Modc C transmission erratic on small aircraft.

Attributes
ACN	126083
Time
Date	198910
Day	Fri
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : stl
State Reference	MO
Altitude	msl bound lower : 3000 msl bound upper : 3000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : stl
Operator	common carrier : air taxi
Make Model Name	Small Transport
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air taxi
Function	flight crew : single pilot
Qualification	pilot : instrument pilot : commercial pilot : cfi
Experience	flight time last 90 days : 250 flight time total : 2300 flight time type : 50
ASRS Report	126083
Person 2
Affiliation	government : faa
Function	controller : departure
Qualification	controller : radar
Events
Anomaly	aircraft equipment problem : less severe other anomaly other
Independent Detector	other other : unspecified cockpit
Resolutory Action	none taken : unable
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

During the climb out, stl departure informed me that my transponder was not replying. I recycled the transponder to no avail. After reaching a higher altitude, departure began to receive my transponder. This is not the first occurrence like this. The same occurrence has happened during the climb out phase and descent phases. Some times the altitude is erroneous or missing (mode C), and most of the time the squawk code is not received. I believe this is due to the fact that the antenna for the transponder is mounted on the aircraft belly and transmission is blocked by the forward or aft portion of the fuselage. Possibly 2 antennas would be the answer. Since the approach and departure phases of flight are the most critical in regards to ATC (for aircraft sep), I believe this problem merits investigation. Callback conversation with reporter revealed the following: (reporter states:) my company is aware of the antenna problem, but does not have a good solution. Newer models of this aircraft have better mode transmission than the older models, so apparently the mfr is also aware of the problem and has modified the newer aircraft.

Original NASA ASRS Text

Title: MODC C TRANSMISSION ERRATIC ON SMA.

Narrative: DURING THE CLB OUT, STL DEP INFORMED ME THAT MY XPONDER WAS NOT REPLYING. I RECYCLED THE XPONDER TO NO AVAIL. AFTER REACHING A HIGHER ALT, DEP BEGAN TO RECEIVE MY XPONDER. THIS IS NOT THE FIRST OCCURRENCE LIKE THIS. THE SAME OCCURRENCE HAS HAPPENED DURING THE CLBOUT PHASE AND DSCNT PHASES. SOME TIMES THE ALT IS ERRONEOUS OR MISSING (MODE C), AND MOST OF THE TIME THE SQUAWK CODE IS NOT RECEIVED. I BELIEVE THIS IS DUE TO THE FACT THAT THE ANTENNA FOR THE XPONDER IS MOUNTED ON THE ACFT BELLY AND XMISSION IS BLOCKED BY THE FORWARD OR AFT PORTION OF THE FUSELAGE. POSSIBLY 2 ANTENNAS WOULD BE THE ANSWER. SINCE THE APCH AND DEP PHASES OF FLT ARE THE MOST CRITICAL IN REGARDS TO ATC (FOR ACFT SEP), I BELIEVE THIS PROB MERITS INVESTIGATION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: (RPTR STATES:) MY COMPANY IS AWARE OF THE ANTENNA PROB, BUT DOES NOT HAVE A GOOD SOLUTION. NEWER MODELS OF THIS ACFT HAVE BETTER MODE XMISSION THAN THE OLDER MODELS, SO APPARENTLY THE MFR IS ALSO AWARE OF THE PROB AND HAS MODIFIED THE NEWER ACFT.

Data retrieved from NASA's ASRS site as of August 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.