What happened
During an experimental flight test intended to evaluate fuel and hydraulic systems, a modified DHC-4 Caribou entered a steep, uncommanded climb shortly after takeoff. The aircraft, which had recently undergone a conversion from piston to turboprop engines, initially climbed normally but began a gradual, steepening pitch-up at approximately 35 feet above ground level. As the aircraft ascended, it entered a slow right-hand roll that progressed into a steep, nose-down, right-wing-low attitude. At roughly 200 feet above ground level, the aircraft struck the ground in a near-vertical dive. The impact caused the onboard fuel to ignite, destroying much of the wreckage. All 3 crew members on board were fatally injured.
The investigation
Investigators examined the aircraft's flight control system, engines, and the wreckage of the gust lock mechanism. Analysis of onboard video and still photography revealed that while the aircraft had elevator authority to pitch up at rotation, the control surfaces remained in a neutral position during the subsequent dive. Notably, the elevator spring tabs were observed deflecting upward without corresponding movement of the actual elevator surfaces.
Examination of the gust lock hardware showed that the rudder locking mechanism was in the engaged position at impact, and damage to the aileron control quadrant suggested the aileron locks were also engaged. Furthermore, the investigation found that the engine fuel flow readings at the moment of impact were consistent with a steady takeoff power setting, suggesting the crew was not adjusting throttles during the maneuver. A knob from the gust-lock handle was even found embedded in the captain's wrist, indicating an attempt to manipulate the controls during the descent.
Findings
- The gust lock system was not fully disengaged prior to the flight.
- One or more locking pins re-engaged after the aircraft became airborne.
- A required six-point control check was likely omitted before takeoff.
- The partially engaged state of the locks allowed for nose-up elevator authority but prevented the nose-down movement necessary to recover from the climb.
- The rudder and aileron locks were effectively engaged, preventing the crew from countering the roll.