A sensible Avatar would use seismic sense to see if that really is a cave that she's heading towards, or might at least earthbend his way down the side of the cliff.

A sensible Avatar would not see the cave, recognize that it's on the face of a cliff, and then gleefully throw himself off said cliff because....cliff.

No one ever said Aang was a sensible Avatar.
He's well into the air and falling by the time he brings his glider into play. The wings snapping open with on command, easily catching and riding the ocean breeze.

At least his joy is somewhat quiet.