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"Lessons Learned" is a series of drop-in articles prepared by the NTSB for use 
in publications of other organizations.  An index of articles is available at
http://www.ntsb.gov/events/journalist/lessons/lessons.htm
05/2006
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Lessons Learned from a Fatal Crash

Truck Air Brake Warning:  Manually Adjusting Automatic Slack Adjusters is Dangerous, 
Can Lead to Deadly Consequences

      Safety officials are warning hundreds of thousands of heavy truck operators, drivers, 
mechanics, and federal and state commercial vehicle inspectors about the dangers of manually 
adjusting automatic slack adjusters on vehicles equipped with air brakes. 
      
      National Transportation Safety Board (NTSB) Acting Chairman Mark V. Rosenker 
bluntly warned: “Manually adjusting automatic slack adjusters is dangerous. It should not be 
done, except during installation or in an emergency to move the vehicle to a repair facility.”  He 
emphasized that manual adjustment of this brake component masks the real reason why the 
brakes are not maintaining adjustment, giving the driver a false sense of security about the 
effectiveness of the brakes, which will likely go out of adjustment again soon.  It also causes 
abnormal wear to the internal adjusting mechanism for most automatic slack adjusters, which 
may lead to failure of this brake component.
      
      The warning comes as a result of an NTSB investigation into a fatal runaway dump truck 
accident in Glen Rock, Pennsylvania, that has shown the deadly consequences of improper 
maintenance of automatic slack adjusters for air brake systems.  In the April 11, 2003, accident, 
a dump truck was traveling on a steep downgrade when the driver found he was unable to stop 
the truck. The truck struck four passenger cars, one of which struck three children who were on a 
nearby sidewalk. A driver and an 11-year-old child from one of the passenger cars were killed. 
      
      The NTSB concluded that the mechanics who worked on this truck did not look for 
underlying problems with the slack adjusters or other brake components.  They misdiagnosed the 
brake problems, probably because they were not properly trained on the function and care of 
automatic slack adjusters and how they relate to foundation brake systems. Consequently, they 
repeatedly manually adjusted the automatic slack adjusters, a dangerous practice.  The NTSB has 
seen similar actions from a driver who worked on a truck involved in a similar accident 
investigated recently in El Cerrito, California.
      
      “The warnings in existing materials available to owners, drivers, mechanics, and 
inspectors of air-braked vehicles equipped with automatic slack adjusters have not been 
successful in communicating the inherent dangers of manually adjusting automatic slack 
adjusters to correct out-of-adjustment brakes,” the NTSB said.
      
      Even organizations that specialize in truck maintenance and repair often give out wrong 
or inadequate information on automatic slack adjusters. During the probe into the Pennsylvania 
accident, investigators found that several private study guides of the ASE’s truck brake test 
inadequately cover the maintenance of automatic slack adjuster–equipped brakes, and some 
contain incorrect information.  NTSB said one study guide wrongly states, “Automatic slack 
adjusters may require periodic adjustment.”  The NTSB is concerned because many mechanics 
use the study guides as a source of general maintenance information as well as for test 
preparation.  “It is imperative that these guides contain thorough and accurate information about 
automatic slack adjustors,” said Rosenker.
      
      The NTSB said many truck operators, who do not consider themselves motor carriers and 
have very little or no interaction with safety regulators or trucking organizations and 
associations, must be alerted to the problem.  These vehicles are used by a diverse cross-section 
of operators, including fire departments, landscaping companies, school bus operators, general 
contractors, and even vacationers who have large recreational vehicles.
      
      In addition, the NTSB investigation found that lack of knowledge and skills in operating 
air-braked vehicles played a role in the accident. The 21-year-old driver of the dump truck had 
been on the job for less than two weeks and had never driven an air brake–equipped vehicle 
before joining the company. He received no training on how to drive an air brake–equipped 
vehicle—an important failure because air brakes on trucks operate differently from hydraulic 
brakes on passenger cars. The rear brakes on the truck were out of adjustment and provided little 
or no braking force. The driver pumped the brakes, reducing the capability of the front brakes 
and exacerbating the loss of braking capability in the out-of-adjustment rear brakes. Until recent 
widespread use of antilock brake systems (ABS) brakes, drivers of hydraulically braked vehicles 
(passenger cars, SUVs, pickups and other light-duty trucks) were taught to pump their brakes in 
emergencies. But in an air-braked vehicle, pumping the brakes depletes the air pressure, thereby 
drastically reducing the brakes’ capability.  
      
      The NSTB estimates there are in excess of 8 million vehicles on the road equipped with 
automatic slack adjusters as every large truck built since 1994 has been required to have them.  
Many of these vehicles may be operated by drivers who have no air brake training and may not 
be able to operate their vehicles safely. “This situation needs to change, and change quickly," 
said the acting NTSB chairman. 
      
      The NTSB found that the probable cause of the accident was the lack of oversight by the 
vehicle’s owner, which had resulted in an untrained driver improperly operating an overloaded, 
air brake–equipped vehicle with inadequately maintained brakes. Contributing to the accident 
was the misdiagnosis of the vehicle’s underlying brake problems by mechanics involved with the 
vehicle’s maintenance. Also contributing was a lack of readily available and accurate 
information about automatic slack adjusters and inadequate warnings about safety problems 
caused by manually adjusting them.
      
      The NTSB issued a series of safety recommendations to address training or regulations 
concerning air brake–equipped vehicles, to the Federal Motor Carrier Safety Administration, 
State governments, the Commercial Vehicle Safety Alliance, automatic slack adjuster 
manufacturers, manufacturers of vehicles equipped with air brakes, the National Institute for 
Automotive Service Excellence, and several publishers of study guides.
      
      A full accident report may be found on the Board’s Web site, www.ntsb.gov, under 
Publications, Highway.






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