Oberfeldwebel Franz-Josef Beerenbrock was among the top German fighter aces when he was shot down, he belonged to 10./JG 51. He achieved his first victory on 24 June 1941. On 1 August 1942 with nine victories he achieved his 100th kill. By then he was the most successful fighter pilot of JG 51. He was awarded the Knight´s Cross with Oak Leaves. But during the fall of 1942, he stopped shooting down so many Russian planes (16 more in three months). He completed over 400 combat missions.
In air combat with numeralically superior Russian fighters over Welish on November 9th, 1942, he downed three Russian fighters (victories 115.-117.), but his Messerschmitt Bf 109 F-2 (WNr.6779, white 12) fighter received a hit in the radiator and went down over Russian-held territory. After emergency landing he became P.O.W.
A few days later, the Russian fighter units in this area suddenly started using the very same tactics as Beerenbrock had used with such success. Beerenbrock´s old friends in JG 51 were certain. Beerenbrock, who had a Russian mother, has gone over to the Russian side: "Da steckt der Beerenbrock dahinter, der wird bei den 'Roten' als Jagdfliegerführer eingesetzt!" (quotation from "Jagdgeschwader 51" by Gebhard Aders and Werner Held).
The truth may never be known. But it is a fact that Beerenbrock in Russian captivity was one of the founders - together with General von Seydlitz and others - of the well-known pro-Soviet German prisoners´ organisation "Bund Deutscher Offiziere" BDO. Several years after the war, in mid-December 1949, Beerenbrock returned to West-Germany. Some years ago, he even supported the writing of the above quoted chronicle of JG 51 with information. Obviously, Beerenbrock kept some secrets to himself!
Maybe the story about Piotr Kozatchenko victory over J.F. Berrenbrock is truth, maybe it looks definitely differnt. Probably you will find answer for this and another questions in the just writed book "Red Star - Black Cross: Russian and German Fighter Pilots in Combat 1941-1945" by Christer Bergstrom!