N5694AEnstrom F-28C · 1980

On June 23, 2011, at 1255 central daylight time, N5694A, an Enstrom F-28C helicopter, was substantially damaged during a forced landing to a field near Blair, Nebraska, following a tail rotor control failure. The commercial pilot and the passenger were not injured. The helicopter was registered to and operated by P and N Corporation, Marion, Iowa. Visual meteorological conditions prevailed and no flight plan was filed for the aerial observation flight conducted under 14 Code of Federal Regulations Part 91.

The pilot said he was flying along the Missouri River at 200 feet when he felt an abrupt "high vibration" in the tail rotor system in conjunction with a sharp jerk in the anti-torque pedals. The pilot made an autorotation to a wet field and landed on the left skid. The skid collapsed and the helicopter rolled on to its left side, damaging the tailboom and the main rotor blades.

A Federal Aviation Administration (FAA) inspector, who had extensive operational experience in Enstrom helicopters, performed an examination of the helicopter and found that the pitch change link retaining bolt for the tail rotor pitch system had come loose in the pitch change link attachment horn. The attachment hole in the pitch link attachment horn showed "excessive wear" and was elongated. The tail rotor control cables were also severed. According to the inspector, the tail rotor pitch links are critical components of the helicopter and they are required to be checked during the pre-flight inspection. The inspector said the pilot "should have caught the loose bolt" during this inspection.

The pilot said he performed a pre-flight inspection of the helicopter the day before the accident and did not recall any problems with the tail rotor pitch links. He then flew from Iowa to Nebraska, and placed the helicopter in a hangar overnight. When the pilot conducted a pre-flight of the helicopter the following morning, he could not recall if he had inspected the pitch links as required per the FAA approved Enstrom F28C Pre-flight Inspection checklist (item 21), for binding or looseness. The pilot said that even though he could not recall specifically checking/touching the pitch links, it was an item that was required to be inspected and something he would normally check. He also reported that the operator operates several Enstrom helicopters and has had previous problems with the pitch links coming loose "in a very short amount of time." After the accident, the operator inspected the remaining seven Enstrom helicopters in their fleet and reported there were no anomalies with any of the pitch link assemblies.

An FAA inspector reviewed the maintenance records and verified that the helicopter underwent a 100-hour maintenance inspection on June 9, 2011, at an airframe total time of 3,706 hours, just two weeks before the accident. Enstrom requires (per the Enstrom Helicopter Maintenance Manual section 3-7) that the feather bearings in the tail rotor be inspected for brinelling and lubrication every 50 hours. To do this inspection, the pitch links have to be disconnected and the tail rotor blades rotated 2 or 3 turns. Then the pitch links and associated hardware have to be reconnected. According to the FAA, this inspection was completed at the last 100-hour inspection. The pitch link assembly failed approximately 14 hours later.

Since the pitch change link retaining bolt and its associated hardware had separated from the helicopter in-flight and were never recovered, it was not determined if the pitch link assembly was installed and torqued correctly. According to the FAA Aircraft Certification Office (ACO) that oversees the type certificate for Enstrom, they were not aware of any instances prior to this accident where the tail rotor pitch link assemblies were becoming loose and failing prematurely if they had been properly installed.

As a result of this accident, Enstrom released a revision to their Service Information Letter (SIL) Number 0165, on November 14, 2011, to include an inspection of the tail rotor rotating controls for proper hardware, modifying the installation hardware, if necessary, and checking the torque of the pitch change link attachment. The SIL also stated that this tail rotor assembly should be inspected at every 50 or 100 hour/annual inspection as recommended in Section 3 of the applicable maintenance manual.