December 7, 2015:
The Russian Su-24 shot down in Syria by a Turkish F-16 on November 24th for being in Turkish air space may have been the victim of self-inflicted communications problems. After talking to the Su-24 pilot and checking the equipment configuration of the downed aircraft if was discovered that the Su-24 was not equipped to receive messages on the frequency Turkish air controllers used to send emergency calls. The Turks insisted their air controllers tried ten times in five minutes to warn the Su-24 to get out of Turkish air space but never got a response. The Turks produced recordings of these messages.
This was not an unknown problem. Russia worked out emergency procedures (for their warplanes over Syria) with the United States, Israel and France but not with Turkey. Apparently the Turkish air controllers were able to warn other Russian aircraft successfully but it turned out that the Su-24s had not had their radio equipment upgraded to receive messages from the Turkish controllers. The Russians still insist that their Su-24 was never inside Turkey and that the incident was deliberate. Russians and Turks will be arguing about this for years to comes, as they do about similar incidents going back centuries.