* Gravity: The primary reason for the speed increase is gravity. When a plane dives, it's essentially falling, and gravity pulls it downwards, accelerating it.

* Air Resistance: While air resistance does act against the plane, it's not strong enough to completely counteract the effect of gravity, especially at higher speeds.

* Engine Power: The plane's engine can also contribute to the speed increase. If the engine is at full power, it will continue to accelerate the plane downwards.

Important Notes:

* Terminal Velocity: There's a limit to how fast a plane can fall. This is called terminal velocity, the point where the force of air resistance equals the force of gravity, and the plane stops accelerating.

* Controlled Descent: Pilots don't usually let planes freefall. They use control surfaces to maintain a controlled descent, which is still faster than level flight, but allows for a safe landing.

In Summary:

A plane's speed increases when it dives because gravity pulls it downwards, accelerating it. The engine can further contribute to this speed increase. While air resistance plays a role, it's not enough to completely negate the effects of gravity at higher speeds.