Incident Overview
A Hiller UH-12E rotorcraft suffered a total loss of engine power while in cruise flight. The pilot maneuvered over trees in an attempt to avoid a water landing, but subsequently ran out of rotor RPM and landed hard.
Post-Crash Examination
Examination of the rotorcraft after the crash revealed that the left-hand exhaust stack tube, located inside a cylindrical stainless steel heat muffler, was cracked and separated circumferentially near an old weld. The left-side muffler was dedicated exclusively to supplying engine-heated air for carburetor heat application. The exhaust stack tube was observed to be thin and heat distorted, with the circumferential split positioned in the vicinity of an old weld.
This split in the tube allowed hot engine exhaust to duct directly to the carburetor via the carburetor heat plenum. Additionally, the carburetor heat valve within the plenum leading to the carburetor was found to be out of rig. This condition meant that even when carburetor heat was fully de-selected in the cockpit (FULL COLD), engine-heated air was still partially flowing to the carburetor intake.
Maintenance History
A 100-hour inspection had been completed and signed off by company maintenance personnel 19.8 hours before the accident. According to Appendix D to Part 43 of the Federal Aviation Regulations, each person performing an annual or 100-hour inspection is required to inspect applicable components of the engine and nacelle group, including exhaust stacks, for cracks, defects, and improper attachment.