Control Issues Doomed Recovery Options

The UK's AAIB has published a final report on the tragic accident that claimed the life of IAC President and US Aerobatic Team member, Vicki Cruse. The 22 August 2009 accident occurred as part of the 2009 World Aerobatic Championships being flown over Silverstone Circuit, Towcester, Northamptonshire. Vicki was flying a leased Zivko Aeronautics Inc Edge 540, N540BW (standard practice for foreign competition due to the complexities and expense of transporting US Team aircraft overseas).

The AAIB summarized their findings by noting that, "The aircraft was being flown in an aerobatic competition during which it failed to recover from a downwards snap roll manoeuvre, initiated at about 2,300 ft agl. The aircraft continued to rotate until it struck the ground, fatally injuring the pilot. The investigation discovered a mechanism whereby rudder pedal extensions used by the pilot could have contributed to a rudder control restriction, but pilot incapacitation was also considered a possible contributory factor."

More specific detail note that, "The accident occurred during the fifth manoeuvre of the sequence, which was the last manoeuvre the pilot had practised minutes beforehand. The manoeuvre called for a pull up to the vertical, followed by half of an eight point roll (rolling through 180°). The aircraft was then to be ‘pushed over’ the top of the manoeuvre until it was pointing vertically nose down, before carrying out a positive snap roll to the left through 1¼ turns and pulling out to the horizontal. The aircraft appeared to perform the initial stages of the manoeuvre normally but it did not recover from the downwards 1¼ snap roll. Instead, the aircraft continued to rotate in a nose-low attitude until it struck the ground."

The AAIB also reports that, "...the aircraft had struck the ground in a nose-down, left wing-low attitude with significant rotation..."

AAIB noted that the control system was pretty much intact at impact but spent considerable time looking at the rudder and rudder pedal control assemblies, noting that Vicki had reported previously that she had experienced a problem 'whilst carrying out a snap roll during practice. During the manoeuvre her right foot had bent the right foot plate downwards, deforming it. The footplate was straightened after the flight. The team engineer prepared a pair of longer, stiffer footplates but these had not been fitted at the time of the accident.' Additionally, "According to information posted by the pilot on her internet site, another pilot had also bent the footplate, whilst getting into the aircraft, and had bent it back."

Edge 540 -- File Photo

AAIB Investigators looked at a number of failure modes based on these concerns as well as observations of video and witness reports regarding the last few seconds of the flight... in which it became clear that the proper corrective control inputs were either not being applied or could not be applied. AAIB personnel used video of the same maneuver performed by Vicki in that airplane during practice against the fatal sequence that occurred a short time later. The AAIB concluded that "...a rudder control restriction could occur if the lower attachment of the rudder pedal extension had moved outboard and the pedal in question, having been pushed forward, was subsequently moved aft towards the neutral position. The range of rudder positions in which a restriction could occur on the left pedal assembly, with the extension fitted, was estimated to be between 6° and 23° left rudder deflection. Similar values were estimated for a restriction on the right rudder assembly. It was concluded that, if such a restriction occurred, it was unlikely that the force a pilot could apply on the pedal extension would be sufficient to overcome the restriction. The way to free the pedals would be to shift the lower extension attachment point inboard."

The AAIB's analysis of the video evidence stated that examination revealed "three anomalies at the expected point of recovery; the inappropriate rudder position, the unusual aircraft pitch attitude (compared to the practice case) and the pilot’s head position, which would be expected to have been central in the cockpit at that point.
As the manoeuvre continued, there were no apparent control inputs made which could be regarded as being part of a positive recovery attempt. Instead, the rudder remained displaced in the direction of roll, driving the rotation. Although aileron deflection did return to near zero for much of the latter part of the descent, it was at times near full deflection in the early stages, again providing a strong driving force for the rotation. There was almost no aileron movement to oppose the rotation.

The rudder position is the most significant anomaly. Either the pilot reacted correctly to the situation but was physically prevented from removing the pro-rotation rudder, or her ability to recognize and/or react to the situation had become impaired.

Edge 540 -- File Photo

The examination of the rudder pedal assemblies revealed that a restriction could occur with this set of extensions fitted. Moreover, such was the design of the extensions that they could readily move outboard with normal operation (a requirement for a restriction to occur). This is corroborated by the wear marks on the outboard ends of the pivot tubes and the witness marks made by the pip-pins on the pivot tubes. Importantly, the witness marks on the base plate of the left rudder pedal were consistent with having been made by the pip-pin. This physical evidence suggested that a jam had occurred at some stage, with the left rudder pedal forward and moving towards neutral.

It was not possible to say when this may have occurred, but there was no report by the pilot of a rudder problem prior to the accident flight. It was not possible, either to say why the restriction, if it did occur on the accident flight, did not occur during the same and similar manoeuvres earlier in the flight.

Had rudder control been affected in such a way, it would account for the continued application of pro-rotation rudder, and perhaps also for the varying amounts and rate of rudder input as the pilot tried to free the unknown restriction. It could also account for a significant distraction during the recovery phase, leading to insufficient forward control column and hence the relatively high pitch attitude.

There was some evidence to suggest that the pilot may have experienced a problem similar to that experienced by another Edge 540 pilot in which his foot became trapped during a snap roll. The accident pilot was not using foot straps and it is known that she had recently had a problem when her right foot became significantly separated from the rudder pedal during a snap roll recovery. However, given that the other pilot quite quickly recognized and corrected the situation, and considering the accident pilot’s extensive aerobatic experience, it would be expected that she would recognize and recover from such a situation before it became critical."

The AAIB's conclusion noted that, "The aircraft did not recover from a downwards snap roll. No recovery action was seen to be taken and the aircraft struck the ground after several rotations. Pro-rotation rudder was applied throughout and pro-rotation aileron applied for part of the descent. A mechanism was identified by which the pilot’s rudder pedal extensions could have caused a restriction of the left pedal in such a way that left rudder could not be fully removed once it had been applied. The circumstances of the accident suggested that the pilot’s ability to recognize or respond to the situation had somehow become impaired, and this must be considered as a possible contributory factor."

Vicki

ANN E-I-C Note: A year later, the feelings are still about as raw as they were in August of 2009... but the detail and research undertaken by AAIB is impressive in both scope as well as the methodology. I can't help but express my appreciation and admiration for their efforts in making sure that the loss of this amazing pilot was given the careful attention she deserved.

FMI: www.aaib.gov.uk