AD 2021-10-06 Prompted By Multiple Reports Of Pressure Sub-System (PSS) Unit Faults

The FAA is adopting a new airworthiness directive (AD) for certain CFM International, S.A. (CFM) LEAP-1B model turbofan engines.

 This AD was prompted by multiple reports of pressure sub-system (PSS) unit faults due to pressure transducer corrosion following extended storage periods. For an engine in service, this AD requires checks for engine maintenance messages related to the pressure transducer and, depending on the results of the check, replacement of the PSS unit before further flight. The AD requires this repetitive check for faults prior to each flight until the PSS has accumulated at least 15 hours of electrical power. For an engine not in service, this AD requires applying electrical power to the PSS unit before further flight. The FAA is issuing this AD to address the unsafe condition on these products. This AD is effective May 10, 2021.

Supplementary Information: In April 2021, the FAA received a report from CFM, the engine manufacturer, of numerous instances of PSS unit faults. The manufacturer reported these faults have been occurring since October 2020 and are a result of pressure transducer corrosion following extended storage periods. The manufacturer's investigation found that certain PSS units, identified by serial number, have been exposed to conditions that make pressure transducers in these units susceptible to an increased rate of faults. These conditions are moisture ingress from long-term on-wing storage, coupled with certain manufacturing processes of the affected pressure transducers. Together these conditions can cause corrosion and subsequent electrical shorting of the pins in the pressure transducer. This short can result in transmittal of erroneous pressure sensor signals to the electronic engine control. Erroneous pressure input from the pressure transducers in the PSS unit has the potential to prevent engine control from meeting thrust demand, thereby resulting in the loss of engine thrust control. Further, the engine manufacturer found that if transmission of erroneous pressure sensor signals were to occur, it would occur within the first 15 hours of electrical power to the PSS unit after the extended storage period is completed. Therefore, the manufacturer recommended that, for engines with fewer than 15 hours of electrical power applied to the PSS unit within the past 90 days, operators either perform a check for engine maintenance messages related to the pressure transducer prior to each flight or apply electrical power to the PSS unit until the PSS unit has accumulated 15 hours or more of electrical power. The FAA is mandating these recommendations in this AD.

This condition, if not addressed, could result in loss of engine thrust control and reduced control of the airplane. The FAA is issuing this AD to address the unsafe condition on these products.

FMI: www.regulations.gov