Narrative:

While deviating around thunderstorms in IMC conditions at FL370; the captain's airspeed began to slowly decay to approximately 140 KIAS while the first officer and standby altimeters descended approximately 500 ft accompanied by airspeed and altitude disagree warnings. There did not appear to be any descent indications on either vsi gauge. Autoplt a was in use and maintained altitude according to the captain's altimeter. We compared the aircraft altitude with the TCAS traffic altitudes with the transponder in both position #1 and #2; which initially seemed to indicate that there may be a problem with the static system for both air data computers. We told ATC that we wanted to descend to FL330 to get in VMC conditions so that we could troubleshoot the problem. ATC subsequently cleared us to FL340 where the airplane leveled off according to the captain's altimeter. We notified ATC that we were unable rvsm and requested FL290 and were subsequently handed off to another center controller. FMC buffet alert. Messages appeared and the autoplt reverted to level change for alpha protection due to the captain's inaccurate low airspeed indication. The FMC and IRS winds were grossly inaccurate as well; which was complicated by the deferral of the right FMC before departure. We decided that; with the remaining FMC compromised and inaccurate air data information; the safest course of action would be to divert to ZZZ which was 70 mi to the north rather than troubleshoot in the air and attempt to continue the flight to the destination with limited and inaccurate display information. We were issued a further descent clearance and later chastised for not descending quickly enough. With the high workload and slew of erroneous indications and warnings; we elected to declare an emergency to get priority handling. The captain disconnected the autoplt and hand flew the aircraft while I finished reviewing the QRH procedures; notified dispatch via ACARS; notified the flight attendants and passenger; computed landing performance; and set the cockpit up for an approach and arrival in ZZZ. Normal airspeed and altitude indications appeared shortly after exiting IMC conditions. We were in a continuous descent from the time we declared the emergency until we were set up on the approach and I felt some self-imposed time pressure. ATC was extremely accommodating and was a tremendous help in managing the situation by allowing us all the time and airspace that we needed. Dispatch was also very helpful by coordinating the diversion with the station and letting us get the airplane down without unnecessary communications which would have further increased our high workload. A similar problem was noted in the logbook previously which was subsequently cleared so enhanced tracking of these problems might help. I feel that the workload management could have been handled more effectively. The captain was under a very high workload; hand flying the airplane essentially partial panel using cross panel instruments; which required me to manage the ancillary duties. I felt that I had to continuously run my thoughts and actions by him for his approval while he was task saturated since he is ultimately responsible how we handled the situation. This led to a lot of unnecessary communication and further increased both of our workloads. I feel that the captain would have been better served by having me fly the aircraft which he managed the situation. I also think that the QRH could be more explicit regarding the potential erroneous transponder and TCAS indications (ie; which adiru would give erroneous readings with which transponder).

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Original NASA ASRS Text

Title: B737-700 DEVELOPED ERRONEOUS AIRSPD AND ALT INDICATION DURING CRUISE. FLT CREW DESCENDED BELOW RVSM AND DECLARED AN EMER; DIVERTING TO A SUITABLE ARPT.

