Commercial, Safety

Tail Rotor Failures, Loss of Control Two of Thorniest Issues for Helo Pilots

By Frank Wolfe | December 13, 2018
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The AW169 at King Power Stadium minutes before the crash.

Tail rotor drive failures and losses of control in the hover mode are two of the thorniest problems for helicopter pilots, and recovery from such failures is a tricky proposition at best.

Authorities investigating the fatal crash of a Leonardo AW169 in Leicester on Oct. 27 are trying to determine what led to damage observed on the tail rotor duplex bearing and to the loss of tail rotor control that led to the crash, according to a Dec. 6 special bulletin by the U.K. Air Accidents Investigation Branch (AAIB).

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The AAIB bulletin said that the AW169's tail rotor actuator continued to change the pitch of the tail rotor blades "until they reached the physical limit of their travel," a situation that led to an uncontrollable right yaw.

"My interpretation is that this was the low pitch limit so a control failure, not a drive failure," Gareth Padfield, emeritus professor of aerospace engineering at the University of Liverpool, wrote in an email. "In Chapter 8 of my book, Helicopter Flight Dynamics, I describe techniques that pilots developed to counteract different kinds of tail rotor failure, but a failure in the hover is very difficult (for a human) to recover from."

The AW169 helicopter crash on Oct. 27 outside King Power Stadium killed the owner of Leicester City football club and four other people. The AW169 appeared to be in a high hover more than 400 feet above the stadium when the pilot made a left pedal command, but the helicopter yawed instead to the right and began spinning downward in what looked to be a state known as "dead man's curve."

"A mechanical control failure limits or prevents control of tail rotor thrust and is usually caused by a stuck or broken control rod or cable," according to Chapter 11, Helicopter Emergencies and Hazards, of the FAA Flying Handbook. "While the tail rotor is still producing anti-torque thrust, it cannot be controlled by the pilot. The amount of
anti-torque depends on the position at which the controls jam or fail...The [recovery] techniques differ depending on the amount of tail rotor thrust, but an auto-rotation is generally not required."

In the past few years, Leonardo has employed simulators to train pilots how to recover from tail rotor failures.

At a Rotorcraft Virtual Engineering conference at the University of Liverpool in November 2016, Riccardo Bianco-Mengotti, a flight mechanics engineer with Finmeccanica, now Leonardo, gave a keynote address on how Leonardo had used virtual engineering in the rotorcraft flight mechanics design process.

Bianco-Mengotti "introduced the triangle of advantages – safety, effectiveness and economy– that VE offers helicopter manufacturers," Padfield, the chairman of the conference, wrote earlier this year in a paper for Aeronautical Journal. "A success story for the safety advantage was described relating to tail rotor failure, considered in the design of the AW169 helicopter, to ensure recovery was possible and to provide guidance on the recovery technique for pilots."

Leonardo has not responded to questions from Rotor & Wing International on what VE design features the AW169 incorporates to improve the helicopter's chances to recover from a tail rotor failure.

 

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