Bear in mind though that 20 minutes really isn't that long to do very much (I'm not a military plane expert but I vaguely remember reading somewhere that a scramble order for a supermarine spitfire in World war II required of the order of about 45 minutes to get airborne and climb to altitude, this is however a very old memory and I don't have the source to hand so it may be completely made up, they may have developed better techniques since then as well but it isn't quick, also don't forget that even oncve you have a plane scrambled and at altitude it still has to get to the point of intercept that takes time even if the pilot is flying at full throttle, I don't know how far the nearest military base is from NYC but its easy to see 20 minutes gone before a military jet could even attend, not to mention whether someone would even be willing to give an order to shoot down a jet liner filled with civilians.