CEN24LA137

The pilot’s improper manual engine mixture management which resulted in a loss of power. Contributing to the accident was the mechanic’s improper setting of the idle mixture adjustment and the pilot’s decision to take off with a known mechanical deficiency.

The pilot reported that he was repositioning the helicopter from his yard to a raised landing platform. When he initiated a hover to maneuver to the platform, the overboost warning light illuminated, the engine subsequently lost power, and the helicopter settled with only the right skid on the platform. The helicopter then rolled to the left, and the main rotor blades contacted the ground. The pilot reported that the overboost warning light had illuminated during a previous liftoff that day but that reducing manifold pressure had effectively extinguished the light.   According to maintenance records, the idle mixture adjustment had been adjusted during the last annual inspection about 7 flight hours before the accident. Although the Rotorcraft Flight Manual stated that the mixture control must be in the full-rich position for takeoff and landing operations (to ensure adequate engine cooling), the pilot reported that, following this maintenance, he had to manually lean the cockpit mixture control during idle and low-power operations to prevent the engine from running excessively rich. Postaccident examination of the engine found that the idle mixture adjustment was set excessively rich. No other preaccident mechanical failures or malfunctions were found with the helicopter that would have precluded normal operation. According to Enstrom Service Information Letter (SIL) No. 0069, excessively rich idle mixture settings can result in engine roughness and engine stoppage following reductions to low-power conditions. These characteristics are consistent with the pilot’s reported need to manually lean the mixture during idle operation after the most recent maintenance. Because the pilot reported routinely leaning the cockpit mixture control during low-power operations after the maintenance was performed, it is likely that the mixture control was not returned to the full-rich position before repositioning to the landing platform landing pad, which resulted in an overly lean condition and power loss. It is likely that the over boost light illuminated due to the lean mixture which resulted in abnormally high exhaust gas temperature and led to turbocharger overspeed.

On March 16, 2024, about 1645 central daylight time, an Enstrom Helicopter Corporation 280C helicopter, N280WR, was substantially damaged when it was involved in an accident near Port Lavaca, Texas. The pilot was not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that, earlier that day, he flew a 2-hour flight and landed at an airport about 5 miles from his home for refueling. He stated that, after refueling, the engine started normally but that, when he initiated a hover, the overboost warning light illuminated. He stated that he adjusted the manifold pressure lower, the overboost warning light extinguished, and he flew the helicopter home and landed in his front yard uneventfully. The pilot reported that, when he initiated a hover to reposition the helicopter to a moveable landing platform, the overboost warning light immediately illuminated. He stated that, while he was “in the process of adjusting to position [the helicopter] on the platform,” the engine lost power, and the helicopter settled with only the right skid on the platform. The helicopter subsequently rolled to the left, and the main rotor blades contacted the ground. The helicopter sustained substantial damage to the main rotor, tail rotor, tailboom, and stabilizers. A postaccident examination of the engine fuel system revealed that the fuel injector servo idle mixture was set in excessively rich position. A review of maintenance records revealed that, during the most recent annual inspection completed in December 2023 (at a helicopter total time of about 7 flight hours before the accident), the “idle mixture adjustment and fuel pressure [were] found to be incorrect,” which was addressed by “set[ting] idle mixture and fuel pressure per Enstrom [Maintenance Manual] and Lycoming [Operator’s Manual]. The pilot reported that, after the maintenance performed during the annual inspection was completed, he had to manually lean the mixture at idle and low power settings to prevent the engine from running excessively rich. (The manual fuel mixture control provides the pilot with the option to manually lean the mixture for best cruise power or best specific fuel consumption by effectively reducing the size of the metering jet. It also provides the means to shut off fuel flow to the engine at engine shutdown.) According to the Enstrom 280C Rotorcraft Flight Manual, the mixture must be full rich for landing and takeoff, regardless of power, for proper engine cooling. The helicopter is equipped with Lycoming HIO-260-E1AD engine and a Rajay turbocharger. The turbocharger installation utilizes no waste gate or mechanical controls, and is equipped with a fixed exhaust pressure bypass. According to the Enstrom 280C Maintenance Manual Supplement, “Pilot operation of the Enstrom turbocharged system is the same as the normally aspirated system except the pilot must not exceed 36.5” of manifold pressure and must monitor the total inlet temperature to the turbine by a proper mixture control schedule” Enstrom Helicopter Corporation SIL No. 0069, “Engine Troubleshooting Procedures for Turbo Installations and Idle Adjustments to Correct Engine Roughness and/or Inadvertent Stoppage,” dated October 7, 1977, provided technical guidance for field adjustments to the fuel system. According to the SIL, an improperly adjusted idle mixture can result in engine roughness or inadvertent engine stoppage, particularly during rapid throttle deceleration or during operations at low power settings. The SIL outlined specific procedures for verifying and adjusting the idle mixture to ensure the proper fuel-to-air ratio is maintained for turbocharged engine installations.