Sikorsky S-76 operators are being advised to carry out immediate checks on their main rotor control systems because of early findings in the investigation of August’s fatal Copterline helicopter crash. The US National Transportation Safety Board tests have found a critical control component from the Finnish-registered aircraft was malfunctioning.
The accident occurred on 10 August just after take-off from Tallin, Estonia on a shuttle to Helsinki, Finland.
The aircraft plunged out of control into the sea and sank immediately, killing all 12 passengers and both pilots. The US agency says it looks as if an uncommanded extension of the forward main rotor control actuator occurred and the unit was not responding properly to pilot inputs. The NTSB has found multiple faults in the forward hydraulic actuator that works with similar aft and lateral actuators to control the main rotor blade.
The agency says: “Ground tests of the [crashed] S-76 control system and additional aerodynamic simulations indicate that an uncommanded extension of the forward actuator will result in a large nose-up pitch upset, a large roll to the left, an aft movement of the cyclic control and an upward movement of the collective control. All of these unexpected aircraft and control movements were recorded by the accident flight data recorder at the time of the accident.”
Examination has shown that in one of the pair of separately supplied forward hydraulic actuators “large pieces of copper/aluminium plasma coating had flaked off the piston and balance tube seals had excessive wear, and pieces of the piston plasma were embedded in the seals and control valve, all of which contributed to internal hydraulic fluid leakage.”
That judgement, and other faults found in the unit, has led the NTSB to recommend that all S-76 opereators inspect the rotor controls immediately, and to check them more frequently than at present required.
It also advises pilots of the importance of pre-flight checks to establish “control movement smoothness and flight control ‘stick-jump’ at every engine start”.
DAVID LEARMOUNT/LONDON
Source: Flight International
