Per the 7th Edition Rule book – Page 26 – Landing at an Airfield
If a plane is able to return to its home field, it can figure its landing chances normally. Since this is not considered an emergency landing, only 5% should be subtracted from each critical hit, rather than the 10% subtracted if forced down elsewhere.
Planes in distress that have made it back to their own lines can land in a back area, but are not considered to be landing at their home field. Home fields are located a considerable distance behind the lines, and the only airfields near the lines are emergency fields.
If a pilot wants to reach such an airfield, he must roll the number of turns it is from his lines or from his current location if he is already behind his own lines. Use the method described for determining the width of the front, but add three to the first roll for number of dice to be rolled. The pilot must travel that additional distance to the field. Or , having gained his lines and reached a back area, the pilot may decide to land without trying to reach an airfield.
I hope this helps Dan.