Date: 201903
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: INS / IRS / IRU
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Human factors: Confusion; Situational Awareness
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Company Policy; Manuals; Aircraft
Primary problem: Aircraft

Narrative: AC arrived with L IRS (Left Inertial Reference System) FAULT. Pilot debrief contained no useful information. Mechanics started an alignment 3X before a successful alignment achieved. They said this was SOP and signed off the fault. AC flew [first trip] uneventfully. I was PF (Pilot Flying) on [the second trip] leg; overhead ZZZ @ FL340; I was startled by the AP disconnect horn. I looked up at my screens and they were blank. I announced 'You got it' and the FO (First Officer) took over flying. QRH procedure IRS FAULT (L) accomplished. At the conclusion of the procedure; there was zero reference in the QRH to what the effect on our aircraft systems were. After a great deal of searching in the FM (Flight Manual); I found wrong information on what effect the failure had on remaining aircraft systems. 'Should either IRS fail; the IRS transfer switch is used to switch all associated systems to the functioning IRS.' and even a diagram that depicted the wrong information. What I found out during the course of the flight and not from [company] procedures; maintenance or dispatch was that we would have no AP (had to leave RVSM); no AB (Auto Brakes) and incomplete Flight Director guidance on my side of the cockpit. The threat here is that if we were going to a short field remote destination in difficult weather; the lack of this awareness would lead to flawed decision making about where to take the aircraft. All of this information needs to be known immediately after the non-normal is complete.In light of the lack of 737X MCAS (Maneuvering Characteristics Augmentation System) information in our Boeing driven manuals and there obvious impact on safety; I strongly advocate that we here at [the company] protect our passengers and crew by supplementing these procedures with more information to increase SA (Situational Awareness). Before you dismiss this suggestion off hand; see the attachment that my jump seater [from another airline with a different aircraft model] showed me after the flight. It is the [other company's] procedure for the same malfunction. Notice the same Boeing procedure; however look at the end of the procedure: 'Autopilots cannot be engaged. Auto-brakes are not available. Only one FD (Flight Director) is available for approach mode. If approach mode is desired for PF (Pilot Flying); turn off the other FD.'FYI: Friend [from yet another airline] showed me their procedure. It is the same as ours; junk.




Synopsis: B737 Captain reported Inertial Reference System failure and flight manual did not state all effects of failure on aircraft systems.