Airspeed


There's more to it than going fast

By David A. Borrows

Many people believe that speed is the essence of life. But pilots should know that airspeed is not about going fast. It's about flying at the velocity that results in the desired performance and safety. One would not, for example, want to land at redline, an airplane's never-exceed speed (VNE), because it would not result in the desired outcome—a safe landing.

Airspeed is the velocity of an airplane relative to the air mass through which it is flying. In simple terms, it's the result of thrust impeded by drag. How fast an airplane can go in level flight depends on the amount of drag and the amount of horsepower. The sleek Grob 115, for example, has less drag per horsepower than a Pitts Special, which has an extra set of wings and their associated struts and wire bracing.

All airspeeds, whether flat-out or economy cruise, are the result of drag equaling the selected amount of power. They are determined by the balance of an aircraft's pitch attitude and power setting, and each results in specific aircraft performance or denote an operating limitation.

Reading speed

While you determine airspeed with pitch and power, you read it on the airspeed indicator (ASI), which may be marked in miles per hour (mph), nautical miles per hour (knots), or both. While this may seem as simple as reading the number to which the needle points, there's more to it because there are three types of airspeed.

Indicated airspeed (IAS) is what you read on the dial; it's uncorrected for variations in atmospheric density and temperature, and installation and instrument error. Calibrated airspeed (CAS) is indicated airspeed corrected for instrument and installation errors. CAS corrections are typically small, depending on aircraft configuration, attitude, and airspeed, and are usually found in the aircraft's pilot operating handbook (POH).

If you're flying an aircraft manufactured before the mid-1970s, being aware of the difference between IAS and CAS is important because performance airspeeds, such as stall speed, are given in CAS, not what you read on the indicator. To know, heed, and use the limiting and performance airspeeds, you have to know what the IAS will be when converted from CAS. Most airplanes built after the mid-1970s give performance and limiting airspeeds in IAS.

True airspeed (TAS) is the final velocity variation. It's IAS that's been corrected for temperature and barometric pressure variations from standard sea level conditions—15 degrees Celsius and a barometric pressure (altimeter setting) of 29.92 inches of mercury (a simple computation made easy with an E6B flight computer).

Because air density decreases with altitude, an airplane must fly faster at higher altitudes to produce the same indicated airspeed it would at sea level. In other words, the plane must fly faster in less dense air to cause the same pressure difference between pitot impact pressure and static pressure. Therefore, if flying the same IAS, TAS will increase with altitude. If flying the same TAS, IAS will decrease with altitude.

This increase in true velocity is the primary reason pilots must compute TAS when planning a cross-country. TAS plus or minus the effects of wind gives you ground speed. And ground speed determines how long it will take to fly from Point A to B. This time determines how much gas you'll need to make the trip, and having enough gas (and a healthy reserve) is the key to a safe, enjoyable trip.

Flying colors

The different airspeeds that either limit or result in specific aircraft performance are known as V speeds—V for velocity. There are a multitude of them, with many, such as VWW, the maximum speed at which you can operate the windshield wipers (honest!), applying only to certain airplanes.

On airplanes with a maximum gross weight of less than 12,500 pounds and certificated after 1945, some of the more important V speeds are color-coded on the ASI. This enables pilots to quickly determine how their aircraft velocity conforms to certain airspeed limitations, and whether they are at a safe speed for their current phase of flight or need to either increase or decrease their speed.

The white arc covers the speeds at which the aircraft can be flown with its flaps fully extended. VSO is at the low end of the white arc. This is the speed at which the airplane will stall in straight flight (turns increase the aircraft's load factor, and thereby its stall speed) when at maximum gross weight with the power at idle, fully extended flaps, landing gear down (if so equipped), and with its center of gravity (CG) at its aft limit. VSO is an important speed to monitor, especially when landing (see below).

VFE, the maximum velocity at which the airplane can be flown with its flaps fully extended, is the high-speed limit of the white arc. Flying at speeds greater than VFE with full flaps can result in damage, perhaps to the point of losing one or both flaps. Not a good thing. A number of airplanes, however, do allow the use of approach flaps, usually around 10 degrees, at speeds higher than VFE. The POH will give the specific details.

The green arc spans the aircraft's normal operating speed range. It starts with VS, the velocity at which the airplane will stall in straight flight when at maximum gross weight, the power at idle, the flaps and gear retracted, and aft CG.

The green arc terminates at VNO, the maximum normal operating velocity or maximum structural cruising speed. The formula for calculating VNO is somewhat complex. But one of the formula's factors is the airplane's ability to withstand a specified vertical gust (30 feet per second for planes certificated before August 1969 and 50 feet per second after this date) and not exceed its maximum load limit. It's important to remember that VNO is a certification value. Only maneuvering speed (VA), which will be addressed shortly, will protect you from harm in turbulence.

VNO is also the start of the yellow arc, often called the caution range. Flight in this speed range should only be considered when the air is glass-smooth because the slightest burble of air may cause you to exceed the aircraft's maximum load factor.

The yellow arc terminates at the red line—VNE—the velocity that should never be exceeded. VNE is 90 percent or less of the demonstrated dive velocity (VD), a calculated value and/or the speed at which a test pilot flew the plane with no vibration or buffeting severe enough to result in structural damage. Don't think there's a 10-percent safety buffer past VNE. A baby's breath will cause the aircraft to exceed its limit load factor, and structural damage will result.

