Final answer:
The trailer is at the right height for backing a tractor under a semitrailer when the trailer will be slightly lifted as the tractor backs under it. This indicates that the kingpin and fifth wheel are correctly aligned.
Step-by-step explanation:
You are about to back your tractor under a semitrailer. The trailer is at the right height when the trailer will be lifted slightly when the tractor backs under it.
This generally means that the kingpin is properly aligned with the fifth wheel, allowing for a secure connection without causing damage to the equipment.
If the landing gear are fully extended or the kingpin is above the fifth wheel by about 1 1/4 inches, it may indicate that the trailer is too high. Conversely, if the end of the kingpin is even with the top of the fifth wheel it may be too low. The correct height is important to ensure a safe and successful coupling of the tractor and the semitrailer.
When backing a tractor under a semitrailer, the trailer is at the right height when the end of the kingpin is even with the top of the fifth wheel.