Aviation safety issues and actions

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.

Proactive Action

Action organisation: ATR
Action number: AO-2014-032-NSA-022
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 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.

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.

Correspondence

Date received: 11 August 2017
Response from: ATR
Action status: Monitor
Response text:

In an update provided on 11 August 2017, ATR briefed the ATSB on the results of:

  • flight testing to determine the pilot input profile following an intentional pitch disconnect
  • a comparison of the dynamic model computation against flight test data
  • the analysis of the pitch system jamming cases.

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.

ATSB response date: 05 September 2017
ATSB response:

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:

  • Cases of inadvertent pitch disconnect events from dual control inputs
  • Evaluation of the effects of variation of the pitch channel stiffness in the fleet

The ATSB will continue to monitor the work carried out by ATR in response to the identified safety issue.

Date received: 11 June 2018
Response from: ATR
Action status: Monitor
Response text:

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.

ATSB response:

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 calculated transient loads at high airspeeds were found to be significantly higher than the static case determined during the risk assessment in December 2016.
  • Some of the cases studied resulted in calculated loads in excess of the ultimate load case determined during the original certification, but were within the residual strength of the aircraft structure.
  • The briefings provided to the ATSB were fairly high level presentation of the basic methodology and results. The ATSB has not been presented with any formal engineering reports that have been through standard engineering review practices. Until then, the ATSB can only consider these results to be preliminary.
  • The analysis to date has examined only the jamming cases. Noting the caveat in the previous dot-point, the safety actions taken so far appear to partially address the ATSB’s concerns regarding the identified safety issue. However, the ATSB has yet to be advised as to how the aircraft is protected from inadvertent pitch disconnects as a result of reasonably foreseeable in‑service dual control inputs.

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.

Correspondence

Date received: 07 July 2017
Response from: European Aviation Safety Agency
Action status: Monitor
Response text:

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.

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.

Correspondence

Date received: 15 June 2017
Response from: Civil Aviation Safety Authority
Action status: Released
Response text:

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 is monitoring ATR's detailed engineering analysis in response to the concerns identified by the ATSB
  • EASA, the State of Design responsible for certification of the aircraft, maintains that based on the available information the aircraft is safe when operated in accordance with the approved operating procedures and within the normal operating envelope
  • ATR has updated their operational and maintenance instructions to address in-flight pitch
    disconnect occurrences, including improved post-event inspections
  • CASA continues to undertake enhanced surveillance of ATR aircraft operators in Australia to ensure appropriate actions have been taken to reduce the likelihood of the aircraft being mishandled in the first instance
  • Both Australian ATR 42/72 operators have documented and implemented an effective safety management system which ensures adequate operational risk mitigation strategies are in place
  • Both Australian ATR 42/72 operators are taking reasonable steps, through their training and checking system, to enhance technical and non-technical skills in relation to the event
  • Both Australian ATR 42/72 operators have amended their flight procedures and pilot refresher training to better equip their crews in preventing an in-flight pitch disconnect
  • CASA has requested that both Australian ATR 42/72 operators provide safety data to CASA on a monthly basis to verify continuing implementation of effective risk mitigation strategies
  • CASA will attend a detailed briefing with EASA and ATR on the technical issues associated with the occurrence.

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.

ATSB response date: 21 August 2017
ATSB response:

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.

Response from: CASA
Action status: Monitor
Response text:

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.

Current issue status: Safety action pending
Status justification:

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.

Last update 24 May 2019