Narrative:

This report being submitted because, although cleared to descend from point of occurrence (approximately FL340), we began descent as part of possible low speed stall recovery, and obtained final altitude confirmation on the way down, rather than waiting at altitude for final flight level assignment. Deviation downward was no more than 700'. We were VMC, not in radar contact. No other reported traffic of any kind. On the leg from rctp to vtbd, the captain's mach/airspeed indicator and the standby mach/airspeed indicator were both erratic, fluctuating from .83 mach (assigned cruise speed) to .79 mach for no apparent reason, with no power or attitude changes. We wrote the captain's system up going into vtbd, but they were unable to duplicate or fix the problem and it was signed off as not being duplicated. On the leg from vtbd to wsss, takeoff and climb out were normal. It was the captain's leg and we were in a climb to FL370. Autoplt #1 (the only one we had) and both automatic throttles were on. Power was set in the climb to maintain .82 mach. After passing FL310, we noticed the first officer's mach/airspeed indicator gradually increase for no reason. Outside the aircraft it was totally dark (except for stars and hundreds of fishing boats with 1 light on each which joined with the stars at the horizon). At FL330, his airspeed pointer exceeded his 'barber pole' and the high speed aural clacker activated. The captain's mach/airspeed indicator was steady at mach .82. I disconnected the automatic throttles. At that point the captain's airspeed indicator began to show a decrease in airspeed (coinciding with the standby mach/airspeed indicator). Because of the problems we had with the captain's mach/airspeed on the way to vtbd, we did not automatically assume that the first officer's system was faulty. In fact, when the first officer's mach/airspeed indicator kept increasing, we instinctively pulled the power back to silence the clacker. Since it was the 'first' abnormal indication we had, we continued to climb for another 2000', the high speed clacker still activated, the first officer's airspeed indicating an increase and the captain's indicating a decrease. Finally, the aircraft began to shake, and despite al the reasons not to assume that we were in a high speed mach tuck, that is exactly what it felt like--and our 'valid' airspeed INS was telling us to slow down. At FL340, I disconnected the autoplt and pulled the nose up, whereupon the stick shaker activated, and we realized that we were, in fact, about to enter a low speed stall! Stall recovery procedures were instantly performed. The aircraft leveled and all of us had an instant case of vertigo, spatial disorientation, because we had no idea what our actual airspeed was. We requested and received descent clearance from bangkok radio to FL290. Shortly, the second officer gave us fuel flow, attitude and N1 figures for .83 mach at FL290. The clacker was still sounding. We finally pulled the engine fire and over-speed aural warning circuit breaker (B3) to silence the clacker. We also had the 'select flap override' and 'select elevator fell manual' yellow caution lights illuminated and the 'hydraulic press reset' cue lights on. We elected to proceed to wsss and considered declaring an emergency, but seemed to have the aircraft under control, so did not. At FL290, we calculated that 6100 pph/engine would hold .83 mach. The first officer's airspeed indicator was at .89 mach plus, and the captain's was anywhere from .81-.77 mach. The WX was VFR. We had figures worked out for descent and traffic pattern airspds based on fuel flow, attitude and N1, and having accomplished all pertinent emergency/abnormal checklists, we began descent/landing preparations, and using all possible navaids and the longest runway available, flew an ILS to 02L. We never lost the tat gauge. We never had any cdac flags or loss indications (i.e., an ADI going 90 degrees sideways). We wrote the aircraft up and were later informed (via phone conversations with mocc) that we had, in fact, lost the first officer's CADC. We had no indication of this in flight.

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Original NASA ASRS Text

Title: ACR WDB LOST FO CADC AT TOP OF CLIMB AND STARTED TO STALL AS FLT CREW RESPONDED TO INCORRECT AIRSPEED READING.

