The answer to the question "how strong should an airplane be" is determined largely by the use to which the airplane will be subjected. This is a difficult problem, because the maximum possible loads are much too high for use in efficient design. It is true that any pilot can make a very hard landing or an extremely sharp pullup from a dive which would result in abnormal loads. However, such extremely abnormal loads must be dismissed somewhat if we are to build airplanes that will take off quickly, land slowly, and carry a worthwhile payload.
The problem of load factors in airplane design then reduces to that of determining the highest load factors which can be expected in normal operation under various operational situations. These load factors are called "limit load factors." For reasons of safety, it is required that the airplane be designed to withstand these load factors without any structural damage. Although Federal Aviation Regulations require that the airplane structure be capable of supporting one and one-half times these limit load factors without failure, it is accepted that parts of the airplane may bend or twist under these loads and that some structural damage may occur.
This 1.5 value is called the "factor of safety" and provides, to some extent, for loads higher than those expected under normal and reasonable operation. However, this strength reserve is not something which pilots should willfully abuse; rather it is there for their protection when they encounter unexpected conditions.
The above considerations apply to all loading conditions,
whether they be due to gusts, maneuvers, or landings. The gust load factor
requirements now in effect are substantially the same as those which have
been in existence for years. Hundreds of thousands of operational hours
have proven them adequate for safety. Since the pilot has little control
over gust load factors (except to reduce the airplane's speed when rough
air is encountered), the gust loading requirements are substantially the
same for most general aviation type airplanes regardless of their operational
use. Generally speaking, the gust load factors control the design of airplanes
which are intended for strictly nonacrobatic usage.
An entirely different situation exists in airplane design with maneuvering load factors. It is necessary to discuss this matter separately with respect to: (1) Airplanes which are designed in accordance with the Category System (i.e., Normal, Utility, Acrobatic); and (2) Airplanes of older design which were built to requirements which did not provide for operational categories.
Airplanes designed under the Category System are readily identified by a placard in the cockpit which states the operational category (or categories) in which the airplane is certificated. The maximum safe load factors (limit load factors) specified for airplanes in the various categories are as follows:
Normal  ------------------------------------- 3.8 -1.52
Utility (mild acrobatics, including spins) ----- 4.4 -1.76
Acrobatic -------------------------------------- 6.0 -3.0
 For airplanes with gross weight of more than 4,000 pounds, the limit load factor is reduced.
To the limit loads given above a safety factor of 50 percent is added.
There is an upward graduation in load factor with the increasing severity of maneuvers. The Category System provides for obtaining the maximum utility of an airplane. If normal operation alone is intended, the required load factor (and consequently the weight of the airplane) is less than if the airplane is to be employed in training or acrobatic maneuvers as they result in higher maneuvering loads.
Airplanes which do not have the category placard are designs
which were constructed under earlier engineering requirements in which
no operational restrictions were specifically given to the pilots. For
airplanes of this type (up to weights of about 4,000 pounds) the required
strength is comparable to present day utility category airplanes, and the
same types of operation are permissible. For airplanes of this type over
4,000 pounds, the load factors decrease with weight so that these airplanes
should be regarded as being comparable to the normal category airplanes
designed under the Category System, and they should be operated accordingly.