Report / May 18, 2007 / Project number: CT114159 - A Category
Location: Malmstrom Air Force Base, Montana
Status: Investigation Complete
The Flight Safety Investigation Report (FSIR) is available from DFS.
The accident occurred during a 431 (AD) Squadron (Snowbirds) practice airshow at Malmstrom Air Force Base (near Great Falls, MT) during a manoeuvre known as the "Inverted Photo Pass" in which Snowbird (SB) Lead flies across the show line upright with SB2, SB3 and SB4 flying inverted in formation on Lead. Shortly after SB2 rolled inverted the aircraft was seen to briefly oscillate in pitch and roll and then begin a negative 'g' climbing roll to the left and move away from the formation. The aircraft reached a maximum altitude of approximately 750 feet above ground level and continued to pitch down. The aircraft struck the ground in a steep nose down wings level attitude and was completely destroyed. The pilot did not eject and was killed on impact.
The investigation found that the pilot's lap belt had come open when he pushed negative 'g', causing him to come out of the seat and be thrown against the canopy. The survival seat pack also became dislodged and interfered with the control stick, preventing the pilot from pulling the stick back to pitch the nose up and arrest the descent. The pilot's lap belt came open because it was in an insidious condition known as "false-lock", caused when the metal clevis on the parachute arming key (see photo above) interferes with the proper closing of the buckle.
This design deficiency first became apparent in January 2002 when, in a similar occurrence, a Snowbird pilot came out of his seat under negative 'g'. In that incident the pilot was able to safely recover the aircraft.
Immediately following the 2002 occurrence, a Record of Airworthiness Risk Management (RARM) was generated that identified the unmitigated risk as HIGH and recommended several measures to reduce the risk, including a re-design of the parachute arming key. A new key design was quickly completed, but, due to a series of delays, it took over three years to complete the operational test and evaluation. Additional delays were incurred in passing the results of the operational test and evaluation to the technical airworthiness staff responsible for implementing the new key into the Tutor fleet. Over this prolonged time period both the aircrew and the airworthiness staffs lost sight of the original risk, resulting in a gradual erosion in the sense of urgency to implement the modification . A lack of false-lock occurrence reporting also contributed to an underestimation of the seriousness of the problem.
Functional checks were introduced in 2002 to ensure the lap belts were not in a false lock condition and the Aircraft Operating Instructions were amended accordingly. Again, over time, the users had become complacent towards the checks and had lost sight of their importance so that by the time of the accident a thorough and standardized briefing was no longer in place. W hile a definitive test for false-lock, the functional checks rely on effective training to ensure the pilots know the proper checks, actually perform them and perform them correctly.
Following the accident, the modified key was finally introduced, numerous changes were made to CF airworthiness processes and aircrew were reminded to report all aircrew life support equipment anomalies.