Crown Copyright 2013
AAIB Report Into The Loss Of L4 Cub At Eilson AFB Alaska. PreambleThis report is intended to establish the causal factors of the aforementioned incident using third party eyewitness accounts, third party photographic evidence, third party examination of the post incident debris and expert witness testimony.
Background The incident occurred at Eilson AFB on the 27th of March 2013. The Handling pilot at the time of the incident was Ianhunt and the aircraft involved was a model L4 Cub with a 1200mm wingspan.
The airframe suffered extensive damage to the fuselage, power section, motor section and empennage during the incident.
Weather ConsiderationsIt is understood that the weather at the time of the incident would be dry with good visibility and wind speeds of less than 15mph.
It is unlikely that weather conditions would be a contributory factor in this incident.
Human FactorsIt has been established the prior to the incident the handling pilot had flown this aircraft several times during the last month, has the appropriate ratings and experience for the aircraft and is considered fully current with regard to the aerial flight of a L4 Cub.
The handling pilot had not consumed alcohol for several hours prior to flight and was free from any serious medical conditions that would affect the handling of the flight.
Deliberate and malevolent acts by the handling pilot can be ruled out due to the consequence of such an incident requiring substantial financial input by the pilot in order to rectify matters.
Eye Witness ReportsA third party witnessed the aircraft manoeuvring in the airspace surrounding Eilson AFB for several minutes. During the flight the aircraft was seen to be conducting several high speed and high G manoeuvres. During one of these manoeuvres the aircraft departed from controlled flight and entered into a steep near vertical spin which was unrecoverable before contact with terrain.
Examination of Third Party Photographic Evidence and Crash Dynamics
Photograph number one shows a collection of broken wing securing bands. It is unclear whether these were recovered from the site of the crash and still attached to the aircraft or were found some distance away. It therefore cannot be established without doubt whether these components failed in flight or were destroyed by extreme stress loadings during the crash.
Photograph number two shows the power section, propeller, cowl, spinner and the detached front portion of the fuselage.
The propeller is of a two blade type and appear to be an owner modification, the spinner is also modified as also is the motor. The effect on weight and balance of these modifications cannot be established at the time of writing, however the it has been established that the person responsible for these modifications has had some experience in these matters and presumably checked this prior to the first test flight.
The propeller is missing one blade which has broken off at the blade root roughly perpendicular to the hub and the other blade is intact, has no apparent scratches, nicks or gouges on the leading edge and this damage is entirely consistent with heavy impact damage while the motor is producing little or no power.
The spinner is displaying some impact damage on the tip, slightly off centre without signs of a high rotational speed at the time of the incident which would be indicated by circular scratches around the rotational axis of the shaft.
It can be concluded that the motor therefore was not under any significant power loading at the time of impact.
Examination of the cowl which shows the heaviest damage to the port underside, along with the evidence provided by the rotating assemblies indicates that the aircraft struck the ground at an almost vertical angle in a left handed bank consistent with a downward spiralling rotational spin to the left.
This corresponds with eyewitness testimony, although the direction of spin was never alluded to.
Photograph three shows the battery box, battery and landing gear strut assembly which became detached from the main fuselage during the crash but was recovered in the close vicinity of the main wreckage.
The battery appears to be a stock Hobbyzone 3 cell type fitted with a stock EC3 connector. The EC3 connector halves fit very tightly together and can handle currents up to 60A at 12v.
Since the connector does not appear to have suffered heat damage it is unlikely that the current rating has been exceeded or the long term integrity of the connection thrown into doubt.
It is not known whether the connection was intact or not post crash.
It is however likely that due to the vertical component of the crash the lateral energy vector was limited in the initial impact and the connector remained connected.
The undercarriage appears to be non standard and mounted directly to the battery box fixing plate in a non standard position.
Closer examination reveals that the battery box plate / fuselage locating pegs had not been glued into position and relied on the screws only for attachment to the fuselage.
The wheels are non standard and appear to be larger rubber tyred wheels rather than the smaller, standard fitment foam wheels. This arrangement would very likely carry a weight penalty.
The fuselage has broken in half at this point with a clear break that does not appear to have a great deal of compression damage.
The weight of the gear and the length of the struts would impart a large inertial coupling between the wheels and the battery mounting plate.
This inertia moment would twist the plate from its mounting screws in the initial impact, breaking the lower fuselage mounting flange foam in the process and severely weakening the lateral strength of the airframe at this point.
Photograph number five shows paint missing from the upper starboard wingtip consistent with the airframe having suffered a heavy initial impact, then using the kinetic energy not expended in deforming the airframe to cartwheel the aircraft along the starboard axis, thus breaking the already weakened fuselage in half at the battery box, allowing it to break free of the fuselage completely.
Photograph Five:
Expert Witness TestimonyReynard 80 offered the opinion:
"I have often wondered how the cold up there in the frozen North affects plane parts.
The fact that a snapped band caused your crash reminds me that some people keep their SC fully assembled at all times, with the wings attached. Being fearful that this constant tension will weaken the elastic bands, so that they might fail in flight, I always remove the bands and wings before putting my plane to bed after a flying session.
Laughing at this practice, a friend once remarked, "I never remove my bands. So what if one does snap? They are cheap enough to replace." I reckon your unfortunate experience is the answer to that!".
Flydiver added:
There certainly are quality differences between bands also. I don't know why but they just seem to disintegrate in my toolbox in spite (or because?) of keeping them in Ziploc. I don't keep any CA or other things that would likely have a nasty off gassing in there.
"I have learned to always give them a stretch to test for rebound, cracking, and feel the texture for degradation. When in doubt....don't."
ConclusionsIn the absence of any further evidence and the limited nature of the evidence available, this board agrees that the aircraft departed controlled flight due to the catastrophic failure of the airframe during a high G manoeuvre.
Since the motor was in a low / no power mode at the time of the crash it can be construed that either the pilot shut down the motor realising a crash was inevitable and thus hoping to save airframe damage or as a speed limiting measure during a high G manoeuvre such as the back side of a loop.
This board finds that the handling pilot acted correctly immediately prior to the incident and the aircraft would not be recoverable with such damage and is therefore found to be free of any negligence.
The maintenance log for this aircraft is not available for inspection although it can be noted that he airframe has been painted and this could possibly have being injurious to the integrity of the bands, along with other factors considered by the expert witnesses such as cold, storage, age, fumes ect.
RecommendationsThis board recommends that rubber band wing attachments should be considered lifed items with an age limit of no more than 6 months where such bands should be discarded and new items procured.
These bands should be inspected closely before flight for cracking, fraying, drying out or losing elasticity.
The board further recommends that the abysmal stock Cub wing struts be modified in such a manner that they are no longer loose fitting and take a share of the load during normal flight.
;D ;D
