Narrative:

Captain's heading seemed to be preceding and required periodic updates with the magnetic compass. Reviewing QRH and cockpit reference sources I felt we had some type of problem with the left IRS. Glancing up at the overhead IRS mode selector unit from the 'new' reference position in the captain's seat (I'm still a fairly low-time captain) it appeared that the left IRS was in att and not navigation. At that point I mistakenly believed the switch had been left in the wrong position. I quickly rotated the switch one position counterclockwise; thinking I was moving it to navigation. After 7 yrs of quick alignments; I was conditioned to rotating both switches; so I rotated the right switch also. Unfortunately I had actually succeeded in moving both switches to align; as they were both indeed already in navigation and it was simply the parallax view which caused me to err. The autoplt disengaged as I returned the switches to the navigation position. We lost map display and LNAV. We advised ATC we had an LNAV failure and were given a heading to fly until we could receive and navigation to the next VOR fix. We continued using VOR navigation while reviewing cockpit resources; ultimately determining that we were not going to be able to regain LNAV capability in the air. I initiated a voice contact with dispatch and requested maintenance get online. I told them the whole story. They concurred that LNAV was lost and for a period of time we continued. Ultimately dispatch contacted us as we were passing just north of ZZZ and advised us that with a line of thunderstorms west of our destination; it would be best if we landed in ZZZ and had maintenance correct the issue. We landed uneventfully in ZZZ. Maintenance corrected the issue; and we departed for an uneventful flight to our destination. Follow boxed memory item on front of QRH; specifically bullet point east; 'take appropriate action.' also follow fom procedures; the first officer should have been advised of what I was thinking of doing before I did it. He probably would not have let me! Supplemental information from acn 794806: within a few mins; we recovered basic attitude and heading information on both pfd's. We were able to engage the autoplt. At all times; aircraft control was maintained. We lost LNAV navigation capability. Our mfd's were blank. The IRS alignment panel on the overhead was blank as well. There were no faults on the overhead panel. Since we were cleared direct to ZZZ1 and we were still about 500 mi away; we asked ATC for a heading to ZZZ1. We used VOR navigation from that point. We also noticed that we had to keep updating our heading information in the FMC. Each time we did this; the aircraft would change heading to match what we entered (we had to update our magnetic heading off the standby compass; every 10 mins).

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Original NASA ASRS Text

Title: A B737-700 CAPT SELECTED BOTH IRS SWITCHES TO ALIGN IN FLT BELIEVING THEM TO BE IN THE INCORRECT POSITION. LNAV AND SLAVED HDG FUNCTIONS WERE LOST FOR THE REMAINDER OF THE FLT WHICH DIVERTED FOR WX.

Narrative: CAPT'S HDG SEEMED TO BE PRECEDING AND REQUIRED PERIODIC UPDATES WITH THE MAGNETIC COMPASS. REVIEWING QRH AND COCKPIT REF SOURCES I FELT WE HAD SOME TYPE OF PROB WITH THE L IRS. GLANCING UP AT THE OVERHEAD IRS MODE SELECTOR UNIT FROM THE 'NEW' REF POS IN THE CAPT'S SEAT (I'M STILL A FAIRLY LOW-TIME CAPT) IT APPEARED THAT THE L IRS WAS IN ATT AND NOT NAV. AT THAT POINT I MISTAKENLY BELIEVED THE SWITCH HAD BEEN LEFT IN THE WRONG POS. I QUICKLY ROTATED THE SWITCH ONE POS COUNTERCLOCKWISE; THINKING I WAS MOVING IT TO NAV. AFTER 7 YRS OF QUICK ALIGNMENTS; I WAS CONDITIONED TO ROTATING BOTH SWITCHES; SO I ROTATED THE R SWITCH ALSO. UNFORTUNATELY I HAD ACTUALLY SUCCEEDED IN MOVING BOTH SWITCHES TO ALIGN; AS THEY WERE BOTH INDEED ALREADY IN NAV AND IT WAS SIMPLY THE PARALLAX VIEW WHICH CAUSED ME TO ERR. THE AUTOPLT DISENGAGED AS I RETURNED THE SWITCHES TO THE NAV POS. WE LOST MAP DISPLAY AND LNAV. WE ADVISED ATC WE HAD AN LNAV FAILURE AND WERE GIVEN A HDG TO FLY UNTIL WE COULD RECEIVE AND NAV TO THE NEXT VOR FIX. WE CONTINUED USING VOR NAV WHILE REVIEWING COCKPIT RESOURCES; ULTIMATELY DETERMINING THAT WE WERE NOT GOING TO BE ABLE TO REGAIN LNAV CAPABILITY IN THE AIR. I INITIATED A VOICE CONTACT WITH DISPATCH AND REQUESTED MAINT GET ONLINE. I TOLD THEM THE WHOLE STORY. THEY CONCURRED THAT LNAV WAS LOST AND FOR A PERIOD OF TIME WE CONTINUED. ULTIMATELY DISPATCH CONTACTED US AS WE WERE PASSING JUST N OF ZZZ AND ADVISED US THAT WITH A LINE OF TSTMS W OF OUR DEST; IT WOULD BE BEST IF WE LANDED IN ZZZ AND HAD MAINT CORRECT THE ISSUE. WE LANDED UNEVENTFULLY IN ZZZ. MAINT CORRECTED THE ISSUE; AND WE DEPARTED FOR AN UNEVENTFUL FLT TO OUR DEST. FOLLOW BOXED MEMORY ITEM ON FRONT OF QRH; SPECIFICALLY BULLET POINT E; 'TAKE APPROPRIATE ACTION.' ALSO FOLLOW FOM PROCS; THE FO SHOULD HAVE BEEN ADVISED OF WHAT I WAS THINKING OF DOING BEFORE I DID IT. HE PROBABLY WOULD NOT HAVE LET ME! SUPPLEMENTAL INFO FROM ACN 794806: WITHIN A FEW MINS; WE RECOVERED BASIC ATTITUDE AND HDG INFO ON BOTH PFD'S. WE WERE ABLE TO ENGAGE THE AUTOPLT. AT ALL TIMES; ACFT CTL WAS MAINTAINED. WE LOST LNAV NAV CAPABILITY. OUR MFD'S WERE BLANK. THE IRS ALIGNMENT PANEL ON THE OVERHEAD WAS BLANK AS WELL. THERE WERE NO FAULTS ON THE OVERHEAD PANEL. SINCE WE WERE CLRED DIRECT TO ZZZ1 AND WE WERE STILL ABOUT 500 MI AWAY; WE ASKED ATC FOR A HDG TO ZZZ1. WE USED VOR NAV FROM THAT POINT. WE ALSO NOTICED THAT WE HAD TO KEEP UPDATING OUR HDG INFO IN THE FMC. EACH TIME WE DID THIS; THE ACFT WOULD CHANGE HDG TO MATCH WHAT WE ENTERED (WE HAD TO UPDATE OUR MAGNETIC HDG OFF THE STANDBY COMPASS; EVERY 10 MINS).

Data retrieved from NASA's ASRS site as of May 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.