Narrative:

Cruising level at FL310. Captain and I noticed mach indication decreased rapidly from .79-.80 to .74-.75. We noticed a few clouds that might indicate possible turbulence. 'Light' had been reported in the area south. I put on the 'fasten seat belt' sign and made a safety announcement. We encountered 1-2 good jolts, but just light to moderate turbulence (mostly smooth to light). However, the mach rapidly increased back toward .80 and reached .82 mach briefly. We had requested FL280 when the autoplt began descending the aircraft from FL310, with no action from the captain. I said 'check altitude.' at approximately FL307 the aircraft began to climb back up toward FL310 and reached approximately FL314. Our initial descent did cause our TCASII to give an RA for the aircraft 2000 ft below us. TCASII indicated the aircraft 1900 ft below us. We both believe that there must have been 'wave action' similar to mountain waves near our altitude and position. For sure the atmospheric conditions had changed quickly and the DC9 autoplt attempted correction. We had no other problems that might indicate autoplt malfunction. I feel that unusual updraft/downdraft wave type action may have been the reason for the autoplt's sudden deviation from altitude. The deviations were approximately 300-400 ft from FL310. The captain, as per SOP for our company, allowed the autoplt to correct. Just as he was about to disengage and return to FL310, the autoplt began to climb the aircraft. Again, the deviations in airspeed and altitude were sudden. Altitude deviation was brief. The captain and I believe our actions were prompt and deliberate and very concerned with the safe operation of the aircraft. No one injured.

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Original NASA ASRS Text

Title: FLC OF DC9 EXPERIENCES A RAPID DECREASE IN MACH SPD AND THEN SEVERAL GOOD JOLTS OF TURB CAUSING AN ALTDEV, BOTH UP AND DOWN.

Narrative: CRUISING LEVEL AT FL310. CAPT AND I NOTICED MACH INDICATION DECREASED RAPIDLY FROM .79-.80 TO .74-.75. WE NOTICED A FEW CLOUDS THAT MIGHT INDICATE POSSIBLE TURB. 'LIGHT' HAD BEEN RPTED IN THE AREA S. I PUT ON THE 'FASTEN SEAT BELT' SIGN AND MADE A SAFETY ANNOUNCEMENT. WE ENCOUNTERED 1-2 GOOD JOLTS, BUT JUST LIGHT TO MODERATE TURB (MOSTLY SMOOTH TO LIGHT). HOWEVER, THE MACH RAPIDLY INCREASED BACK TOWARD .80 AND REACHED .82 MACH BRIEFLY. WE HAD REQUESTED FL280 WHEN THE AUTOPLT BEGAN DSNDING THE ACFT FROM FL310, WITH NO ACTION FROM THE CAPT. I SAID 'CHK ALT.' AT APPROX FL307 THE ACFT BEGAN TO CLB BACK UP TOWARD FL310 AND REACHED APPROX FL314. OUR INITIAL DSCNT DID CAUSE OUR TCASII TO GIVE AN RA FOR THE ACFT 2000 FT BELOW US. TCASII INDICATED THE ACFT 1900 FT BELOW US. WE BOTH BELIEVE THAT THERE MUST HAVE BEEN 'WAVE ACTION' SIMILAR TO MOUNTAIN WAVES NEAR OUR ALT AND POS. FOR SURE THE ATMOSPHERIC CONDITIONS HAD CHANGED QUICKLY AND THE DC9 AUTOPLT ATTEMPTED CORRECTION. WE HAD NO OTHER PROBS THAT MIGHT INDICATE AUTOPLT MALFUNCTION. I FEEL THAT UNUSUAL UPDRAFT/DOWNDRAFT WAVE TYPE ACTION MAY HAVE BEEN THE REASON FOR THE AUTOPLT'S SUDDEN DEV FROM ALT. THE DEVS WERE APPROX 300-400 FT FROM FL310. THE CAPT, AS PER SOP FOR OUR COMPANY, ALLOWED THE AUTOPLT TO CORRECT. JUST AS HE WAS ABOUT TO DISENGAGE AND RETURN TO FL310, THE AUTOPLT BEGAN TO CLB THE ACFT. AGAIN, THE DEVS IN AIRSPD AND ALT WERE SUDDEN. ALTDEV WAS BRIEF. THE CAPT AND I BELIEVE OUR ACTIONS WERE PROMPT AND DELIBERATE AND VERY CONCERNED WITH THE SAFE OP OF THE ACFT. NO ONE INJURED.

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.