Narrative:

Flight abcd from ZZZ to ZZZ1: the before start checklist was accomplished, which includes checking altimeter setting against indicated altitude between captain's side, first officer's side, and standby instruments. Both the captain and first officer are involved in this altimeter setting part of the before start checklist. On this particular night, the altimeter setting was 30.51. ZZZ departure cleared us to 10000 ft. Upon leveling near 10000 ft, ZZZ departure stated that our altitude was 10500 ft and the altimeter setting was 30.51. I looked at the altitude tape and it was indicating 10000 ft, but then I noticed that the altimeter setting was at 29.92. (The captain was talking to company while all this happened.) I reset the altimeter to 30.51 and immediately went down to 10000 ft. ZZZ departure confirmed 10000 ft and switched us to ZZZ1 approach control. Somehow, the altimeter setting had been changed to 29.92 without knowing it. There is a button on the knob that changes the altimeter, which, when pressed, sets 29.92. I did not, as far as I can recall, ever touch that knob after the before start checklist was complete. So, it's a mystery as to how that altimeter setting changed. Later that night, in a different aircraft, flight efgh ZZZ direct ZZZ2, I selected ZZZ2 altimeter 30.46 from 30.47, but the altimeter setting skipped 30.46 and went to 29.92 and then 29.91. All that occurred with just one click of altimeter setting knob.

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Original NASA ASRS Text

Title: A MEDIUM TRANSPORT TURBOJET FO RPTS UNCOMMANDED SWITCHING OF ALTIMETER SETTINGS FROM CORRECTED TO 29 PT 92.

Narrative: FLT ABCD FROM ZZZ TO ZZZ1: THE BEFORE START CHKLIST WAS ACCOMPLISHED, WHICH INCLUDES CHKING ALTIMETER SETTING AGAINST INDICATED ALT BTWN CAPT'S SIDE, FO'S SIDE, AND STANDBY INSTS. BOTH THE CAPT AND FO ARE INVOLVED IN THIS ALTIMETER SETTING PART OF THE BEFORE START CHKLIST. ON THIS PARTICULAR NIGHT, THE ALTIMETER SETTING WAS 30.51. ZZZ DEP CLRED US TO 10000 FT. UPON LEVELING NEAR 10000 FT, ZZZ DEP STATED THAT OUR ALT WAS 10500 FT AND THE ALTIMETER SETTING WAS 30.51. I LOOKED AT THE ALT TAPE AND IT WAS INDICATING 10000 FT, BUT THEN I NOTICED THAT THE ALTIMETER SETTING WAS AT 29.92. (THE CAPT WAS TALKING TO COMPANY WHILE ALL THIS HAPPENED.) I RESET THE ALTIMETER TO 30.51 AND IMMEDIATELY WENT DOWN TO 10000 FT. ZZZ DEP CONFIRMED 10000 FT AND SWITCHED US TO ZZZ1 APCH CTL. SOMEHOW, THE ALTIMETER SETTING HAD BEEN CHANGED TO 29.92 WITHOUT KNOWING IT. THERE IS A BUTTON ON THE KNOB THAT CHANGES THE ALTIMETER, WHICH, WHEN PRESSED, SETS 29.92. I DID NOT, AS FAR AS I CAN RECALL, EVER TOUCH THAT KNOB AFTER THE BEFORE START CHKLIST WAS COMPLETE. SO, IT'S A MYSTERY AS TO HOW THAT ALTIMETER SETTING CHANGED. LATER THAT NIGHT, IN A DIFFERENT ACFT, FLT EFGH ZZZ DIRECT ZZZ2, I SELECTED ZZZ2 ALTIMETER 30.46 FROM 30.47, BUT THE ALTIMETER SETTING SKIPPED 30.46 AND WENT TO 29.92 AND THEN 29.91. ALL THAT OCCURRED WITH JUST ONE CLICK OF ALTIMETER SETTING KNOB.

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.