cannot be exceeded through the control column input (excessive effort and mechanical stops). At high speed, the differential elevator deflection has margin to accommodate the transient load.
  • The probability of a repeat occurrence of the MSN1058 [VH-FVR] event defeating all the barriers inherent in the design and standard operating procedures.
  • The quantitative analysis results showing no immediate action is required.

Detailed engineering analysis of transient elevator deflections

On 11 April 2017, ATR provided the ATSB with an update on the detailed engineering analysis of the transient elevator loads. The briefing included an overview of the analysis methodology and preliminary results.

The analysis being conducted is based upon an analytical model supported by both ground and flight testing. The analytical model represents the ATR pitch control system and has system component masses and stiffness represented as group blocks. This includes a block representing the pitch uncoupling mechanism (PUM), which was modelled to represent the behaviour of the PUM before, during and after activation.

ATR has compared the model to the behaviour of the system recorded during ground test and has identified a favourable correlation. The results of the model showed that, following activation of the PUM on the ground, without aerodynamic loads, the flight control system responded in an underdamped oscillatory manner.

For analysis of the inflight situation, ATR has used the aerodynamic model that was developed during certification. Preliminary results for the jamming scenarios was provided. Those results showed that the inflight system response is also that of an underdamped oscillatory system. It also indicates that the magnitude of the system response is dependent upon the pilot input to the control column, and how quickly the flight crew respond to PUM activation. The system has margin for jams at the elevator. ATR are continuing the analysis of jams at the control column.

ATR are continuing with the detailed analysis. Further work includes:

  • Flight testing to determine a suitably realistic pilot response to activation of the PUM
  • Verification of the analytical model with data recorded during the flight tests
  • Modelling of the dual input case
  • Modelling of other cases required by the European Aviation Safety Agency.

ATSB response:

The ATSB acknowledges the efforts of ATR to resolve the safety issue. The ATSB also notes that, while the short‑term risk assessment does not account for the transient elevator deflections associated with a pitch disconnect, until the results of the detailed engineering analysis are available it is not possible to accurately quantify the transient elevator loads. Consequently, it is not possible to fully determine the magnitude of the risk associated with continued operation of ATR42/72 aircraft until the engineering analysis is complete.

Noting the above, the ATSB’s retains a level of ongoing concern as to whether the aircraft has sufficient strength to withstand the loads resulting from a pitch disconnect. Consequently, while the ATSB accepts that the current level of safety action partially addresses the safety issue; the ATSB makes the following safety recommendations.

   
Current issue status: Safety action pending
Status justification:

The ATSB acknowledges the efforts of ATR with regard to the detailed engineering analysis of the transient elevator deflections. The preliminary results have shown that the system responds in an underdamped oscillatory manner, resulting in elevator deflections greater than those identified by the static analysis previously carried out by ATR. The ATSB is encouraged by the level of detail into which ATR have developed the analysis and will continue to monitor their progress. Until such time that the analysis has satisfactorily shown that the aircraft has sufficient strength to withstand the loads resulting from a pitch disconnect, the identified safety issue will remain open.

 
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Last update 05 May 2017