An LJ60 first officer experienced mode C problems during departure at aus and experienced an near midair collision with a cessna while trying to rectify the problem for ATC.

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Attributes
ACN	453253
Time
Date	199910
Day	Sun
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : aus.airport
State Reference	TX
Altitude	msl single value : 3000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : aus.tower
Operator	general aviation : corporate
Operating Under FAR Part	Part 91
Flight Phase	climbout : takeoff
Route In Use	departure : on vectors
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tower : aus.tower
Make Model Name	Cessna Single Piston Undifferentiated or Other Model
Flight Phase	climbout : takeoff
Person 1
Affiliation	company : corporate
Function	flight crew : first officer
Qualification	pilot : atp pilot : multi engine
Experience	flight time last 90 days : 40 flight time total : 4200 flight time type : 140
ASRS Report	453253
Person 2
Affiliation	company : corporate
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe non adherence : required legal separation
Independent Detector	other controllera other flight crewa
Resolutory Action	controller : issued advisory
Miss Distance	horizontal : 500 vertical : 100
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance ATC Human Performance
Primary Problem	Aircraft

Narrative:

In the morning of oct/xa/99, we were doing a repositioning leg to austin, tx, out of addison airport, tx. A C172 or maybe a C182 had been cleared for takeoff before us. After a short delay, we got takeoff clearance behind the cessna with a distance sufficient to provide separation between both aircraft. Shortly after takeoff the tower advised us that they were not receiving our mode C. After a few tries, we were to change encoders, and tower received our mode C. During this time, we were aware of the cessna that departed before us being displaced on TCASII and later, visual contact. By the time we had the mode C working, tower questioned us about our close proximity to the cessna and if we had visual, which we did! We got within approximately 500 ft of each other, with us having visual contact the whole time and throttle back to avoid too much of a proximity to other aircraft.

Original NASA ASRS Text

Title: AN LJ60 FO EXPERIENCED MODE C PROBS DURING DEP AT AUS AND EXPERIENCED AN NMAC WITH A CESSNA WHILE TRYING TO RECTIFY THE PROB FOR ATC.

Narrative: IN THE MORNING OF OCT/XA/99, WE WERE DOING A REPOSITIONING LEG TO AUSTIN, TX, OUT OF ADDISON ARPT, TX. A C172 OR MAYBE A C182 HAD BEEN CLRED FOR TKOF BEFORE US. AFTER A SHORT DELAY, WE GOT TKOF CLRNC BEHIND THE CESSNA WITH A DISTANCE SUFFICIENT TO PROVIDE SEPARATION BTWN BOTH ACFT. SHORTLY AFTER TKOF THE TWR ADVISED US THAT THEY WERE NOT RECEIVING OUR MODE C. AFTER A FEW TRIES, WE WERE TO CHANGE ENCODERS, AND TWR RECEIVED OUR MODE C. DURING THIS TIME, WE WERE AWARE OF THE CESSNA THAT DEPARTED BEFORE US BEING DISPLACED ON TCASII AND LATER, VISUAL CONTACT. BY THE TIME WE HAD THE MODE C WORKING, TWR QUESTIONED US ABOUT OUR CLOSE PROX TO THE CESSNA AND IF WE HAD VISUAL, WHICH WE DID! WE GOT WITHIN APPROX 500 FT OF EACH OTHER, WITH US HAVING VISUAL CONTACT THE WHOLE TIME AND THROTTLE BACK TO AVOID TOO MUCH OF A PROX TO OTHER ACFT.

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.