By Patrick Veillette jumprsaway@aol.com

Christmas Eve, 2009 in Decatur, Texas, was marked by low visibility with blowing snow. The flight crew of a Bell 407 helicopter hunkered down in the hospital's crew room while waiting for the miserable weather to subside, which it finally did. At that point the dispatcher called with a release for a cross-country positioning flight. The crew climbed aboard, strapped in and the pilot cranked up the Rolls-Royce 250 turbine.

As the helicopter ascended to a height of about 50 to 60 ft., the pilot heard two warning horns sound, followed by the Bell yawing 90 deg. to the left. The pilot immediately lowered the collective in an effort to preserve rotor rpm and maneuvered the helicopter back over the helipad. When about 5 to 8 ft. over the pavement, the pilot increased collective pitch, but the helicopter continued its rapid descent and skids hit the surface so hard that two of the three people aboard were seriously injured.

According to the NTSB's official accident report, the helicopter had been parked outside for approximately 5 hr. in blowing snow conditions without any plugs or covers over the engine inlets or the exhaust. The plugs and covers were later installed and the helicopter remained outside in temperatures ranging from 16F to 34F for the next 19 hr. A video surveillance camera at the scene revealed that at no time did anybody look at the exhaust stack, right-side engine intake, or open any access panels prior to the operation of the helicopter.

An examination of the recorded Engine Control Unit (ECU) data revealed that the helicopter's powerplant experienced a momentary flameout followed by a successful relight. However, due to the helicopter's close proximity to the ground, the pilot was unable to recover main rotor rpm before ground impact. As part of the post-accident examination, the engine was removed from the airframe and an engine run was performed. There were no anomalies found with either the engine or airframe that would have contributed to the loss of engine power.

The operator's procedures manual stated that a pilot must ensure that all ice, snow and frost is removed from the engine inlet area prior to flight. The helicopter manufacturer's flight manual stated that as part of the preflight inspection, the pilot must check that the engine inlet area is cleaned of all debris, accumulated snow and ice. The engine manufacturer's operation and maintenance manual included a precaution that the accumulation of snow or ice may result in the engine experiencing a flameout.

The NTSB determined the probable causes of this accident were the pilot's inadequate preflight inspection and the momentary loss of engine power due to snow or ice ingestion.

It's a given that gas turbine engines operate best when the air is flowing smoothly through the engine. In reality, the airflow within a compressor is quite complex and tends to be especially unsteady near the compressor blade tips. Typical flow phenomena in rotating machinery can include vortices, separations, secondary flows, shock and boundary layer interactions, and turbulent wakes.