§393.70 covers coupling devices for tractor–semitrailer and truck–trailer combinations: fifth wheels, kingpins, pintle hooks, drawbars, safety chains, and the mounting hardware that ties them to the chassis. The rule requires that each be of adequate strength, properly secured, and equipped with a positive locking mechanism that cannot release unintentionally.
Severity weight + OOS
Severity weight 5 in the Vehicle Maintenance BASIC. Cracked or damaged fifth-wheel plates, mounting bolts that are missing or loose, kingpin damage, or a locking mechanism that cannot fully engage are OOS conditions under the NAS Out-of-Service Criteria. The failure mode is uncoupling under load — the most consequential equipment failure on a combination vehicle.
How it gets cited
The two patterns: (1) visible cracks at the fifth-wheel mounting brackets or in the plate itself, found by inspector visual; (2) a coupling test that reveals the jaws do not fully close around the kingpin — the trailer is "secured" but the locking pin is not seated.
How to prevent it
- Coupling inspection on every drop-and-hook — visual of the kingpin in the jaws, tug test against the brakes, look for the locking pin position.
- Annual fifth-wheel inspection should pull the plate, inspect mounting bolts, replace torn or thinning bushings, and re-grease.
- Treat any §393.70 finding as a drop-everything event — coupling failures are the highest-severity equipment incidents in the fleet.
How Roadworthy HQ helps
§393.70 findings link to both the tractor and the trailer record, trigger a critical-severity alert, and route through the §396.9(d)(3) repair certification before either vehicle can be dispatched.