The U.S. Federal Aviation Administration (FAA) (Washington, DC, USA) has adopted a new airworthiness directive for certain CFM International, S.A. (CFM) LEAP-1B model turbofan engines. This directive 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, the directive 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. This repetitive check is required 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 directive requires applying electrical power to the PSS unit before further flight.
The directive was published on May 4, 2021 in the U.S. Federal Register.
Background on the Faults
In April 2021, the FAA says it received a report from CFM regarding 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, the FAA writes. This short can result in the transmittal of erroneous pressure sensor signals to the electronic engine control.
According to the FAA, erroneous pressure input from the pressure transducers in the PSS unit has the potential to prevent engine control from meeting thrust demand. In turn, this results in the loss of engine thrust control.
Manufacturer’s Recommendation
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.
As such, the FAA is mandating these recommendations in its directive. This condition, if not addressed, could result in loss of engine thrust control and reduced control of the airplane.
Source: Federal Register, www.federalregister.gov.