When you look at the specs for an aerial, you often see terms like tip load, structural safety factor and jack spread. These terms have very specific definitions, and you need to know what they really mean in order to understand how an aerial will perform.

Many of the terms used to define aerial performance are found in the NFPA 1901 Standard for Automotive Fire Apparatus. Here is a summary of some of the more important ones.

Rated capacity or load. This is the total amount of weight of all personnel and equipment that can be supported at the outermost rung of an aerial ladder or on the platform of an elevating platform. It is measured with the waterway uncharged. NFPA 1901 further states that it is measured with the aerial fully extended in the horizontal position. The minimum rated load is 250lb (1 person) for aerial ladders and 750lb (3 persons) for aerial platforms. This is sometimes called the tip load, although different people may use that term in different ways. To ensure you get an aerial with a sufficient capacity for your application, always ask for the NFPA 1901 rated capacity at zero degrees elevation and full extension.

Distributed load. All aerial ladders, and those elevating platforms equipped with ladderways to the platform, have a distributed load rating that is used when two or more persons are positioned along the length of the aerial. This load rating varies according to the angle of elevation and is usually significantly greater than the rated capacity on the tip. It often appears as a graphic chart affixed to the side of the aerial near the base and is useful to determine how many personnel can safely be on the aerial at one time. Remember that all loads include the weight of both firefighters and rescue victims.

Structural safety factor. To ensure that aerials are strong enough to handle loads without failing, the NFPA 1901 standard requires that the stress required to permanently deform the aerial, called the yield point, must be at least 2 times the stress caused by the rated load. This is commonly called a 2-to-1 structural safety factor (sometimes written 2:1). It does not mean that the aerial can safely support twice the rated load. When an aerial reaches the yield point, it starts to bend and fail. The actual yield point of a particular aerial depends on age, localized damage, heat exposure, corrosion and many other factors. Departments should never load aerials beyond the rated capacity. Aerials that experience overloads, even momentary ones, should be taken out of service immediately and have the aerial load tested. Some manufacturers offer aerials with 2.5-to-1 or 3-to-1 structural safety factors. These aerials offer additional strength for an additional margin of safety, but they should never be loaded beyond their rated capacity.

Stability safety factor. To ensure that apparatus will not tip over when the aerial is operated to one side, the NFPA 1901 standard has two additional safety factor requirements. One requires the aerial must be able to operate at 1.5 times the rated capacity in every position in which the aerial can be placed when the apparatus is on a level surface. The other requires the aerial must be able to operate at 1.33 times the rated capacity in every position in which the aerial can be placed when the apparatus is on a 5-degree downward side sloping surface. Most manufacturers use jacks, outriggers or other stabilizing devices to meet these requirements. Side stability is especially important in areas with high-crowned roadways or sloped terrain. It is also important in situations where aerials have to operate at low elevations and full extension, such as when firefighters need to reach the upper windows towards the rear of a two- or three-story structure.

Vertical reach. This distance is determined with the aerial at maximum vertical elevation and extension. It is measured from the ground to the outermost rung for aerial ladders and to the top surface of the platform handrail for aerial platforms. When building setbacks require operation at angles below the maximum elevation, departments may have to specify aerials with higher vertical reaches in order to reach out and up. A performance specification stating the required vertical height and horizontal setback may be advisable.

Horizontal reach. This distance is determined with the aerial at maximum horizontal reach. It is measured from the centerline of the aerial turntable to the outermost rung for aerial ladders and to the outer edge of the platform handrail for aerial platforms. It is usually achieved when the aerial is at 0-degree elevation and full extension, but it may vary from one manufacturer to another. Some aerials with the same vertical reach may vary in horizontal reach by 4-5 feet. This is another place where a performance specification may be advisable.

Jack spread. This is the distance between the centerlines of the side supporting jacks, or outriggers, when they are in the fully extended and lowered position. The horizontal distance from the centerline of the apparatus to each jack is half this distance. Some aerials can operate without fully extending the jack on one side in order to avoid parked cars or other obstacles. This is called short jacking. Not all aerials have this capability, and those that do may have limitations on the range of movement. The narrower the jack spread of an aerial, the less likely you will need short jacking.

Overall or travel height. This seemingly obvious dimension is often the cause of much station renovation or repair. Consider the door heights on your station when you select an aerial. Ditto for overpasses in the area. Hitting anything with the aerial is cause for immediate removal from service and testing. In many cases, it spells the end of the aerial and a very expensive retrofit. To paraphrase the old carpenter's rule, "measure twice before you drive once."

Angle of departure. Simple stated, this is the angle between the ground and the lowest point at the rear of the apparatus when measured from the rear wheels. NFPA 1901 requires a minimum of 8 degrees. Some station aprons, access driveways, or cross streets with high centers and low gutters may require greater angles for more clearance. Unless your shop enjoys doing body work, this is something you should consider doing before you buy an aerial. Apparatus with midmount aerials are a special concern because they typically have a longer rear overhangs.