1994 Ford F-150 AIR

No, it isn't normal. What you are looking for is a vacuum leak. WHat happens is this. The vent doors that direct air flow are actuated by vacuum. If you have a small vacuum leak, it will still work. However, when you go up a hill, you press harder on the gas whick opens the throttle plate thus lowering the intake vacuum. When the vacuum is lowered along with a leak, the vent door changes directions and stops coming from the dash.

Check under the dash and the hood for vacuum lines that are damaged, dry rotted, disconnected. If you are lucky, with the engine running, you may hear a hissing from a leak.

Let me know if you have other questions.
Joe