The Onderzoeksraad voor Veiligheid (Dutch Safety Board, OVV) has issued several recommendations in its final accident report into a contained engine failure on a converted Boeing 747-400 freighter (B747-400BCF) in which engine parts dropped on a village in the Netherlands.
The incident took place on February 20, 2021, when the Boeing 747-400BCF, registered as VQ-BWT, suffered a contained engine failure shortly after its departure from Maastricht Aachen Airport (MST) to New York John F. Kennedy International Airport (JFK). The aircraft, operated by Longtail Aviation, suffered damage to engine 1 (outer wing left-hand side engine).
While none of the crew onboard the 747 were not hurt, two people on the ground suffered injuries. Further damage was recorded to cars and houses in the village of Meerssen located approximately two kilometers (1.2 miles) south of runway 21 at MST.
Following the contained engine failure, the aircraft diverted to Liege Airport (LGG) in Belgium, where the Boeing 747-400F landed without further incident. The freighter was powered by four Pratt & Whitney PW4000 family turbofan engines that accumulated 73,995 flight hours (FH) and 9,964 flight cycles (FC) since they entered service in January 1993. Since their last overhaul in 2010, the four turbofans had 11,516 FHs and 1,998 FCs at the time of the incident.
Contained engine failure of the Boeing 747-400BCF
Throughout its investigation, the OVV focused on finding the cause of the contained engine failure and whether the risks to the people on the ground had been properly managed.
According to the Dutch investigators, the risks were looked into because the OVV “received e-mail messages and letters from residents who expressed their safety concerns and that injuries to residents and damage to property had occurred as a result of this occurrence”.
The final report depicted that apart from the auxiliary power unit (APU) being unserviceable due to the inlet door being stuck in a partially open position, resulting in the crew compensating for the additional drag during the takeoff performance calculation, the aircraft was in normal operating condition. The operator’s Minimal Equipment List (MEL) allowed the Boeing 747-400BCF to be operated in such a condition.
The OVV noted in its final report that as the aircraft began to taxi to runway 21, “the flight crew started the engines; the start up times were within limits and all engine indications were normal”. Investigators added that the taxi time “provided sufficient time for the engines to comply with the minimum warm-up time, as recommended by the engine manufacturer”.
While the take-off roll was uneventful, at an altitude of 1,150 feet (350 meters) above mean sea level (AMSL), the pilots of the aircraft heard a banging noise and the 747 began to roll and yaw slightly to the left. Furthermore, the exhaust gas temperature of engine no 1 was above the maximum temperature, with the power plant failing and losing thrust shortly after.
The first officer, who was pilot flying (PF), corrected for the asymmetric trust and performed the first item of the “engine limit or surge or stall” procedure, investigators noted, reducing the number 1 engine’s thrust to flight idle. At the same time, “the runway controller informed the flight crew that flames were observed from the number 1 engine”. The captain, who was pilot monitoring (PM) declared an emergency, telling Air Traffic Control (ATC) that they were shutting down the engine.
Even though there was no engine fire indication in the cockpit, the report claimed that “the crew decided to treat the event as an engine fire and performed the memory items for the ‘engine fire’ procedure”. Subsequently, the pilots switched “the number 1 fuel control switch to cutoff, pulled the engine 1 fire switch and rotated it to its stop to exert a fire suppressing agent into the engine”, isolating the engine from the hydraulic system and cutting off the flow of fuel into the now-failed turbofan.
Since the aircraft was above its maximum landing weight, the flight crew and ATC coordinated to dump fuel and reduce weight. At this point, the captain became the PF, while the first officer was now the PM. Less than an hour after taking off from MST, the crew dumped over three tons of fuel and landed uneventfully at LGG. Notably, the report found that “the fire brigade reported to the flight crew that there were no signs of fire from engine number 1, nor that there was external damage visible”.
Prior to landing, ATC informed the flight crew that there were already reports about debris on the ground from the engine, which is why the pilots “took into consideration that possible damage to the wing or flaps might have happened”, the report added.
