Landing upside down:
Upside down landings are allowed on level 6. Pressing Option 2 + Left
or Right to perform a barrel-roll. Keep the plane upside-down and
press Up while over an airstrip to land. After landing, the cockpit
will open from the fuel tank and you may take off again by flying
backwards.

Bonus points:
Activate the afterburners as you enter the canyons on level 4 to
receive a "Gutsy" bonus of 30,000 points. Activate the afterburners
again immediately before entering the second set of canyons to
receive a "Lunatic" bonus of 65,000 points. The bonus points are
added to your score after landing.

Level Codes
Level Password
1 AAAA
2 PLAN
3 ALFA
4 BELL
5 NINE
6 LOCK
7 HAND
8 FLEA
9 LIFE