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.
The ATSB acknowledges the efforts of ATR and EASA with regard to the detailed engineering analysis of the transient elevator deflections. Based on the information provided to the ATSB, this safety issue has been partially addressed.
However, until the effects of a pitch disconnect resulting from dual control inputs has been fully assessed, and the ATSB been provided with evidence of a fully verified engineering report into the transient dynamic behaviour assessment, the ATSB does not consider that the safety issue has been addressed and will continue to monitor the work of ATR and EASA.
The ATSB continues to monitor the recommendations and will continue to update the website after release of the final report.
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.
CASA's position is informed by the ongoing certification status of the aircraft together with an adequate response from the two Australian ATR 42/72 operators in terms of their safety management and training and checking systems. In response to the ATSB safety recommendation, CASA advises the following:
CASA and EASA are maintaining close and regular communication in relation to this matter. EASA continues to liaise regularly with ATR as part of their continued investigation and CASA will review each new development on its merits, and subsequently determine whether any further action may be required. In particular, if the completed ATR engineering analysis identifies any ongoing concerns, CASA will take appropriate action, taking into account any measures taken by EASA, in consultation with the Australian operators of this aircraft.
CASA will continue to provide updates to the ATSB as our own investigations progress.
The ATSB accepts that the Civil Aviation Safety Authority (CASA) has been monitoring ATR’s engineering assessment of the transient elevator deflections following an in-flight pitch disconnect.
The ATSB acknowledges that CASA will continue to monitor the ATR engineering assessment and consult with EASA regarding the implications of the assessment results. If the completed ATR engineering results identify any ongoing concerns, the ATSB expects that CASA will take appropriate action, taking into account any measures taken by the European Aviation Safety Agency (EASA), and in consultation with Australia operators of the aircraft type.
Due to the ongoing nature of the engineering assessment, the ATSB will continue to monitor the actions taken by CASA with regard to Safety Recommendation AO-2014-032-SR-016. Any further progress updates will be made on the ATSB website.
CASA advised that since 10 February 2016, they have been involved in a comprehensive dialogue with ATR and EASA regarding the assessment of the transient elevator deflections associated with pitch disconnect to address this safety recommendation. CASA has also engaged with the ATSB throughout the investigation. CASA provided an interim response to the ATSB safety recommendation on 15 June 17. CASA intends to provide a further response to the ATSB safety recommendation following the release of the final report. That response, in part, depends on EASA’s response to the same safety recommendation.
Since February 2016, CASA has been monitoring ATR and EASA detailed engineering analysis of the aircraft’s pitch control system. However due to inherent conservatism in design ATR have now demonstrated the aircraft has sufficient strength to withstand an intentional pitch disconnect at almost any reasonable operating condition and the aircraft meets its certification requirements. For a worst possible case of factors, the conditions of which have never been encountered in the life of the type, it is possible the maximum load can be marginally exceeded during an intentional pitch disconnect. ATR propose to remove this risk by implementing a further updated pitch disconnect procedure. EASA is have provided that the review on ATR's engineering assessment of transient elevator deflections associated with a pitch disconnect is finalised to EASA's satisfaction. EASA are in the process to compile a final submittal regarding the results. EASA plan to provide this submittal by latest Q2/2021.
CASA's position is informed by the ongoing certification status of the aircraft together with an adequate response from the two Australian ATR 42/72 operators in terms of their safety management and training and checking systems. CASA and EASA are maintaining close and regular communication in relation to this matter. EASA continues to liaise regularly with ATR as part of their continued investigation and CASA will review each new development on its merits, and subsequently determine whether any further action may be required. In particular, if the completed ATR engineering analysis identifies any ongoing concerns, CASA will take appropriate action, taking into account any measures taken by EASA, in consultation with the Australian operators of this aircraft.
As CASA is not the State of Design for the ATR 42/72 aircraft, CASA issued a Type Acceptance Certificate (TAC) for the ATR 42 and 72 models and does not hold any type design data for these aircraft. Therefore, CASA relies on EASA as the NAA for the State of Design to conduct the assessment and correct any identified deficiencies in the aircraft. EASA has been working with ATR on resolution of the transient elevator deflection with pitch disconnect and providing CASA with updates when necessary.
