Particularly if you're trying to get a goon in without being noticed, if you don't have grass stains on the underside of your wings, you're flying too high.
I've gotten a couple kills in an Il-2 on the pizza map when I was sailing down a canyon and picked up someone up above the canyon who dove in on me and didn't think about where they were going to get the airspace to pull out of their dive while I was skating along trying not to scrape a wingtip on the canyon wall.
Of course, you can't expect them all to be that obligingly stupid.