lenbrazil- If a plane loses airspeed without the pilot noticing while it's in trimmed flight, intially it will fly level but eventually it will start to lose altitude for two reasons. First, after some time has elapsed, friction will slow the plane down and if you try to maintain altitude at slower speeds, you will descend. Second, because the plane is trimmed for level flight at that specific airspeed, a decrease in airspeed will cause the plane to nose down. An unnoticed decrease in airspeed will cause the plane to lose altitude, especially if it's trimmed for cruising at that speed. I don't think that the loss in altitude will be directly proportional to loss in speed, but they are certainly related.
cybercat- Trim is something pilots use to facilitate level flight. The physics of the wing is designed to create natural lift, this is how flight is possible. At higher speeds, the force if lift is greater, creating a natural upward force on the plane. Planes have a mechanism called a trim wheel, which controls a panel on the elevator located in the rear of the aircraft. At higher speeds, pilots 'trim the plane down' meaning the panel on the elevator is raised so that their is now a downward force on the plane as well. This force cancels out the effect of the lift from the wings and level flight is made simple. Without the trim wheel, pilots would have constantly pitch the nose of the plane downwards to counteract lift forces of the wing.