Recommendation Date
Recipient Name
Boeing
Text
Carry out an investigation, and take any necessary action, to ensure the design requirement for an IRU not to be faulted by an external failure, such as a shorted or open circuited data bus, is met.
Reply Text
According to Boeing specifications, the IRU wraparound BITE test failure should not be a critical failure which causes the box to transmit data with FW Fail Warning SSMs Sign Status Matrix on all outputs and annunciate IRS failure in-flight on EICAS.
Accordingly, Boeing is currently working with the IRU supplier, Honeywell, to incorporate a software change to the ARINC output wraparound BITE test to change it from a critical to a non-critical fault and eliminate this type of discrepancy. In addition the IRU transmitter circuit designs are being reviewed to determine if sufficient transmitter to transmitter isolation exists to tolerate a worst case external bus fault voltage that could knock out one transmitter. We anticipate that corrective action for the IRU may be available during 1st quarter 1998.
Accordingly, Boeing is currently working with the IRU supplier, Honeywell, to incorporate a software change to the ARINC output wraparound BITE test to change it from a critical to a non-critical fault and eliminate this type of discrepancy. In addition the IRU transmitter circuit designs are being reviewed to determine if sufficient transmitter to transmitter isolation exists to tolerate a worst case external bus fault voltage that could knock out one transmitter. We anticipate that corrective action for the IRU may be available during 1st quarter 1998.
Related Investigation(s)