The "invincible aircraft" setting is
available in the controls. When
this is on, press [Ctrl][Tab]. This adds
500 feet to your altitude.
While in flight or after you die.

[+][s] Go into slow mode.
[+][x] Turn on your joystick. Now, you can
move with your keyboard.
Here are the keys:

8 -- hit _ repeatedly _ to go forward
2 -- hit _ repeatedly _ to go backwards
4 -- hit _ repeatedly _ to go left
6 -- hit _ repeatedly _ to go right
7 -- hit _ repeatedly _ to go up
1 -- hit _ repeatedly _ to go down