To keep your pilots alive forever, make a copy of the ROSTER.DAT file before going on a dangerous mission. If you get killed, you can copy the file back and your pilot is resurrected - as though he had never flown the fatal mission.
For example, to copy the file, change to the Aces directory and type:
COPY ROSTER.DAT BACKUP.XXX
To resurrect a pilot, change to the Aces directory and then type:
COPY BACKUP.XXX ROSTER.DAT
Make sure you copy the ROSTER.DAT file every time you successfully complete a mission.