2025-07 · NASA ASRS report 2265388
B737-800 air carrier crew reported a malfunction in the IRS system during climb which resulted in improper instrument data information on the flight instruments. The crew diverted the flight back to the departure airport and landed.
The First Officer (FO) was the pilot flying (PF); and I was the pilot monitoring (PM). During climb at approximately 6000 ft.; FO stated I have pitch on my Primary Flight Display (PFD). I also noticed a pitch amber flag on my PFD. I stated out loud 'your aircraft' and started to assess the problem by verbally stating my pitch att on my PFD; which was 2.5 degrees and saw the FOs was approximately 9 degrees and he verified by stating it was 8.5 degrees. The SDBY ATT was showing 8.5 degrees. I quickly realized that the FOs PFD was reliable and stated he cont. to fly the aircraft. At that point I went to the QRH to find the appropriate checklist and told the FO I didn't want any pitch changes and to cont. to climbing. I was in the QRH trying to find a checklist for pitch flag when we both got roll amber flag on our PFDs. My PFD was showing a bank of 30 degrees; and the FOs was showing correct bank. I COULDNT FIND ANY CHECKLIST FOR PITCH OR ROLL FLAG. I quickly went to airspeed unreliable checklist to see if I could gather any insight or info. when I saw on step 6-the following is reliable -ATTITUDE (which I did not have); that's when I decided there must be a problem with the ADRUs and decided to return to ZZZ and requested priority handling. ATC told us to stop the climb at FL190. As ATC was giving us RV to Runway XXL and descending through 17;000 ft. we got master warning annunciation Inertial Reference System (IRS) light and the L IRS fault light. I accomplished the checklist; and we landed 600 lbs. overweight at a normal rate touch down.Cause: There is NO checklist for pitch and roll flags/disagree on PFD. Not having any reference or guidance in the QRH can cause confusion to someone that doesn't have a good foundation on system knowledge or might be less experience Captain or crew.
During climbout from ZZZ; passing 6000 feet I noticed an amber PITCH on my Primary Flight Display (PFD). I was the flying pilot. the Captain also got the PITCH indication and we noticed a difference in our pitch angles. The sby ATT was showing the same as mine so we concluded the Captain indication was incorrect. I continued hand flying. the Captain started checking the QRH for a procedure. At that point a ROLL amber appeared also in both PFDs and it was obvious that the Captain PFD was also showing an incorrect bank . The Captain could not find any procedure in the QRH similar to what we were experiencing. I continued hand flying and we decided since we were very close to the departure airport and with no specific guidance from the QRH to return and land. At FL190 the Captain requested priority handling and started our return to ZZZ.In our descent to start the approach into XXL we got an Inertial Reference System (IRS) L fault light. we accomplished that checklist and landed safely in ZZZ. the Autopilot was never engaged and by the checklist the autothrotles were disconnected for the approach. we were 600 lbs overweight for landing with a normal rate of descent in touchdown.Cause: Everything was done as required. One of the things that affected our decision to return was the inability to find a procedure similar to the PITCH/ROLL indications that would direct us to the right checklist. By the time we got the IRS fault from the beginning of the problem at least 10 minutes had elapsed. My suggestion would be to include some kind of guidance for these indications in the QRH.
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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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