History of Flight
On March 2, 2011, at 1756 central standard time, a Bell 206L4, registration N154MW, operated by Omniflight Helicopters Incorporated, performed a forced landing within the city limits of Birmingham, Alabama. The helicopter had departed from Carraway Medical Center Heliport (AL49) at about 1741 for a local maintenance test flight. Day visual meteorological conditions prevailed, and a company visual flight rules flight plan was filed. The certificated airline transport pilot, the sole occupant, was not injured. The helicopter sustained substantial damage to the tail boom section just aft of the cabin. The flight was conducted under 14 Code of Federal Regulations Part 91.
According to the pilot, the engine governor had been replaced earlier that day, and the test flight aimed to confirm proper inflight settings. Before departure, maintenance personnel adjusted the linear actuator to the correct ground setting. During the flight, the pilot made several collective adjustments and noted some lag in the N2 (engine core speed) gauge readings, though the rpm returned to the selected value with collective adjustment. While turning from downwind to base leg at about 500 feet above ground level and 40 to 50 knots, the pilot reported hearing a "loud bang and the airframe lurched." He entered an autorotation and located a vacant parking garage deck within gliding range. The engine-out audio sounded and segment lights illuminated. After landing, the helicopter skidded approximately 15 feet. The pilot performed the emergency shutdown procedure, but the engine had already stopped.
Personnel Information
The pilot held an airline transport pilot certificate with ratings for multiengine airplane and helicopter, a commercial pilot certificate for airplane single-engine land, and a flight instructor certificate with ratings for rotorcraft helicopter and instrument helicopter. He reported 10,599 total flight hours, including 1,684 hours in the accident helicopter make and model, all as pilot in command, and 360 hours as a flight instructor in that make and model.
Aircraft Information
The helicopter's most recent inspection occurred on February 8, 2011, under the operator's Approved Airworthiness Inspection Program. At the time of the accident, the helicopter had 4,613 total hours time in service. It was equipped with an Allison 250-C30P engine with 8,785 total hours, 19,028 cycles, and 138 hours since its most recent inspection.
Maintenance logbook entries within the two weeks before the accident included: on March 2, 2011, the power turbine governor was removed and an overhaul serviceable governor installed, with all rigging adjustments checked and no leaks or defects noted; on February 24, 2011, a fuel control outlet line leak was repaired by retorquing the nut, with no other issues; and on February 20, 2011, the fuel control unit was replaced per the Rolls Royce maintenance manual, followed by a test flight with no further defects.
Meteorological Information
The 1753 recorded weather observation at Birmingham Regional Airport (BHM) included wind from 110 degrees at 3 knots, visibility 10 miles, clear skies, temperature 20°C, dew point 3°C, and barometric altimeter 30.21 inches of mercury.
Wreckage and Impact Information
According to an FAA inspector, the helicopter came to rest on the top level of a parking garage. The tail boom separated aft of the cabin, and the tail rotor impacted the right rear door. External examination of the engine revealed no abnormalities; the power turbine governor had been replaced before the flight. No eyewitnesses were located, and local authorities were unaware of the accident until called to help secure the helicopter overnight.
A follow-on examination on March 29, 2011, by the NTSB included cursory external and internal checks of the airframe and engine, with no component failure noted. Hoses, fittings, and lines showed no damage or scrape marks, though several lines were in contact with each other. N2 continuity to the main and tail rotor output drive shaft was confirmed by hand rotation, and N1 continuity to the starter generator was confirmed, with smooth rotation. The throttle was found closed and operated by hand.
The airframe fuel filter bowl was removed; it was full of fuel and free of debris. The filter element was separated and clean. The fuel nozzle line fitting at the combustor case released fuel, and the line contained moisture consistent with fuel. Fuel was present in the fuel pump line.
Collective movement revealed the power turbine governor range was 40 to 61 units. With electrical power applied, collective movement via beeper adjustment showed a range of 39 to 70 units. The droop compensator measured 10.625 inches in length at the linear actuator, with about 0.125 inches of adjustment on the rod end, correlating to a lower rpm setting.
Approximately 10 gallons of fuel were added to the main tank, and fuel pump pressure tested at about 8 psi on the cockpit gauge, at the bottom of the green operating range. The Turbine Outlet Temperature gauge showed no over-temperature indication. The freewheeling unit exhibited wear step marks on drive side splines and chatter damage on coast side splines, but operated normally when checked by hand.
The engine was removed and mounted in a test cell. A pneumatic leak check revealed a leak at the Pc line between the air filter and power turbine governor at the aft connection B-nut, with no torque stripe observed. After starting, the engine operated below normal specifications. During deceleration and cool-down, operation appeared normal. Post-run shutdown required no hydro braking, and N2 shutdown seemed quicker than normal. The engine was restarted to simulate deceleration then rapid acceleration; the first two attempts failed, but the third succeeded with normal response.
The N1 Pc line was sent to the NTSB Materials Laboratory, where examination showed superficial longitudinal tool marks that did not extend to interior surfaces. One flared end exhibited localized fretting damage. A Zyglo dye penetrant test revealed no cracks.
The power turbine governor was sent to the manufacturer for testing. External inspection found soot/carbon at the air regulator outlet. Drive shaft movement was normal, and the Pg orifice was clear. The Pr diaphragm and spring appeared normal. Teflon shavings were observed at the bottom of the post in the inner diameter of the bushing. The Teflon sleeve on the drive shaft had a longitudinal split along its total length. After replacing the sleeve, the governor was dynamically tested under elevated temperatures and operated normally, with no condition preventing normal operation.