sentences:  Later, he went on to study in Nazi Germany, where he graduated from the fighter pilot school in Werneuchen in 1938.In the summer of 1943, he was sent to the English Channel with another Bulgarian pilot to observe the German pilots and master aerial warfare tactics.On 20 December 1943, a group of 200 American B-24 Liberator bombers and fighter planes headed to Sofia in order to bomb the Bulgarian capital once again.