Transponder code on departure small aircraft indicates 900 ft high on radar scope, then gives correc...

Attributes
ACN	196302
Time
Date	199111
Day	Thu
Local Time Of Day	0001 To 0600
Place
Locale Reference	airport : bil
State Reference	MT
Altitude	msl bound lower : 12000 msl bound upper : 12900
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Controlling Facilities	tower : bos
Operator	common carrier : air taxi
Make Model Name	Small Aircraft, High Wing, 1 Eng, Retractable Gear
Flight Phase	climbout : initial
Route In Use	departure other enroute airway : bil
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : departure
Qualification	controller : radar
Experience	controller radar : 10
ASRS Report	196302
Person 2
Affiliation	company : air taxi
Function	flight crew : single pilot
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	other anomaly other
Independent Detector	other controllera
Resolutory Action	none taken : unable
Consequence	Other
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	other
Situations
ATC Facility	other physical facility

Narrative:

I was working the midshift on 11/91. The traffic consisted of routine air freight departures. An small aircraft X departed and climbed. At a point where I normally change the aircraft to the center I noticed the aircraft's altitude 900 ft above his assigned altitude. He (the pilot) informed me that he was just level at 120. His mode C indicated 129. On the next sweep it indicated triple X's (XXX). The next sweep it indicated 120, the assigned altitude. This has been an ongoing problem at billings and it's getting worse. I am filing this report simply as another way of bringing attention to this problem before something drastic happens here.

Original NASA ASRS Text

Title: TRANSPONDER CODE ON DEP SMA INDICATES 900 FT HIGH ON RADAR SCOPE, THEN GIVES CORRECT ALT.

Narrative: I WAS WORKING THE MIDSHIFT ON 11/91. THE TFC CONSISTED OF ROUTINE AIR FREIGHT DEPS. AN SMA X DEPARTED AND CLBED. AT A POINT WHERE I NORMALLY CHANGE THE ACFT TO THE CTR I NOTICED THE ACFT'S ALT 900 FT ABOVE HIS ASSIGNED ALT. HE (THE PLT) INFORMED ME THAT HE WAS JUST LEVEL AT 120. HIS MODE C INDICATED 129. ON THE NEXT SWEEP IT INDICATED TRIPLE X'S (XXX). THE NEXT SWEEP IT INDICATED 120, THE ASSIGNED ALT. THIS HAS BEEN AN ONGOING PROBLEM AT BILLINGS AND IT'S GETTING WORSE. I AM FILING THIS RPT SIMPLY AS ANOTHER WAY OF BRINGING ATTN TO THIS PROBLEM BEFORE SOMETHING DRASTIC HAPPENS HERE.

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.

Transponder code on departure small aircraft indicates 900 ft high on radar scope, then gives correct altitude.