Narrative:

Captain with the standby gyro. Troubleshot to find which system was correct. Glimpsed VMC, recaged standby gyro. Navigation display frozen. No valid heading information. Assessed situation, lack of navigation capability, hostile terrain, weight and WX. ATC offered 5500 runway option and later ZZZ1. Both were declined as mountainous. Coast had onshore rain and thunderstorms. We felt that blue sky, good WX, flat terrain, long runways option of ZZZ even though it was 400 miles away was our best option. Attempted to use att mode of the inertial reference unit, but gave us red flags and warnings on primary flight display and navigation display. Returned switches to navigation. Notified flight attendants and passenger of unscheduled landing. Did no gyro start turn/stop turn heading changes with ATC. Local vectoring put us on a 90 degree right base just a few miles southeast of the airport. We had expected a turn on from the southwest 30 degrees standard to a long final. Gained visual on the airport and shot an uneventful landing. Maintenance personnel found the database and bite checks to show a dual inertial reference unit failure. Further talk with air carrier technician on a three way call with a dispatcher at desk xx, showed there was a problem with the comparator system as well. Our statistically improbable dual inertial reference unit failure led us to the most basic of flying skills through lack of automation. We were down to a 2 inch standby gyro and a wet compass that we view through a lighted slot by way of a makeup sized mirror. We flew that way toward known good WX and flat terrain for over an hour in IMC conditions. Attempting to troubleshoot a system with a complete failure and no procedure is an impossible task. Along with first officer and excellent help from ATC, we applied our years of experience and brought about a safe, prudent, effective solution to a desperate situation. In talking with first officer, we found out today that the inertial reference units can shut down and go to a reversionary state of operation. We also found out today that the aircraft has had a history in the past couple weeks of digital flight guidance system #2 problems. Callback conversation with reporter revealed the following information: the reporter stated in cruise at FL330, the aircraft lost navigation capability due to both navigational displays becoming frozen. The reporter said the standby gyro was caged after a brief glimpse of VMC due to attitude displays having different indications. The reporter stated att mode was switched to go to a reversionary attitude mode but the displays went blank with red flags and warnings and was immediately switched back to navigation mode. The reporter said the flight was operated with the standby attitude gyro and the wet compass for directional data that was viewed through a lighted slot by way of a small mirror. The reporter stated the operations manual does not indicate when att mode is selected, a 20 to 30 second delay is encountered when the displays go blank. The reporter said the diversion airport was selected on good WX, flat terrain, and long runways. The reporter stated on the ground maintenance found a dual inertial reference and comparator failure. A check of the maintenance history reveals #2 flight guidance chronic problem and as recently as early november had a failure of the #2 air data computer and #2 navigation system. The reporter said he still feels there is a problem in this airplane.

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Original NASA ASRS Text

Title: AN MD80 IN CRUISE AT FL330 DECLARED AN EMER AND DIVERTED DUE TO LOSS OF ALL NAVIGATIONAL CAPABILITY. NAVIGATION DISPLAY FROZEN. REQUIRED USE OF STANDBY GYRO AND WET COMPASS.

