VARA F100 at Geraldton on Jul 9th 2019, engine failure
Last Update: February 4, 2021 / 06:03:24 GMT/Zulu time
The Australian Transport Safety Bureau (ATSB) rated the occurrence an incident and opened a short investigation.
On Feb 4th 2021 the ATSB released their final report concluding the probable causes of the incident were:
- About 5 minutes after take-off, while climbing away from Geraldton, the engine 1 fuel flow regulator failed, resulting in fuel starvation to the engine and the total loss of thrust (flameout).
- The fuel flow regulator seized due to internal gearing wear, despite being maintained within the recommended service life limits of the Rolls-Royce Tay 650-15 engine management programme.
Other factors that increased risk
- Following the failure of engine 1 the crew did not adjust the aircraft speed or thrust, electing to maintain the aircraft’s incidental (cruise climb) speed of 250 knots. This degraded the single-engine climb performance resulting in increasing the time that the aircraft was outside controlled airspace and the glide range of an emergency airport.
- The decision to continue to Perth following the engine failure resulted in a longer exposure to one engine inoperative flight risks, compared to a return to the nearest suitable airport (Geraldton).
- Thrust variation from engine 1 (over about 45 seconds) due to the failing fuel flow regulator went undetected by the crew due to the effects of automation/clutch tie, focused attention on other cockpit tasks and the absence of any alert prior to the engine failure.
- Flight crew competency regarding execution of a manual thrust approach had recently been assessed as part of the operator’s F100 cyclic training and assessment program and was found to have assisted during the approach to land.
The ATSB reported the captain (ATPL, 8,191 hours total, 2,313 hours on type) was pilot flying, the first officer (ATPL, 5,724 hours total, 730 hours on type) was pilot monitoring. The flight had been planned to climb to FL330.
Following departure the pilot flying selected autothrust into CLB mode and LVLCH in the autopilot's vertical channel. About 4 minutes after departure the aircraft was identified by the Melbourne Center controller and provided a slightly shorter route. About 5 minutes after departure the left hand engine's thrust began to slightly increase beyond the limit commanded by the flight augmentation computer and the autothrottle, which remained unnoticed by the crew. The pilot flying reported, just when they climbed through the transition level (usually FL115 to FL130) both thrust levers began to move abnormally without crew input developing a minor split between the thrust levers due to the clutch tie mechanism. There were no vibrations and all indications for the right hand engine remained normal. The aircraft's nose dropped sharply and a minor yaw to the right developed. The pilot monitoring did not notice the unusual thrust lever movement. About 45 seconds after the initial thrust increase of the left engine and some time after the pilot flying detected the abnormal thrust lever movements the left engine's fuel flow regulator (FFR) ceased to effectively meter fuel causing the left hand engine's rotation to decay and thrust to reduce. As designed flight augmentation computer and autothrust increased the thrust of the right hand engine, both thrust levers moved towards the Max Continuous Thrust position. About 30 seconds later the left engine flamed out due to lack of fuel, the crew received a "ENG 1 FAIL" message on the MFDU.
Due to a pre-existing engine #2 autothrottle system unservicability autothrust disconnected reverting to manual thrust control. The pilot flying aligned the thrust levers towards the upright/normal climb position and decided to maintain the 250 KIAS, two engine climb speed, which was above the recommended single engine climb speed.
At the time of the left hand engine flaming out the aircraft was 22nm southeast of Geraldton, which was the nearest suitable airport at that point. The crew perceived the aircraft was struggeling to climb, the pilot flying thus elected to arrest the climb at FL150. The crew discussed and agreed to their plan to continue to Perth on their cleared flight track. The crew attempted to relight the left engine. The ATSB wrote: "Initially the PF thought the engine had restarted, however after discussion with the PM it was agreed the relight had been unsuccessful. As there were several indications of continued engine rotation, the crew assessed the engine was not mechanically damaged and therefore, in accordance with the QRH, did not activate the engine 1 fire suppression system."
The crew secured the left engine, the FMS calculated the engine out maximimum flight level at FL205. The crew decided to not strain the good engine and decided to descend to FL140. About 3 minutes after the engine failure the crew made a PAN PAN call to Melbourne center and advised of the revised flight level. The crew adjusted TCAS functionality as required and assessed their gliding capability with respect to various airfields around their flight track in case the right engine would also fail. The pilot monitoring felt comfortable with the gliding capability assessment because the emergency aerodromes were all visible from the aircraft.
