This statement from the organisers is unclear about whether or not he has a Tracker.
Contrary to our advice to return to Khartoum, Mr. Kirk, 72, departed Ad-Damazin, Sudan at approximately 1400 local time (1100 GMT) on the +/- 3 hour flight to Gambella. No radio communications or satellite tracking were received at any time during the flight, and his location is unknown. Mr. Kirk is a very experienced pilot, and so it is believed that he has made a precautionary landing before sunset, somewhere in Ethiopia. The Ethiopian authorities, and his family, have been informed and search and rescue procedures will begin at first light.