MOUNTAIN TURBULENCE AND MANEUVERING SPEED
The science of meteorology, with all its knowledge, technical equipment (including super computers) and experience, should be able to pinpoint mountain turbulence. Let’s face it. Predicting mountain turbulence is an inexact science. Meteorologists have established guidelines and they actually do a good job forecasting for large areas. But the fact remains, the exact location and severity of disturbed air cannot be foretold.
Turbulence is not limited to mountainous regions and its classification is the same for all areas where you can fly.
An understanding of turbulence and maneuvering speed is important because airplanes operated at a speed greater than the turbulent air penetration speed or maneuvering speed, in moderate or greater turbulence, risk structural failure.
Mountain pilots are often exposed to more and usually greater intensity mechanical turbulence than their flatland counterparts, because the obstructions causing the turbulence are larger.
Mountain pilots are also at the mercy of mountain wave turbulence (wind shear). The majority of mountain wave phenomena produces light to moderate turbulence, but some waves produce severe or greater turbulence.
Weather may be fickle, changing at will; whereas, engineering is nearly an exact science that allows engineers to predict the amount of stress necessary to cause the structural failure of an airplane.
An airplane’s strength is basically measured by load factor, that is, the total load the wings and tail are capable of carrying without distortion, permanent damage or structural failure. Load factor is the actual load on the wings divided by the weight of the airplane. In straight-and-level flight the wings support a weight equal to the weight of the airplane and its contents, one g load factor.
The normal category airplane is generally stressed for a positive limit load factor of 3.8 Gs, a negative limit load of –1.52 Gs, and a 50-percent safety factor that is added to the positive limit load resulting in +5.7 Gs for the ultimate load factor.
Regulations for the certification of the normal category airplane require that it be able to withstand a derived gust velocity of 30 feet per second (fps) at maximum level flight speed and normal-rated power.
Lenticulars with rotor turbulence near Lake Tahoe.
Moderate turbulence is classified as a derived gust velocity of 20-35 fps. This means anytime you encounter moderate or greater turbulence, you must, for safety sake, slow to maneuvering speed.
When operating at a speed greater than maneuvering speed vertical gusts can cause a sudden increase in the angle of attack. This results in large wing loads that are resisted by the inertia of the airplane. The structure is not designed to withstand this load and may become permanently deformed or worse, it may fail. When operating at or less than maneuver speed, the airplane will stall before breaking. Gusts are momentary features, so the stall is a brief stall and normally does not require pilot-initiated stall recovery.
Maneuvering speed, abbreviated Va, is called the rough air speed. It is defined as the maximum speed at which full abrupt control deflection can be made without exceeding the design load factor.
Some pilots mistakenly believe it is proper to fly at the ultimate load factor. The ultimate load factor is designed to compensate for vagaries in materials, workmanship, and age of the airplane. The structure is required to support ultimate loads without failure for at least three seconds (FAR § 23.305 Strength and deformation).
Maneuvering speed is a value of 1.9 Vs. It changes with a change in gross weight; decreasing as the weight decreases.
Turbulence producer – lenticular clouds
Pilots notice that a heavily loaded airplane rides smoother in turbulent air. They perceive this as an indication that the airplane should be loaded to its maximum whenever turbulence is expected. This is a bad assumption.
Consider an airplane that has a maximum allowable gross weight of 3,000 pounds. If it encounters a +30 fps gust that results in an additional 2-g load factor, the airplane experiences a total of 3 Gs load factor. Multiply the 3-g load factor by 3,000 pounds and the wings are supporting 9,000 pounds.
Assume the airplane is loaded to 1,500 pounds and that it is subjected to the same gust. With half the inertia, the gust acceleration is doubled, causing the airplane to experience a 5-g load factor (4-g force plus 1-g level flight). Multiply 1,500 pounds by 5 gs and the wings are supporting 7,500 pounds.
The lightly loaded airplane is subjected to 1,500 pounds less load when encountering the same gust. Even though the heavy airplane realizes less load factor, it incurs more strain. The pilot recognizes load factor; the airplane recognizes load.
IS VA ALWAYS SAFE?
The diligent pilot who adheres strictly to the maneuvering speed when in turbulence can experience an unexpected problem.
Maneuvering speed is based on multiplying the power off stall speed (VS1) by the square root of the limit positive maneuvering load factor, n, used in design (usually 3.8 Gs). For most normal category airplanes this is 1.949 VS. Most of the time an airplane is flown with the power on. The power on stall speed is significantly less than with the power off. When the stall speed decreases, maneuvering speed decreases.
A pilot flying across a mountain range may encounter the rotor cloud (roll, rotor, arcus) that subjects the airplane to a gust from the front. This will increase the headwind shear and has the effect of increasing the airspeed beyond the value of maneuvering speed.
The reduced stall speed (power on) and increasing airspeed (shear) may cause the wing loading to exceed the limit load factor.
My recommendation, when required to slow to maneuvering speed, is to use the value of 1.7 Vs instead of the 1.9 Vs allowed by regulation. Further, if the airplane doesn’t have an owner’s manual or pilot’s operating handbook, determine the reduced weight maneuvering speed in the following manner:
The primary source of information concerning turbulence is pilot reports. Many of these reports are exaggerated because of different reporting criteria used by the pilot and the pilot’s judgment, usually based upon the amount of time spent in the turbulence. Brief encounters with turbulence are not likely to be considered significant. Turbulence lasting several minutes or more will be considered important.
Rotor turbulence, Aspen, Colorado
The primary source of information concerning turbulence is pilot reports; however, these reports are often grossly exaggerated.
A pilot's judgment of turbulence and its severity is influenced by his experience level, the type of aircraft being flown and the length of time he experiences the turbulence. Brief encounters are not likely to be considered significant, but turbulence lasting several minutes or longer will be considered to the pilot.
Turbulence is divided into four degrees of intensity.
Fly above cumulus clouds (if possible) to avoid convective turbulence. If the air is too dry to form clouds, determine the base of convective activity by subtracting the dew point from the temperature in degrees Fahrenheit. Multiply this figure by two. The result is the altitude, in hundreds of feet (add two zeros to your answer), of the base of convective activity above the ground. Fly 1,000 feet or so above this altitude to avoid convective turbulence.
Destructive mountain wave turbulence can be avoided by flying half again as high as the mountain tops. This doesn’t mean when flying over Colorado, with 54 peaks over 14,000 feet that you have to fly at 21,000 feet. Subtract the surrounding terrain elevation from the mountain top and use half this value added to the mountain top.
Rotor cloud, Calgary, Alberta