Narrative: WHILE DEVIATING AROUND TSTMS IN IMC CONDITIONS AT FL370; THE CAPT'S AIRSPD BEGAN TO SLOWLY DECAY TO APPROX 140 KIAS WHILE THE FO AND STANDBY ALTIMETERS DSNDED APPROX 500 FT ACCOMPANIED BY AIRSPD AND ALT DISAGREE WARNINGS. THERE DID NOT APPEAR TO BE ANY DSCNT INDICATIONS ON EITHER VSI GAUGE. AUTOPLT A WAS IN USE AND MAINTAINED ALT ACCORDING TO THE CAPT'S ALTIMETER. WE COMPARED THE ACFT ALT WITH THE TCAS TFC ALTS WITH THE XPONDER IN BOTH POS #1 AND #2; WHICH INITIALLY SEEMED TO INDICATE THAT THERE MAY BE A PROB WITH THE STATIC SYS FOR BOTH AIR DATA COMPUTERS. WE TOLD ATC THAT WE WANTED TO DSND TO FL330 TO GET IN VMC CONDITIONS SO THAT WE COULD TROUBLESHOOT THE PROB. ATC SUBSEQUENTLY CLRED US TO FL340 WHERE THE AIRPLANE LEVELED OFF ACCORDING TO THE CAPT'S ALTIMETER. WE NOTIFIED ATC THAT WE WERE UNABLE RVSM AND REQUESTED FL290 AND WERE SUBSEQUENTLY HANDED OFF TO ANOTHER CTR CTLR. FMC BUFFET ALERT. MESSAGES APPEARED AND THE AUTOPLT REVERTED TO LEVEL CHANGE FOR ALPHA PROTECTION DUE TO THE CAPT'S INACCURATE LOW AIRSPD INDICATION. THE FMC AND IRS WINDS WERE GROSSLY INACCURATE AS WELL; WHICH WAS COMPLICATED BY THE DEFERRAL OF THE R FMC BEFORE DEP. WE DECIDED THAT; WITH THE REMAINING FMC COMPROMISED AND INACCURATE AIR DATA INFO; THE SAFEST COURSE OF ACTION WOULD BE TO DIVERT TO ZZZ WHICH WAS 70 MI TO THE N RATHER THAN TROUBLESHOOT IN THE AIR AND ATTEMPT TO CONTINUE THE FLT TO THE DEST WITH LIMITED AND INACCURATE DISPLAY INFO. WE WERE ISSUED A FURTHER DSCNT CLRNC AND LATER CHASTISED FOR NOT DSNDING QUICKLY ENOUGH. WITH THE HIGH WORKLOAD AND SLEW OF ERRONEOUS INDICATIONS AND WARNINGS; WE ELECTED TO DECLARE AN EMER TO GET PRIORITY HANDLING. THE CAPT DISCONNECTED THE AUTOPLT AND HAND FLEW THE ACFT WHILE I FINISHED REVIEWING THE QRH PROCS; NOTIFIED DISPATCH VIA ACARS; NOTIFIED THE FLIGHT ATTENDANTS AND PAX; COMPUTED LNDG PERFORMANCE; AND SET THE COCKPIT UP FOR AN APCH AND ARR IN ZZZ. NORMAL AIRSPD AND ALT INDICATIONS APPEARED SHORTLY AFTER EXITING IMC CONDITIONS. WE WERE IN A CONTINUOUS DSCNT FROM THE TIME WE DECLARED THE EMER UNTIL WE WERE SET UP ON THE APCH AND I FELT SOME SELF-IMPOSED TIME PRESSURE. ATC WAS EXTREMELY ACCOMMODATING AND WAS A TREMENDOUS HELP IN MANAGING THE SITUATION BY ALLOWING US ALL THE TIME AND AIRSPACE THAT WE NEEDED. DISPATCH WAS ALSO VERY HELPFUL BY COORDINATING THE DIVERSION WITH THE STATION AND LETTING US GET THE AIRPLANE DOWN WITHOUT UNNECESSARY COMS WHICH WOULD HAVE FURTHER INCREASED OUR HIGH WORKLOAD. A SIMILAR PROB WAS NOTED IN THE LOGBOOK PREVIOUSLY WHICH WAS SUBSEQUENTLY CLRED SO ENHANCED TRACKING OF THESE PROBS MIGHT HELP. I FEEL THAT THE WORKLOAD MGMNT COULD HAVE BEEN HANDLED MORE EFFECTIVELY. THE CAPT WAS UNDER A VERY HIGH WORKLOAD; HAND FLYING THE AIRPLANE ESSENTIALLY PARTIAL PANEL USING CROSS PANEL INSTS; WHICH REQUIRED ME TO MANAGE THE ANCILLARY DUTIES. I FELT THAT I HAD TO CONTINUOUSLY RUN MY THOUGHTS AND ACTIONS BY HIM FOR HIS APPROVAL WHILE HE WAS TASK SATURATED SINCE HE IS ULTIMATELY RESPONSIBLE HOW WE HANDLED THE SITUATION. THIS LED TO A LOT OF UNNECESSARY COM AND FURTHER INCREASED BOTH OF OUR WORKLOADS. I FEEL THAT THE CAPT WOULD HAVE BEEN BETTER SERVED BY HAVING ME FLY THE ACFT WHICH HE MANAGED THE SITUATION. I ALSO THINK THAT THE QRH COULD BE MORE EXPLICIT REGARDING THE POTENTIAL ERRONEOUS XPONDER AND TCAS INDICATIONS (IE; WHICH ADIRU WOULD GIVE ERRONEOUS READINGS WITH WHICH XPONDER).

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.