Narrative:

The problem in question arose while we were at entering en route cruise at FL330. The aircraft accelerated and power was reduced as per procedure to maintain mach 0.76. As the aircraft accelerated the forces on the control surfaces increased and the aircraft began to fly in an uncoordinated attitude. I disengaged the autoplt and manually trimmed the aircraft to a proper attitude, found it satisfactory, and reengaged the autoplt. In this aircraft the autoplt vertical speed knob detents itself back to the 0 position if the rate of climb is less than 500 FPM. While the aircraft was being retrimmed the vertical speed never exceeded 500 FPM, but upon autoplt re-engagement the autoplt vertical speed wheel apparently never detented to 0, thus giving the aircraft a descent rate of approximately 300 FPM. Immediately following re-engagement the captain and I immediately turned our attention to plotting the next segment of the flight's course on the arwy charts. I assumed that since there was a minimal vertical speed the vertical speed knob did what it should and the rate sprung to 0. I will not make this assumption twice. At approximately the same instant the altitude alerter sounded and ATC instructed us to turn 300 degrees left. ATC also asked for verification of altitude. At this time I had already initiated an autoplt disengagement and climbed back up to FL330. ATC then cancelled the request for the turn and to proceed on course. I believe that situations like these could be averted through pilots not relying so much on technology, planned designs and simple expectations. Also, altitude alerters should not be calibrated to sound at 300' from the assigned altitude for the simple fact that most aircraft, particularly jet aircraft, can generate a significant vertical speed in much less space than 300'. 1 simple solution that should be instituted is that these devices should alert the pilot when the aircraft is deviating from its altitude by no more than 150'. This would allow the pilot to correct the deviation in a more timely manner and to allow a greater margin of safety. A device which alerts the crew of a deviation at the same time ATC must issue an evasive vector is not a safety device. The flight crew, and no other personnel, should be the first to be advised of an irregularity of their own aircraft.

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Original NASA ASRS Text

Title: ALT DEVIATION WHEN 500' LOST.

Narrative: THE PROB IN QUESTION AROSE WHILE WE WERE AT ENTERING ENRTE CRUISE AT FL330. THE ACFT ACCELERATED AND PWR WAS REDUCED AS PER PROC TO MAINTAIN MACH 0.76. AS THE ACFT ACCELERATED THE FORCES ON THE CTL SURFACES INCREASED AND THE ACFT BEGAN TO FLY IN AN UNCOORDINATED ATTITUDE. I DISENGAGED THE AUTOPLT AND MANUALLY TRIMMED THE ACFT TO A PROPER ATTITUDE, FOUND IT SATISFACTORY, AND REENGAGED THE AUTOPLT. IN THIS ACFT THE AUTOPLT VERT SPD KNOB DETENTS ITSELF BACK TO THE 0 POS IF THE RATE OF CLB IS LESS THAN 500 FPM. WHILE THE ACFT WAS BEING RETRIMMED THE VERT SPD NEVER EXCEEDED 500 FPM, BUT UPON AUTOPLT RE-ENGAGEMENT THE AUTOPLT VERT SPD WHEEL APPARENTLY NEVER DETENTED TO 0, THUS GIVING THE ACFT A DSNT RATE OF APPROX 300 FPM. IMMEDIATELY FOLLOWING RE-ENGAGEMENT THE CAPT AND I IMMEDIATELY TURNED OUR ATTN TO PLOTTING THE NEXT SEGMENT OF THE FLT'S COURSE ON THE ARWY CHARTS. I ASSUMED THAT SINCE THERE WAS A MINIMAL VERT SPD THE VERT SPD KNOB DID WHAT IT SHOULD AND THE RATE SPRUNG TO 0. I WILL NOT MAKE THIS ASSUMPTION TWICE. AT APPROX THE SAME INSTANT THE ALT ALERTER SOUNDED AND ATC INSTRUCTED US TO TURN 300 DEGS LEFT. ATC ALSO ASKED FOR VERIFICATION OF ALT. AT THIS TIME I HAD ALREADY INITIATED AN AUTOPLT DISENGAGEMENT AND CLBED BACK UP TO FL330. ATC THEN CANCELLED THE REQUEST FOR THE TURN AND TO PROCEED ON COURSE. I BELIEVE THAT SITUATIONS LIKE THESE COULD BE AVERTED THROUGH PLTS NOT RELYING SO MUCH ON TECHNOLOGY, PLANNED DESIGNS AND SIMPLE EXPECTATIONS. ALSO, ALT ALERTERS SHOULD NOT BE CALIBRATED TO SOUND AT 300' FROM THE ASSIGNED ALT FOR THE SIMPLE FACT THAT MOST ACFT, PARTICULARLY JET ACFT, CAN GENERATE A SIGNIFICANT VERT SPD IN MUCH LESS SPACE THAN 300'. 1 SIMPLE SOLUTION THAT SHOULD BE INSTITUTED IS THAT THESE DEVICES SHOULD ALERT THE PLT WHEN THE ACFT IS DEVIATING FROM ITS ALT BY NO MORE THAN 150'. THIS WOULD ALLOW THE PLT TO CORRECT THE DEVIATION IN A MORE TIMELY MANNER AND TO ALLOW A GREATER MARGIN OF SAFETY. A DEVICE WHICH ALERTS THE CREW OF A DEVIATION AT THE SAME TIME ATC MUST ISSUE AN EVASIVE VECTOR IS NOT A SAFETY DEVICE. THE FLT CREW, AND NO OTHER PERSONNEL, SHOULD BE THE FIRST TO BE ADVISED OF AN IRREGULARITY OF THEIR OWN ACFT.

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.