To inform affected operators of RDMI/RMI inability to automatically select valid IRS heading data during standby power operation and provide temporary Quick Reference Handbook (QRH) replacement pages.
Boeing has discovered some airplanes received an incorrect wiring configuration, which affects Radio Distance Magnetic Indicator (RDMI/RMI) operation on standby power. The incorrect wiring exists only on airplanes with production installation of Hydraulic Driven Generator (HDG) wiring provisions. This Operations Manual Bulletin is not effective for airplanes with an HDG installed.
On airplanes without an HDG installed, failure of both engine Integrated Drive Generators (IDG) combined with an Auxiliary Power Unit (APU) or APU electrical generator failure results in the main battery providing the sole source of electrical power. The standby busses are powered from the main battery and are certified to provide a minimum of 30-minutes of power to essential equipment. On some airplane configurations with the main battery and APU battery paralleled, a minimum of 90-minutes of standby electrical power is available.
Normally, the right IRU provides heading data to the captain’s RDMI/RMI resulting in proper compass card and VOR-bearing pointer operation. On HDG equipped airplanes during HDG operation, the right IRU de-powers after a 5-minute time delay with the left and center IRUs continuously remaining powered. In addition, the source input to the captain’s RDMI/RMI automatically switches to the center IRU, to sustain normal operation of the RDMI/RMI during HDG operation.
On airplanes with only HDG wiring provisions and without an HDG installed, automatic IRU source switching to the captain’s RDMI/RMI is not provided during standby power operation. As a result, the captain’s RDMI/RMI compass card "HDG" flag and the left VOR-bearing pointer flag will display due to the loss of valid IRU heading data.
Boeing is in the process of issuing a Service Bulletin to all operators with the incorrect wiring configuration. With the Service Bulletin incorporated on the affected airplanes, the captain’s RDMI/RMI will be capable of automatic selection of valid IRS heading data during operation on standby power. As an alternative to retrofitting this Service Bulletin, operators can elect to retrofit an HDG as the existing IRU wiring configuration on the affected airplanes addressed by this Operations Manual Bulletin is compatible with an HDG-equipped airplane.
Remove and replace the applicable AC BUS OFF checklist in the QRH with the checklist provided by this Operations Manual Bulletin. The revised checklist includes a step to push the first officer’s IRS instrument source select switch to the ALTN position. By accomplishing this additional step during execution of the AC BUS OFF checklist, the captain’s RDMI/RMI compass card and left VOR bearing pointer will continue to operate normally.
This anomaly is corrected by Boeing Service Bulletin 757-34-0295. This Operations Manual Bulletin will be canceled after Boeing is notified that all affected airplanes in the operator’s fleet have been modified.