SIKORSKY S-97A near Jupiter, FL — 2017-08-02

Probable cause

A design change in the helicopter's fly-by-wire control system that resulted in an increase in the sensitivity of the cyclic stick while transitioning from ground to flight mode, magnifying the pilot's control inputs, which resulted in unintentional over control in the roll axis, upper and lower main rotor contact, and damage to the rotor blades.

NTSB narrative

The airline transport pilot of the experimental, fly-by-wire-equipped helicopter was taxiing to the runway for a test flight. During the taxi, he had commanded a high collective pitch and a slight forward cyclic pitch stick, and the helicopter was light on its wheels; as a result, the flight control computer transitioned from ground mode to flight mode. The slight forward pitch stick in combination with flight control mode resulted in a nose down pitch rate. The pilot took corrective action, inputting aft cyclic stick. The initial pitch axis correction led to a small initial left roll. The pilot countered the small left roll rate with an appropriate magnitude right stick input. In conjunction with the right stick input, the pilot raised the collective to increase the helicopter's altitude. The helicopter's right roll response to the right stick input was larger than expected, and the pilot countered with a large left stick input. The helicopter's left roll response was also larger than expected. The helicopter bank angles quickly increased, and the pilot rapidly lowered the collective to land the helicopter. The rapid left and right rolls resulted in upper and lower rotor contact, which damaged all rotor blades. The investigation determined that the larger-than-expected roll response to the pilot's cyclic stick input was the unintended result of a flight control system design change that resulted in an increase in cyclic stick sensitivity in the roll axis during the transition from ground control mode to flight mode.

Analysis

Primary failure mode

Loss of control

First missed decision gate

Pilot could have avoided excessive collective and forward cyclic inputs during taxi.

NTSB coding

• Flight control sys malf/fail · Takeoff-rejected takeoff

Evidence available

• FDR / data
• Photos
• 8 docket documents

Docket documents8

• Evidence Control Formform
• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• Release of Aircraft Wreckage, NTSB Form 6120.15form
• Pilot's Statementsinterview
• Statement of Party Representatives to NTSB Investigationinterview
• Witness Statementsinterview
• Lightweight Data Recorder & Onboard Image Recorder - Specialist's Factual Reportphotos
• Helicopter Specialist Reportreport

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