Emerald AT72 at Belfast on Sep 19th 2022, partial electrical failure and control difficulties
Last Update: February 29, 2024 / 16:12:02 GMT/Zulu time
Incident Facts
Date of incident
Sep 19, 2022
Classification
Incident
Airline
Emerald Airlines
Flight number
EI-3671
Departure
Leeds, United Kingdom
Destination
Belfast City, United Kingdom
Aircraft Registration
EI-GPN
Aircraft Type
ATR ATR-72-200
ICAO Type Designator
AT72
The AAIB rated the occurrence a serious incident and opened an investigation.
The aircraft returned to service 4 days after landing.
On Sep 19th 2023 the AAIB released an interim statement reporting: "The investigation has identified that the loss of electrical systems was caused by the failure of an electrical connector which resulted in the loss of DC power to the standby and emergency bus bars." The investigation is still ongoing, the final report will be released in due course.
On Feb 29th 2024 the UK AAIB released their final bulletin concluding the probable causes of the serious incident were:
The aircraft was operating under EASA Emergency Airworthiness Directive E-AD 2021-0120-E which had been introduced to de-risk potential problems within the battery master toggle switch and the 1PA contactor. This meant that the TRU was selected on and the 95PA and 96PA contactors were energised throughout all flights.
In this case the 95PA contactor failed when the TRU was selected ON and thereby not properly energising, disconnecting the DC output from the TRU to the STBY and EMER BUS bars. This caused a number of vital aircraft systems to drop off-line during the aircraft’s approach to Belfast City Airport.
The extreme, unforeseen, and dynamic nature of the failure caused characteristic startle and surprise effects in the flight crew. Without a specified procedure to follow, they continued flying the approach, successfully restoring systems by turning off the TRU which restored electrical power to the affected systems by de-energising failed relay 95PA.
The effect of a malfunction of the 95PA or the 96PA contactor when the AD was issued may not have been fully understood, and so an in-flight rectification procedure had not been introduced into the QRH to allow the flight crew to identify, troubleshoot and restore systems. The report considers situation management methods for helping crew mitigate startle and rebuild situation awareness before completing an approach.
Technical cause of the contactor failure
The failure of this 95PA contactor fitted in EI-GPN was caused by a slippage between the plunger and pin assembly operating under increased cycles. The slippage resulted in poor connection, localised arcing and subsequent loss of continuity across its A1 and A2 terminals. A definitive cause of the pin and plunger slippage with the 95PA contactor fitted to EI-GPN and several other contactors manufactured during 2015, could not be fully determined.
The AAIB analysed:
Electrical system failure
The electrical system failures during the approach to Belfast City Airport was caused by a malfunction within the SM150D19 contactor designated as the 95PA contactor. Within the circuit, it supplies 28 V DC output from the TRU to the STBY and EMER BUS bars. On this occasion, whilst 95PA was in the energised condition continuity was lost between the A1 and A2 terminal contacts. This was caused by the pin carrying the contact plate slipping out of its correct position as a result of its movement within the plunger.
The results of the metallurgical and dimensional tests were inconclusive. However, the combination of tiny differences may have coalesced to allow the slippage of the pin and plunger in a batch of contactors made during 2015 and two outliers made in 2019 and 2020.
Based on the year that EI-GPN was built, 2015, and the contactor date of manufacture, it is likely this was the original item fitted at aircraft build. Until the introduction of the E-AD 2021-0120-E, ordinarily the 95PA and 96PA contactors remained in a de-energised condition other than for occasional routine testing of the TRU. However, after introduction of the AD, these contactors were now required to remain in an energised condition throughout the flight.
There are numerous SM150D19 contactors used in this aircraft type and many of them are regularly held in the energised condition whilst in service without any problems. However, in cases where they have failed, the result has generally not been as noticeable because fewer systems have tripped off-line and have been routinely dealt with by referring to a procedure set out in the QRH. In these cases, the aircraft then continues with the flight and lands safely with the malfunctioning contactor being replaced as required and the aircraft released to service. In this case, the 95PA contactor affected quite a broad range of seemingly unrelated systems via the STBY and EMER BUS BARS and the failure of this combination was not covered by a specific procedure in the QRH. It therefore was more difficult for the pilots to troubleshoot and eventually resolve.
E-AD 2021-0120-E was developed and issued to address the potential problems with the 7PA battery master toggle switch the and FIN 1PA contactor. It is possible the in service experience of the reliability of the SM150D19 contactors, meant that the effect of the failure of the 95PA or 96PA was not considered a significant risk at the time.
It is likely the 95PA contactor on EI-GPN had a dormant anomaly, which was perhaps a less tightly assembled pin and plunger. As this was now under increased use, it started to slide out of position. It eventually reached the point where the contactor plate between terminals A1 and A2 could no longer make proper contact. The presence of arcing on the contact terminals suggests the slippage was gradual and took place over a number of cycles. The total number of cycles this contactor had carried out at the time of failure is unknown.
However, this did not affect the de-energised condition whereby the internal spring takes over and the contact plate between terminals B2 and B3 was re-established. Therefore, the DC power distribution system returns to normal with systems properly powered when the TRU was selected OFF. This is what happened when the aircraft commander deselected the TRU thereby deenergised the 95PA contactor and returning power to the affected aircraft systems.
