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.
On 21 February 2017 EASA released Safety Information Bulletin (SIB) 2016-20R1. On that SIB, EASA noted:
'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 under Regulation (EU) 748/2012, Part 21.A.3B.'
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.