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On February 5, 2003, at 0815 mountain standard time, a Cobb Rotorway Exec 90, N9876D, made a hard landing after experiencing a loss of tail rotor authority at the Phoenix Regional Airport, Casa Grande, Arizona. Cobb International operated the helicopter under the provisions of 14 CFR Part 91. The helicopter was substantially damaged. The certified flight instructor (CFI) and the private pilot undergoing rotorcraft category training received minor injuries. Visual meteorological conditions prevailed for the local area instructional flight, and no flight plan had been filed. The flight originated from Stellar Airpark, Chandler, Arizona, about 0750.
According to the CFI, the purpose of the flight was to practice pattern work and autorotations. The CFI stated that the student had done approximately four landing approaches. During the accident approach they had made the base turn and had leveled off when they heard a "bang" in the aft section of the helicopter. The CFI took the flight controls, and initiated a turn to an open area for an emergency landing. During the autorotation, he raised the collective to check for controllability. The helicopter yawed to the left, at which point the CFI lowered the collective and continued with the autorotation. As he pulled collective to "cushion the landing" the helicopter yawed to the left, landed firmly on the ground, and rolled over on to its right side.
According to a Federal Aviation Administration (FAA) inspector who examined the helicopter, the secondary drive shaft appeared to have sheared about midspan.
Helicopter main rotor and tail rotor drive system:
The engine and secondary drive shaft are mounted vertically and parallel to each other behind the cockpit. The engine drives four v-belts, which in turn drive the secondary drive shaft. The secondary drive shaft drives the main rotor via a chain drive on its upper end. Attached to the lower end is an engine cooling fan and a pulley that drives a series of three v-belts. The v-belts extend to two pulleys mounted in series along the interior length of the tail boom; the last belt drives the tail rotor. Two bearings, one above and one below the main drive pulleys, support the secondary drive shaft. A lock collar is attached to the shaft directly below the lower bearing. The drive shaft extends below the lower bearing where the tail rotor pulley and the engine cooling fan attach. The secondary drive shaft fractured between the lock collar and the inner race of the lower bearing.
Examination of the helicopter's maintenance records revealed that the airframe had 2,937 operational hours, and the secondary drive shaft had 561 hours of service. The secondary drive shaft had been visually inspected as part of the 100-hour scheduled servicing. At 2,697 airframe hours the helicopter was in a rollover mishap. The secondary drive shaft (315 service hours) was removed and inspected during the ensuing helicopter repair activity. The logbook also recorded that this airframe had been in involved in two tail rotor strikes, which affected this secondary drive shaft; the first at 2,678 airframe hours and the second at 2,804 airframe hours.
Tail rotor failure procedures:
The FAA Rotorcraft Flying Handbook (FAA-H-8083-21) discusses general emergency procedures appropriate to an antitorque system failure. "If a tail rotor failure occurs, power has to be reduced in order to reduce main rotor torque. The techniques differ depending on whether the helicopter is in flight or in a hover, but will ultimately require an autorotation. If a complete tail rotor failure occurs while hovering, enter a hovering autorotation by rolling off the throttle. If the failure occurs in forward flight, enter a normal autorotation by lowering the collective and rolling off the throttle. If the helicopter has enough forward airspeed (close to cruising speed) when the failure occurs, and depending on the helicopter design, the vertical stabilizer may provide enough directional control to allow you to maneuver the helicopter to a more desirable landing site."
The Rotorway Exec 90 Flight Manual states four steps for a "Tail rotor failure during forward flight."
"K. Tail rotor failure during forward flight:
1. Failure is usually associated by right or left yaw which can not be corrected by applying pedal.
2. Immediately enter a shallow descent into the wind.
3. Adjust the collective and the throttle to extend the glide if sideslip is not excessive and the aircraft does not tend to spiral. Cyclic and collective are used to limit sideslip angle.
4. Select landing site and perform a run-on landing using throttle to maintain heading."
There are no steps or discussions in the manual that addresses what to do if the sideslip is excessive or the helicopter tends to spiral.
TESTS & RESEARCH:
Secondary drive shaft:
The secondary drive shaft has been under scrutiny by the manufacturer. Before February 2000, the secondary drive shaft was on a 100-hour inspection interval and a 1000-hour recommended change out schedule. After February 2000, the manufacturer modified the recommended maintenance to a 100-hour inspection interval and a 500-hour recommended change out schedule. This was a result of previous secondary drive shaft concerns associated with the upper portion of the shaft. The 100-hour inspection procedure is a visual inspection only. The manufacturer released three Advisory Bulletins (A-32, A-34, A-38) since July 1998 addressing installation, inspection, and shaft design change issues.
The failed secondary drive shaft had a total of 561 service hours. The manufacturer's recommended change out time is 500 service hours. The FAA does not issue type certificates for experimental category aircraft and 14 CFR Part 43 - Maintenance, Preventative Maintenance, Rebuilding, and Alternation regulations do not apply. According to the manufacturer, the 500-hour recommended change out is over two factors of safety below the engineering analyzed operational safe life of the shaft. The manufacturer was operationally performing an expanded shaft life test program utilizing this secondary drive shaft.
The Safety Board Materials Laboratory, Washington, DC, examined the secondary drive shaft, and prepared a factual report. The secondary drive shaft fractured between the lock collar (located just above the tail rotor drive belt pulley) and the inner race of the lower bearing. The lock collar was held onto the shaft by a set screw, and the lower bearing had been assembled with Locktite between the inner race and the secondary shaft. Examination of the shaft in the area contacted by the lock collar (on the lower portion of the shaft) revealed fretting and corrosion that was primarily concentrated in two circumferential bands. One was associated with the upper edge of the lock collar, and one was associated with the lower edge of the lock collar. Examination of the mating faces of the shaft fracture revealed that almost the entire fracture surface was on a flat plane. It intersected the exterior surface of the shaft at a 90-degree angle, and most of the fracture contained crack arrest positions, indicative of fatigue cracking. The origin area was at the lower edge of the fretting and corrosion area on the inner race of the lower bearing.