NYC98LA076

The helicopter was under the control of a student pilot. Departing from a hover, and while climbing through an altitude of 50 to 60 feet above the ground, the pilot heard a loud bang and a high pitched whine. The pilot performed an autorotation. He reported that he touched down hard and started sliding on the grass. The nose pitched down, and the main rotor blades contacted the tail boom. Examination of the helicopter revealed a blade had separated from the engine-cooling fan and penetrated the shroud. The fan blades were held to the fan disk by 1 bolt and 2 rivets. Evidence of corrosion was found on the fan disk and the failed rivets.

On March 12, 1998, about 1625 eastern standard time, an Enstrom F-28A, N4884, was substantially damaged during a precautionary landing at the Blue Ash Airport (ISZ), Blue Ash, Ohio. The student pilot was not injured. Visual meteorological conditions prevailed for the local solo training flight. No flight plan had been filed for the flight that departed from Blue Ash about 1545, and was conducted under 14 CFR Part 91.

In the NTSB Pilot/Operator Aircraft Accident Report, the pilot stated:

"...We flew dual pattern work of .9 tenth of an hour. I landed at Blue Ash Airport and let...[flight instructor] out. I was instructed to make several solo pattern work and landings. I flew the [pattern] once and came in to land and stopped in a hover...[The flight instructor] motioned for me to continue with his hands. I started forward...[to] gain altitude. At approx. 1625 I was 3/4 of the way down the east-west taxiway at roughly 50-60 feet in the air when I heard and felt a bang. Instantly I heard a high pitch whine sound and the helicopter began yawing to the left. I began to try and get the helicopter safely to the ground. I pulled up on the collective but there was no response or power. I landed hard and was thrown forward in my seat. The helicopter lunged forward and stopped. At this time I push back against the seat. Loud noises of metal ripping apart was heard all around me. I locked the collective down and shut the master switch off."

An inspector from the Federal Aviation Administration (FAA) reported that following the loud bang, the flight instructor observed the helicopter yaw nose left, recover heading, and land. The helicopter's nose pitched down while the helicopter was sliding on the grass and the main rotor blades contacted the tail boom. The aft portion of the tail boom was severed, and the helicopter came to rest upright.

Examination of the helicopter by the FAA Inspector revealed that a blade had separated from the cooling fan, and penetrated the surrounding shroud. No other evidence of a mechanical failure was found.

The failed blade and fan disk were examined in the NTSB Metallurgical laboratory in Washington, DC. There were 19 blades on the disk, and each blade was held in place by two rivets and one bolt. According to the metallurgists factual report, the fracture face of the leading edge rivet was damaged and the fracture features were not identified. Heavy accumulations of oxide and other deposits were found on the mating halves of the rivet. The bolt had features consistent with fatigue cracking that initiated from multiple origins around the majority of the circumference. The trailing edge rivet had failed in tensile overload.

The helicopter had previously been involved in a water submersion. A check of records by the FAA inspector failed to find any records that could indicate if the fan disk involved in the accident was installed on the helicopter at the time of the water submersion.

According to an accident investigator from Enstrom Helicopters, the main rotor blades will contact the tail boom under conditions of low rotor RPM, or high "G" vertical loading.