BY MIKE NASTA
The apparatus operator is one of the most important positions in the fire department. But does your department do enough to properly train the firefighters who are assigned to drive apparatus or could be in the future? Many apparatus operators do not understand the position’s tremendous responsibilities. Do they understand the effects of chassis dynamics, weight distribution, the center of gravity, and other physical forces on driving and handling?
Most departments spend a lot of time and effort to train operators in pumping water or raising aerial devices, but these extremely important skills are not the only areas in which drivers need to be proficient. Driving itself sometimes takes a backseat to pump operations and aerial operations. Considering the alarming rise in apparatus accidents, properly training our operators to consider the forces below is essential for safety.
Velocity or speed is the rate at which something changes position. For example, a vehicle traveling at 60 miles per hour (mph) has a velocity of 60 mph. Speed is one of the leading causes of accidents. Ignorance of how speed and other physical forces affect driving can have deadly consequences; a large percentage of apparatus accidents are attributable to speed.
Fortunately, we can change this by simply training our operators to slow down and respect speed limits. Unfortunately, most drivers and their officers choose not to address this issue, which is where the problems begin. Operating an emergency vehicle does not exempt you from the laws governing motor vehicles or physics. Officers must immediately correct and retrain drivers who speed excessively.
Speed kills! It’s as simple as that. Slow down, and the risk of an accident diminishes accordingly.
Potential energy is the amount of energy a body has at rest. The horsepower capabilities of newer apparatus can easily accelerate most apparatus to highway speeds in a relatively short distance, creating the kinetic energy of motion. The vehicle’s velocity multiplied by its mass (weight) creates a tremendous amount of kinetic energy in the form of momentum. The truck’s braking system must exert a tremendous amount of force to deal with this momentum and bring the vehicle to a stop (assuming two trucks are traveling at the same velocity, a truck with twice the mass of the other truck will also have twice as much momentum). The higher the velocity, the more momentum a moving vehicle develops; thus, the greater the stopping distance it requires.
Consider that a three-axle truck traveling at 55 mph (81 feet per second) requires a minimum of 390 feet to stop. The operator’s reaction time—the time between seeing the need to slow down and applying the brake—will allow the vehicle to travel another 61 feet, resulting in a total stopping distance of 451 feet.
Keep in mind that other factors will contribute to the minimum stopping distance needed, such as the vehicle’s weight, the driver’s reaction time (everyone’s is different), the condition of the vehicle’s braking system and tires, the road surface’s type (concrete or asphalt) and quality (smooth or badly potholed), and weather-related road conditions (wet or icy). Remember, the slower you go, the easier you can stop.
CENTER OF GRAVITY
The vehicle’s center of gravity depends on how the weight is distributed on it. Simply stated, the higher the weight is located on the vehicle, the easier it will be for the vehicle to overturn, especially when the weight is shifted to one side when executing a turn. For example, a ladder truck’s aerial device places a great amount of weight on the upper part of the vehicle, as does the water tank on an engine. Does your operator understand and respect this weight distribution?
Additionally, the tank water puts a live load inside the vehicle. This weight moves with the motion of the vehicle, thus constantly changing the weight distribution. Water weighs approximately eight pounds per gallon, so a tank holding 750 gallons adds 6,000 pounds of movable weight to the vehicle. That amount of weight will greatly change the chassis dynamics and create an unstable load, which will make it very easy for the vehicle to overturn.
To add to this problem, centrifugal force will cause the water to lean to the outside of the curve when the apparatus makes a turn, pulling the vehicle in that direction and thus increasing the chance of a rollover.
Vehicle brake systems include drum and disc type brakes. With a drum brake, the brake shoe is in contact with 90 percent of the drum’s braking surface when the brakes are applied. This leaves only about 10 percent of the drum available for cooling. The lack of cooling will cause the brake to overheat. As the heat builds, the brake drums will expand; as this expansion continues, it will become almost impossible for the shoes to maintain good contact with the drum. Hence, the hotter the brake gets, the more the braking power is reduced, causing the brakes to “fade.”
The effects of brake fade depend on the condition of the brake shoes and brake drum, the vehicle’s speed, how long the operator maintains pressure on the brakes, and whether the vehicle is traveling uphill or downhill. Each of these factors will contribute to the heat buildup in the shoe and drum, thus diminishing the stopping power.
Disc brakes work and cool differently than drum brakes. With disc brakes, the brake pad makes contact with about 15 percent of the disc braking surface, leaving 85 percent of the disc surface available for cooling. However, as the disc braking surface heats up and expands, it makes better contact with the brake pad. Consider this when writing specifications for new apparatus.
