Air carrier loses TCASII display on MTR directly below due location of transponder antenna on MTR.

Attributes
ACN	211027
Time
Date	199205
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : ama
State Reference	TX
Altitude	msl bound lower : 39000 msl bound upper : 41000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zab
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	cruise other
Flight Plan	IFR
Aircraft 2
Operator	other
Make Model Name	Military Trainer
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 150 flight time total : 16000 flight time type : 450
ASRS Report	211027
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

While cruising at FL410 with the TCASII situation display on, an air force MTR passed directly under us at FL390. The ARTCC reported his transponder normal with proper altitude displayed. I have observed many aircraft at lower altitudes not displayed until within about 15 NM of my aircraft. I suspect that this is caused by the other aircraft not being converted to TCASII and thus not having the top transponder antenna. I am concerned that many of the small military aircraft will not have this antenna modification and thus may be totally invisible to TCASII in other aircraft at higher altitudes.

Original NASA ASRS Text

Title: ACR LOSES TCASII DISPLAY ON MTR DIRECTLY BELOW DUE LOCATION OF TRANSPONDER ANTENNA ON MTR.

Narrative: WHILE CRUISING AT FL410 WITH THE TCASII SITUATION DISPLAY ON, AN AIR FORCE MTR PASSED DIRECTLY UNDER US AT FL390. THE ARTCC RPTED HIS TRANSPONDER NORMAL WITH PROPER ALT DISPLAYED. I HAVE OBSERVED MANY ACFT AT LOWER ALTS NOT DISPLAYED UNTIL WITHIN ABOUT 15 NM OF MY ACFT. I SUSPECT THAT THIS IS CAUSED BY THE OTHER ACFT NOT BEING CONVERTED TO TCASII AND THUS NOT HAVING THE TOP TRANSPONDER ANTENNA. I AM CONCERNED THAT MANY OF THE SMALL MIL ACFT WILL NOT HAVE THIS ANTENNA MODIFICATION AND THUS MAY BE TOTALLY INVISIBLE TO TCASII IN OTHER ACFT AT HIGHER ALTS.

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.