What happened
On 4 August 2004, a Hughes 369 D helicopter, registration SE-HSI, was performing a commercial transport flight near Ammarnäs, Sweden. The aircraft was carrying a pilot and four passengers when, approximately 30 seconds after takeoff, the pilot noticed a significant pull to the left in the roll plane.
In an attempt to neutralize the aircraft, the pilot utilized the cyclic stick trim controls, but the lateral force increased progressively. The situation became so severe that the pilot was forced to use his left hand and knee to physically counteract the stick's movement. Realizing the aircraft was becoming uncontrollable, the pilot attempted to land at a nearby open area. During the first landing attempt, the intense lateral force caused the aircraft to roll violently when the pilot momentarily released the stick. During a second attempt, the helicopter struck the ground hard on its left landing skid and overturned. While the four passengers and the pilot sustained only slight injuries, the helicopter suffered extensive damage.
The investigation
The Swedish Accident Investigation Board (SHK) conducted a technical examination of the aircraft's control and trim systems. Investigators discovered that the lateral trim system had moved to its maximum left stop position at the time of the accident.
Upon disassembling the components, technicians found silver-colored metal filings within the four-way trim switch. Furthermore, the investigation of the lateral trim actuator gearbox revealed heavy wear. The findings suggested that the input drive pinion or an intermediate gearwheel had likely become misaligned or worn, causing the gears to unmesh. This mechanical degradation likely caused the trim system to move erratically and eventually seize at the end position during the pilot's attempts to correct the flight attitude.
Findings
- The accident was caused by a technical fault in the lateral trim system that drove the controls to their left limit.
- The trim actuator gearbox was heavily worn, and metallic debris was present in the trim switch.
- The trim system is capable of generating cyclic stick forces of up to 14 kp, which is far beyond the level a pilot can effectively manage during critical maneuvers.
- Existing design regulations for helicopters do not establish maximum permitted limits for cyclic stick or pedal forces.
- There are currently no operating time restrictions for the trim actuators, unlike other components such as the trim switch.