Robinson R22 N2306T accident near Del Ray Beach, Florida

Probable cause

An in-flight failure of the main drive vee belts.

NTSB analysis

According to the certified flight instructor (CFI), they were at 3000 feet with an airspeed of 65 -70 knots when he felt a low frequency vibration throughout the helicopter. He described the vibration as being a lateral vibration as if coming from the tailrotor. He did not remember feeling the vibration in the pedal or cyclic, but more of an overall vibration. According to the student pilot, during a change in cruise altitude from 3,500 feet mean sea level (MSL) to 4,000 msl, they felt an airframe vibration. The vibration increased in intensity to a point where they could not read the instrument panel and they both observed the rotor brake light and clutch light illuminate. The CFI took control of the helicopter and initiated an emergency autorotation. Examination of both main drive vee belts showed they were off their pulleys. The forward vee belt left black residue on the front side of the pulley and shaft. The rear vee belt was broken in two and also had breaks in the center between the two V sections. Other than the breaks, the belts had no evidence of deterioration. There was no indication that the belts had visible damaged prior to the flight. A review of the helicopters logbooks found no entries as to when the belts were last changed.

Text from the NTSB. Open the NTSB docket for ERA09LA360 for the full record.