Narrative:

The VNAV capture function of autoplt is potentially hazardous during conditions of high aircraft climb performance (ie, light gross weight and/or low temperature). It is not uncommon for the autothrottles to pull back power below clean speed while in a turn, at 3000 ft MSL with level off at 6000 ft MSL. Even climbing at 250 KTS passing through 10000 ft en route to 12000 ft it is not uncommon for the autothrottles to pull back power and then jam it on again to accelerate. This erratic nature of the autothrottles requires the pilot's full attention, or will be missed while performing other flight duties and result in airspeed falling below safe levels. The inclusion of the 6000 ft initial climb altitude in the FMS for the redss one departure inhibits VNAV when, while on vectors, we are given clearance to a higher altitude. Please remove this restr from the FMS data bank for the departure. Callback conversation with reporter revealed the following information: this reporter said that since he has started operating this aircraft in line operations he has discovered that it does not always smoothly and correctly capture the selected altitude. The MD88 autothrottles will start reducing thrust well before they should as the aircraft approachs its altitude he said. Also he complained that in some extreme cases the autothrottles would allow the airspeed to decrease to a critical level if he were not watchful. He also complained that if the level off altitude was around 10000 ft or slightly above, the autothrottles would reduce as the altitude was captured and then go back to full power after leveling at or passing 10000 ft. Lastly, the reporter complained about a recent change in the software that was anticipating a normal clearance that is often given on a departure. However, the altitude restr is not always given yet, with the software change, the FMC always tries to meet the restr.

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Original NASA ASRS Text

Title: AN ACR CAPT RPTS THAT THE AUTOFLT SYS ON THE MD88 HAS SOME 'ERRATIC AND POTENTIALLY HAZARDOUS' CHARACTERISTICS ASSOCIATED WITH THE VNAV AND AUTOTHROTTLE FUNCTIONS. THE RPTR ALSO COMPLAINS ABOUT A NAV PROGRAM IN THE FMC.

Narrative: THE VNAV CAPTURE FUNCTION OF AUTOPLT IS POTENTIALLY HAZARDOUS DURING CONDITIONS OF HIGH ACFT CLB PERFORMANCE (IE, LIGHT GROSS WT AND/OR LOW TEMP). IT IS NOT UNCOMMON FOR THE AUTOTHROTTLES TO PULL BACK PWR BELOW CLEAN SPD WHILE IN A TURN, AT 3000 FT MSL WITH LEVEL OFF AT 6000 FT MSL. EVEN CLBING AT 250 KTS PASSING THROUGH 10000 FT ENRTE TO 12000 FT IT IS NOT UNCOMMON FOR THE AUTOTHROTTLES TO PULL BACK PWR AND THEN JAM IT ON AGAIN TO ACCELERATE. THIS ERRATIC NATURE OF THE AUTOTHROTTLES REQUIRES THE PLT'S FULL ATTN, OR WILL BE MISSED WHILE PERFORMING OTHER FLT DUTIES AND RESULT IN AIRSPD FALLING BELOW SAFE LEVELS. THE INCLUSION OF THE 6000 FT INITIAL CLB ALT IN THE FMS FOR THE REDSS ONE DEP INHIBITS VNAV WHEN, WHILE ON VECTORS, WE ARE GIVEN CLRNC TO A HIGHER ALT. PLEASE REMOVE THIS RESTR FROM THE FMS DATA BANK FOR THE DEP. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THIS RPTR SAID THAT SINCE HE HAS STARTED OPERATING THIS ACFT IN LINE OPS HE HAS DISCOVERED THAT IT DOES NOT ALWAYS SMOOTHLY AND CORRECTLY CAPTURE THE SELECTED ALT. THE MD88 AUTOTHROTTLES WILL START REDUCING THRUST WELL BEFORE THEY SHOULD AS THE ACFT APCHS ITS ALT HE SAID. ALSO HE COMPLAINED THAT IN SOME EXTREME CASES THE AUTOTHROTTLES WOULD ALLOW THE AIRSPD TO DECREASE TO A CRITICAL LEVEL IF HE WERE NOT WATCHFUL. HE ALSO COMPLAINED THAT IF THE LEVEL OFF ALT WAS AROUND 10000 FT OR SLIGHTLY ABOVE, THE AUTOTHROTTLES WOULD REDUCE AS THE ALT WAS CAPTURED AND THEN GO BACK TO FULL PWR AFTER LEVELING AT OR PASSING 10000 FT. LASTLY, THE RPTR COMPLAINED ABOUT A RECENT CHANGE IN THE SOFTWARE THAT WAS ANTICIPATING A NORMAL CLRNC THAT IS OFTEN GIVEN ON A DEP. HOWEVER, THE ALT RESTR IS NOT ALWAYS GIVEN YET, WITH THE SOFTWARE CHANGE, THE FMC ALWAYS TRIES TO MEET THE RESTR.

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.