Mi-4A crash of 2 February 1988

Cause

The commission established that after 40 minutes of flight the helicopter was put into a descent from an altitude of 2700 m. In the 52nd minute of flight, at a distance of 1.5 — 2 km from the landing site, while making an approach to the site in order to inspect it, the helicopter developed spontaneous pitch oscillations, which led to an emergency situation. In order to make a forced landing on a relatively level site located in the river valley, the crew made a left turn. However, owing to the loss of longitudinal control, the helicopter, simultaneously with entering the left turn, spontaneously pitched up with the swashplate tilting fully aft, which was accompanied by a vigorous loss of forward speed. Subsequently the helicopter, uncontrollable in pitch, under the action of aerodynamic and inertial forces and moments went into a dive, struck the ground with a high vertical speed and a negative pitch angle of about 90° and was destroyed.

An inspection of the helicopter's structural elements at the accident site revealed a disconnection, in the pre-booster section, of the longitudinal control bellcrank from the control bellcrank of the command spool valve of the BU-10P longitudinal control hydraulic booster. The disconnection occurred because the connecting bolt had fallen out. Organised searches did not find the bolt, washer, nut or cotter pin.

On 26 November 1987, on this helicopter at the Chara operating site, the longitudinal control hydraulic booster was replaced by an aviation technician who was not authorised to perform this work independently, and without the appropriate instructions and supervision from the lead aviation technician, the technical control department (OTK) and the engineering staff of the aviation technical base (ATB). At the operating site there was no technical card for the replacement of the hydraulic booster and no torque wrenches for tightening the connections. Another lead aviation technician who arrived at the operating site (on 3 December) to supervise the performance of F-3 maintenance did not promptly detect the gross violations of the requirements of the technological instructions for replacing the hydraulic booster, which led to the failure of the longitudinal control in flight. The operating time of the hydraulic booster after replacement was 128 hours 35 minutes.

The commission considered the most likely cause of the connecting bolt falling out to be the installation in the joint of a cotter pin of a smaller diameter than required, but no confirmation of this was found, and it was decided not to include this wording in the commission's conclusion.

Findings: in flight a failure of the helicopter's longitudinal control occurred due to disconnection, in the pre-booster section, of the longitudinal control bellcrank from the control bellcrank of the command spool valve of the BU-10P longitudinal control hydraulic booster. The cause of the disconnection in this unit was the low level of technological discipline of the personnel when replacing the BU-10P longitudinal control hydraulic booster and the lack of supervision over the performance of work at the ATB of the Nizhneudinsk OAO.

Conclusion: the helicopter crash occurred due to the loss of longitudinal controllability in flight because the connecting bolt fell out, in the pre-booster section, of the longitudinal control bellcrank with the control bellcrank of the command spool valve of the BU-10P longitudinal control hydraulic booster. The bolt fell out as a result of a violation of paragraph 10.2.3 of the “Manual on the technical operation and repair of civil aviation equipment of 1983” and the “Technological instructions for performing scheduled maintenance on the Mi-4 helicopter”.