Significance of V-Speeds

I am not a big proponent of memorizing airspeeds. It can be a risky practice if you fly many different types of aircraft. Most of the important airspeeds can be determined from the airspeed indicator (ASI), though there are a few you probably should commit to memory, or at least keep on a placard.

Eight speeds not found on the ASI are listed in the associated illustration. These are not annotated because they vary with weight and other factors. Yet we are often asked to memorize these. This is an interesting anomaly given they are based on maximum gross weight (MGW)—a condition you should never experience legally once airborne. Let's take a closer look.

This is one V-speed I wish pilots would never be exposed to. The non-thinking pilot seems to reason that at V_R the airplane will magically fly and many inappropriately try to force it into doing so by yanking back on the stick at the designated airspeed. A smooth transition from takeoff into the climb portion of fight is essential on an instrument departure.

Forcing an airplane into the air before it's ready to fly is anything but smooth— V_R is determined by the airplane. All the pilot needs to do is apply a little backpressure on the yoke to set the appropriate pitch and the plane will fly on its own when sufficient lift is available based on prevailing conditions. In general, V_R equals 1.15 times V_S1 (the bottom of the green arc). The only reason to know V_R is if you find yourself significantly above this number and still on the tarmac—then it might be time to abort the takeoff. Leaving snow or frost on the wings is an example.

Knowing the general performance numbers for your aircraft and the length of the runway, you should be able to visually determine if the aircraft is not performing as it should.

Use this speed for a stabilized short final instrument approach. It assumes all maneuvering has been completed. While not indicated on the ASI, it can be calculated as 1.3 times V_S0 (bottom of the white arc). The catch being it is based on MGW, which we will typically never be anywhere near during the landing phase. A general rule of thumb is to reduce V_Ref by half the percentage you are below your MGW.

From the weight and balance completed before departure you know the current relationship to MGW and should be able to estimate the weight reduction from fuel burn. Thus, a reasonable V_Ref can be calculated even before departure. How important is V_Ref? If you fly just 10 percent over V_Ref you can expect your ground roll to be over 20 percent longer. Another common rule of thumb used is each knot above V_Ref adds another 100 feet to the ground roll. We tend to become complacent about the landing roll flying light general aviation aircraft from runways that are three or four times longer than we need. This can bite us when popping out of the overcast with too much speed and a tailwind.

Calculated by the manufacturer, V_A is the speed at which the aircraft will stall before exceeding design maximum G loading. This is a good speed to know when flying in turbulence, something quite common in IMC, since it will help prevent damaging the airframe. Just like V_Ref you can calculate V_A. It is typically determined by multiplying the flaps-up, power-off stall speed (V_S1—typically the bottom of the green arc) by 1.95 which is the square root of the normal category load limit of 3.8 Gs.

A more conservative thumb rule uses 1.7 times V_S1, which is generally associated with turbulence penetration airspeed, V_B, not normally specified for general aviation aircraft. V_A can then be reduced in the same manner as V_Ref is based on weight. If you anticipate turbulence, then create a timeline of what V_A (or V_B) should be over the course of the flight as fuel is consumed.

These can be calculated but would require knowing a host of factors related to your aircraft and some complicated math. Best angle of climb (V_X) is useful for takeoff over an obstacle, an ODP with a challenging climb gradient, flying a delayed missed approach, or need to perform a short field landing.

Best rate of climb (V_Y) is an airspeed to get you efficiently to the enroute altitude. Further complicating things a bit is the fact that V_X increases slightly with altitude while V_Y decreases about a half to one knot per thousand feet until they are equal at an airplane's absolute ceiling. Fortunately, both of these values are normally needed at low altitude so the published figures are good to use if corrected for weight.

This is one of the most important airspeeds you should know. For most airplanes, it's about halfway between V_X and V_Y. Once again, most manufacturers establish the best glide speed at MGW. This means your best glide speed will always be lower than the book value should the need arise to use it. V_Y, V_X and V_G should all decrease about a half-knot for each 100 pounds under MGW.

Recall that weight itself has no effect on the glide range or ratio, only the proper airspeed to attain the max glide range. However, a tailwind allows you to decrease V_G further by about one-third of the tailwind component while a headwind requires an increase in glide speed—typically by one-half the headwind component—to maximize your range. V_G can also be approximated by keeping the wing cord parallel with the horizon—provided you can see the horizon—not typical on an IFR flight. Another trick using V_X and V_Y is to use them to calculate cruise climb (V_CC) by taking the difference between the two and add it to V_Y.

Knowing V_X and V_Y and using the ASI color coding can allow you to determine all the important airspeeds for instrument flight. Naturally, consult the POH for more specific guidance for your aircraft.

Did you ever wonder why they are called V speeds? The "V" is from the French word ‘Vitesse' which means ‘speed' or ‘rate.'

A word of caution when using airspeeds based on the color coding of the ASI. Airplanes manufactured before the mid-1970s had their color coding based on calibrated airspeeds (CAS), and were primarily in mph (some may show knots as a secondary indication). However, airplanes built after this period primarily had their ASIs marked in indicated airspeed (I AS).

For most of us, the differences between CAS and IAS will be small but you need to be cognizant of this difference. CAS is important when calculating True Airspeed (TAS) and Equivalent Airspeed (EAS). As always, follow the guidelines specified in your POH for your particular airplane.

Richard Lanning Ph.D. is a graduate of the U.S. Naval Academy and a pilot for more than 30 years. He is an ATP, CAP Check Pilot Examiner, and CFII.

This article originally appeared in the January 2018 issue ofIFR Refreshermagazine.

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