The US Federal Aviation Administration (FAA) is giving Sikorsky S-92A operators 20 "time in service hours" from 21 August to perform a borescope inspection of the pitch change mechanism of 34 helicopters in the US fleet.
The Airworthiness Directive (AD) comes after an incident where the "shaft and bearing assembly and servo clevis shaft" failed in an S-92, resulting in loss of tail rotor control.
In the worst case, the FAA says such failures can cause "loss of tail rotor pitch and yaw control, and subsequent loss of control of a helicopter".
Sikorsky on 3 August had issued an alert service bulletin calling for the inspections within 50h time in service. The FAA slashed the compliance time by 30h based on the "ease of the inspection, the availability of borescopes, the flight hours per day for high-time helicopters [about 8h], and the potential for a helicopter to ditch while servicing the oil rig industry".
The AD also requires that S-92s having received a new shaft and bearing assembly be inspected after 10-15h time in service.
The FAA says the inspections are an interim step in addressing the problem as Sikorsky continues to investigate the mechanism.
According to FAA records, there have been four previous S-92 ADs dealing with the main gear box and transmission and the tail gear box.