SMA PLT HAD ALTDEV.

Narrative

THE XPONDER IN MY ACFT; THOUGH ALLEGEDLY TSO'ED; HAS ALWAYS BEEN PAINFULLY SLOW TO ACCEPT NEW CODE INPUTS. IT HAS BEEN BACK TO THE MANUFACTURER SEVERAL TIMES; AND I HAVE BEEN ASSURED THAT THIS IS NORMAL FOR THIS MODEL; A TERRA TRT250D. ALSO; THERE IS NO STANDBY SETTING ON THIS MODEL XPONDER; SO I HAVE 2 CHOICES. I CAN EITHER TURN THE XPONDER ON; DIAL IN THE ASSIGNED CODE; AND SQUAWK IT DURING RUNUP AND TKOF (WHICH I UNDERSTAND ANNOYS CTLRS WITH GND CLUTTER); OR WAIT UNTIL I TAKE OFF; AND THEN TURN ON THE XPONDER; ALLOW IT TO WARM UP; DIAL IN; AND SQUAWK THE ASSIGNED CODE. ON THIS OCCASION; I ELECTED TO DIAL IN AND SQUAWK THE CODE BEFORE TKOF. I WAS GIVEN AN ASSIGNED HDG AND ALT TO EXIT THE CLASS B AIRSPACE BY CLRNC DELIVERY; WHICH I FOLLOWED AFTER TKOF. UPON SWITCHING TO DEP; AND ARR AT MY ASSIGNED ALT; I WAS INFORMED THAT MY MODE C WAS NOT RESPONDING. THIS IS NOT AN UNUSUAL OCCURRENCE WITH THIS XPONDER. IT SEEMS TO TAKE SEVERAL MINS FOR THE MODE C TO BECOME OPERATIVE. DEP ASKED ME TO RECYCLE AND RESET THE XPONDER. IN ADDITION TO ITS OTHER STERLING FEATURES; THIS XPONDER (WHICH I WOULD USE FOR A DOORSTOP; EXCEPT IT'S THE ONLY ONE THAT WILL FIT IN MY AIRPLANE) TAKES THE PLT'S FULL ATTN TO INPUT THE CODE. ONE OTHER THING I SHOULD MENTION IS THAT THE AIRPLANE IN QUESTION IS AN UNLIMITED AEROBATIC AIRPLANE; AERODYNAMICALLY UNSTABLE BY DESIGN. WHILE I WAS ATTEMPTING TO RECYCLE THE XPONDER; I DRIFTED UP 300 FT. THIS; OF COURSE; IS THE MOMENT THAT THE MODE C CHOSE TO BECOME ACTIVE; AND DEP NOTED THAT I WAS 300 FT HIGH AND POLITELY REQUESTED ME TO RETURN TO MY ASSIGNED ALT AT ONCE. I RECOGNIZE THAT 300 FT ALTDEV IS A SERIOUS INCONVENIENCE TO ATC; THOUGH AS FAR AS I COULD TELL FROM THE CHATTER ON THE RADIO; AND LOOKING AROUND; THERE WAS NO OTHER TFC WITHIN SEVERAL MI OF ME AT THE TIME. I ALSO RECOGNIZE THAT IT IS THE RESPONSIBILITY OF THE PLT TO HAVE APPROPRIATE EQUIPMENT IN HIS AIRPLANE; AND TO MAINTAIN CTL OF HIS ACFT AT ALL TIMES. HOWEVER; IN MY OPINION; FURTHER ISSUES CONTRIBUTED TO THIS INCIDENT. FIRST; THIS XPONDER SHOULD NEVER HAVE BEEN APPROVED FOR USE IN ACFT. IT IS IMPOSSIBLY DIFFICULT TO TUNE; THE GAS DISCHARGE DISPLAY IS SO DIM THAT IT CAN'T BE READ IN SUNLIGHT; THE KNOBS ARE TOO SMALL TO MANIPULATE WITH ANY PRECISION; ESPECIALLY IN THE SLIGHTEST TURB; IT HAS NO STANDBY POS; AND ITS ELECTRONIC PERFORMANCE LEAVES A GREAT DEAL TO BE DESIRED. I HAVE BEEN TOLD ON MANY OCCASIONS BY ATC THAT MY MODE C IS NOT WORKING; OR IS GIVING ERRONEOUS READINGS; AND YET NUMEROUS TRIPS TO THE AVIONICS SHOP AND BACK TO THE MANUFACTURER HAVE FAILED TO RECTIFY THE SIT. OTHER USERS OF THIS XPONDER; WHICH IS HANDY BECAUSE OF ITS SMALL SIZE; HAVE RPTED THE SAME PROBS. I CAN'T BLAME ATC IN ANY WAY FOR THIS INCIDENT. MY INSTRUCTIONS WERE SIMPLE; AND HAD I BEEN FLYING A NORMAL AIRPLANE WITH A NORMAL XPONDER; THERE WOULD HAVE BEEN ABSOLUTELY NO PROB COMPLYING. IN THIS AIRPLANE; THE SLIGHTEST INATTN OR DISTR CAN EASILY RESULT IN AN ALTDEV OF 500 FT OR EVEN 1000 FT. I AM NOT SURE THAT MOST ATC SPECIALISTS EVEN THINK ABOUT THE SPECIAL NEEDS OF THE UNLIMITED AEROBATIC ACFT. ALSO; PERHAPS THE FAA SHOULD BE A LITTLE MORE DEMANDING IN THE APPROVAL OF AVIONICS FOR GA AIRPLANES. AFTER HAVING SPENT SEVERAL TENS OF THOUSANDS OF DOLLARS ON THIS RADIO SYSTEM; I CAN'T AFFORD TO CHANGE IT; BUT I'LL CERTAINLY NEVER BUY ANOTHER ONE LIKE IT.