What happened
The aircraft was en route to a nearby forest fire when the pilot observed that the takeoff roll had been longer than usual, an issue attributed to high temperatures. During flight, the pilot noted lower propeller RPM and manifold pressure, as well as a descent rate of 50 to 100 feet per minute. The pilot attempted to use the emergency jettison handle to release the retardant load, but several attempts were unsuccessful. While the pilot's attention was focused on attempting to jettison the load, the aircraft collided with terrain.
The investigation
Examination of the accident site revealed that the slurry mixture had a very thick consistency. The pilot had mixed and loaded the Fire-Trol slurry, which contains ammonium phosphate, a clay thickener, corrosion inhibitor, and colorant, into the aircraft's 400-gallon hopper. A sample taken from the sealed pump hose was tested and identified as LCA-R, which is concentrated retardant that has not been mixed with water. The sample weighed 12.2 pounds per gallon, whereas pure concentrate weighs 12.1 pounds per gallon and a properly mixed slurry should weigh 9.13 pounds per gallon. While a properly mixed load should weigh 3,652 pounds, it was computed that the payload in the hopper at the time of the accident weighed 4,880 pounds, representing an excess of 1,228 pounds.
Findings
- The pilot failed to disengage the emergency release handle-locking lever prior to takeoff as required by the Pilot Operating Handbook.
- The pilot did not arm the hydraulic power for the retardant gate prior to takeoff.
- The aircraft was carrying an overweight load due to the use of concentrated retardant without sufficient water dilution.