Accident Overview
On December 7, 2012, at approximately 1545 central standard time, an Enstrom F-28C helicopter, registration N574H, impacted terrain during an autorotation following a loss of engine power while on approach to Abilene Regional Airport (ABI) near Abilene, Texas. The certified flight instructor and student pilot reported no injuries. The helicopter sustained substantial damage to the tailboom. The aircraft was registered to and operated by the student pilot under Title 14 Code of Federal Regulations Part 91 as an instructional flight. Visual flight rules (VFR) conditions prevailed, and no VFR flight plan was filed. The flight originated from Brownwood Regional Airport (BWD) near Brownwood, Texas, at an unknown time and was destined for ABI.
Preflight and Flight Details
According to the flight instructor's report, he and the student pilot met at 0930 outside a fixed base operator at ABI. They discussed the helicopter, its logbooks, maintenance history, airworthiness, preflight actions, the student's experience, and other topics for about an hour. No pending maintenance issues were detected, and after an extensive preflight inspection, the instructor considered the helicopter airworthy. The fuel tanks were filled completely, providing an estimated 40 gallons of total fuel. The student requested a round-robin flight to Albany, Eastland, Brownwood, and back to Abilene. The first three legs were uneventful with landings at each location.
On the final leg, the instructor noted the fuel level and decided to continue to ABI. The fuel gauge indicated 3/8 full. The instructor estimated that, out of 40 gallons, the helicopter passed over BWD with 15 gallons remaining. He calculated that with a fuel burn rate of 13.5 gallons per hour and a flight time of 30–35 minutes to ABI, the helicopter should land with the required 20-minute reserve.
Engine Power Loss and Autorotation
Approximately four miles from ABI, the flight was cleared to pass behind a Beechcraft on final to runway 17L. At two miles from ABI, the helicopter yawed left briefly. The instructor shifted his attention from the Beechcraft to the engine instruments and the student pilot's actions. The helicopter yawed left again, and engine and rotor RPM began to decay. The instructor lowered the collective, informed the student he was taking control, and leveled the helicopter for a 70-knot attitude. This caused rotor RPM to decay further, below the minimum for autorotative descent. The instructor pushed the nose over and then back to level, which caused the blades to cone upward and increase rotor RPM, but still below the red line. He nosed the helicopter over again and turned left, coning the rotor to unload it, but this maneuver was only slightly effective in regaining rotor speed.
The instructor suspected that the overrunning clutch had not disengaged the engine from the transmission. He flared at 30 feet above ground level (AGL), slowing the helicopter but loading the rotor system, which dropped to 200 RPM. At 3 feet, he leveled the helicopter and applied collective to cushion the landing, but had only about one-third of the normal rotor RPM. The helicopter landed level with minimal forward movement. One main rotor blade struck the tailboom, causing the helicopter to spin left approximately 60 degrees. A subsequent blade contacted tail rotor debris, and the third blade sustained damage from sudden stoppage.
Student Pilot's Account
The student pilot reported voicing concern about the low fuel quantity on board. He stated that the instructor informed him that the fuel gauges are often faulty and that they had sufficient fuel to complete the return leg to ABI.
Postaccident Examination
Federal Aviation Administration inspectors conducted a postaccident examination. A mechanic assisted, draining less than one gallon of fuel from each of the right and left fuel tank sumps. The fuel strainer and fuel injection servo screen were examined and reinstalled. The helicopter was serviced with four gallons of 100 low-lead aviation gasoline, and the engine started and ran smoothly for five minutes. The overrunning clutch operation was examined, and no anomalies were detected.
Relevant Manual and Directive Information
The Enstrom F-28C rotorcraft flight manual (FM) preflight inspection checklist requires visual verification of fuel quantity in both tanks and confirmation that fuel tank caps are secured, as well as draining both tank sumps. The FM lists rotor limitations for power-off flight: maximum 385 rpm, minimum 332 rpm. It also includes a ground check procedure to close the throttle gently to verify overrunning clutch operation. For engine failure, the FM directs entering normal autorotation at 58 mph and notes that maximum glide distance is attained at 80 mph and 332 rotor rpm.
Enstrom Service Directive Bulletin No. 0092, regarding fuel quantity system calibration, indicates that one gallon per tank is unusable fuel. It recommends pilots monitor fuel quantity indications and use a fuel dipstick during preflight to verify fuel quantity.
Neither the flight instructor nor the student pilot visually checked the fuel level at any of the stops during the round-robin cross-country flight.