After coupling a tractor to a semitrailer, how do you verify the fifth wheel connection is secure?

THE TUG TEST is the definitive coupling verification because visual inspection alone cannot confirm the fifth wheel jaws are fully engaged around the kingpin. A partially latched fifth wheel can appear visually connected but release under load — with catastrophic results. CORRECT TUG TEST PROCEDURE: With the trailer brakes set (pull the trailer air supply valve to apply emergency brakes), shift into the lowest forward gear; apply steady, gentle forward traction — the idea is not to pull hard but to apply enough force to test the connection; if the tractor moves forward and the trailer does not follow (separates at the fifth wheel), the connection is NOT secure; if the whole combination moves together, the connection held. ADDITIONAL VERIFICATION: Reach under the trailer and attempt to feel the locking jaws around the kingpin through the gap — properly coupled, the jaws should be fully closed; look for the visual indicator (most fifth wheels have a safety latch pin or indicator that shows when fully engaged); the trailer should sit level with the fifth wheel, not tilted. WHAT PARTIAL COUPLING LOOKS LIKE: The kingpin is resting on top of the closed jaws rather than locked inside them — this is called 'riding on the fifth wheel'; it looks connected from outside but will release when forward pull exceeds a threshold.