Human Factor Maintenance: Error Led to Jammed Cyclic


Sikorsky helicopter inflight

The pilot of a Sikorsky S76 helicopter was left with only lateral cyclic control when a loose screw lodged at the base of the cyclic stick.

During transition from normal cruise flight to the approach to land the pilot found that the cyclic could not be moved aft. He also found that with any further forward movement of the cyclic stick it could not be moved aft of the new position.

The pilot froze the cyclic longitudinal position and the helicopter stabilised in a level pitch attitude at about 85 knots indicated airspeed. Using only lateral cyclic movements to manoeuvre the helicopter, the pilot conducted an 80-knot run-on landing on the runway at Barrow Island. A run-on landing utilises the aircraft's weathervane effect to streamline the fuselage until landing.

An inspection discovered that a panhead type screw was lodged at the base of the cyclic stick. The screw had lodged between the lower protrusion on the casting on the end of the cyclic stick torque tube and the lugs on a support bracket.

The cyclic stick base hardware is accommodated in a tub-like area formed by the cabin structure supports. A leather boot mounted at the base of the cyclic normally prevented foreign objects from entering the tub. Further inspection found the leather boot on this helicopter to be intact. With the boot in place, the only possible entry points for a screw is through a rigging pin hole in the aft mid-height position of the boot-halves joint, or vertically through an opening provided for the cyclic stick electrical wiring loom.

It was unlikely that the screw would have entered the tub area with the boot fitted. It was more probable that it was introduced during previous maintenance when the boot was removed.

After this incident and a similar incident experienced by another Australian S76 operator in 1995, an ATSB investigation was begun. The operator issued an alert message for its fleet of S76 helicopters to undergo an inspection of the subject area. A defect report was also submitted to the Civil Aviation Safety Authority.

The ATSB worked with the manufacturer to develop an acceptable solution that would eliminate the hazard.

The manufacturer conducted a design engineering review of the cyclic stick base hardware. It was agreed that an engineering design change, although extensive in nature, would more effectively reduce the effects of human factor maintenance error in this area.

The manufacturer advised that a field modification of the pilot's side bracket was being prepared to increase the gap between the torque tube rig boss and the bracket foot.

In October 1999, the manufacturer issued Alert Service Bulletin 76-64-44 outlining an inspection for foreign objects and procedures to modify both the composite controls cover and the cyclic stick support tube assembly. These modifications eliminate the foreign object interference problem.

Type: Educational Fact Sheet
Author(s): Sam Webb
Publication date: 9 September 2000
Related: Human factors
Last update 07 April 2014