According to the "109 Myths" page, at least one source is Flight Journal magazine. We'd need to read that article to find out what their source was.
However, I do have a copy of Williamson Murray's "Luftwaffe" (Nautical & Aviation Publishing Co. of America, Inc., Baltimore, 1985, ISBN 0-933852-45-2). I consider Murray to be one of the best WWII aviation historians. At page 177 we find this:
The disastrous rate of attrition was a reflection both of combat losses and numerous aircraft losses through noncombat causes. In fact, the Luftwaffe seems to have almost been in a race with its opponents to see who could destroy the most German aircraft. After a fairly respectable showing in 1940, from 1941 through 1944 the Luftwaffe lost between 40 percent and 45 percent of its total losses through noncombat causes. [footnote 1, which I'll type below.] The surprising element in such an accident rate is the fact that until the spring of 1944, few in the general staff seem to have been particularly worried about the implication of such a level of noncombat losses. At that point, however, a number of authorities awoke and began to examine the problem in detail. [footnote 2] The German safety record, however, deserves no smugness from an American audience. The Army Air Forces managed in 1943 to have no less than 20,389 major accidents in the continental United States with 2,264 pilots and 3,339 other aircrew members killed. The record for 1944 was not much better with 16,128 major accidents (1,936 pilots and 3,037 other aircrew killed. [footnote 3]
Footnote 1: Based on the figures in BA/MA, RL 2 III/1025, Gen. Qu. 6 Abt. (IIIA), "Front-Flugzeug-Verluste," 1941-1944. The percentages of noncombat losses work out as follows: Jan-Jun 1941, 44.5 percent; July-Dec 1941, 39.5 percent; Jan-Jun 1942, 45 percent; July-Dec 1942, 40.9 percent; Jan-Jun 1943, 45 percent; July-Dec 1943, 44.6 percent; Jan-Jun 1944, 37.2 percent. The decrease in the last period seems to have been the result of the fact that Allied fighters were shooting down German aircraft faster than their pilots could crash them.
Footnote 2: Among other items, see BA/MA, RL 2 II/181, OKL, Fuhrungsstab,, Ia/Ausb. Nr. 999/44, 11.4.44., "Herabsetzung von Flugzeugunfallen"'; Ia/Ausb., 25.7.44., "Verhutung von Flugzeugverlusten ohne Feindeinwirkung"; OKL Generalquartiermeister, A2 52 b 10 Nr. 1370/44, "Tote und Verletzte der Luftwaffe im Flugbetrieb ohne Feindeinwirkung"' Ia/Ausb. (IIIA), "Studie, Herabsetzung der Flugzeugverluste ohne Feindeinwirkung," 30.9.44.
Footnote 3: I'll type it tomorrow, have to leave the office now.
- oldman