1/7 www.bea.aero INVESTIGATION REPORT The BEA investigations are conducted with the sole objective of improving aviation safety and are not intended to apportion blame or liabilities. BEA2015-0491/November 2018 Accident to MX Aircraft MX2 experimental aircraft registered N88MX on 16 August 2015 at Châteauneuf sur Cher (Cher) Time Around 19:05 (1) Operator Private Type of flight General aviation, aerobatics Persons on board Captain Consequences and damage Aircraft destroyed This is a courtesy translation by the BEA of the Final Report on the Safety Investigation. As accurate as the translation may be, the original text in French is the work of reference. (1) Unless otherwise stated, all times given in this report are in local time. 1 - HISTORY OF THE FLIGHT The pilot was carrying out a training flight for his participation in the World Aerobatic Championships in the “unlimited” category scheduled to take place four days later. At the end of his roughly 10-minute program, the pilot left the judging line and headed towards the runway for the last fly-pass in front of the public. He started his descent, accelerating to 200 kt, with a view to a vertical climb. When the pilot started the pull-out at 8 g, he said that he heard a “loud explosion”. The right wing broke and the engine separated from the airframe. The pilot managed to evacuate the aircraft and deploy his parachute. He landed in a field of sweetcorn. Figure 1: photo taken by witness Failure in flight while performing a pull-out in an aerobatics display, evacuation of pilot
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INVESTIGATION REPORT
The BEA investigations are conducted with the sole objective of improving aviation safety and are not intended to apportion blame or liabilities.
BEA2015-0491/November 2018
Accident to MX Aircraft MX2 experimental aircraftregistered N88MXon 16 August 2015at Châteauneuf sur Cher (Cher)
Time Around 19:05(1)
Operator PrivateType of flight General aviation, aerobaticsPersons on board CaptainConsequences and damage Aircraft destroyedThis is a courtesy translation by the BEA of the Final Report on the Safety Investigation. As accurate as the translation may be, the original text in French is the work of reference.
(1)Unless otherwise stated, all times
given in this report are in local time.
1 - HISTORY OF THE FLIGHT
The pilot was carrying out a training flight for his participation in the World Aerobatic Championships in the “unlimited” category scheduled to take place four days later. At the end of his roughly 10-minute program, the pilot left the judging line and headed towards the runway for the last fly-pass in front of the public. He started his descent, accelerating to 200 kt, with a view to a vertical climb. When the pilot started the pull-out at 8 g, he said that he heard a “loud explosion”. The right wing broke and the engine separated from the airframe. The pilot managed to evacuate the aircraft and deploy his parachute. He landed in a field of sweetcorn.
Figure 1: photo taken by witness
Failure in flight while performing a pull-outin an aerobatics display, evacuation of pilot
The BEA investigations are conducted with the sole objective of improving aviation safety and are not intended to apportion blame or liabilities.
BEA2015-0491/November 2018
2 - ADDITIONAL INFORMATION
2.1 Site and wreckage information
The rupture in flight occurred over a 60-hectare circular field planted with sweetcorn measuring approximately 2.5 meters high. The engine, still integral with its mount, was found at the edge of the field at around 100 meters from the threshold of runway 09. The fuselage and debris from the wing were found in the central part of the field.
Figure 2: position of wreckage
The main aircraft debris was found and it was possible to piece the wings together. The firewall was missing. It was found in December of the same year after the field had been harvested.
The findings on the accident site were that:
� the roll controls were continuous from the stick up to the area of rupture of the right wing and continuous for all of the left wing;
� the engine and its mount had separated from the airframe at the firewall; � there were no remains of the firewall on the forward section of the fuselage; � the main spar of the right wing had broken, in flight, in two places; � there were no signs of an impact in flight (e.g. from a bird, debris from propeller or
engine) on the right wing leading edge.
It was not possible to determine the rupture sequence from the visual examination of the debris.
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2.2 Aircraft information
The MX2 is a tandem two-seat aircraft designed for aerobatics and entirely built from composite material (principally carbon fibre and epoxy resin). It has been operated since 2005 in the United States as an “Experimental Aircraft”. The maximum load factors indicated by the manufacturer are +/-12 g. The MX2 is equipped with a Lycoming engine(2) providing 310 hp at a speed of 2,700 revolutions per minute. N88MX, 11th aircraft in the series, was equipped with a Whirlwind 400C composite, variable-pitch, three-blade propeller.
