Hi!
Firstly, i will say that Mechjeb is a Great autopilot as it is, especially for rockets, however it lacks a few very basic needs for spaceplanes.

One of the major issues is in relating to the Speed hold function. While using ground speed is fine for rockets, it is basically USELESS for flying aircraft such as spaceplanes. It would be absolutely GREAT if mechjeb could be configured to maintain a set AIRSPEED ( EAS ) when being used with spaceplanes (make it user selectable). the FAR Mod provides us with EAS as it is so it shouldnt be hard to implement this small but very handy change.

Because Mechjeb can maintain groundspeed, then when ever the pilot needs to change altitude in the spaceplane, he must manually control the throttle until the altitude stabilises so the correct groundspeed can be entered. This is not a bad problem at low to medium altitude, but when flying at very high altitudes, the available airspeed envelope becomes VERY SMALL. TO slow and you stall, try to go to fast and your jet engines flame out and you possibly loose control.

If we could have the ability to maintain a selected Airspeed then that would be Fantastic.
Thanks.

I use the Smart a.s.s. to set the a heading for my planes and if you change the accent auto pilot it can easily fly you into orbit once you get to about 30km.