Bell Textron Canada Ltd 505 N945TC accident near Marianna, Florida

Probable cause

The student pilot’s failure to maintain rotor rpm during a simulated autorotation, which allowed for a resonance phenomenon to develop. Contributing to the accident was the flight instructor’s inadequate monitoring of the student pilot’s collective control inputs.

NTSB analysis

After completing several training maneuvers during a helicopter instructional flight with a flight instructor, the student pilot was tasked to perform a simulated autorotation and to land on the runway. Data recovered from the helicopter’s avionics revealed that, during the simulated autorotation, the collective initially remained relatively steady during the descent, and the rotor rpm remained at or above 100%.

Both the flight instructor and the student pilot reported that the touchdown on the runway was soft with no bounce or drift and good heading control as the helicopter entered the forward slide. However, as the helicopter continued to slide down the runway with decreasing groundspeed, it began to bounce laterally and shake violently with increasing severity before it came to a complete stop.

A postaccident examination of the helicopter revealed substantial damage on the tailboom assembly. Examination of the helicopter revealed no mechanical anomaly, and the flight instructor reported no mechanical malfunction or failure that would have precluded normal operation.

The data showed also that, as the helicopter descended through about 30 ft above ground level (agl), the collective began to increase sharply, and the rotor rpm began to drop below 100%. After touchdown on the runway, the collective continued to be increased, resulting in the rotor rpm continuing to decline. This increase in collective control was inconsistent with helicopter’s flight manual, which stated that the collective should be lowered after landing.

Although the data showed that the helicopter’s horizontal speed, rate of descent, attitude, and vertical acceleration at touchdown were consistent with the parameters for a power-off landing, the data also showed that, as the helicopter subsequently slid along the runway, rapid oscillations in helicopter pitch, roll, and yaw developed. Divergent oscillations in vertical, lateral, and longitudinal accelerations also occurred, the peak of which was about 3.6 G in the vertical direction when the rotor system had slowed to about 45%.

Further review of the data determined that, as the rotor slowed to about 40%, the rotor vibration frequency started to match with the oscillation frequencies of the helicopter’s pitch and roll angles and the lateral, longitudinal, and vertical accelerations as it slid along the runway. It is likely that a resonance developed between the vibrations of the slow-turning rotor system and the vibrations of airframe, resulting in increasing loads on the airframe that exceeded the airframe structural limitations and damaged the tailboom. Had the collective been lowered after touchdown per the rotorcraft flight manual’s procedure, it is unlikely that resonance would have occurred.

Text from the NTSB. Open the NTSB docket for ERA24LA399 for the full record.