M-939 5-TON TRUCK
The M-939 series comes in six body styles: cargo, dump, wrecker, van and long wheel base cargo. As of late 2000, the most common versions of the M939 Series vehicles in service were the M923 Cargo Truck, M925 Cargo Truck W/Winch, M927 XLWB Truck, the M931, 5-ton Tractor, and the M936 Medium Wrecker.
The M-939A2 tactical truck is a five-ton capacity, six-wheel drive cargo truck used for transportation of all types of supplies. The basic M939A0 model featured improvements such as automatic transmission, improved power steering, a complete airbrake system, an improved cooling system, an improved electrical system, a three-crew member cab, a tilt hood, a hydraulically powered front winch and a simplified test equipment/internal combustion engine diagnostic connector. The M939A1 model added super single radial tires while a new CTIS, a new diesel engine and chemical-agent resistant paint were the main additional features of the M939A2 model.
The M939 Series vehicles are powered by a NHC-250 Cummins diesel engine. The truck's central tire inflation system enables the crew to increase or decrease the air pressure in the tires to improve mobility on or off roads. It can tow 21,000 pounds.
The M939 truck was introduced in 1983. It's a general-purpose military vehicle, primarily designed for tactical, off-road use, with a top speed of 65 mph and an automatic transmission. It augmented the older M809 series tactical truck. The major differences between the M939 Series vehicles and the M809 Series vehicles are as follows:
- Automatic transmission;
- Improved power steering system;
- Complete air brake system;
- Improved cooling system;
- Improved electrical system;
- Three crew member cab;
- Tilt hood; and
- Hydraulically powered front winch.
The M-939A2 is a fitting replacement for the famed Army "deuce-and-a-half" truck. The Army received the M-939A2 in 1989.
There were 10,807 in the Army's inventory in the early 1990s. As of mid-1998, there were about 32,000 M-939 trucks in the Army fleet.
The US Army settled on adding computer-controlled brakes to its M-939 five-ton tactical trucks to make them safer for soldier-operators. It began purchasing ABS hardware starting in fiscal year 1999, with retrofitting to begin in 2000, and the project to be completed in 2003.
The Medium Tactical Vehicle Replacement program was set to replace the Marine Corps' aging fleet of M-939/M-809 five-ton trucks with trucks with greater mobility, lift, and reliability. The MTVR is set to carry an increased payload of 7.1 tons cross-country; 15 tons on hard-surface roads while simultaneously being able to tow an 11-ton load.
The M-931 and M-932 are the tractor truck variants of the M939 Series vehicles. The M-931 and M-932, like the M818 tractor truck, is equipped with a fifth wheel used to haul semitrailers with loads up to 37,000 pounds cross country and 55,000 pounds on the highway. The only difference between the M-931 and the M-932 is that the M-932 has a front mounted winch which has a pulling capacity of 20,000 pounds. The medium wrecker/recovery vehicle variant of the M-939 Series vehicles is the M936. The winch and crane capacities of the M936 are identical to the M816 wrecker/recovery vehicle.
In FY95 the M939 series 5-Ton truck was responsible for 26% of the total Army Military Vehicle (AMV) accidents and 53% of the total AMV fatalities. In 1999, GAO report GAO/NSIAD-99-82 analysis indicated that from January 1987 through June 1998 accident data showed that while M939s made up an average of only 9 percent of the AMV fleet, the M939 accounted for 34% of the fleet's accidents resulting in fatalities. Comparison of U.S. Department of Transportation accident statistics to M939 accident statistics showed that over a 10-year period, the fatality rate of occupants of the M939 averaged about 30 times higher than the fatality rate for occupants of comparably sized commercial trucks. Also noted in the GAO report were the results of a TACOM tire study conducted from 1995 - 1997. That study concluded that the M939s were being used on road more than originally planned. The original intent was to drive the M939s on highway and secondary roads 60% of the time and 40% off road. Current data indicates the M939 trucks are being driven on roads 80-90% of the time and only 10-20% off road. For the past four years, the M939 Series trucks have been operating under Safety of Use Message 98-07 (SOUM 98-07) limiting the highway speed to 40 miles per hour in an attempt to limit accidents, injuries and fatalities occurring under this highway operational scenario.
