LAX00LA024

The pilot was practicing autorotations to a power-on recovery. When he attempted to recover the power, the engine did not respond as quickly as anticipated and the helicopter landed hard, hitting on the rear heels of both skids. The flight instructor said that when he saw the pilot was late adding power, he attempted to correct the situation but was unsuccessful. The surface of the dry riverbed was mostly soft sand. The left skid heel contacted a log that was embedded in the sand and the helicopter pitched forward onto the skid toes and rolled over onto its left side. Both the flight instructor and pilot reported that there were no problems with the engine during prior autorotations, and, it was running after the helicopter came to rest. The engine was run in a test cell and met all of the manufacturer's perimeters. No discrepancies were found with the control or fuel systems.

History of flight

On October 23, 1999, at 1115 hours Pacific daylight time, a Bell 206-L4, N36R, landed hard and rolled over in the Lake Piru riverbed near Santa Clarita, California. The helicopter, operated by MG Aviation, Teterboro, New Jersey, under the provisions of 14 CFR Part 91, was substantially damaged. Neither the private pilot/owner nor the certified flight instructor was injured. Visual meteorological conditions prevailed and no flight plan was filed. The local training flight originated from the Van Nuys airport about 1030.

The CFI stated, in the pilot operator report, that they were practicing autorotations to a power-on recovery within a dry riverbed. The pilot reported that based on their briefing, he would initiate the autorotation by rolling off the throttle and lowering the collective, and he would roll the throttle back on, to a hovering position, at the termination of the maneuver.

The private pilot stated that they had successfully completed one practice autorotation and entered a second approximately 600 feet agl. When he attempted to recover power, the engine did not respond as quickly as anticipated and the helicopter settled hard into the soft creek bed.

The CFI reported that the pilot had been late bringing in the power and he had gotten on the controls during the flare to help the pilot and level the helicopter. The pilot reported that when he realized there was a lack of power, he tried to push the nose over. Both the pilot and CFI applied full collective, and reported that there was not sufficient rotor rpm to effectively cushion the landing, causing the helicopter to land on the heels of both skids.

The surface of the dry riverbed was mostly soft sand; the aft end of the left skid contacted a 12-inch-diameter log that was embedded within the riverbed. The tip of the left skid nosed into the sand and the helicopter rolled over onto its left side. The CFI stated that just before the helicopter impacted the terrain, the power came back in and the engine torque caused the aircraft to yaw to the right. Both the CFI and the pilot reported that the engine was running after the helicopter came to rest.

The pilot reported that the engine performance seemed normal prior to the accident, including the start-up and accelerations/decelerations tests. Both the pilot and the CFI stated that they were familiar with the engine spool-up time, and that they had not noted any redline excedances, or any abnormalities with N1 or N2.

Crew information

Review of the Federal Aviation Administration (FAA) airman certification database revealed that the CFI held a commercial certificate for rotorcraft-helicopters, an airline transport pilot certificate for single and multiengine airplanes, and, a CFI certificate for single and multiengine airplanes, instrument airplanes, and rotorcraft-helicopters. He also held advanced and instrument ground instructor certificates. He reported that he had 3,859 hours of total flight time, including 394 hours in rotorcraft, and 75 in the Bell 206-L4.

According to the FAA airman certification database, the pilot held a private pilot certificate with airplane single engine land and rotorcraft helicopter ratings. He reported that he had accumulated about 994 hours of total flight time, including 332 hours in rotorcraft and 235 hours in the Bell 206-L4.

Tests and research

The tail boom and landing gear were removed from the fuselage to facilitate recovery. The basic structure of the cockpit and cabin was intact and no evidence of distortion or deformation was noted. The front seat occupant restraints were intact and neither the seat bottoms nor seat pans showed signs of deformation and the cockpit controls appeared undamaged. All switches were in found in the "OFF" position, all instrument static positions appeared normal, and all overhead circuit breakers were in.

The battery was reconnected and the engine igniter and starter circuit breakers were pulled out. When the battery switch was placed in the "ON" position the "engine out" and "rotor rpm low" audio warnings were heard, and the following caution lights were observed illuminated: "GEN FAIL," "TRANS OIL PRES," "ENG OUT," and "ROTOR LOW RPM." The fuel gauge indicated 335 pounds of fuel. When the caution panel test switch was depressed, all caution segments illuminated.

The cockpit controls were manipulated through their entire range of travel and no abnormalities were noted. The cyclic and collective controls were found to operate normally up to the hydraulic servo actuators. The tail rotor linkage was operable aft to the fractured control tube at the tail boom separation point. The throttle and collective droop linkages were intact and operable.

Both main rotor blades were fractured just outboard of the blade doublers. One of the blade tips displayed blue paint transfer, which matched damage on the upper portion of the vertical fin. The hub assembly feathering and flapping bearings operated normally. The pitch horns on both main rotor blades were intact. One of the main rotor pitch links was intact and the other pitch link was fractured.

The tail boom exhibited buckling around the entire circumference of the tail boom structure. Drive continuity was established through the 90-degree gearbox to the tail rotor. The tail rotor hub and blade assembly were intact and the blades did not exhibit damage.

The landing gear assembly had been removed from the airframe during recovery of the helicopter. The forward cross tube was fractured at the left skid attachment fitting. The front and rear cross tubes of the landing gear exhibited downward compression at the left attachment saddle area. With the fractured left skid attachment fitting placed back together, measurements were taken on the cross tubes with the landing gear assembly upright on the hangar floor. The rear cross tube measured 24 inches from the top of the tube at the left saddle down to the floor, and 24.5 inches from the top of the tube at the right saddle to the floor. The front cross tube measured 16.5 inches from the top of the tube at the left saddle down to the floor, and 21 inches from the top of the tube at the right saddle down to the floor. According to the representative from Bell Helicopters, this height measurement on an undamaged helicopter is approximately 25 inches.

Disassembly of the fuel filter revealed the element to be full of fuel, clean, and absent of debris. With both fuel boost pump circuit breakers pulled out, the battery switch was turned to the "ON" position. Both fuel boost pump warning segments illuminated on the caution panel. Each of the fuel boost pump circuit breakers were pushed in individually, and each of the respective fuel boost caution lights extinguished. The fuel line from the airframe to the inlet of the airframe filter was removed at the filter and placed in a 5-gallon container. With both LEFT and RIGHT fuel boost pump circuit breakers pulled out, the battery switch was turned on. The LEFT fuel boost pump circuit breaker was depressed and fuel began flowing into the container at a rate estimated in excess of 65 gallons per hour. The LEFT boost pump breaker was pulled out and the RIGHT fuel boost pump breaker was depressed. Fuel flowed into the container at a rate estimated to be in excess of 65 gallons per hour.

The engine remained mounted to the airframe. The left aft engine mount tube was bent outward approximately 2 inches. External examination of the engine and accessories revealed no damage. All fuel system and airlines were tight and secure at their fittings. Control linkages were intact and secure. Hand rotation of the steel segment of tail rotor drive shafting verified proper operation of the freewheeling unit and also produced rotation of the power turbine rotor as observed through the exhaust stack.

The engine was removed for further testing.