Longtail B744 at Maastricht on Feb 20th 2021, rain of engine parts
Last Update: April 19, 2023 / 10:14:36 GMT/Zulu time
A number of cars on the ground received damage as result of debris falling.
The police in Meerssen requested local residents to leave any debris in place and inform police.
The airport reported one of the engines lost pieces of its turbine after takeoff, the aircraft subsequently diverted to Liege on three engines. Metal pieces fell down in the vicinity of Meerssen, there are several reports of damages.
A resident in Meerssen reported he heard a loug bang, spotted the aircraft with streaks of flames from one of the right hand engines, then metal rained from the sky.
A ground observer video shows one of the engines was pulling black smoke behind.
The Dutch Onderzoeksraad/Dutch Safety Board (DSB) opened an "exploratory investigation" into the occurrence.
On Feb 22nd 2021 the DSB reported: "Shortly after take-off from Maastricht Aachen Airport, one engine of the cargo aeroplane encountered a failure. The crew then diverted to Liege Airport in Belgium and made a safe landing. The aeroplane lost some parts of the engine above the village of Meerssen. This resulted in two people suffering from minor injuries and, among other things, damage to some cars. The Dutch Safety Board has started an investigation into the cause of this occurrence." (Editorial note: according to local media the second minor injury occurred, when a boy touched one of the still hot blade fragments).
On Apr 19th 2023 the Onderzoeksraad/DSB released their final report concluding the probable causes of the serious incident were:
The investigation into this contained engine failure with the departing engine debris, revealed that the turbine of the number one engine of the aeroplane had failed. This engine –turbine- failure was caused by elevated gas temperatures that existed for an extended period of time in the turbine of the engine causing wear and deformation of outer transition duct panels. This resulted in one outer transition duct panel coming loose and one being fractured, which subsequently caused severe damage to the turbine. Consequently, engine debris -turbine parts- exited the tail pipe of the engine and came down in the village of Meerssen.
The manufacturer of the engine was aware of the problem with the outer transition ducts coming loose since the nineteen-eighties. To prevent the failure of the outer transition ducts and turbine section, several service bulletins were issued since 1993. Also airworthiness directives were issued to improve the reliability of the outer transition ducts and the safe working of the engine. These improvements concerned among others additional cooling features for the high pressure turbine and the installation of new outer transition duct panels. The investigation revealed that the engine was equipped with those new panels; however, the engine was not modified with the additional cooling features. The lacking additional cooling features were supposed to prevent a too high level of gas temperature. The installation of these cooling features, as adviced by a service bulletin, was not mandatory.
The operator, who had been using the aeroplane in its fleet for three months at the time of the incident, was not responsible for decisions not to embody Service Bulletin 72-462 at the shop visits in 1999 and 2009. Despite this, the operator was not able to present the documented reasoning regarding the non-incorporation of this service bulletin.
Having an adequate record keeping of maintenance documentation enables the operator and its maintenance organisation to make sound risk management decisions about the continuing airworthiness of their aeroplanes. This is crucial for the safe operation throughout the operating life of, in this case, the engine.
With the convergence of air traffic over areas surrounding airports, there is an increased risk of occurrences in these areas. In this case, the area of convergence runs over the villages of Meerssen, Geverik and Beek. This means that their residents are involuntarily exposed to a risk of departing engine debris that is likely higher than in other residential areas surrounding the airport. The engine failure showed that the hazard of departing engine parts is real, resulting in injured people and damaged property. The present case contributed to the feelings of unsafety by the residents.
The DSB analysed:
The cause of the engine failure
The investigation revealed that the second stage blade outer air seal of the high pressure turbine (HPT), as well as the HPT itself had deteriorated. This exposed the outer transition ducts to elevated temperatures. These two factors contributed to the slow deformation of the outer transition 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. These panels then damaged the turbine blades, resulting in fragments of the turbine leaving the engine via the exhaust pipe. 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.
Implementation of service bulletins
The failure mode as described above was known to Pratt & Whitney. This manufacturer proposed solutions to prevent reoccurrence since the beginning of the nineties. These solutions have been introduced via service bulletins (SB) and airworthiness directives (AD) (see Appendix F). At the publication date of this report, engines equipped with the additional cooling features and redesigned outer transition ducts, as prescribed by the SBs and ADs, have not exhibited this failure mode. Therefore, the measures appeared to be effective to prevent this failure mode. However, information provided by Pratt & Whitney during the investigation indicated that (given certain conditions) it is still possible to liberate an outer transition duct with both SB 72-488 and SB 72-462 incorporated, but these upgrades add significant margin and greatly minimize the probability of these events.
The event engine was not modified according to SB 72-462, which advises additional cooling features in the HPT. This service bulletin aims to reduce the elevated temperatures in the HPT, which prevent the failure of the outer transition ducts and subsequently parts of the HPT and LPT. 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.
As mentioned above, the engine was not modified with the additional cooling features, as prescribed by SB 72-462. However, it was modified with outer transition ducts of the redesigned type, as prescribed in SB 72-488. 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. It should be noted that according to Pratt & Whitney, the present failure was the first known time that outer transition ducts failed that were of the redesigned type in the PW4000-94” engine family. The configuration standard in which the engine was found was permissible, as SB 72-462 was not mandatory to be incorporated. Normally, operators weigh on factors such as financial, operational and safety aspects to incorporate a service bulletin. This incorporation can be done either during a scheduled shop visit or during a separate maintenance action.
More specifically, the service bulletin had a compliance category code 552, which indicates that incorporation can be accomplished when the engine is disassembled sufficiently during scheduled maintenance and therefore considered not time critical. Scheduled maintenance was accomplished in 1999 and 2009 during which the engine was disassembled (see the timeline in Appendix F). During this time, the aeroplane and engine were used by another operator. The incorporation of a service bulletin is an operator’s decision. Therefore, the initial decision whether (not) to embody SB 72-462 would have been taken (and should have been recorded) by that operator. Despite not being responsible for the decision not to embody SB 72-462 at the shop visits in 1999 and 2009, the operator at the time of the occurrence was not able to present the documented reasoning regarding the non-incorporation of SB 72-46. The content of the service bulletin was not considered an urgent safety issue.
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This article is published under license from Avherald.com. © of text by Avherald.com.
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