ATC offered a more direct track to Perth, which the crew declined due to work load considerations and the necessity to perform required procedural steps to prepare the single engine approach into Perth. The captain handed aircraft control to the first officer, briefed cabin crew, made an announcement to the passengers and completed the non-normal checklists. The crew was prepared for the single engine approach about 70nm before Perth.
The aircraft followed the standard arrival route into Perth for an ILS runway 21 single engine approach. The aircraft landed safely and was accompanied by emergency services to the terminal.
The ATSB described post flight maintenance activities:
Once the passengers deplaned the operator’s maintenance engineering team attempted to restart engine 1. An engine re-light occurred, however the engine would not accelerate beyond ground idle. Troubleshooting resulted in removal of the engine 1 FFR unit - also known as a combined acceleration and speed control (CASC).
The unit was subsequently examined by Rolls-Royce, who found that the internal gear within the drive end assembly was significantly worn.
The FFR was replaced, however this unit was also found to be faulty. Another FFR was then fitted and the aircraft was successfully returned to service.
The ATSB analysed:
About five minutes after take-off, the engine 1 FFR seized due to internal gearing wear that was associated with the unit’s high time since overhaul (14,334.1 hours). This resulted in failure of the FFR to regulate fuel to engine 1, which then consequently flamed out.
As a result of previous, similar occurrences, Rolls-Royce was aware, and had advised operators of the potential for wear related FFR failure due to extended service life. Despite this, the subject FFR failed while being maintained within the service life limits of the Rolls-Royce engine management programme. Since this occurrence, Rolls-Royce have introduced a 10,000 hour midlife inspection and rework requirement for the FFR maintenance option selected by VARA (see the section titled Safety action). This requirement, which includes replacement of the sun and planet gear assemblies, would likely have prevented the subject failure and should reduce the future frequency of this failure type.
Presentation of the failure
In complex multiengine aircraft, one engine can rollback unnoticed by the flight crew, due to the autopilot and autothrottle masking the thrust asymmetry created. In this case, the PF noticed unusual thrust lever movement (due, in part, to the clutch tie), but did not associate that movement with the uncommanded increase in engine 1 thrust (due malfunctioning FFR). Further, although the PF noticed the movement of thrust lever 2, the progressively significant reduction in thrust (rollback) of engine 2 was not detected.
It is likely that the deteriorating health of engine 1 went undetected by the crew due to the effect of the automation/clutch tie and the absence of any cockpit alert. Additionally, at the time, the PF reported being focussed on the unusual (slightly decoupled) and uncommanded movement of the thrust levers due to the action of the clutch-tie mechanism. The PM was preoccupied with various support duties including cabin crew coordination, communicating with air traffic control and the completion of transition procedures.
Single engine operation
The operator’s F100 aircraft operations manual, provided clear guidance on the management of an engine failure throughout the varying phases of flight. These procedures were designed to maximise the aircraft’s single-engine climb performance in order to ensure terrain clearance, allow flight in controlled (higher) airspace, avoid low level weather, and to optimise glide range should the remaining engine fail. In a limited fuel situation, these procedures also served to maximise single-engine range and/or endurance.
For this event, at the point of engine flameout, fuel remaining and terrain clearance were not limiting factors. However, by not slowing the aircraft towards the best angle of climb (green dot) speed, and by not increasing engine 2 thrust from CLB towards MCT the aircraft’s single-engine climb performance was degraded, resulting in the crew’s decision to descend to FL 140. The crew recalled this collective decision was due to concern with ‘straining the good engine’ associated with any attempt to increase altitude, being ‘well above the lower safe altitude, with clear skies’ and ATC coverage. That is, they couldn’t see any advantage to going higher.
That the aircraft was only capable of manual thrust control was significant for the crew, as the autothrottle system provides automatic speed and gust protection, inputs to the angle of attack (alpha)51 mode protection, and windshear protection. If required, a missed approach, regardless of landing point (Perth or Geraldton), would need to be carefully executed manually to prevent exceeding any engine limitations. Both flight crew reported that they had practiced a manual thrust approach in the operator’s preceding cyclic (simulator) assessment session and this assisted their conduct during the single-engine recovery into Perth.
Post incident - course of action
Decision to continue to Perth
Regardless of experience, flight crew require time to initially identify a non-normal condition (engine failure) and consider their ‘immediate plan’. Once the aircraft was in the vicinity of waypoint IRWIN (Figure 1), the crew had a number of options:
- return to the point of departure and the nearest suitable airport (Geraldton)
- maintain their incidental altitude (FL 140) and original track continuing to avoid active military airspace, or
- optimise their recovery path to Perth.
The decision to continue, divert or return can be complex and requires crew to have a comprehensive understanding of the characteristics of the route, including the suitability of proximate airports, the forecast and actual weather, airspace factors and their respective aircraft’s systems.
