Turbulence in the form of vertical air currents can, under certain conditions,
cause severe load stress on an airplane wing.
When an airplane is flying at a high speed with a low angle of attack, and suddenly encounters a vertical current of air moving upward, the relative wind changes to an upward direction as it meets the airfoil. This increases the angle of attack of the wing.
If the air current is well defined and travels at a significant rate of speed upward (15 to 30 feet per second), a sharp vertical gust is produced which will have the same effect on the wing as applying sudden sharp back pressure on the elevator control.
All certificated airplanes are designed to withstand loads imposed by turbulence of considerable intensity. Nevertheless, gust load factors increase with increasing airspeed. Therefore it is wise, in extremely rough air, as in thunderstorm or frontal conditions, to reduce the speed to the design maneuvering speed. As a general rule, when severe turbulence is encountered, the airplane should be flown at the maneuvering speed shown in the FAA-approved Airplane Flight Manual, Pilot’s Operating Handbook, or placard in the airplane. This is the speed least likely to result in structural damage to the airplane, even if full control travel is used, and yet allows a sufficient margin of safety above stalling speed in turbulent air.
Placarded “never exceed speeds” are determined for smooth air only. High dive speeds or abrupt maneuvering in gusty air at airspeeds above the maneuvering speed may place damaging stress on the whole structure of an airplane. Stress on the structure means stress on any vital part of the airplane. The most common failures due to load factors involve rib structure within the leading and trailing edges of wings.
The cumulative effect of such loads over a long period of time may tend to loosen and weaken vital parts so that actual failure may occur later when the airplane is being operated in a normal manner.
Determining Load Factors in Flight
The leverage in the control systems of different airplanes varies; some types are balanced control surfaces while others are not. (A balanced control surface is an aileron, rudder, or elevator designed in such a manner as to put each side of its hinged axis in balance with the other side.) Therefore the pressure exerted by the pilot on the controls cannot be used as a means to determine the load factor produced in different airplanes. Load factors are best judged by feel through experience. They can be measured by an instrument called an accelerometer, but since this instrument is not commonly used in general aviation-type airplanes, developing the ability to judge load factors from the feel of their effect on the body is important. One indication the pilot will have of increased load factor is the feeling of increased body weight. In a 60° bank, the body weight would double. A knowledge of the principles outlined above is essential to estimate load factors.
In view of the foregoing discussion on load factors, a few suggestions can be made to avoid overstressing the structure of the airplane:
• Operate the airplane in conformance with the Pilot’s Operating Handbook.
• Avoid abrupt control usage at high speeds.
• Reduce speed if turbulence of any great intensity is encountered in flight, or abrupt maneuvers are to be performed.
• Reduce weight of airplane before flight if intensive turbulence or abrupt maneuvering is anticipated.
• Avoid turns using an angle of bank in excess of 60°.