Accident Summary
On April 20, 2017, about 1030 eastern daylight time, a Robinson R44 II, registration N899GB, was substantially damaged during an autorotative landing near Orlando, Florida. The commercial pilot and two passengers were not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the local revenue sightseeing flight, which originated about 5 minutes earlier from Air Orlando Heliport (2FD7), Orlando, Florida. The flight was conducted under Title 14 Code of Federal Regulations Part 91.
During the first flight of the day, while returning to 2FD7, the pilot stated he was flying southbound and descending out of 1,000 feet. At 600 feet, he heard and felt a loud "pop" accompanied by a yaw to the right, shaking, vibration, and engine roughness. At that time, the rotor tachometer dropped to zero, while the engine tachometer indicated about 60-70%. He immediately entered an autorotative descent to a median between two roads. The pilot did not recall his airspeed during the maneuver and landed "a little hard." After touchdown, the helicopter began to roll forward, which he attempted to correct with aft cyclic input. Once stable on the ground, he secured the engine and evacuated the passengers after the main rotor stopped.
Postaccident Examination
Examination of the helicopter revealed the aft section of the tailboom was partially detached. The leading edge of the main rotor blades exhibited slight damage. At the tailboom separation point, the tail rotor drive shaft was fractured and the tail rotor push-pull tube assembly was deformed. The arm associated with the damper assembly was bent about 90° and fractured, while the damper remained connected to the support, which was torn and deformed. No deformation was noted internally to structural members adjacent to the damper area, and no other damage to the tail rotor drive system was found.
In the engine compartment, one magnet assembly (part number B570-1) associated with the main rotor tachometer indicating system was separated from the yoke assembly (part number C908-1) and not recovered. The other magnet assembly remained secured to the yoke assembly. The yoke assembly was not damaged, but there was evidence of slight damage to one sender assembly, consistent with the magnet separation. The lowest numeric marking on the dual tachometer associated with the main rotor is 50, with two graduated lines beneath.
Engine Examination
The engine was removed and examined at a repair facility, then operated on a test stand twice. The first and second runs lasted 15 and 25 minutes, respectively, with no discrepancies noted. During a third run, about 10 minutes in, the left magneto failed the magneto check. The defective magneto was removed and replaced, after which the engine started and operated normally.
Additional Information
Robinson Helicopter Company personnel reported previous instances of a magnet separating from a magnet assembly, prompting Service Bulletin (SB) 86; however, no prior instances of the magnet assembly separating from the yoke assembly were reported. According to the Pilot's Operating Handbook, a caution in the emergency procedures section related to power failure specifies to not apply aft cyclic during touchdown or ground slide to prevent possible blade strike to the tailcone. The maintenance manual states that the sensor for the main rotor tachometer is an electronic Hall Effect device that senses passage of two magnets attached to the main rotor gearbox input yoke assembly. Robinson personnel reported that with only one magnet installed, the main rotor tachometer RPM would indicate about 50% of the actual rotor RPM.