<< >> NASA ASRS Report 511940, May 2001

B190 flight crew are questioned by ZMA to verify altitude, ZMA showing up to 400 ft low. Flight crew later determines malfunctioning altitude encoder.

Attributes
ACN	511940
Time
Date	200105
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : zma.artcc
State Reference	FL
Aircraft 1
Controlling Facilities	tower : arb.tower
Operator	common carrier : air taxi
Make Model Name	Beech 1900
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air taxi
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : cfi
Experience	flight time last 90 days : 300 flight time total : 5300 flight time type : 1800
ASRS Report	511940
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot other anomaly other
Resolutory Action	aircraft : automation overrode flight crew controller : issued alert flight crew : returned to assigned altitude flight crew : overcame equipment problem
Consequence	faa : reviewed incident with flight crew other
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Aircraft

Narrative:

Flight to pbi. We were given a descent from 12000 ft to 10000 ft by ZMA. Upon leveling at 10000 ft, center asked (rather urgently) our altitude. We responded 10000 ft. He then asked to confirm 10000 ft and said traffic ahead and below. We confirmed level at 10000 ft and asked if anything was wrong. He said he showed us at 9600-9700 ft. We confirmed again level at 10000 ft. He said ok, no problem. (It did not occur to us yet at this time that we were experiencing a malfunctioning altitude encode.) we were discussing the incident and finally realized we had a malfunctioning altitude encoder. Why it took us this long to figure it out is a mystery! Maybe fatigue. Through a series of exchanges with ZMA, we determined our #1 encoder was transmitting 300-400 ft lower than the #2 encoder, which was transmitting 10000 ft -- which agreed with both the captain's and first officer's altimeters. We switched to #2 and continued.

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Original NASA ASRS Text

Title: B190 FLT CREW ARE QUESTIONED BY ZMA TO VERIFY ALT, ZMA SHOWING UP TO 400 FT LOW. FLT CREW LATER DETERMINES MALFUNCTIONING ALT ENCODER.

Narrative: FLT TO PBI. WE WERE GIVEN A DSCNT FROM 12000 FT TO 10000 FT BY ZMA. UPON LEVELING AT 10000 FT, CTR ASKED (RATHER URGENTLY) OUR ALT. WE RESPONDED 10000 FT. HE THEN ASKED TO CONFIRM 10000 FT AND SAID TFC AHEAD AND BELOW. WE CONFIRMED LEVEL AT 10000 FT AND ASKED IF ANYTHING WAS WRONG. HE SAID HE SHOWED US AT 9600-9700 FT. WE CONFIRMED AGAIN LEVEL AT 10000 FT. HE SAID OK, NO PROB. (IT DID NOT OCCUR TO US YET AT THIS TIME THAT WE WERE EXPERIENCING A MALFUNCTIONING ALT ENCODE.) WE WERE DISCUSSING THE INCIDENT AND FINALLY REALIZED WE HAD A MALFUNCTIONING ALT ENCODER. WHY IT TOOK US THIS LONG TO FIGURE IT OUT IS A MYSTERY! MAYBE FATIGUE. THROUGH A SERIES OF EXCHANGES WITH ZMA, WE DETERMINED OUR #1 ENCODER WAS XMITTING 300-400 FT LOWER THAN THE #2 ENCODER, WHICH WAS XMITTING 10000 FT -- WHICH AGREED WITH BOTH THE CAPT'S AND FO'S ALTIMETERS. WE SWITCHED TO #2 AND CONTINUED.

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.