N88MX had a special airworthiness certificate(3) issued by the American authority (FAA(4)). On this basis, the DGAC(5) issued a temporary pass for the N88MX on 9 June 2015, for the ferry flights and flights to be performed as part of the World Aerobatic Championships at Châteauroux.
The pilot purchased this aircraft, assembled by MX Aircraft, new in 2009. He was the sole owner and user. He said that he had never reached an acceleration in excess of 10 g with this aircraft.
The aircraft had logged around 280 ferry flight hours and 140 aerobatic flight hours.
The pilot stated that the routine maintenance inspections were carried out on N88MX every 50 flight hours and every six months. No specific maintenance of the airframe or wing was required other than a visual inspection. The pilot specified that these visual inspections were regularly carried out and had never revealed anything.
2.3 Pilot information
The pilot, aged 42 years, held an airline transport pilot license. At the time of the accident, he had logged around 7,000 flight hours. He had been performing aerobatics for approximately 18 years and was taking part in the World Aerobatic Championships for the sixth time. He had logged approximately 140 aerobatic flight hours on the MX2.
2.4 Witness statements
Several witnesses, including experienced aerobatic pilots, reported that when they heard the explosion, they saw the engine and right wing separate from the airframe. Witnesses specified that the pitch of the aircraft was slightly positive when the rupture occurred. It was not possible to determine from all the witness accounts gathered, the order in which the rupture of the wing and the separation of the engine mount from the airframe had occurred.
The pilot said that he had not detected any vibration or forewarning for the in-flight rupture, which had been very sudden. He specified that the aircraft had never been involved in a crash or been damaged.
2.5 Meteorological information
The aviation routine weather report (METAR) at 19:00 for the Châteauroux aerodrome situated approximately 26 NM from the accident site, was CAVOK and a 5 kt wind from 320°.
Several witnesses specified that there was no turbulence.
(2)AEIO540-EXP engine derived from AEIO540
and optimized by a performance tuner
for the competition.
(3)Within the scope of an “Experimental
Aircraft” rating
(4)Federal Aviation Administration
(5)Direction Générale de l’Aviation Civile
(French civil aviation authority)
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BEA2015-0491/November 2018
2.6 Technical Examinations
2.6.1 Wings
In-depth examinations of the wings were carried out at the Nantes CETIM (6) to try and determine the nature and cause of the wing ruptures.
The results from this work showed that the build-up of the carbon fabric plies was compliant with the manufacturer’s specifications. The epoxy resin was correctly cured. The porosity rate (3 to 4%), even if a little high in places (up to 9% locally), was globally consistent with the manufacturing process and above all, did not significantly affect the mechanical compression strength of the spar flanges.
No sign of fatigue was found despite the examination of numerous samples with a SEM(7). The bonding was of good quality. The ruptures observed were characteristic of a rupture from overloading.
2.6.2 Propeller, engine and engine mount
The engine and its mount were entrusted to DGA-EP(8) in order to examine the propeller, and in particular, to try to determine if substantial unbalance could be the cause of the separation of the powerplant assembly.
The results of the examinations established that at the time of the separation, the propeller was in good condition, the three blades were complete and set to the same pitch. Consequently, no abnormal load linked to an unbalance of the propeller was transmitted to the structure.
On the engine mount, the top right housing for the bolt attaching the mount to the fuselage was not deformed whereas the three other housings and in particular, the two lower housings were deformed, attesting that an abnormally high stress had been applied to them. The engine mount was deformed by torsion in the clockwise direction(9) around the longitudinal axis of the aircraft and by a downwards bending movement. The engine mount probably pivoted around the two lower attachment points.
2.6.3 Attachment of firewall and engine mount to airframe
The engine mount was attached to the fuselage via four composite structural support brackets. They were composed of a bracket reinforced by two gussets. The brackets were drilled with a hole to hold a bolted fastener attaching the engine mount to the airframe. The washer of the bolted fastener was in direct contact with the composite bracket. The firewall sealed the front section of the fuselage. The periphery of the firewall was attached to the fuselage by bonding of the flanged-edge over a width of approximately 4 cm. The firewall was also bonded to the four support brackets.
