On Oct. 21, 1999, a school bus on a field trip failed to stop at an intersection. It was hit by a dump truck. Seven bus passengers were seriously injured, 28 bus passengers and the truck driver received minor injuries.
After the accident, a mechanical inspection of the dump truck air brake system revealed the absence of a tractor protection system. This system is required to protect the air supply of the towing vehicle in case of catastrophic failure in the trailer brake system.
The truck had been modified from its original chassis form, adding the dump truck body and a hitch and altering the air brake system so it could tow a trailer with an air brake system.
Commercial Vehicle Safety Alliance inspection procedures for a tractor protection system require the inspectors to instruct the driver to release the vehicle's emergency or parking brakes, exit the vehicle, then disconnect both air lines from the towing vehicle. After both lines are disconnected, the inspector is supposed to check the trailer gladhands for escaping air. A second check is to occur when the air stops flowing from the supply line. The inspector then asks the driver to return to the tractor and apply the service brakes.
In the dump truck involved in the crash, NTSB investigators found that if an inspection skipped the last stop, it would have looked as if the tractor protection valve existed and was operating correctly.
The vehicle underwent as many as 15 separate inspections in its lifetime, and apparently this equipment deficiency was never discovered.
As a result, the NTSB recommended that the Federal Motor Carrier Safety Administration advise its staff, and that the CVSA and The Maintenance Council of the American Trucking Associations advice their members, of the importance of requiring a brake application during inspections of tractor protection systems.