Flight crew decision making surrounding the TRU
Having previously queried operating with the TRU on, the commander appeared conscious of how it affected the applicability of the QRH procedure during the first flight. In those circumstances, the QRH procedure required the crew to keep the TRU on and arrange maintenance action on arrival. With no persisting fault after arriving at Leeds, the commander – in liaison with the operator’s engineering department – took the practical decision of returning the aircraft to Belfast where company engineers could inspect it.
During the incident, with the QRH checklist still in his mind, the commander selected TRU off, attempting to resolve the failure by returning the aircraft to what he termed its “original design” (in reference to its electrical configuration prior to OEB 56).
Effects of the failure
Significance
Some anomalies existed between the manufacturer’s simulator session and the flight crews’ recollection of the event – for example, EFCP and engine instrumentation functionality – but the overall significance of the failure was similar. The manufacturer and the operator recognised the challenging nature of the 95PA contactor failure, which caused unforeseen and significant problems with major systems. This included its effect on, instrumentation, primary and secondary flying controls, landing gear, autoflight, engines, communications, navigation, and operational procedure access for flight crew.
Startle and surprise The CVR and information from the flight crew indicated supportive teamworking during the event between crew members. The sudden magnitude of the failure caused a degree of natural startle and surprise, which characteristically affected aspects of their communication and information processing. Accordingly, their experience provides opportunity to explore startle management.
The ambiguous nature, and unclear source, of the technical symptoms caused persisting stress and surprise while the flight crew attempted to re-evaluate the situation. Information from both pilots alluded to the disorientating effects of flight instrumentation and EWD abnormalities. Being apparently unable to access electronic procedures removed part of their habitual rule-based response, which could have helped them to recover from the effects of startle and surprise.
The dynamic nature of the incident perpetuated the surprise, probably raising any potential for plan continuation bias, as the crew commenced then continued with the approach while focussing on the salient features of the failure but without some systems necessary for landing. The commander had experienced electrical smoke during previous simulator sessions which possibly exacerbated a ‘fight or flight’ response. He described feeling time pressured and focussed on, what could be considered, the active solutions of landing the aircraft and resolving the technical failure. Continuing the approach without the landing gear and flaps deploying, the pressure on the commander continually increased. He perceived there was insufficient time to use landing gear gravity extension while also feeling concern over already difficult pitch control forces worsening during a go-around manoeuvre.
Consequently, he intuitively turned off the TRU at around 1,900 ft amsl, fortunately restoring lost systems. Some plan continuation bias might have persisted while the flight crew attempted to achieve approach stabilisation, which occurred around 500 ft agl. That was somewhat below the operator’s revised stabilisation height of 1,000ft but coincident with its previously specified figure.
Situation management
As part of the manufacturer’s safety action, an abnormal procedure has been included in OEB 56 Issue 3, which its simulator pilots used to demonstrate resolving the incident failure.
That particular procedure was unavailable to the incident crew leaving them with no clear course of action. While recognising the challenging nature of that situation, the manufacturer recommended pilots consider taking time to complete any outstanding procedures before flying an approach. Relevant QRH procedures during the incident would probably have resulted in the TRU being selected off. Otherwise, standby pitch trim and ADC switching were functioning, and the ‘Landing gear gravity extension’ and ‘Reduced flaps landing’ procedures were available. Such rule-based responses alleviating any startle and surprise.
Notwithstanding any need to refer to offside instruments, giving control of the aircraft to the co-pilot could reduce the commander’s physical and cognitive workload, assisting with rebuilding situation awareness for completing any briefing and decision-making processes.
EI-GPN’s crew intuitively, though successfully, resolved the startling and complex failure whilst flying the approach. However, given that pressure on them had been rising as the approach continued, making time to regain situation awareness might have weakened the fight or flight response, and revealed other options ahead of reaching a stage where an intuitive response became necessary.
As well as being used to prioritise tasks, situation management tools like ‘Fly, navigate and communicate’ and ‘Plane, path and people’ can help crew update their situation awareness, and manage startle and surprise. Performing a simple, linear rule together can help crews structure their analysis, manage workload, lower stress, and generate options for any subsequent decision-making process. Possibly suited to EI-GPN’s unforeseen failure, where certain normal cues were removed and there was no clear course of action.
Such processes can preclude plan continuation bias if alternative options include delaying commencing an approach to troubleshoot a technical failure, complete checklists and configure the aircraft; or discontinuing an approach after resolving a failure to rebuild situation awareness, particularly if stability criteria cannot be met.
Wishing to proactively assist its crew during potentially challenging circumstances, the operator stated its intent to adopt a rule-based structure for situation management, helping crew to manage startle and rebuild situation awareness.
Aircraft Registration Data
Incident Facts
Date of incident
Sep 19, 2022
Classification
Incident
Airline
Emerald Airlines
Flight number
EI-3671
Departure
Leeds, United Kingdom
Destination
Belfast City, United Kingdom
Aircraft Registration
EI-GPN
Aircraft Type
ATR ATR-72-200
ICAO Type Designator
AT72
This article is published under license from Avherald.com. © of text by Avherald.com.
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