Narrative:

After holding 1 1/2 hours at paleo on the lga minks 1 arrival at FL290, we were cleared direct dupont for the arrival. Told to cross somto at 13000 ft, we put the altitude in both the FMS and up on the MCP. Autoplt engaged, VNAV and LNAV selected (full automation) we started down. All was normal until the altitude capture took over and dropped VNAV because we were held high, the sink rate was such that altitude capture happened around 15000 ft and disabled VNAV. The autoplt shallowed the rate of descent to the point that we crossed somto at 14300 ft instead of the 13000 ft assigned. Factors contributing: 1) design of system that changes a mode without alerting the pilot. It drops the programmed restr at altitude capture. 2) during descent all looked normal, so captain looking at approach to be flown, first officer listening to and writing down new ATIS, neither pilot noted the mode change till queried by ZNY about altitude. Supplemental information from acn 343695: the bottom line is someone must always fly the aircraft. Callback conversation with reporter revealed the following information: the reporter said that the B757-200 was his first commercial glass cockpit and he was not expecting the autoflt system to react as it did. He had flown the F15 and felt that its cockpit was friendlier because of its 'voice' advisory system in which the computer tells the pilot if something changes or happens to the system. He suggested that this type of system be incorporated in the commercial aircraft rather than have another bell or chime alert the flight crew that something, unspecified, had happened. This analyst discussed with the reporter the various system now in use and the procedure used in their operation. The reporter knows that regardless of the system the pilot must monitor its performance and take over if it should not perform the needed tasks. The reporter recognized his error and from his own study, now better understands the autoflt system's limitations.

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Original NASA ASRS Text

Title: AN ACR FLC CROSSES A NAV POINT TOO HIGH WHEN THE AUTOFLT SYS CAPTURES THE LEVELOFF ALT 2000 FT ABOVE AND REVERTS TO A NORMAL DSCNT RATE VERSUS THE HIGH (OFF PROFILE) RATE USED IN THE EARLIER PORTION OF THE DSCNT.

Narrative: AFTER HOLDING 1 1/2 HRS AT PALEO ON THE LGA MINKS 1 ARR AT FL290, WE WERE CLRED DIRECT DUPONT FOR THE ARR. TOLD TO CROSS SOMTO AT 13000 FT, WE PUT THE ALT IN BOTH THE FMS AND UP ON THE MCP. AUTOPLT ENGAGED, VNAV AND LNAV SELECTED (FULL AUTOMATION) WE STARTED DOWN. ALL WAS NORMAL UNTIL THE ALT CAPTURE TOOK OVER AND DROPPED VNAV BECAUSE WE WERE HELD HIGH, THE SINK RATE WAS SUCH THAT ALT CAPTURE HAPPENED AROUND 15000 FT AND DISABLED VNAV. THE AUTOPLT SHALLOWED THE RATE OF DSCNT TO THE POINT THAT WE CROSSED SOMTO AT 14300 FT INSTEAD OF THE 13000 FT ASSIGNED. FACTORS CONTRIBUTING: 1) DESIGN OF SYS THAT CHANGES A MODE WITHOUT ALERTING THE PLT. IT DROPS THE PROGRAMMED RESTR AT ALT CAPTURE. 2) DURING DSCNT ALL LOOKED NORMAL, SO CAPT LOOKING AT APCH TO BE FLOWN, FO LISTENING TO AND WRITING DOWN NEW ATIS, NEITHER PLT NOTED THE MODE CHANGE TILL QUERIED BY ZNY ABOUT ALT. SUPPLEMENTAL INFO FROM ACN 343695: THE BOTTOM LINE IS SOMEONE MUST ALWAYS FLY THE ACFT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID THAT THE B757-200 WAS HIS FIRST COMMERCIAL GLASS COCKPIT AND HE WAS NOT EXPECTING THE AUTOFLT SYS TO REACT AS IT DID. HE HAD FLOWN THE F15 AND FELT THAT ITS COCKPIT WAS FRIENDLIER BECAUSE OF ITS 'VOICE' ADVISORY SYS IN WHICH THE COMPUTER TELLS THE PLT IF SOMETHING CHANGES OR HAPPENS TO THE SYS. HE SUGGESTED THAT THIS TYPE OF SYS BE INCORPORATED IN THE COMMERCIAL ACFT RATHER THAN HAVE ANOTHER BELL OR CHIME ALERT THE FLC THAT SOMETHING, UNSPECIFIED, HAD HAPPENED. THIS ANALYST DISCUSSED WITH THE RPTR THE VARIOUS SYS NOW IN USE AND THE PROC USED IN THEIR OP. THE RPTR KNOWS THAT REGARDLESS OF THE SYS THE PLT MUST MONITOR ITS PERFORMANCE AND TAKE OVER IF IT SHOULD NOT PERFORM THE NEEDED TASKS. THE RPTR RECOGNIZED HIS ERROR AND FROM HIS OWN STUDY, NOW BETTER UNDERSTANDS THE AUTOFLT SYS'S LIMITATIONS.

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.