The lack of a complete firewall between a school bus's engine compartment and its passenger compartment led to the rapid spread of superheated gases, smoke and fire following a minor accident in Iowa in December 2017, ultimately killing the driver and his only passenger, a 16-year-old female student.

The bus driver mistakenly backed the bus too far across a rural road, stranding the bus in a ditch, blocking the bus exhaust and igniting a turbocharger fire.

Compounding the incident, interior parts of the bus were flammable when exposed to ignition sources greater than those used in tests under Federal Motor Vehicle Safety Standard 302 and in fire block tests.

The accident scene. Credit: NTSBIn reaching its conclusion, the National Transportation Safety Board (NTSB) said that the Iowa school bus fire, along with other fires reported nationally and as shown in school bus fire demonstrations, illustrates that once a school bus compartment is breached, a fire can spread quickly, and smoke, toxic gases and heat make the interior "untenable for occupancy."

The NTSB called on school bus manufacturers to install fire suppression systems on all new vehicles, that at a minimum address engine fires. It also called on manufacturers to ensure that, for any opening or penetration of the engine firewall, no hazardous amount of gas or flame can pass through the firewall from the engine compartment to the passenger compartment.

NTSB also recommended that the National Highway Traffic Safety Administration revise Federal Motor Vehicle Safety Standard 302 to adopt the more rigorous performance standards for interior flammability and smoke emissions characteristics already in use through the Department of Transportation for commercial aviation and rail passenger transportation (H-15-12).

Accident timeline

According to NTSB investigators, on Dec. 12, 2017, just before 6:50 a.m., a 2004 International 65-passenger school bus, operated by the Riverside Community School District, was traveling south on rural 480th Street outside Oakland, Iowa.

The bus driver turned right onto a residential driveway for the first student pickup on his route. After the 16-year-old female student boarded, the driver reversed out of the driveway, as was his normal practice for the location, backed across 480th Street, and continued reversing until the bus’s rear wheels ran off the road and dropped into a 3 ft deep ditch next to the road.

While the driver was attempting to drive the bus out of the ditch, a fire began in the engine compartment and spread throughout the bus. The driver and the 16-year-old passenger died in the fire.

The NTSB said the fire likely originated at the exterior of the turbocharger in the engine compartment. It said that the blocked exhaust pipe, resulting in turbocharger overload with significant heat output during repeated engine acceleration, was the primary contributing factor to the fire's start.

It said that fluids in the engine compartment fueled the fire. NTSB said that the initial fuel source could not be determined because of extensive damage to the engine compartment.

Investigators speculated that the passenger was possibly trying to help the driver, whose limited mobility due to medical conditions might have prevented him from evacuating the bus. NTSB said the passenger may not have perceived the immediate danger before being overcome by smoke and superheated gases as a result of the fire.

The school bus driver had "progressive chronic pain and stable mild right dorsal flexion leg weakness." Even so, NTSB said there was no evidence that the driver’s medical conditions and medications would have affected his ability to perform the driving functions required.

The investigators said it is likely that the bus driver’s progressive chronic back disease, which caused severe chronic pain, impaired his ability to evacuate the school bus himself or to help the passenger to evacuate.