ERA15FA096

The student pilot’s failure to maintain rotor rpm while maneuvering in the airport traffic pattern, which resulted in the helicopter’s uncontrolled descent to the water. Contributing to the accident was the student’s distraction with other aircraft operating in the traffic pattern.

The student helicopter pilot was on a solo training flight in the airport traffic pattern. He had completed eight approaches via a right downwind approach to the runway, when the air traffic controller advised him that he was number three for his next approach. About 1 minute later, the student pilot requested a left 360-degree turn. The controller then instructed him to hold at his current location and expect to be number four in sequence. During the next 6 minutes, the controller made three attempts to have the student pilot report the traffic to follow on final approach in sight, and the student pilot advised that it was hard for him to hear the controller's instructions due to wind noise. The controller then advised the student pilot to follow an airplane on short final approach, and the student pilot reported the traffic in sight. About 1 minute later, the controller advised the pilot of another airplane to follow the helicopter on the approach. The airplane pilot observed the helicopter ascend in a series of right, 360-degree turns for about 100 to 200 ft. As it climbed, white smoke consistent with a rapid increase in engine rpm and an engine overspeed trailed the helicopter. When the helicopter climbed to an apex of about 800 ft, the ends of both rotor blades coned upward to where the tips were nearly vertical, consistent with a low rotor rpm condition. The helicopter then entered a right, spiraling descent until it impacted the water. A postaccident examination of the airframe and engine revealed no evidence of mechanical malfunctions or failures with the helicopter that would have precluded normal operation. The main rotor blade elastomeric teeter stops were missing, consistent with low rotor rpm blade flapping. Although the temperature and dew point were conducive to carburetor icing, its formulation likely would not have allowed the helicopter to climb as high as it did just before the accident. More likely, the student pilot became distracted while he attempted to track other aircraft in the traffic pattern and sequence the helicopter for the approach, which led to his failure to maintain rotor rpm. Toxicological testing performed on specimens from the pilot identified butalbital in liver (1.24 ug/g) and in muscle (0.468 ug/g). Estimated corresponding blood levels were likely below the therapeutic window for butalbital, and unlikely to have been directly impairing at the time of the accident.