Narrative: CAPT WITH THE STANDBY GYRO. TROUBLESHOT TO FIND WHICH SYSTEM WAS CORRECT. GLIMPSED VMC, RECAGED STANDBY GYRO. NAVIGATION DISPLAY FROZEN. NO VALID HDG INFO. ASSESSED SIT, LACK OF NAV CAPABILITY, HOSTILE TERRAIN, WEIGHT AND WX. ATC OFFERED 5500 RWY OPTION AND LATER ZZZ1. BOTH WERE DECLINED AS MOUNTAINOUS. COAST HAD ONSHORE RAIN AND TSTMS. WE FELT THAT BLUE SKY, GOOD WX, FLAT TERRAIN, LONG RWYS OPTION OF ZZZ EVEN THOUGH IT WAS 400 MILES AWAY WAS OUR BEST OPTION. ATTEMPTED TO USE ATT MODE OF THE INERTIAL REFERENCE UNIT, BUT GAVE US RED FLAGS AND WARNINGS ON PRIMARY FLT DISPLAY AND NAVIGATION DISPLAY. RETURNED SWITCHES TO NAV. NOTIFIED FLT ATTENDANTS AND PAX OF UNSCHEDULED LNDG. DID NO GYRO START TURN/STOP TURN HDG CHANGES WITH ATC. LOCAL VECTORING PUT US ON A 90 DEG RIGHT BASE JUST A FEW MILES SE OF THE ARPT. WE HAD EXPECTED A TURN ON FROM THE SW 30 DEGS STANDARD TO A LONG FINAL. GAINED VISUAL ON THE ARPT AND SHOT AN UNEVENTFUL LNDG. MAINT PERSONNEL FOUND THE DATABASE AND BITE CHECKS TO SHOW A DUAL INERTIAL REFERENCE UNIT FAILURE. FURTHER TALK WITH ACR TECHNICIAN ON A THREE WAY CALL WITH A DISPATCHER AT DESK XX, SHOWED THERE WAS A PROB WITH THE COMPARATOR SYSTEM AS WELL. OUR STATISTICALLY IMPROBABLE DUAL INERTIAL REFERENCE UNIT FAILURE LED US TO THE MOST BASIC OF FLYING SKILLS THROUGH LACK OF AUTOMATION. WE WERE DOWN TO A 2 INCH STANDBY GYRO AND A WET COMPASS THAT WE VIEW THROUGH A LIGHTED SLOT BY WAY OF A MAKEUP SIZED MIRROR. WE FLEW THAT WAY TOWARD KNOWN GOOD WX AND FLAT TERRAIN FOR OVER AN HOUR IN IMC CONDITIONS. ATTEMPTING TO TROUBLESHOOT A SYSTEM WITH A COMPLETE FAILURE AND NO PROC IS AN IMPOSSIBLE TASK. ALONG WITH FO AND EXCELLENT HELP FROM ATC, WE APPLIED OUR YEARS OF EXPERIENCE AND BROUGHT ABOUT A SAFE, PRUDENT, EFFECTIVE SOLUTION TO A DESPERATE SIT. IN TALKING WITH FO, WE FOUND OUT TODAY THAT THE INERTIAL REFERENCE UNITS CAN SHUT DOWN AND GO TO A REVERSIONARY STATE OF OP. WE ALSO FOUND OUT TODAY THAT THE ACFT HAS HAD A HISTORY IN THE PAST COUPLE WEEKS OF DIGITAL FLT GUIDANCE SYSTEM #2 PROBS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED IN CRUISE AT FL330, THE ACFT LOST NAVIGATION CAPABILITY DUE TO BOTH NAVIGATIONAL DISPLAYS BECOMING FROZEN. THE RPTR SAID THE STANDBY GYRO WAS CAGED AFTER A BRIEF GLIMPSE OF VMC DUE TO ATTITUDE DISPLAYS HAVING DIFFERENT INDICATIONS. THE RPTR STATED ATT MODE WAS SWITCHED TO GO TO A REVERSIONARY ATTITUDE MODE BUT THE DISPLAYS WENT BLANK WITH RED FLAGS AND WARNINGS AND WAS IMMEDIATELY SWITCHED BACK TO NAV MODE. THE RPTR SAID THE FLT WAS OPERATED WITH THE STANDBY ATTITUDE GYRO AND THE WET COMPASS FOR DIRECTIONAL DATA THAT WAS VIEWED THROUGH A LIGHTED SLOT BY WAY OF A SMALL MIRROR. THE RPTR STATED THE OPS MANUAL DOES NOT INDICATE WHEN ATT MODE IS SELECTED, A 20 TO 30 SECOND DELAY IS ENCOUNTERED WHEN THE DISPLAYS GO BLANK. THE RPTR SAID THE DIVERSION ARPT WAS SELECTED ON GOOD WX, FLAT TERRAIN, AND LONG RWYS. THE RPTR STATED ON THE GND MAINT FOUND A DUAL INERTIAL REFERENCE AND COMPARATOR FAILURE. A CHECK OF THE MAINT HISTORY REVEALS #2 FLT GUIDANCE CHRONIC PROB AND AS RECENTLY AS EARLY NOVEMBER HAD A FAILURE OF THE #2 AIR DATA COMPUTER AND #2 NAVIGATION SYSTEM. THE RPTR SAID HE STILL FEELS THERE IS A PROB IN THIS AIRPLANE.

Data retrieved from NASA's ASRS site as of July 2007 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.