Narrative:

The transponder in my aircraft, 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 transponder, so I have 2 choices. I can either turn the transponder on, dial in the assigned code, and squawk it during runup and takeoff (which I understand annoys controllers with ground clutter), or wait until I take off, and then turn on the transponder, 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 takeoff. I was given an assigned heading and altitude to exit the class B airspace by clearance delivery, which I followed after takeoff. Upon switching to departure, and arrival at my assigned altitude, I was informed that my mode C was not responding. This is not an unusual occurrence with this transponder. It seems to take several mins for the mode C to become operative. Departure asked me to recycle and reset the transponder. In addition to its other sterling features, this transponder (which I would use for a doorstop, except it's the only one that will fit in my airplane) takes the pilot's full attention 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 transponder, I drifted up 300 ft. This, of course, is the moment that the mode C chose to become active, and departure noted that I was 300 ft high and politely requested me to return to my assigned altitude at once. I recognize that 300 ft altitude deviation 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 traffic within several mi of me at the time. I also recognize that it is the responsibility of the pilot to have appropriate equipment in his airplane, and to maintain control of his aircraft at all times. However, in my opinion, further issues contributed to this incident. First, this transponder should never have been approved for use in aircraft. 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 turbulence, it has no standby position, 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 situation. Other users of this transponder, which is handy because of its small size, have reported the same problems. 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 transponder, there would have been absolutely no problem complying. In this airplane, the slightest inattn or distraction can easily result in an altitude deviation 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 aircraft. 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.

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Original NASA ASRS Text

Title: 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.

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.