2025-05 · NASA ASRS report 2243545
B737 flight crew reported a series of component malfunctions during cruise to include the autoflight system; autopilot; and left Inertial Reference System. The flight diverted to an enroute alternate and landed safely.
The loss of the left IRS (Inertial Reference System) wasn't indicated right away. I estimate it took 5-10 minutes for the fault light to finally illuminate which led me down multiple QRC procedures.I was PF had just started to eat; so with the tray in my lap I selected the data button on the EFIS (Electronic Flight Instrument System) control panel. Simultaneously; the autopilot disconnected so I made sure the yoke was free and I tried unsuccessfully to reselect it. Then I noted the loss of my attitude indicator. I handed the controls over to the FO; switched the flight director to the right side and attempted to couple his AP (Autopilot). No luck. Then I noticed he didn't have lateral navigation. I selected his prog page and told him to monitor for lateral deviation. I reported the loss to ATC and started with non-normal checklists. We had just made radar contact so I said we would divert to ZZZZ. ATC said they were providing [priority handling] and I agreed. The divert was uneventful.
While in cruise at FL380; approaching ZZZZZ; the autopilot unexpectedly kicked off. After the CA attempted to re-engage it; I noticed his attitude indicator had failed. As PM; due to the failure the captain gave me the controls and I became PF. As the captain began to work the issue; more issues began to arise; including Mach trim fail; EEC's (Electronic Engine Control) in alternate; and eventually left IRS (Inertial Reference System) failure displayed after a period of 5-10 minutes. As the aircraft had just entered radar service and we were unable to engage an autopilot it became necessary to exit RVSM airspace. The crew agreed a diversion to ZZZZ was appropriate. ATC provided vectors initially; and cleared the aircraft to descend to 5;000; shortly thereafter informing us they were [providing priority handling]. I began a gradual descent and flew the vector as the captain coordinated with dispatch; FA's (Flight Attendant); passengers; and ATC. After setting the aircraft up for the ILS Z to XX; we transferred controls so I could brief the approach; then I resumed flying. Aircraft landed safely and taxied in without further incident.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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