The operator’s procedures in the event of an engine failure or shutdown required the crew to land as soon as practicable.52 Notionally, the concept of ‘land as soon as practicable’ versus land as soon as possible53 is predicated on the crew considering the existence and/or risk of further complications such as a fuel leak, fuel contamination and/or engine fire.
With respect to the relevant factors outlined in CAO 20.6 for continuation of the flight to Perth, the crew reported considering the following:
- although above alternate minima, a deteriorating cloud base on return to Geraldton may have necessitated a non-precision approach, at least for the initial segment
- anticipation of increased workload at Geraldton due to local traffic (two aircraft)
- the aircraft’s thrust control system had reverted to manual thrust only
- Perth runway 21 was ILS capable and significantly longer than the runway 21 in Geraldton
- Perth airport was supported by category 9 ARFF services.
These factors supported their course of action to continue to Perth. However, the ATSB considered that the following factors supported returning to Geraldton Airport following the engine power loss, consistent with the operator’s procedural requirement to ‘land as soon as practicable’:
- it was the nearest suitable airport
- as a regularly used VARA port, the crew were thoroughly familiar with it
- the weather at the time would probably have permitted an approach in visual flight conditions.
While continuation to Perth was a permitted option, it also resulted in longer exposure to one engine inoperative flight risks, compared to a return to the nearby Geraldton Airport.
Additionally, the route selected, combined with the decision to adopt a cruise altitude 6,500 ft below the recommended (FMS generated ENG OUT MAX) single engine altitude, resulted in the aircraft remaining outside of:
- controlled airspace, or in class E airspace until about 167 km (90 NM) north of Perth
- glide range to Perth Airport or an emergency airport for about 222km (120 NM) track miles, or about 30 minutes flight time.
In accordance with the operator’s single engine procedures, the crew also degraded the functionality of the TCAS to prevent the production of unachievable climb advisories (RA) when operating on one engine. This reduced the mechanisms by which the crew could avoid conflicting traffic.
Technically, the continuation to the planned destination (Perth) was also a ‘recovery’, as the aircraft was operating under a non-normal (single-engine) condition. The PF reported that the crew’s key priorities for the recovery included maintaining a safe aircraft configuration, availing the crew planning time and managing the increased workload associated with single-engine and manual thrust operations.
Although the recovery track to Perth remained unaltered, the adopted cruise level (FL 140), below the recommended single engine altitude, limited the aircraft’s potential glide distance by about a third. Continuing the climb closer to, or at the green dot speed with increased engine 2 thrust would have availed the crew more time and glide distance in the exceptionally unlikely54 event that remaining engine also failed.
The crew recalled being confident that in the event of the failure of their remaining (right) engine they could have recovered to either RAAF Pearce or Gingin, ATSB analysis found that this would not have been possible for a further 30 minutes after the initial engine failure. Prior to this point, the crew discussed the option of Jurien Bay Airport. While Jurien Bay did not meet the operator’s requirements as an emergency airport for flight continuation decision making, once the decision to continue to Perth had been made, it would have provided a viable option in preference to an off-field landing in the event of a second engine failure. However, by maintaining their diverging (as cleared) track away from the coastline and around military airspace, the aircraft actually tracked away from Jurien Bay and Gingin (Figure 1) for a period.
Following the application of the operator’s failure management procedures, the crew were required to reassess (evaluate) their understanding of the evolved risk profile of continued flight via the SAFE model. The crew probably assessed that the likelihood of a second engine failure was remote, but may not have fully contemplated the operational risks associated with continued single-engine flight at the lower altitude. Consequently, opportunities were missed to further mitigate operational risk via repositioning the aircraft into controlled airspace, more expeditious (direct) tracking to Perth and the optimisation of their glide range. The crew reported their overriding concern was workload management. Naturally, due to the abnormal situation, workload was increased. However, a direct track to Perth Airport or the runway 21 ILS initial approach fix could have further reduced their workload (given visual conditions).
In any event, a prompt landing is a key precaution against the risk associated with failure of the second engine, or further issues arising from the original engine failure, including effects on other aircraft systems. Should it occur, the resulting consequence (a forced landing on an unprepared area or a short runway) could be significant. Therefore, when faced with a one engine inoperative situation, consideration should be given to minimising the remaining flight time through track shortening and maximising the glide range of the aircraft. In this case the recovery path selected did not mitigate risk to as low as reasonably practical (ALARP).
Aircraft Registration Data
This article is published under license from Avherald.com. © of text by Avherald.com.
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