Narrative: THIS RPT BEING SUBMITTED BECAUSE, ALTHOUGH CLRED TO DSND FROM POINT OF OCCURRENCE (APPROX FL340), WE BEGAN DSNT AS PART OF POSSIBLE LOW SPD STALL RECOVERY, AND OBTAINED FINAL ALT CONFIRMATION ON THE WAY DOWN, RATHER THAN WAITING AT ALT FOR FINAL FLT LEVEL ASSIGNMENT. DEVIATION DOWNWARD WAS NO MORE THAN 700'. WE WERE VMC, NOT IN RADAR CONTACT. NO OTHER RPTED TFC OF ANY KIND. ON THE LEG FROM RCTP TO VTBD, THE CAPT'S MACH/AIRSPD INDICATOR AND THE STANDBY MACH/AIRSPD INDICATOR WERE BOTH ERRATIC, FLUCTUATING FROM .83 MACH (ASSIGNED CRUISE SPD) TO .79 MACH FOR NO APPARENT REASON, WITH NO PWR OR ATTITUDE CHANGES. WE WROTE THE CAPT'S SYS UP GOING INTO VTBD, BUT THEY WERE UNABLE TO DUPLICATE OR FIX THE PROB AND IT WAS SIGNED OFF AS NOT BEING DUPLICATED. ON THE LEG FROM VTBD TO WSSS, TKOF AND CLBOUT WERE NORMAL. IT WAS THE CAPT'S LEG AND WE WERE IN A CLB TO FL370. AUTOPLT #1 (THE ONLY ONE WE HAD) AND BOTH AUTO THROTTLES WERE ON. PWR WAS SET IN THE CLB TO MAINTAIN .82 MACH. AFTER PASSING FL310, WE NOTICED THE F/O'S MACH/AIRSPD INDICATOR GRADUALLY INCREASE FOR NO REASON. OUTSIDE THE ACFT IT WAS TOTALLY DARK (EXCEPT FOR STARS AND HUNDREDS OF FISHING BOATS WITH 1 LIGHT ON EACH WHICH JOINED WITH THE STARS AT THE HORIZON). AT FL330, HIS AIRSPD POINTER EXCEEDED HIS 'BARBER POLE' AND THE HIGH SPD AURAL CLACKER ACTIVATED. THE CAPT'S MACH/AIRSPD INDICATOR WAS STEADY AT MACH .82. I DISCONNECTED THE AUTO THROTTLES. AT THAT POINT THE CAPT'S AIRSPD INDICATOR BEGAN TO SHOW A DECREASE IN AIRSPD (COINCIDING WITH THE STANDBY MACH/AIRSPD INDICATOR). BECAUSE OF THE PROBS WE HAD WITH THE CAPT'S MACH/AIRSPD ON THE WAY TO VTBD, WE DID NOT AUTOMATICALLY ASSUME THAT THE F/O'S SYS WAS FAULTY. IN FACT, WHEN THE F/O'S MACH/AIRSPD INDICATOR KEPT INCREASING, WE INSTINCTIVELY PULLED THE PWR BACK TO SILENCE THE CLACKER. SINCE IT WAS THE 'FIRST' ABNORMAL INDICATION WE HAD, WE CONTINUED TO CLB FOR ANOTHER 2000', THE HIGH SPD CLACKER STILL ACTIVATED, THE F/O'S AIRSPD INDICATING AN INCREASE AND THE CAPT'S INDICATING A DECREASE. FINALLY, THE ACFT BEGAN TO SHAKE, AND DESPITE AL THE REASONS NOT TO ASSUME THAT WE WERE IN A HIGH SPD MACH TUCK, THAT IS EXACTLY WHAT IT FELT LIKE--AND OUR 'VALID' AIRSPD INS WAS TELLING US TO SLOW DOWN. AT FL340, I DISCONNECTED THE AUTOPLT AND PULLED THE NOSE UP, WHEREUPON THE STICK SHAKER ACTIVATED, AND WE REALIZED THAT WE WERE, IN FACT, ABOUT TO ENTER A LOW SPD STALL! STALL RECOVERY PROCS WERE INSTANTLY PERFORMED. THE ACFT LEVELED AND ALL OF US HAD AN INSTANT CASE OF VERTIGO, SPATIAL DISORIENTATION, BECAUSE WE HAD NO IDEA WHAT OUR ACTUAL AIRSPD WAS. WE REQUESTED AND RECEIVED DSNT CLRNC FROM BANGKOK RADIO TO FL290. SHORTLY, THE S/O GAVE US FUEL FLOW, ATTITUDE AND N1 FIGURES FOR .83 MACH AT FL290. THE CLACKER WAS STILL SOUNDING. WE FINALLY PULLED THE ENG FIRE AND OVER-SPD AURAL WARNING CB (B3) TO SILENCE THE CLACKER. WE ALSO HAD THE 'SELECT FLAP OVERRIDE' AND 'SELECT ELEVATOR FELL MANUAL' YELLOW CAUTION LIGHTS ILLUMINATED AND THE 'HYD PRESS RESET' CUE LIGHTS ON. WE ELECTED TO PROCEED TO WSSS AND CONSIDERED DECLARING AN EMER, BUT SEEMED TO HAVE THE ACFT UNDER CTL, SO DID NOT. AT FL290, WE CALCULATED THAT 6100 PPH/ENG WOULD HOLD .83 MACH. THE F/O'S AIRSPD INDICATOR WAS AT .89 MACH PLUS, AND THE CAPT'S WAS ANYWHERE FROM .81-.77 MACH. THE WX WAS VFR. WE HAD FIGURES WORKED OUT FOR DSNT AND TFC PATTERN AIRSPDS BASED ON FUEL FLOW, ATTITUDE AND N1, AND HAVING ACCOMPLISHED ALL PERTINENT EMER/ABNORMAL CHKLISTS, WE BEGAN DSNT/LNDG PREPARATIONS, AND USING ALL POSSIBLE NAVAIDS AND THE LONGEST RWY AVAILABLE, FLEW AN ILS TO 02L. WE NEVER LOST THE TAT GAUGE. WE NEVER HAD ANY CDAC FLAGS OR LOSS INDICATIONS (I.E., AN ADI GOING 90 DEGS SIDEWAYS). WE WROTE THE ACFT UP AND WERE LATER INFORMED (VIA PHONE CONVERSATIONS WITH MOCC) THAT WE HAD, IN FACT, LOST THE F/O'S CADC. WE HAD NO INDICATION OF THIS IN FLT.

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.