You could, in a REAL combat situation, destroy a slow attack aircraft like the Su-25 or A-10 if it was heading at a speed slower the 500 mph, right in front of you, and at a very LOW altitude. Then there is the excellent chance that the rocket will miss, or be dodged by the slow but nimble plane and your position would be compromised and you would have a 20mm round through your skull. Attack a big, slow, stupid tank or a fast but equally stupid helo seems much more attractive then shoting a small target going at 300 mph at least.