About after Luke landed. Quentin Reynolds'
They Fought for the Sky quotes a sworn document by the Mayor of Murvaux and thirteen townsmen.
"The undersigned, living in Murvaux... certify to have seen on 19th September 1918 towards evening an American aviator followed by an escadrille of Germans in the direction of Liny... (this describes the action with the balloons)... From there he came back over Murvaux and still with his guns he killed six German soldiers and wounded as many more. Following this he landed and got out of his machine, undoubtedly to quench his thirst at the stream (they say he was apparently wounded) He had gone fifty yards when, seeing the Germans come towards him, he still had the strength to draw his revolver to defend himself. A moment after he fell dead following a serious wound he received in the chest. The undersigned themselves placed the body of the aviator on the wagon and conducted it to the cemetery."
I dont understand the bit about wanting a drink, I suspect he had damage or a bad wound but they werent to know that.
I dont know how reliable this source is, I guess no one can ever really know, but apart from the German soldiers, they are probably the only eyewitnesses so I'm inclined to believe them.
They don't say anything about any Germans killed in the firefight so maybe that info came from a German report. But anyway there's my answer - although there can be confusing elements, in my opinion you can't get much more reliable than eyewitness accounts, and in those days, a man's word meant a lot more than it does today. So I believe in their honesty with regard to the facts, even if I wonder at the accuracy of their interpretation. Maybe someone else can spread more light on the subject, I'm not claiming I know that much about it.