<< >> NASA ASRS Report 356060, December 1996

Twin corporate turboprop appeared to be experiencing an altitude deviation altitude undershot in climb. Upon further investigation it was found that #1 transponder altitude encoder was faulty, pilot switched to #2 transponder.

Attributes
ACN	356060
Time
Date	199612
Day	Tue
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : mvy
State Reference	MA
Altitude	msl bound lower : 3750 msl bound upper : 4000
Environment
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zbw tracon : fmh
Operator	general aviation : corporate
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	Part 91
Flight Phase	cruise other cruise other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : atp pilot : cfi pilot : commercial
Experience	flight time last 90 days : 75 flight time total : 13000 flight time type : 600
ASRS Report	356060
Person 2
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Events
Anomaly	aircraft equipment problem : less severe other anomaly other
Independent Detector	other controllera
Resolutory Action	flight crew : overcame equipment problem
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

I had been cleared to climb to 4000 ft. I arrived at 4000 ft on the altimeter. After a few mins otis approach asked me if I was showing 4000 ft. I acknowledged that I was. They asked if I had 29.95 on the altimeter. I acknowledged yes. They said they were showing 3750 ft. I said I was at 4000 ft indicated. They said have the altimeter checked. They changed me over to ZBW to check my altitude on the second altimeter and second transponder. They said they were showing 4000 ft. I concurred with them. The remaining flight was ok.

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Original NASA ASRS Text

Title: TWIN CORPORATE TURBOPROP APPEARED TO BE EXPERIENCING AN ALTDEV ALT UNDERSHOT IN CLB. UPON FURTHER INVESTIGATION IT WAS FOUND THAT #1 XPONDER ALT ENCODER WAS FAULTY, PLT SWITCHED TO #2 XPONDER.

Narrative: I HAD BEEN CLRED TO CLB TO 4000 FT. I ARRIVED AT 4000 FT ON THE ALTIMETER. AFTER A FEW MINS OTIS APCH ASKED ME IF I WAS SHOWING 4000 FT. I ACKNOWLEDGED THAT I WAS. THEY ASKED IF I HAD 29.95 ON THE ALTIMETER. I ACKNOWLEDGED YES. THEY SAID THEY WERE SHOWING 3750 FT. I SAID I WAS AT 4000 FT INDICATED. THEY SAID HAVE THE ALTIMETER CHKED. THEY CHANGED ME OVER TO ZBW TO CHK MY ALT ON THE SECOND ALTIMETER AND SECOND XPONDER. THEY SAID THEY WERE SHOWING 4000 FT. I CONCURRED WITH THEM. THE REMAINING FLT WAS OK.

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.