I was recently on two flights on MD-80 aircraft seated behind the wing and I noticed different flap settings were used for the two landings. I have been told previously that MD-80s use either a Flap setting of 28 or 40 for landings. However, the conditions for the landings were similar--clear day, winds were light and variable, dry runway, and runway length was similar (one landing was at ATL; the other was at DFW). The only thing I don't know, of course, was the aircraft weight.
Can someone tell me what criteria would a pilot use, or under what conditions would a pilot choose, to use one flap setting or the other--i.e., what conditions dictate to an MD-80 pilot which flap setting to use for landing?