History of the Flight
The pilot, a private licensee with 113 total flying hours (30 on type), hired the helicopter for a local flight from Sywell Aerodrome with three passengers, culminating in a landing at a private site in Flecknoe, near Daventry. Prior to the flight, he completed a 28-day check flight with an instructor covering normal and abnormal procedures, including autorotations. Weather was CAVOK with surface wind at Sywell from 030° at 6 kt.
During the initial engine start, after depressing the starter button and opening the throttle as N1 reached 15%, the engine accelerated normally. However, as N1 reached 50%, the engine began to run down. The pilot maintained starter engagement until N1 reduced to approximately 20%, then shut down. Having never experienced this, he pulled the starter circuit breaker and sought assistance from a more experienced commercial pilot, who observed a second start that was normal.
After a normal departure, the pilot climbed to 1,500 ft and flew locally for about 20 minutes. En route to Flecknoe, the low voltage light blinked twice; the pilot depressed the light to test it, and after blinking three times it extinguished. The light did not illuminate again during the flight. Upon reaching Flecknoe, fuel quantity was 50 USG, and engine temperatures and pressures were normal. The pilot conducted a low pass to check for wires, then approached into wind, reducing speed to 70 KIAS and lowering the collective to establish a rate of descent.
At about 100 ft, the pilot heard an intermittent audio warning tone followed by a continuous tone. He noted the ROT LOW RPM warning light illuminated on the Central Warning Panel (CWP) and immediately entered autorotation. At about 20 ft, he flared and raised the collective aggressively. A loud bang and violent shuddering occurred, coinciding with the large control inputs. The pilot lowered the collective and allowed the aircraft to run along the grass. Smoke appeared to his left, and he evacuated the passengers and shut down the helicopter.
Aircraft and Warning Systems
The Bell 206B Jet Ranger III, manufactured in 1981, was powered by a single Allison 250-C20B turbine engine. The ‘Engine Out’ warning system activates when N1 drops below 55%, producing an intermittent audio signal and an illuminated ENG OUT caption on the CWP. The ‘Rotor Low RPM’ audio warning activates when the collective pitch lever is off the down stop and rotor RPM is less than 90%, producing a steady audio signal and an illuminated ROT LOW RPM caption.
Post-Accident Examination
After the accident, the tail pylon was found completely severed immediately aft of the horizontal stabiliser by the main rotor blades. No fire was observed. The pilot notified the operator, and a staff member arrived in another helicopter. Witnesses reported that the helicopter destabilised and the rotors coned upward during the descent, likely when the pilot entered autorotation.
The engine was removed and sent to an approved overhaul facility for inspection and bench testing. Engine acceleration and deceleration checks were accomplished satisfactorily, with parameters within specified limits. No hesitation during starts or power transients, no magnetic chip light indications, and nil oil consumption were noted. The engine did not smoke during operation, and the exhaust collector was dry. The test data indicated the engine met manufacturer’s minimum specification requirements. No technical reason for a loss of power was identified.
Analysis from the Bulletin
The pilot did not recall seeing the ENG OUT caption illuminated. His prompt collective lever lowering would have minimised rotor RPM decay but initiated a high rate of descent at low height. The aggressive flare and collective reduction of descent rate ensured a safe run-on landing, avoiding heavy landing hazards, but the large control movements caused the main rotor blades to sever the tail pylon. The engine was still running on the ground and had to be shut down. The engineering investigation revealed no reason for a power loss, but the pilot had warning symptoms and performed an emergency landing.