Reporter says he changed the barometric setting for the mode C encoder. Did not think this was possi...

Attributes
ACN	194498
Time
Date	199111
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : sea
State Reference	WA
Altitude	msl bound lower : 10000 msl bound upper : 10000
Environment
Flight Conditions	IMC
Light	Dusk
Aircraft 1
Controlling Facilities	tracon : sea
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	cruise other descent other
Route In Use	arrival star : star
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : flight engineer pilot : instrument
Experience	flight time last 90 days : 180 flight time total : 7800 flight time type : 20
ASRS Report	194498
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe other anomaly other
Independent Detector	other flight crewa
Resolutory Action	flight crew : overcame equipment problem
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	other

Narrative:

While on the chins 1 arrival to seattle, wa, approaching auburn intersection at 8 DME, I leveled of at 10000 ft as instructed by ATC. I noticed that the aural warning on this particular altitude alerter was quite different from that of other altitude alerters. I reached for the adjustment knob on the encoder (barometric adjustment knob). I gave it a turn thinking it was a backgnd light for the transponder. It was actually the barometric adjustment for the encoder. This may have caused my encoded altitude to be lower than 10000 ft. Right away I adjusted the barometric knob to the proper altimeter setting. Nothing was said by ATC. The remainder of the flight was normal. This is a classic case of fleet nonstandardization. Picking an large transport type B after flying only type a series. Yes, there is a differences section, and no, I can't remember every difference when I see a dash type B every 90 days.

Original NASA ASRS Text

Title: RPTR SAYS HE CHANGED THE BAROMETRIC SETTING FOR THE MODE C ENCODER. DID NOT THINK THIS WAS POSSIBLE.

Narrative: WHILE ON THE CHINS 1 ARR TO SEATTLE, WA, APCHING AUBURN INTXN AT 8 DME, I LEVELED OF AT 10000 FT AS INSTRUCTED BY ATC. I NOTICED THAT THE AURAL WARNING ON THIS PARTICULAR ALT ALERTER WAS QUITE DIFFERENT FROM THAT OF OTHER ALT ALERTERS. I REACHED FOR THE ADJUSTMENT KNOB ON THE ENCODER (BAROMETRIC ADJUSTMENT KNOB). I GAVE IT A TURN THINKING IT WAS A BACKGND LIGHT FOR THE TRANSPONDER. IT WAS ACTUALLY THE BAROMETRIC ADJUSTMENT FOR THE ENCODER. THIS MAY HAVE CAUSED MY ENCODED ALT TO BE LOWER THAN 10000 FT. RIGHT AWAY I ADJUSTED THE BAROMETRIC KNOB TO THE PROPER ALTIMETER SETTING. NOTHING WAS SAID BY ATC. THE REMAINDER OF THE FLT WAS NORMAL. THIS IS A CLASSIC CASE OF FLEET NONSTANDARDIZATION. PICKING AN LGT TYPE B AFTER FLYING ONLY TYPE A SERIES. YES, THERE IS A DIFFERENCES SECTION, AND NO, I CAN'T REMEMBER EVERY DIFFERENCE WHEN I SEE A DASH TYPE B EVERY 90 DAYS.

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.

Reporter says he changed the barometric setting for the mode C encoder. Did not think this was possible.