Cobb ROTORWAY EXEC 90 N9876D

Summary

On 5 February 2003 at about 15:15 local time, a Cobb ROTORWAY EXEC 90 registered N9876D was involved in an accident near Casa Grande, Arizona, United States. 2 people were on board and 2 had minor injuries. The aircraft was substantially damaged. The NTSB has published a probable cause for this accident; it is quoted in full below.

Date

5 February 2003 at 15:15

Classification

Accident

Location

Casa Grande, Arizona, United States

Nearest airport

Phoenix Regional (A39)

Coordinates

32.9972, -111.9231

Aircraft

Cobb ROTORWAY EXEC 90

Registration

N9876D

Category

Helicopter

Engines

1

Operator

Not recorded

Operating rule

Part 91: General Aviation

Purpose of flight

Instructional

Phase of flight

Not recorded

Route

Chandler → Casa Grande

Aircraft damage

Substantial

Weather

VMC

Light

Day

NTSB number

LAX03LA084

2 people had minor injuries.

the flight instructor's failure to maintain directional control of the helicopter during the flare and landing. Factors in the accident were the failure of the secondary drive shaft and inadequate tail rotor failure procedures specified by the manufacturer.

Quoted verbatim from the NTSB record. This site does not paraphrase or interpret it.

Read the full NTSB narrative

The helicopter made a hard landing following a failure of the antitorque system. While in the landing pattern, the student and CFI heard a "bang." The CFI took the flight controls, initiated an autorotation, and turned to an open area for an emergency landing. During the autorotation the CFI noticed a loss of his ability to maintain directional control. He 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. The CFI's execution of an autorotative landing failed to adjust for the engagement of the engine when he pulled in collective to cushion the landing. Engagement of engine power in the autorotative flare caused the helicopter to yaw left, which was not compensated for by the failed tail rotor. The helicopter encountered ground resistance when it landed, and the force of the uncontrolled left yaw rolled it over onto its right side. The FAA Rotorcraft Flying Handbook describes the appropriate response to an antitorque failure as an entry into an autorotation profile by reducing the collective and rolling off the throttle. Rolling off the throttle will prevent the engine from engaging during the autorotative landing. The manufacturer's flight manual does not address the issue of performing an autorotative landing with a tail rotor failure. Examination of the helicopter revealed that the secondary drive shaft appeared to have fractured between the lower bearing and the lock collar. Examination of the shaft revealed fretting and corrosion in the area contacted by the lock collar. The fracture was a result of fatigue cracking that initiated in the area of fretting and corrosion. This secondary drive shaft had been involved in one helicopter rollover and two tail rotor strikes. It is possible that excessive loads generated during these previous events was sufficient to loosen the lower bearing on the shaft and initiate the fretting and corrosion process that eventually led to the initiation of a fatigue crack.

Quoted verbatim from the NTSB record.