Narrative:

Experienced #2 (right) IRS failure in-flight. Prior to IRS fault light illuminating; right IRS began drifting. CDU message 'verify position' displayed. Navigation status/position shift page revealed right IRS drift 12 mi and increasing. Left IRS and VOR/DME position keeping tight as was rnp/anp values on legs page. Right IRS drift continued to increase to approximately 37 mi. HUD was down and large 'pitch' caution annunciated in middle of HUD. Unable to locate the meaning of this display anywhere: QRH; fom; frm; etc. #2 IRS fault light finally illuminated; autoplt abruptly disconnected; master caution on; flags on first officer's ADI (frozen) and RMI; captain's ADI ok with flag on RMI; standby ADI ok. Got our attention quick. Rnp/anp values still tight; hand flew rest of flight following LNAV course guidance; leveloffs and approach; etc. Followed QRH guidance; heading input on CDU; fault light went out; flags went away; navigation status/position shift page showed right IRS offline; continued without autoplt uneventfully. 1) event occurred during a high task load portion of flight. Night/IMC could have been disastrous. Autoplt disconnect came unexpectedly and without notice during a critical phase. 2) have no idea what the 'pitch' caution was in the HUD. Are there any other displays that could be seen in the HUD that are not in our books? Very interested to know more details on these 'new' HUD displays; etc. 3) we were very aware that the right IRS was providing bad information to the FMC for 20 mins before it failed; but had no guidance to do anything with it until all the horns and lights came on. There needs to be a standard way to deselect a bad IRS/GPS (-700) once it drifts a certain amount so as to avoid a dangerous disorientation malfunction in night/IMC. 4) lastly; I think we need more QRH type guidance on some of the more important CDU messages were to rectify it was in the QRH; etc. Callback conversation with reporter revealed the following information: even though the crew knew the right IRS was drifting and might fail; when it did; it caught them off guard and was quite dramatic. The QRH procedure was followed and the right IRS was operated in attitude mode.

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Original NASA ASRS Text

Title: B737 CREW EXPERIENCES IRS FAILURE IN CRUISE WITH SUBSEQUENT AUTOPLT DISCONNECT.

Narrative: EXPERIENCED #2 (R) IRS FAILURE INFLT. PRIOR TO IRS FAULT LIGHT ILLUMINATING; R IRS BEGAN DRIFTING. CDU MESSAGE 'VERIFY POS' DISPLAYED. NAV STATUS/POS SHIFT PAGE REVEALED R IRS DRIFT 12 MI AND INCREASING. L IRS AND VOR/DME POS KEEPING TIGHT AS WAS RNP/ANP VALUES ON LEGS PAGE. R IRS DRIFT CONTINUED TO INCREASE TO APPROX 37 MI. HUD WAS DOWN AND LARGE 'PITCH' CAUTION ANNUNCIATED IN MIDDLE OF HUD. UNABLE TO LOCATE THE MEANING OF THIS DISPLAY ANYWHERE: QRH; FOM; FRM; ETC. #2 IRS FAULT LIGHT FINALLY ILLUMINATED; AUTOPLT ABRUPTLY DISCONNECTED; MASTER CAUTION ON; FLAGS ON FO'S ADI (FROZEN) AND RMI; CAPT'S ADI OK WITH FLAG ON RMI; STANDBY ADI OK. GOT OUR ATTN QUICK. RNP/ANP VALUES STILL TIGHT; HAND FLEW REST OF FLT FOLLOWING LNAV COURSE GUIDANCE; LEVELOFFS AND APCH; ETC. FOLLOWED QRH GUIDANCE; HDG INPUT ON CDU; FAULT LIGHT WENT OUT; FLAGS WENT AWAY; NAV STATUS/POS SHIFT PAGE SHOWED R IRS OFFLINE; CONTINUED WITHOUT AUTOPLT UNEVENTFULLY. 1) EVENT OCCURRED DURING A HIGH TASK LOAD PORTION OF FLT. NIGHT/IMC COULD HAVE BEEN DISASTROUS. AUTOPLT DISCONNECT CAME UNEXPECTEDLY AND WITHOUT NOTICE DURING A CRITICAL PHASE. 2) HAVE NO IDEA WHAT THE 'PITCH' CAUTION WAS IN THE HUD. ARE THERE ANY OTHER DISPLAYS THAT COULD BE SEEN IN THE HUD THAT ARE NOT IN OUR BOOKS? VERY INTERESTED TO KNOW MORE DETAILS ON THESE 'NEW' HUD DISPLAYS; ETC. 3) WE WERE VERY AWARE THAT THE R IRS WAS PROVIDING BAD INFO TO THE FMC FOR 20 MINS BEFORE IT FAILED; BUT HAD NO GUIDANCE TO DO ANYTHING WITH IT UNTIL ALL THE HORNS AND LIGHTS CAME ON. THERE NEEDS TO BE A STANDARD WAY TO DESELECT A BAD IRS/GPS (-700) ONCE IT DRIFTS A CERTAIN AMOUNT SO AS TO AVOID A DANGEROUS DISORIENTATION MALFUNCTION IN NIGHT/IMC. 4) LASTLY; I THINK WE NEED MORE QRH TYPE GUIDANCE ON SOME OF THE MORE IMPORTANT CDU MESSAGES WERE TO RECTIFY IT WAS IN THE QRH; ETC. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: EVEN THOUGH THE CREW KNEW THE R IRS WAS DRIFTING AND MIGHT FAIL; WHEN IT DID; IT CAUGHT THEM OFF GUARD AND WAS QUITE DRAMATIC. THE QRH PROC WAS FOLLOWED AND THE R IRS WAS OPERATED IN ATTITUDE MODE.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.