Using my Thrustmaster Hotas X throttle, assigned to Collective Analogue Raise and Lower, I can set it to a certain position and it will maintain my altitude as I fly forward at high speed with the nose pitched down.

Trying this with the keyboard and I start losing altitude as soon as I stop pressing my raise collective key (W), so that I have to keep tapping it to avoid flying into the side of the hill in front of me and causing me to bob up and down instead of flying at a constant altitude.