(6)Technical Centre for Engineering Industries
(7)Scanning electron microscope
(8)Direction Générale de l’Armement – Essais
Propulseurs (French Defence Procurement
Agency – Power Plant Test Centre)
(9)View from pilot seat (propeller turns in counterclockwise
direction)
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Figure 3: firewall with four support brackets looking forward
Figure 4: rear view of firewall with left and right lower support brackets
The firewall and the four support brackets were sent to CETIM to try to determine the nature and cause of their rupture.
The examinations found that:
� the upper right support bracket had bonding defects on the firewall. Detailed examinations with a SEM found signs characteristic of fatigue rupture perpendicular to the washer of the bolted fastener. In particular, fatigue striations were observed in the resin at this area. In addition, translaminar fractures were observed in the gusset-to-bracket front coupling area;
� damage observed on the three other support brackets was more diffused (translaminar and interlaminar fractures) and had the characteristics of a rupture from overloading. It was the consequence of the initial failure of the upper right support bracket.
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Figure 5: chiefly adhesive failure of the upper right support bracket - front view
Figure 6: crushing damage from washer of upper right support bracket - rear view
Figure 7: sample from Figure 6 cut up for SEM examination
Figure 8 : fatigue striations observed with SEM on part “a” of Figure 7
Figure 9: fatigue striations observed with SEM on part “b” of Figure 7
The ruptures observed on the peripheral flanged edge at the junction of the firewall with the fuselage showed that the firewall had separated from the fuselage by tearing from the upper right support bracket.
Translaminar fracture areas
b
a
Resin
Fatigue striations
FibresFibres
Resin
Fatigue striations
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BEA2015-0491/November 2018
2.7 Similar failure
A few days after the accident to N88MX, the manufacturer sent a letter to all MX owners informing them of the event. In this letter, they were asked, in particular, to carry out a visual inspection (and, if necessary, a “Tap Test”(10)) of the wing and the four support brackets of the engine mount and to transmit the results of these inspections to the manufacturer. The manufacturer did not transmit these results to the BEA.
Nevertheless, the BEA was informed of the failure of an engine mount support bracket in June 2017 after a change in ownership of the MX Aircraft company. This failure was found during an inspection of a MXS-R (single-seat version). The support bracket had, in particular, bonding defects. Following this finding, in December 2015, MX Aircraft wrote a document setting out a measure for reinforcing the support brackets on this aircraft. The manufacturer recommended this measure without it being compulsory. This incident was not reported to the FAA. This is not a requirement for aircraft in the “experimental aircraft” category.
3 - LESSONS LEARNED AND CONCLUSION
3.1 In-flight rupture sequence
The investigation showed that the upper right support bracket attaching the engine mount to the fuselage had failed during a pull out of around 8 g. This rupture generated excessive loading stresses in the three other support brackets which instantaneously failed under overload, causing the firewall to tear.
The engine and its mount were thus separated from the airframe. Due to the loss of the engine, the aircraft’s centre of gravity instantly shifted beyond the aft limit and, still subject to the action of the elevator to perform the pull out, the aircraft suddenly pitched up. The right wing then broke mid-span under the abnormally high loading.
3.2 Cause of primary rupture
The failure of the upper right support bracket attaching the engine mount to the airframe occurred due to fatigue, even though the aircraft had always been operated in the flight envelope defined by the manufacturer.
The fatigue crack appeared because of a bonding fault, probably linked to the bonding procedure, associated with a non-optimum design of these support brackets. These support brackets are loaded at the bolted fastener in a direction which does not correspond to the best tensile strength of a composite(11). In addition, the washer of the bolted fastener was in direct contact with the composite, crushing the composite in service. This generated local stress concentrations perpendicular to the washer and facilitated the initiation of the cracks.
3.3 Manufacturer safety action
Following the accident to N88MX, the manufacturer determined by calculation that on the MX2 aircraft, it was necessary to replace the washers of the bolted fasteners used to attach the engine mount to the fuselage, the level of associated stresses exceeding the authorized loads for composite. In addition, for aircraft subject to more than 8 g, the manufacturer also determined that it was necessary to install a metal insert between the washer and the composite surface of the rear face of the bracket.
(10)Test to search for separations or cavities in bonded assemblies
(11)The composite is subject to tensile
loading crosswise to the fabric planes.
The transverse mechanical properties
of the composite are significantly less than the properties