The accident scenario for all M939 trucks occurs during panic stop situations and is worsened on wet pavement. In panic stop situations the trucks wheels lock up causing engine stall. This causes loss of power steering resulting in uncontrolled skidding creating accident and roll-over situations. Extensive testing of ABS for this truck has shown that ABS will eliminate 100% of the engine stalls and wheel lock-up regardless of the skill level of the drivers. The accident scenario for M939 basic vehicles with NDCC tires occurs during panic stop situations on wet pavement. The front wheels lock up, the NDCC bias tires react like ice skates and stopping distance is increased by 245-320 feet over trucks with radial tires. The M939 truck is expected to support the Army's transformation effort through FY30. Recently the program looked at removing radial tire applications to avoid cost. Recent test data concludes that ABS with NDCC tires is a deadly combination. The tires react like ice skates on wet roads and braking distance is increased by 245 to 320 feet over a truck with radial tires and the ABS kit installed. Once the ABS kit and radial tires are installed, SOUM 98-07 can be lifted, allowing the vehicles to once more be capable of safe operation up to their required operational capability and full mission requirements.
The nondirectional cross country (NDCC) tire design on the M939 basic truck was engineered for cross-country applications prior to World War II. Changes in vehicle speeds, road construction, mission requirements, as well as advances in tire technology have made this NDCC bias tire obsolete and unsafe. This modification will change the tires from the current bias ply NDCC tire to a radial tire designed for on/off road. Recent improvement in radial tire design will provide better traction and mobility, which will enhance system safety.
There are 32,000 M939 and 350 M945 trucks worldwide that must have ABS applied. Of those 32,000 trucks, 11,700 basic trucks are having their bias tires upgraded to radial tires to further improve vehicle safety.
The Army identified certain hazards associated with the M939 series truck in 1995 and informed units in the field through Ground Precautionary Message (GPM) 96-04. Army leadership applied more controls to reduce hazards by identifying a maximum speed limit of 40 mph in Tank-Automotive Command (TACOM) Safety of Use Message (SOUM) 98-07. This change was incorporated into Technical Manual (TM) 9-2320-272-10, with Change 1.
The Army determined the fix for reducing these hazards across the force was a modification work order (MWO) to place antilock braking systems on the entire M939 fleet. For the basic M939 fleet, the Army also decided to replace existing non-directional, cross-country tires with radials (TACOM SOUM 98-07). Yet, this endeavor took time to complete. Until its completion, leaders MUST be informed and proactive about addressing the hazards associated with this common system used by numerous units across the Army EVERY time they use it.
Despite these efforts, as of 2002 units continued to operate M939 series vehicles in the very conditions the messages warn against. Until the MWO is complete, M939 trucks are not to be driven above 40 mph, which means 40 mph is the extreme limit. Driving too fast for conditions creates an environment for compounding the effects of the other hazards listed below. Unit leaders must evaluate and re-evaluate the conditions the truck will be used in and apply the appropriate controls.
Tailgating can create an extremely hazardous condition when drivers overreact to vehicles braking to their front. Over-braking can lock up the wheels, causing the engine to stall. This can lead to loss of control of the vehicle.
Damp or wet conditions contribute to the vehicle losing traction when the brakes are applied suddenly and with too much pressure. Drivers must slow down when damp or wet conditions exist, and leaders MUST re-evaluate the need to operate the truck in these conditions and, at a minimum, implement additional control measures and inform their drivers of the increased risks.
The M939 series truck was developed for heavy loads and off-road conditions. The accidents seen so often are M939 series trucks operating on asphalt roads. The trucks are generally hauling cargo on or around post, or they are hauling soldiers to and from training and details (see Army Regulation (AR) 385-55, Prevention of Motor Vehicle Accidents, for guidance on hauling soldiers). This does not mean you need to add weight to the truck to operate it safely, but it does mean that leaders need to recognize the increased risk of operating in these conditions and enforce speed limits and safe distances between vehicles, as well as inform drivers of the increased risks. Information and knowledge about the system is half the battle of operating any equipment safely. Soldiers will not know if leaders do not.
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