Accident Details
On June 23, 2011, at 1255 central daylight time, an Enstrom F-28C helicopter (registration N5694A) was substantially damaged during a forced landing in a field near Blair, Nebraska. The commercial pilot and passenger were not injured. The helicopter was operated by P and N Corporation of Marion, Iowa, under 14 CFR Part 91 for an aerial observation flight. Visual meteorological conditions prevailed; no flight plan was filed.
The pilot reported flying along the Missouri River at 200 feet when an abrupt high vibration in the tail rotor system occurred, accompanied by a sharp jerk in the anti-torque pedals. He executed an autorotation to a wet field, landing on the left skid. The skid collapsed, causing the helicopter to roll onto its left side, damaging the tailboom and main rotor blades.
Examination
An FAA inspector with extensive Enstrom helicopter operational experience examined the wreckage. The pitch change link retaining bolt for the tail rotor pitch system had come loose from the pitch change link attachment horn. The attachment hole in the horn showed excessive wear and was elongated. The tail rotor control cables were severed. The inspector noted that the tail rotor pitch links are critical components and are required to be checked during pre-flight inspection, stating the pilot "should have caught the loose bolt" during that inspection.
The pilot stated he performed a pre-flight inspection the day before the accident and did not recall problems with the pitch links. He flew from Iowa to Nebraska and hangared the helicopter overnight. The following morning, he conducted a pre-flight but could not recall if he inspected the pitch links for binding or looseness per the FAA-approved Enstrom F28C Pre-flight Inspection checklist (item 21). He said it was an item he normally would check. The pilot also reported that the operator has several Enstrom helicopters and has experienced previous issues with pitch links coming loose quickly.
Maintenance History
Maintenance records showed the helicopter underwent a 100-hour inspection on June 9, 2011, at an airframe total time of 3,706 hours. Enstrom's maintenance manual requires inspection of tail rotor feather bearings for brinelling and lubrication every 50 hours, which involves disconnecting pitch links and rotating blades, then reconnecting hardware. This inspection was completed at the last 100-hour inspection. The pitch link assembly failed approximately 14 flight hours later.
The pitch change link retaining bolt and associated hardware separated in flight and were not recovered, so it could not be determined if the assembly was properly installed and torqued. The FAA Aircraft Certification Office overseeing the Enstrom type certificate reported no prior instances of tail rotor pitch link assemblies becoming loose and failing prematurely when properly installed.
Aftermath
As a result of this accident, Enstrom revised Service Information Letter (SIL) Number 0165 on November 14, 2011, to include inspection of tail rotor rotating controls for proper hardware, modifying installation if necessary, and checking torque of the pitch change link attachment. The SIL recommended this inspection at every 50- or 100-hour/annual inspection as per the maintenance manual.
The operator inspected the remaining seven Enstrom helicopters in the fleet after the accident and reported no anomalies with any pitch link assemblies.