CASA has been monitoring the progress of the review and assessment by EASA of ATRs analysis. Details provided below:
1. EASA and ATR have been working on the issue to understand the loads and demonstrate compliance with the relevant airworthiness standard. Certification Review Item (CRI) D-24 Issue 3 dated 25 January 2019 was raised. This CRI relates to demonstrating compliance with airworthiness requirements JAR 25.671(C (3) and CS 25.671(c)(3) and has been finalise, and closed. This CRI was intended as the method to be used to address AO-2014-032-SI-02.
2. EASA advised CASA on 18 March 2020 the following:
a. That they had reviewed preliminary data that indicates the aircraft can safely withstand the loads from a pitch disconnect within the operational envelope.
b. In context of the preliminary data review, ATR and EASA found potential for improvement of the pitch uncoupling function at high speeds. Consequently the priority shifted to work on an improved the pitch uncoupling AFM procedure. This AFM procedure review is currently in the final phase.
c. In parallel, EASA is progressing the review on ATR’s engineering assessment of transient elevator deflections associated with a pitch disconnect. Finalisation of the assessment is planned for the first half of 2020.
d. Routinely, the status of the pitch uncoupling occurrences is reviewed at regular Airworthiness Review Meetings three times per year.
3. CASA contacted EASA on 16 March 2021 for an update on their review of the ATR engineering assessment of transient elevator deflections associated with a pitch disconnect.
4. EASA responded on 16 March 2021 advised that the review on ATR's engineering assessment of transient elevator deflections associated with a pitch disconnect is finalised to EASA's satisfaction. EASA are in the process of compiling a final submittal regarding the results. EASA plan to provide this submittal by latest Q2/2021.
CASA will continue to monitor the actions of EASA on the closure of this safety recommendation as part of its business as usual.
If EASA issues corrective action, such as an airworthiness directive, under Part 39 of the Civil Aviation Safety Regulations this corrective action would be automatically mandatory for all operators of the affected aircraft in Australia. Any other action taken by the State of Design will be handled by the usual processes and procedures CASA already have in place.
As the monitoring of this issue is now business as usual, CASA recommends that the ATSB closes AO-2014-032-SR-016.
The ATSB accepts that CASA will continue to monitor the actions of EASA relating to this safety issue as part of their business as usual and take appropriate action when EASA take corrective action.
On 1 December 2016, in response to the identified safety issue, ATR advised the ATSB that they intended to:
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:
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 accepted that the current level of safety action partially addresses the safety issue; the ATSB made the following safety recommendations on 5 May 2017, as released in the second interim report.
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.
In an update provided on 11 August 2017, ATR briefed the ATSB on the results of:
The flight testing identified a consistent post-disconnect pilot input profile for use in the dynamic model and indicated that there was no discernible difference in the profile across the tested speed range. Also, the results from the dynamic (engineering) model compared well with the flight test results, indicating that the dynamic model satisfactorily represents the aircraft behaviour during an in-flight pitch disconnect.
ATR applied the dynamic model to assess the effect of an in-flight pitch disconnect at the maximum operating speed (VMO) in two representative pitch system jamming cases. The results indicate that there is a margin between the peak elevator deflection during the pitch disconnect and the deflections required to generate the ultimate loads, at VMO.
The ATSB accepts that ATR has completed part of the engineering assessment of the transient elevator deflections following an in-flight pitch disconnect.
The ATSB notes that to date, we have only been provided with basic analysis results and that those results have been presented to EASA in a similar timeframe. The ATSB has not yet been provided with documentation showing an independently reviewed engineering assessment, but acknowledges that this would not be practical until the engineering assessment has been completed.
The ATSB also notes that the following engineering analyses will be required to meet the intent of this Safety Recommendation:
The ATSB will continue to monitor the work carried out by ATR in response to the identified safety issue.
