This is a basic maneuver load diagram. This diagram is not specific to a particular airplane type, but it does represent a Vn flight envelope that is common to most airplanes. In this example, the horizontal axis represents the airspeed. Equivalent airspeed is used here, which is defined as the airspeed at sea level in the international standard atmosphere. The difference between equivalent airspeed and calibrated airspeed is negligible at lower speeds, but at higher speeds and altitudes, the difference emerges due to compressibility.

The vertical axis represents the load factor, which is defined as the ratio of lift generated by the airplane to its weight. The load factor is equal to ‘1’ in straight and level flight and is a good measure of the structural stresses the airplane experiences during flight. Simply put, this diagram represents the maximum load factor that can be exerted on the airplane at a particular airspeed to avoid structural damage or failure.

Now let's identify some features of this diagram. The speed and load factor range within the OACDE section of the envelope represents the positive maneuver boundary, also known as the limit positive load factor. The negative maneuver boundary, or the limit negative load factor, is a near reflection of the positive side and is represented by the OHFE section.

Point O represents zero speed and zero load factor. Point S is the stall speed at level flight with a load factor of one. The V-speed designator for Vs is the stall speed. Point A is the positive load factor limit, and the V-speed designator for Va is the maneuvering speed. Maneuvering speed is the airspeed corresponding to point A. It is important not to exceed the load factor limit at speeds slower than point A because the airplane will stall first. However, at speeds faster than point A, the load limit can be exceeded, posing a risk of deformation or structural failure.

If the load factor limit is exceeded, the safety factor on the airframe is 1.5. Failure of the structure will not occur at a load factor of 2.5, but it can occur at 3.75. Permanent deformation of the structure may occur at load factors between 2.5 and 3.75. Therefore, it is not safe to assume that the load factor may be increased above the limiting value just because there is a safety factor.

The lines AC and HF represent the maximum positive and negative load factor limits, as specified in certification specification 23 or 25. The airplane can fly within the speed range of line CD and FE. However, the higher aerodynamic loads due to higher speeds require a reduction in the maximum permissible load factor. This speed range should be flown with caution. The V-speed designator for points C and F is Vc, which stands for design cruise speed. DE is the maximum speed allowed for the airplane, and the V-speed designator is Vdd, which stands for design diving speed. This is the highest speed achieved during testing of the airplane.