Hop! AT42 at Aurillac on Mar 25th 2018, loud noise in flight, cracks in fuselage
Last Update: February 12, 2021 / 14:34:15 GMT/Zulu time
Passengers reported that they were asleep in flight when they were woken up by large noise. Cabin crew indicated nothing serious had happened, the flight crew suggested a goose or block of ice may have struck the aircraft. After landing the large hole was discovered in the fuselage just below the left main wing.
The airline suggested a part may have separated in flight when the landing gear was lowered for landing.
The occurrence aircraft has been removed from service and is still on the ground about 20 hours after landing.
On Mar 2th 2018 the BEA reported investigators are on site, the BEA is investigating the occurrence. A left main gear door was missing from the aircraft and probably separated when the gear was lowered, the part likely impacted the airframe just underneath the left main wing.
On Mar 29th 2018 the BEA reported the aircraft was descending towards Aurillac when the flight crew heard the noise of impact. At the apron it was observed the left main landing gear door was missing and the wing root was damaged. The occurrence was rated a serious incident stating the damage was minor and is being investigated by the BEA.
On Feb 10th 2021 the BEA released their final report in French only tweeting at the same time that an English version of the report will be released in a couple of days.
On Feb 12th 2021 the BEA released their final report in English concluding the probable cause of the probable causes of the serious incident were:
The loss of the nut from the rear hinge assembly of the left main landing gear door was behind the door’s separation from the aircraft. It was not possible to establish the exact reason for the loss of the nut.
The examination of the main landing gear door hinge assemblies on F-GPYF and the ATR42 HOP! fleet has, however, highlighted non conformities with these assemblies.
The investigation brought to light errors in the generic maintenance documentation that could lead to the installation of screw/nut combinations outside of state-of-theart criteria. The consequences of these deviations from good practices, with respect to the mechanical performance of the screwed joint were not precisely determined during the investigation but could result in the fastener no longer fulfilling its function.
Thus, the following combinations of organisational factors and human factors may have contributed to the nut of the rear hinge assembly of the left main landing gear door becoming loose and then unscrewing:
- The ATR maintenance documentation concerning the choice of nuts to be used as an alternative to the IPC P/Ns was complex and contained errors which could lead to the choice of a nut with an unsuitable P/N.
- The tightening torque to be applied to this fastener and to the associated alternatives was missing from the ATR documentation.
- The ATR generic maintenance documentation allowed nuts to be reused without checking their self-locking capability. The landing gear door removal/installation job card did not specify that it was a self-locking nut and did not require a check for its self-locking capability or its replacement with a new nut.
The BEA analysed:
The crew took off at around 18:30 from Paris-Orly and during the climb, initially levelled off at FL110.
During the investigation, the observations made concerning the attachment points of the aeroplane’s left main landing gear door made it possible to establish that the nut of the door’s rear hinge assembly had unscrewed in service before coming off. The screw then moved backwards which allowed the male hinge to come out of the female hinge resulting in the landing gear door being slightly misaligned with the fuselage.
This misalignment increased the aeroplane’s drag. When the conditions for the activation of the CRUISE SPEED LOW alert were all present (the first time at FL110 and then in cruise at FL190), the associated light came on in the cockpit. The crew carried out the associated procedure and continued the flight. The analysis of the FDR data found that the increased drag and thus the misalignment of the door was present at least from the first level flight at FL110 and that it probably did not exist on the previous flights.
This abnormal position of the door also resulted in additional loads on the other landing gear door attachment points, in particular the link which failed under fatigue at the start of the descent.
The loss of the link and rear hinge assembly attachment points then introduced substantial loads on the remaining main attachment point, the front hinge assembly, until failure under overload of the male hinge.
Once the three main attachment points were no longer effective (link, front hinge assembly and rear hinge assembly), the door swung upwards around hinge No. 1 and struck the fuselage and window. This situation generated the noise heard and jolt felt by people on the plane followed by the failure under overload of this same hinge.
The door then separated from the aeroplane, causing the other damage observed on the aircraft:
- damage to wing root fairings;
- scratches to cabin window and surrounding skin;
- tear on lower surface skin of left flap;
- scratches on lower surface skin of left wing;
- small dents on vertical stabilizer.
Not understanding what had just happened but seeing that the flight parameters were normal, the crew decided to continue the descent and then landed at Aurillac in a nominal manner. It was only on the tarmac that the damage to the aircraft could be seen.
An occurrence in 2013 reported by ATR (see paragraph 1.18.6) showed significant similarities with the observations made during the examination of the F-GPYF parts (absence of the nut and washer from the rear hinge assembly, failure under fatigue of the link, marks at the bottom of the fairing, damage in various places on the airframe including the wing root, the flaps and the vertical stabilizer). In this occurrence, it was surmised that the lost nut might not have been tightened during a maintenance operation carried out shortly before the occurrence flight.
Although the hinge assemblies of the main landing gear doors are not considered as critical parts, these two occurrences show that the simple loss of the nut from the rear hinge assembly can result in the loss of a main landing gear door weighing around 15 kg leading to risks for the aeroplane and also constituting a danger for people and infrastructures on the ground which cannot be ignored.
Factors which can lead to the loss of a nut
As the lost nut was not found, the investigation was unable to establish the exact reason for its loss. However, given the analysis of the manufacturer’s maintenance documentation, the results of the inspection of the operator’s fleet and the examination of the three other F-GPYF nuts which were recovered (nut from the front hinge assembly of the left main landing gear door and nuts from both the front and rear hinge assemblies of the right main landing gear door), the investigation brought to light several factors which, individually or together, might have potentially contributed to the unscrewing of the nut.
The mechanical performance of a screwed joint depends on multiple parameters defined during the initial design (see paragraph 1.18.1). The modification of one of these parameters may compromise the design. However, the actual impact of these modifications on the mechanical performance of the fasteners could not be quantified during the investigation (no test plan).
Choice of nut
The examination of the fasteners of the HOP! ATR 42 fleet found that around 50% of the nuts (and in particular the three nuts from the hinge assemblies of the left and right main landing gear doors of F-GPYF which were recovered) did not correspond to the material specified by ATR during the initial design of the assembly.
During the investigation, it emerged that there were differences between the configuration specified by ATR during the initial design of the hinge assemblies of the main landing gear doors and the alternatives authorized by the maintenance documentation. In fact, errors in transcribing standards and/or the omission of information in various documents (SRM, AMM, SPET), meant that it was possible to choose unsuitable alternative P/Ns.
The missing nut behind the separation of the door was lost during the occurrence and the investigation was not able to determine the operation which led to this nut first being installed or its characteristics. The hypothesis of an unsuitable nut cannot, however, be excluded given the number of unsuitable nuts on the rest of the HOP! ATR 42 fleet and the characteristics of the three remaining nuts on F-GPYF.
A defective nut (initially or after use) can also make a screwed joint ineffective.
Detection of unscrewing
Partial loosening or unscrewing of the nut does not result in the immediate loss of the hinge assembly’s functionality (pivot link to open and close the door), this can in fact occur several years later as the incident to F-GPYF shows. Conversely, once the nut has been lost, the door may separate from the aeroplane in the relatively short time of a few flights.
At the time of the occurrence, only the removal of the fairings during maintenance operations would allow an agent to see whether the hinge fastener was loose.
The frequency of these operations (in particular, every nine years or 20,000 landings for the removal of the landing gear) and the perception of their criticality meant that the possible progressive loosening of the nut on F-GPYF was not detected.
Aircraft Registration Data
This article is published under license from Avherald.com. © of text by Avherald.com.
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