Skidding is the most inefficient way to stop a vehicle. When a tire slides or skids on the road surface, the friction between the tire and the road surface ceases to exist, greatly increasing the stopping distance. The operator may also lose control of the vehicle, especially when negotiating a turn. Newer vehicles are equipped with antilock brake systems, which cause the brakes to apply and release when wheel sensors detect that a wheel is skidding.
Auxiliary braking systems include the engine brake. The most popular type of engine brake works by converting the engine, which normally generates power, to an energy-absorbing air compressor. The engine brake uses a series of slave valves and mechanical devices that open the engine cylinder exhaust valves just prior to the cylinder’s firing. When this occurs, the cylinder, instead of producing power, absorbs power. This force is then transferred through the driveline to the rear wheels. Realize that when this occurs, unlike in regular braking, the braking force is applied only to the rear wheels, not to all wheels equally, which can result in a skid under slippery conditions. Operators should not use these auxiliary devices during wet, snowy, or icy conditions.
Tires are often overlooked until they go flat. However, tire condition and inflation play an important role in how the vehicle handles and stops. Worn and underinflated tires will reduce the vehicle’s braking power. Check tires routinely with an accurate tire gauge to ensure they have proper inflation, which will also increase fuel economy, an important consideration in these trying economic times.
TURNS AND PHYSICS
To stop the vehicle, at what point you apply the brakes makes a big difference. Brake while the vehicle is traveling in a straight line. When a vehicle makes a turn, the turning action actually helps reduce the vehicle’s speed. Now this may sound like a good thing at first, but applying the brakes while making a turn at the same time drastically changes the vehicle’s dynamics and weight distribution.
As your vehicle enters a left turn, you will first probably let off the accelerator, which will slow the vehicle. Centrifugal force takes over here and pulls the vehicle toward the outside of the turn. In a left turn, these forces will transfer weight to the right front tire; all the dynamic forces (e.g., the tank water) increase the force on the tire. As the operator applies the brakes, this puts even more force on that tire. If the tire is in poor condition, it may fail at this point, making it nearly impossible for the operator to maintain control of the vehicle. This is the reason you should do all braking in a straight line.
If the combination of all these forces is too great, it will cause the vehicle to roll over. When posted, the speed limits for turns are intended for cars, not for a 60,000-pound fire apparatus. A vehicle of this size, weight, and center of gravity must negotiate these turns at a much lower speed.
By increasing these forces through excessive speed, hard braking, or turning the vehicle too sharply, the driver also increases the possibility of a rollover.
The driver can manage most of these forces through better operator training that focuses on awareness of these forces and their effect on apparatus driving and handling.
We have so far discussed several items related to the laws of physics, over which we have no control. From this point, we will look at what we can control, which involves the operator’s training. Before entering the fire service, many firefighters may have driven nothing larger than the family car. Educating these novice apparatus drivers is the only way to get them used to driving larger vehicles. It is an unsafe practice to allow new drivers to drive apparatus in emergency situations until they have had sufficient practice and are familiar with driving apparatus under nonemergency conditions.
The physical and emotional state of the firefighter operating the apparatus can affect the vehicle’s operation. How quickly the driver reacts to situations that require stopping and steering will vary among operators. Consider age, physical condition, eyesight, and depth perception when choosing prospective drivers. Age influences all of these factors. I know we all hate to admit it, but as we age, our reactions and other physical limitations become an issue. I’m not saying that there should be an age cutoff for apparatus drivers, but we should periodically retest or retrain drivers to certify their ability to drive apparatus.
The driver’s emotional state influences how he drives. Angry, tired, or distracted drivers will not focus their full attention on driving. The company officer should be the first to notice these conditions and must take action to safeguard other members and the general public.
Your apparatus operators should be aware of how the laws of physics affect apparatus operation, in addition to basic vehicle training. Drivers must fully train in all these considerations early and continue training until they are competent and until controlling these forces becomes second nature.
Remember, fire apparatus is not like the family sedan or the sports car you drove before you became an apparatus operator. Fire apparatus handles much differently. In addition, you are responsible for all the lives riding on that apparatus as well as the general public, so drive accordingly.
MIKE NASTA, a 25-year veteran of the Newark (NJ) Fire Department, is deputy chief in charge of the Training Division and the department’s safety officer. He is also serving his third term as chief of the South Hackensack (NJ) Volunteer Fire Department. Owner of Nasta Trucking LLC, he has driven tractor-trailers for 29 years and has accumulated approximately two million driving miles. He is a New Jersey-certified level II fire instructor and a senior fire instructor at the Bergen County (NJ) Fire Academy, a H.O.T. coordinator for FDIC, and a member of Fire Engineering ’s editorial advisory board. He coauthored Fireground Officer Development with Anthony Avillo and has written numerous fire service articles.