As Moo directed to, I can explain in this post;
There is a blue altimeter located right next to your steering-wheel and velocity-meter. If you carefully look, you'll see the blue liquid inside increasing in volume as you go higher, and decreasing in volume / shrinking as you go lower.
That's a form of altimeter, and it is sufficient for any pilot (or non-pilot) controlling the ship.
Good suggestion, though! But it is already ingame.