Narrative:

Flying in light turbulence ATC advises TA. Autoplt is on with altitude hold but unknown to me it disconnects (either I inadvertently disconnected or some other means). I spend a couple of mins in concentrated outside scanning for the traffic. Chicago then advises me of my altitude deviation (+700 from 7000 ft). Noticing the problem I misread the single needle altimeter (an instrument I find difficult to quickly interpret), and thought I had only lost 300 ft. I began to climb back to what I thought was 7000 ft, but was actually 8000 ft. At 7900 ft I caught my mistake (as did chicago) and quickly returned to 7000 ft. The mistake and altitude deviation can be contributed to human error and aircraft system design weakness. First, I obviously was fixating outside the cockpit. Of course I was right to be concerned about a potential traffic conflict, but having just recently completed the cockpit chores for the cruise phase of flight and placed the autoplt on altitude hold I too readily neglected my job as monitor of the autoplt and flight instruments. Secondly, scenarios like this could be avoided if an aural indication of pitch mode change were incorporated in the system. The annunciator light is also not easily seen. The altimeter (which uses a needle to point out hundreds of ft and numbers on a drum to indicate thousands), is not as clear to me as the conventional double (clock type) altimeter. Clearly, a greater understanding of a diligence toward the aircraft system was necessary.

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Original NASA ASRS Text

Title: CORP ACFT LOOKING FOR TFC HAS ALTDEV.

Narrative: FLYING IN LIGHT TURB ATC ADVISES TA. AUTOPLT IS ON WITH ALT HOLD BUT UNKNOWN TO ME IT DISCONNECTS (EITHER I INADVERTENTLY DISCONNECTED OR SOME OTHER MEANS). I SPEND A COUPLE OF MINS IN CONCENTRATED OUTSIDE SCANNING FOR THE TFC. CHICAGO THEN ADVISES ME OF MY ALT DEV (+700 FROM 7000 FT). NOTICING THE PROBLEM I MISREAD THE SINGLE NEEDLE ALTIMETER (AN INST I FIND DIFFICULT TO QUICKLY INTERPRET), AND THOUGHT I HAD ONLY LOST 300 FT. I BEGAN TO CLB BACK TO WHAT I THOUGHT WAS 7000 FT, BUT WAS ACTUALLY 8000 FT. AT 7900 FT I CAUGHT MY MISTAKE (AS DID CHICAGO) AND QUICKLY RETURNED TO 7000 FT. THE MISTAKE AND ALTDEV CAN BE CONTRIBUTED TO HUMAN ERROR AND ACFT SYS DESIGN WEAKNESS. FIRST, I OBVIOUSLY WAS FIXATING OUTSIDE THE COCKPIT. OF COURSE I WAS RIGHT TO BE CONCERNED ABOUT A POTENTIAL TFC CONFLICT, BUT HAVING JUST RECENTLY COMPLETED THE COCKPIT CHORES FOR THE CRUISE PHASE OF FLT AND PLACED THE AUTOPLT ON ALT HOLD I TOO READILY NEGLECTED MY JOB AS MONITOR OF THE AUTOPLT AND FLT INSTS. SECONDLY, SCENARIOS LIKE THIS COULD BE AVOIDED IF AN AURAL INDICATION OF PITCH MODE CHANGE WERE INCORPORATED IN THE SYS. THE ANNUNCIATOR LIGHT IS ALSO NOT EASILY SEEN. THE ALTIMETER (WHICH USES A NEEDLE TO POINT OUT HUNDREDS OF FT AND NUMBERS ON A DRUM TO INDICATE THOUSANDS), IS NOT AS CLR TO ME AS THE CONVENTIONAL DOUBLE (CLOCK TYPE) ALTIMETER. CLRLY, A GREATER UNDERSTANDING OF A DILIGENCE TOWARD THE ACFT SYS WAS NECESSARY.

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.