FAA issues directive to prevent Boeing 737 pilot-induced oscillations

By John Croft

Material defects in one batch of wire that Boeing purchased to make springs is at the root of a proposed airworthiness directive (AD) for dozens of next generation Boeing 737 models issued by the US FAA today.

The springs were used to build "rudder feel and centering" (RFCU) units on approximately 70 Boeing 737 NG models sold in the US. The devices are meant to artificially create rudder pedal forces that are representative of aerodynamic loading on the rudder.

The AD results from reports of low rudder pedal forces from operators, problems that were traced to a broken inner spring in the RFCU.

FAA says a broken inner spring combined with a broken outer spring "could result in increased potential for pilot-induced oscillations and reduce the ability of the flight crew to maintain safe flight and landing of the airplane."

Along with repetitive inspections or force measurements on the RFCUswithin 30 days of aircraft identified by Boeing as possibly having the defective springs, FAA is also mandating replacement of the spring assembly in the RFCU within three years of the effective date of the AD. Replacing the springs will cost less than $4,000 per aircraft, says FAA, and will remove the need for further inspections.

The agency is also seeking to prevent the suspect springs from being retrofitted into other next generation 737 aircraft in the future.