The US Federal Aviation Administration plans to issue an airworthiness directive (AD) requiring mandatory inspections for potential valve cracks in some Boeing 737 rudder power control units (PCUs). The FAA has also issued a directive covering the Rolls-Royce Allison AE3007 affecting Embraer RJ-145s and Cessna Citation X business jets.

The 737 AD would apply to -100 up to -500 series models - 2,900 aircraft worldwide. The inspection on each aircraft will take 1h, at an estimated cost of about $60 per aircraft. The tests would be have to be completed within 30 days or four months, depending on the model.

The AD would mandate tests on the PCU to detect cracks in a joint in the servo valve that regulates the intake of hydraulic fluid to the unit. Cracks have been discovered in PCU valve assembly components. The ADs result from unexplained accidents involving a US Airways 737-300 in September 1994, and a United Airlines 737-200 in March 1991. In both cases, an uncommanded rudder hard-over is suspected, but not proven, as the cause of the accidents.

Meanwhile, the FAA has issued an AD requiring preflight engine run-ups and inspections for the R-R Allison AE3007A and AE3007C-powered ERJ-145s and Citation Xs during extreme cold weather. The AD is designed to prevent inflight engine shutdowns due to loss of engine oil. It affects 120 twin-engined Citation Xs and ERJ-145s.

The agency found that starting these engines in very cold temperatures can cause the starter shafto-ring seal to allow oil to leak from the engine's accessory gearbox.

The FAA says the AD is an interim measure pending possible additional rulemaking. Allison says that a simple, inexpensive fix has been devised that will be the subject of a pending service bulletin. The company advisory may then be mandated by another FAA AD.

Source: Flight International