Case Synopsis: Safely descending a long grade in a commercial truck requires more than just a properly functioning braking system: it also requires careful operating procedures and familiarity with the truck and surroundings. A recent case study demonstrates the very worst that can happen when all of these factors contribute in all the wrong ways.
The operator of a commercial tank truck was involved in a fatal, two-vehicle collision when his truck overturned at the bottom of a long grade. As he began his descent, the operator immediately began experiencing braking problems. Although he was able to limit his speed, he was unable to reduce it sufficiently to avoid overturning at the unexpected curve at the bottom of the grade. His vehicle struck an oncoming passenger vehicle, fatally injuring its operator.
After the incident, investigators discovered that several brake chamber push rod stroke lengths exceeded their readjust limits. Although there was debate over how and when the brakes became misadjusted, the findings were consistent with significantly reduced braking. Other deficiencies such as air leaks and improperly matched brake components were also consistent with a reduction in braking performance. Accordingly, mechanical deficiencies were identified as potential contributing factors; however, there was additional data that suggested possible driver error.
Expert Analysis: The incident truck was equipped with an engine control module (ECM) capable of recording various vehicle parameters for approximately the last 60 seconds before a recording event. After imaging (downloading) the ECM data and affirming its relevance to the subject incident, engineers began to study the various parameters within it to evaluate operator actions and decisions during the long descent.
In addition to recording vehicle speed – which showed a speed consistent with the operator’s testimony, as well as that which would result in the vehicle overturning – the ECM also recorded engine speed, clutch and brake pedal application, and the position of the engine brake (aka, Jake brake) switch. This data, along with other vehicle data such as rear axle gear and transmission gear ratios and tire size, permitted calculating which transmission gear, or gears, the operator utilized during the descent. These calculations showed the operator selected a transmission gear several ranges higher than what would normally be required for safe descent. Furthermore, the ECM data showed that the switch for the engine brake – which provides substantial braking effects independent of the primary brake system – was in the OFF position.
Result: Modern truck braking and drivetrain systems are engineered such that appropriate transmission gear selection and engine brake usage should allow an operator to safely descend a long grade with little, if any, brake pedal application; however, as with most engineered systems, proper operation is often predicated on adequate maintenance and careful use. This case settled prior to trial.
For additional information, please contact R. Scott King, BSME,
Mechanical / Automotive expert with DJS Associates, at experts@forensicDJS.com.