Damage to the high pressure and low-pressure turbines of the P&W PW4000
Following a post-landing inspection, the OVV pointed out there was “significant damage was visible to the aft stages of the low pressure turbine when looking forward into the tail pipe”.
Further inspections showed that the number 1 engine “encountered a contained failure and sustained internal damage of among others the high pressure turbine and low pressure turbine”. No other parts of the aircraft were harmed during the event.
The Dutch investigators found “that the second stage blade outer air seal of the high pressure turbine (HPT), as well as the HPT itself had deteriorated”, resulting in the outer transition ducts being exposed to elevated temperatures. As a result, these two factors contributed to the slow deformation of the ducts. “The outer transition duct panels distorted, the attachment hooks deformed and backed away from the case, which led to liberation of one panel and one being fractured,” the OVV continued. The panels damaged the turbine blades, fragments of which left the engine via the exhaust pipe and rained down on the Dutch village.
The report noted: “The investigation ruled out that runway foreign object debris or a bird strike or drone strike had led to the engine failure. The meteorological conditions played no part in this failure either.”
Pratt & Whitney, the manufacturer of the PW4000 engine, was aware that such a failure could occur, and issued two Service Bulletins (SB) in 1993. Furthermore, the Federal Aviation Administration (FAA) issued two airworthiness directives (AD) to the power plant.
The OVV highlighted that while the engine had the SB 74-488 incorporated, SB 72-462, which is meant to add additional HPT cooling features, did not. While one AD was not applicable due to the incorporation of SB 72-488, the other AD, requiring operators to inspect LPT vanes, had not been incorporated because the engine’s last shop visit had been in 2009, before the publishing date of the directive.
“According to Pratt & Whitney’s analytical modelling, the outer transition ducts failure would not have occurred if SB 72-462 had been incorporated. “This claim seems credible, as the investigation did not find similar failure modes with engines that had been modified with the additional cooling features,” the Dutch investigators stated.
“Despite the fact that the engine was equipped with the redesigned outer transition ducts, the temperature could rise to a level that it caused damage over a long period of time, which led to liberation of outer transition duct panels and finally failure of the engine, whereas the lacking additional cooling features were supposed to prevent this from happening,” the OVV added.
However, while the SB was published in 1993, it was not incorporated during the engine’s visit to the shop in 1999 and 2009, when the engine was disassembled. Longtail Aviation was also not the operator of the aircraft at the time, rather it was Singapore Airlines (operated between 1991 and 2004), and Martinair, a subsidiary of KLM (between 2007 and 2010).
As a result, investigators found that “the content of the service bulletin was not considered an urgent safety issue” and operation of the engine is allowed without the incorporation of SB 72-462.
Reconsider whether SB should be a mandatory AD
The Dutch investigators issued three recommendations to various parties.
First, the OVV recommended that Longtail Aviation “keep the record keeping of the (non-)implementation of service bulletins for leased engines of your fleet of commercial air transport aeroplanes complete and accessible”.
Second, the Dutch investigators advised the FAA to reconsider whether SB 72-462 should be made mandatory through an AD.
And finally, the institution recommended that the Dutch Minister of Infrastructure and Water Management “perform and publish an assessment for residential areas around Maastricht Aachen Airport of the risks of parts departing the aircraft, such as departing engine debris”. The report noted that while the failure caused no risk to the safe operation of the aircraft, the fragments that exited the engine “caused a hazard to persons and property on the ground”.
“The engine failure showed that the hazard of departing engine parts is real, resulting in injured people and damaged property,” the OVV said, adding that “residents around airports are at least exposed to two types of risks: first, parts departing the aircraft, and second an accident with an aircraft”.
At the time of the report, no study had been conducted to assess the residential areas around MST for the two risks.
“Based on the results of such an assessment, an informed decision about the acceptability of these local risks should be made,” the OVV concluded.
The OVV marked the event as a serious incident due to the “potential for an accident, as the departing engine debris that came down in a village could have seriously injured people, besides the injuries that had taken place”.