GA SMA APPARENT OVERSHOOT OF ALT ON CLIMB. TRANSPONDER MODE C NO OUTPUT.

Narrative

WHEN DEPARTING SNA THE CLB PORTION OF THE IFR CLRNC WAS GIVEN AS; 'MAINTAIN 3000'; EXPECT 6000' 10 MINS LATER.' AT TKOF THE TWR ADVISED THAT MY CLRNC WAS AMENDED TO 'MAINTAIN 2000'; EXPECT 6000' 10 LATER.' AFTER TKOF AND RPTING TO COAST DEP CLBING THROUGH 600' COAST REPLIED WITH; 'NOT READING YOUR TRANSPONDER.' I IMMEDIATELY RECYCLED THE TRANSPONDER; BOTH THROUGH THE CODE AND ACTIVITY MODES. THE IMMEDIATE READOUT THEN WAS 2400' WITH AN ALTIMETER READOUT OF 1400'. THE TRANSPONDER ALT OF 2400' WAS RPTED WITH AN IMMEDIATE COAST INSTRUCTION TO CLB TO 3000'. IT IS FELT THAT LAST MINUTE AMENDMENTS TO A CLRNC; ESPECIALLY DURING THE INITIAL TKOF OR ANY OTHER CRITICAL PHASE OF FLT; IS DEFINITELY NOT A SAFE PROC. IN THIS PARTICULAR INSTANCE RESETTING THE ALT ALERT SYS; THE TRANSPONDER AND ACKNOWLEDGING THESE REQUESTS WAS AN ADDITIONAL BURDEN ON THE PLT. ALL THIS; ALONG WITH THE RECYCLING OF THE TRANSPONDER ALT INDICATION AND SUBSEQUENT ERRONEOUS ALT CALLOUT BY THE PLT. ALTHOUGH IN THIS CASE THERE WAS NO INVOLVEMENT OF ALT VIOLATION; OTHER ACFT OR SAFETY OF FLT; THE OVERALL SITUATION IN ANOTHER INSTANCE COULD HAVE BEEN SERIOUS. I WOULD HIGHLY RECOMMEND NOT ISSUING AMENDMENTS TO A CLRNC WHEN A PLT IS INVOLVED IN ANY CRITICAL PHASE OF A FLT; AND ESPECIALLY NOT DURING TKOF. IN THIS INSTANCE THE CHANGE IN ALT COULD JUST AS EASILY HAVE BEEN ISSUED BY COAST DEP UPON INITIAL CONTACT WITH THEM AFTER TKOF.