Flying the book

There are a number of important performance and limiting airspeeds that are not shown on the airspeed indicator. They are in the POH and, in many cases, on the cockpit placards. One of the most important is VA, maneuvering speed. Also known as the rough air penetration speed, this is the speed to fly if you're in turbulence—or you think you will be. VA is an aerodynamic safety valve. If the airplane hits a very turbulent bump, the airplane will stall before its limit load factor is reached.

If VA is given for max gross weight only, beware. Like stall speeds, VA decreases with gross weight. If your POH just gives VA for max gross, you can compute a weight-specific figure quite easily. Just divide the airplane's actual weight (you performed a weight and balance, didn't you?) by its maximum gross weight. Then find the square root of this number and multiply the square root by the max gross VA number.

It's not as complicated as it sounds, but there is an easier, although less accurate, way of determining the needed number. Reduce the max gross VA by a percentage equal to half the weight reduction. If a 20-percent weight reduction is made, reduce the speed by 10 percent.

If your aircraft has retractable landing gear, two additional velocities will be of interest. VLO is the airspeed at which the landing gear can be safely operated—extended or retracted. VLE is the maximum speed at which you can fly with the gear extended.

VLE is often higher than VLO because operating the gear causes abrupt and changing air loads to hit parts of the landing gear and doors when they are other than up or down and locked. Attempting to lower the gear at speeds greater than VLE could result in damage, such as losing a gear door.

If you're in an emergency situation, however, one where airspeed is increasing toward redline and the ground is getting rapidly closer, don't worry about the gear doors. Extend the landing gear. It creates a lot of drag and is one sure way to slow your progress toward VNE so you can regain control of the craft.

Because the gear does create a lot of drag, most POHs attach a note to the best glide speed—"Gear and flaps up." Regardless of whether your gear moves up and down, commit the best glide speed to memory and fly it precisely. Any variation from it will result in a reduced glide ratio, which will mean the difference between making and almost making your chosen emergency landing site. For this reason, never try to stretch your glide by gliding at speeds below the book figure.

Tree speeds

At some point during their training, all pilots learn about VX and VY. And in many cases, because both are climb speeds, they spend the rest of their flying careers trying to remember which is which.

VX is the velocity that will give you the most altitude in the shortest distance. It's your aircraft's best angle of climb speed, the speed to use when there is a sequoia or mountain at the end of the runway. It will take more time to gain altitude at VX because you're flying at a slower speed. But gaining altitude in a short amount of time isn't your goal. Gaining altitude in the shortest horizontal distance, like before you hit that tree or mountain, is.

When faced with such a situation, don't pull the airplane off the ground early and try to force it over the obstacle. Airplanes fly at slower speeds in ground effect, and when you climb out of it, you may settle back to earth. Not the thing to do when you want to gain a lot of altitude in a short horizontal distance (the distance between the tree and where you start your takeoff).

And don't hold the airplane on the ground past VX, thinking that the extra speed will rocket you up and over the obstacle. Assuming that the acceleration is essentially unaffected, takeoff distance varies as the square of the takeoff velocity. In other words, 10 percent excess airspeed would increase the takeoff distance 21 percent.

If obstacle X is not impeding your takeoff path, VY is your post-takeoff climb speed because it gives the best rate of climb. You'll gain a lot of altitude in a short amount of time—and you'll also cover a lot a ground. VY also provides for better visibility and engine cooling because of its lower pitch angle when compared to VX.

Whether you're going up, coming down, or going from Point A to B, heeding your aircraft's speed limitations and flying at the airspeeds that will result in the desired performance is a key to safety. While speed may be life in certain situations, it can also kill. Use it wisely.

Getting down
VREF for light aircraft

Because stall speed varies with aircraft weight, the pilots of large, heavy airplanes approach and land using reference speeds—VREF—that are based on the stall speed and other factors at the aircraft's landing weight.

Although the differences between takeoff and landing weights are not as great as they are with large airplanes, there's no reason why pilots of light aircraft can't benefit from VREF speeds. VREF gives you a landing speed that provides a margin of safety above stall speed but is not so fast that the plane will float the length of the runway.

Before you modify your landing speeds, consult and follow POH recommendations. Your VREF speeds are based on your aircraft's VSO. You should use the figure that represents the aircraft's landing weight (or as close to it as you can get, erring on the high side). If your VSO is given for maximum gross weight only, you can adjust it for lighter weights with the same formula used to determine VA at less than max gross weight (10 percent reduction in weight, 5 percent reduction in VSO).

It's important to know whether your aircraft's VSO is given in IAS or CAS when determining VREF speeds. CAS should be used when applicable and converted to IAS for practical use because the difference between the two can be quite large.

Unless your POH recommends otherwise, fly the pattern no faster than VFE and no slower than 1.4 times VSO. This keeps your speed up in the pattern and gives you full use of your flaps and a safety margin over stall speed. Maintain this speed until you turn final. Then let your speed decay to 1.3 times VSO once the landing gear and full flaps are deployed. Remember, VSO is only an accurate stall speed in this specific landing configuration.

If the wind is blowing, add one-half the gust factor to your landing speed. If the wind is 10 knots gusting to 20, add half the difference (5 knots) to your speed. Remember, 1.3 VSO gives you a safety margin, but only after all maneuvering is completed and full flaps and gear are down. So use 1.3 VSO on short final only.


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