On 11 June 2018, ATR reported to the ATSB that:
Since August 11, 2017, further analyses using the dynamic model were performed to evaluate the effect of variation of the pitch channel stiffness on the results previously shared with EASA and ATSB. Stiffness variations considered were based on stiffness measurements made on several new production aircraft. Considering the sampling taken, and the evaluation of control channel stiffness potential variation with ageing, it was agreed by EASA that the data used are representative of the in‐service fleet. In the worst scenario considered in terms of jamming location (control column jamming) and with an extreme low stiffness value, the loads resulting from transient elevator deflections associated to a pitch disconnect at VMO would slightly increase with regards to certification ultimate loads. Structural assessment performed for the jamming loads envelope demonstrates that the structure is capable to sustain this load increase with positive strength margins.
The engineering analysis conducted evidenced that under the scenario of control column jamming, the elasticity of the cables provides a level of control on aircraft pitch axis through elevators deflection before PUM activation. As a result, the effort required to activate the PUM is increased compared to the 50 daN necessary to activate the mechanism when the elevators are totally blocked. Indeed, the pilot effort to activate the pitch uncoupling mechanism can reach up to 78 daN at VMO.
On the other side, this residual pitch authority can enable the crew to decelerate the aircraft. At lower airspeed the effort required to activate the PUM is lowered and margin with regards to the certification ultimate loads is increased. Based on this analysis and several simulations and flight tests, ATR proposes a revised pitch channel jamming procedure providing effective guidance to slow down the aircraft prior to PUM activation. This procedure has been successfully verified in flight test with ATR and EASA test pilots.
All the above activities have been performed under the oversight of EASA. The consolidation of the applicable requirements and EASA position has been formalized through a Certification Review Item (CRI) that is still open at the moment. Next iteration of the CRI with EASA is scheduled end of June 2018.
Also note that with regards to the risk associated to dual input, the following actions have been taken at various industry levels:
- ATR released the AOM42/72/2016/03 and revised the FCOM to raise crew awareness regarding the potential detrimental effect of uncoordinated crew input and/or large and aggressive flight control input at high speed.
- EASA released the SIB 2016‐20 to highlight the risks associated to rapid and large alternating control inputs.
- EASA added the ‘Inappropriate Flight Control Inputs’ item to its risk portfolio in the frame of their risk management system, recognizing this is an industry concern. It will cover the issue of simultaneous inputs, as well as inputs of large amplitude or frequency inadequate for the flight phase at the event.
- Paragraph 5.3 of the ICAO Airplane Upset Prevention and Recovery Training Aid revision 3 (AUPRTA https://www.icao.int/safety/LOCI/AUPRTA/index.html) highlights the risk of upset induced by pilot excessive input.
ATR subsequently advised that, from their perspective, at this point the continued airworthiness of the ATR 42/72 fleet is assured, but recognise they must continue to analyse threats such as these. Indeed, ATR is also part of a working group at EASA level re-examining industry wide experience.
The ATSB notes the work carried out by ATR in assessing the post-PUM activation transient behaviour of the pitch control system. To date, the ATSB has received a number of briefs from ATR on the results of their detailed engineering work regarding the assessment of the certification jamming scenarios. From this, the ATSB notes:
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.
EASA advised they were in regular contact with ATR about the second step of the analysis regarding scenarios that will be evaluated based on current certification practices with regards to CS 25.671. It was found that this has a repercussion on the first step which was focused on the certification practices at time of initial type certification.
The plan is expected to be completed in by end 2018. Should an unsafe condition be identified then ATR and EASA will take action as per Annex I paragraph 21.A.3 of Commission Regulation (EU) No 748/2012 to ensure the ongoing safe operation of the ATR42/72 aircraft.
The European Union Aviation Safety Agency (EASA) has reviewed the ATR assessment of transient elevator deflections associated with a pitch disconnect. It has been based on a study of the control cable elasticity, the results from a full-scale static test as well as a dedicated flight testing campaign, on top of other analysis. EASA agrees with ATR that the aircraft can safely withstand the loads resulting from a pitch disconnect within the entire operational envelope.
The ATSB considers that this recommendation has been adequately addressed.