RHC Safety Notice 24: LOW RPM ROTOR STALL CAN BE FATAL Excerpts include: "Rotor stall due to low RPM causes a very high percentage of helicopter accidents, both fatal and non-fatal. [It] can occur at any airspeed and when it does, the rotor stops producing the lift required to support the helicopter and the aircraft literally falls out of the sky. Rotor stall is very similar to the stall of an airplane wing at low airspeeds. As the airspeed of an airplane gets lower, the nose-up angle, or angle-of-attack, of the wing must be higher for the wing to produce the lift required to support the weight of the airplane. At a critical angle (about 15 degrees), the airflow over the wing will separate and stall, causing a sudden loss of lift and a very large increase in drag. The airplane pilot recovers by lowering the nose of the airplane to reduce the wing angle-of-attack below stall and adds power to recover the lost airspeed. The same thing happens during rotor stall with a helicopter except it occurs due to low rotor RPM instead of low airspeed. As the RPM of the rotor gets lower, the angle-of-attack of the rotor blades must be higher to generate the lift required to support the weight of the helicopter. Even if the collective is not raised by the pilot to provide the higher blade angle, the helicopter will start to descend until the upward movement of air to the rotor provides the necessary increase in blade angle-of-attack. As with the airplane wing, the blade airfoil will stall at a critical angle, resulting in a sudden loss of lift and a large increase in drag. The increased drag on the blades acts like a huge rotor brake causing the rotor RPM to rapidly decrease, further increasing the rotor stall. As the helicopter begins to fall, the upward rushing air continues to increase the angle-of-attack on the slowly rotating blades, making recovery virtually impossible, even with full down collective. When the rotor stalls, it does not do so symmetrically because any forward airspeed of the helicopter will produce a higher airflow on the advancing blade than on the retreating blade." An autopsy was performed on the pilot at El Instituto de Ciencias Forenses de Puerto Rico, San Juan Puerto, where the cause of death was determined to be "severe body trauma." Toxicological testing was performed by the FAA Forensic Toxicology Research team, Oklahoma City, Oklahoma. Results noted no ethanol, but did find 1.24 (ug/ml, ug/g) of butalbital detected in the liver, 0.468 (ug/ml, ug/g) of butalbital detected in muscle, and losartan detected in the liver. According to the FAA Aerospace Medical Research web site, butalbital is a short- to intermediate-acting barbiturate. It is commonly used in combination with other drugs such as acetaminophen and caffeine to treat mild to moderate pain, migraines and tension headaches. Losartan is used in the treatment of hypertension. The helicopter was recovered from San Juan Bay in the vicinity of 18 degrees, 26.53 minutes north latitude, 66 degrees, 07.16 minutes west longitude. The tailboom was initially not recovered. The wreckage was subsequently taken to a secure facility where it was examined. Cabin crush patterns were consistent with a nose-down, right-side-down water entry. The doors were not installed for the flight. The instrument console was found separated from the lower console, but tethered by wires. All five flight instruments had water inside of them. The keyed ignition switch was in the "Both" position and the rotor RPM gage indicated 76%. Other instrument indications moved as the wreckage was moved. The clutch switch was in the "Engaged" position, and the "Master Battery" and "Alternator" switches were in the "On" position. The cyclic was found jammed in the neutral position with the friction off. The collective was jammed about two-thirds of the way up with the friction off. The left tail rotor control pedal was jammed forward and the right tail rotor control pedal was jammed aft. The removable controls (in case a second pilot was onboard) were found stowed under the left seat. The fuel mixture knob was jammed in the "Full Rich" position, and the throttle grip was jammed in the "Idle" position. The carburetor heat control was jammed .20 inches up from full down (heat off) position. The carburetor heat control wire sheathing was stretched, and the mounting bracket was separated from the air box. The carburetor heat sliding door was deformed and jammed open 1.7 inches, or about 70% heat on. Both drive V-belts remained on their sheaves and appeared undamaged. The sprag clutch was operated without any anomalies noted. The engine cooling fan was bent slightly and the upper half of the scroll sustained impact damage. The lower half of the scroll was not recovered. There was a single scuff mark on the edge of the cooling fan inlet adjacent to the scroll lip and several static contact marks on the leading edge of two of the fins. The exhaust system was bent and deformed with the tailpipe up and around the lower sheave and starter ring gear. The alternator cooling fan was deformed on one side, but with no rotational scoring noted. The alternator belt remained in position. The forward face of the upper sheave had a scuff mark adjacent to the lower frame tube. The aft face of the upper sheave had no contact marks. There were no rotational scoring marks noted at any observed contact points. The engine did not exhibit any preexisting mechanical anomalies that would have precluded normal operation. Approximately 3 gallons of water and oil were drained from the oil sump. The cooling fan was rotated and crankshaft continuity was confirmed with no anomalies noted to the valve train or accessory gears. Thumb compression and suction were observed on all four cylinders. Visual examination of the rocker arms, push rods, valve caps, valve stems, valve springs and lower spark plugs revealed no anomalies. Both magnetos produced spark after internal components were dried. All oil lines and fittings were secure with no indications of the oil system being compromised. Visual inspection of the oil screen and oil filter element revealed no debris. The fuel mixture arm on the carburetor was separated from its shaft and the accelerator pump shaft was slightly bent. Carburetor removal and disassembly revealed no preexisting anomalies. The float bowl was full of liquid consistent in appearance with 100LL aviation fuel and water. The brass floats were undamaged. The gascolator bowl was also full of liquid consistent with 100LL aviation fuel and a small amount of water. Recovery personnel had also noted a fuel-type sheen on the water's surface and an odor of fuel at the crash site. The forward flex coupling was undamaged, and the intermediate flex coupling was bent and disconnected at the yoke consistent with overload. The main rotor driveshaft was rotated by hand more than 360 degrees with no anomalies noted. Oil was visible in the main rotor gearbox sight gauge. One main rotor blade was bent slightly upward about 7 feet from the tip, and bent downward and aft slightly about 3 feet from the tip. It had several creases running mostly chordwise from the trailing edge. The trailing edge was bent upward near the tip. There were no visible contact marks on the leading edge. The pitch control bearing rotated smoothly. The other main rotor blade was bowed upward at midspan. There were no visible contact marks on the leading edge although there was a scuff mark on the upper skin near the tip that appeared to be yellow paint. The paint could not be matched to specific point on the helicopter; however, the only yellow paint on it was on the main rotor blades. The pitch control bearing rotated with resistance. Both main rotor blade elastomeric teeter stops were missing, consistent with low rpm blade flapping. On February 27, 2015, the operator learned that portions of the tail had washed onshore