“Man, my airplane is fast,” boasts a pilot. “I was getting 150 knots true groundspeed.”
“I was descending at 180 knots indicated when I felt a vibration,” reported another to the National Transportation Safety Board. Another pilot comments that this would result in a true airspeed above the airplane’s Never Exceed speed.
With so many technical things to learn in a short time when earning a Private certificate, it’s not surprising that some of the details sometimes get glossed over, and pilots are certificated without a complete understanding of some concepts. The seemingly trivial differences in “type” of airspeed measurement are important, though; let’s review the different types of airspeed, and why it’s important to know the difference.
FIVE AIRSPEEDS
There are five different ways commonly used to quantify airspeed. They are:
- Indicated airspeed: This is simply the airspeed read directly off the airspeed indicator.
- Calibrated airspeed: Calibrated airspeed is indicated (or equivalent) airspeed adjusted for errors introduced by the pitot / airspeed indicator hardware and installation.
- Equivalent airspeed: Not a player for most pilots, equivalent airspeed is calibrated airspeed adjusted for compressibility, the “cramming” of air into the pitot tube at very high air speeds.
- True airspeed: True airspeed is the actual speed of the airplane through the air. It is calibrated (or equivalent) airspeed adjusted for nonstandard atmospheric pressure and/or temperature.
- Groundspeed: Groundspeed is the speed of the airplane over the ground. Groundspeed is true airspeed adjusted for headwind or tailwind.
Indicated airspeed equals true airspeed only at sea level at standard temperature and pressure, and then only if there is no instrument error. Ground speed equals true airspeed only in absolutely zero wind at the altitude flown. Most times, however, each airspeed would be different for a given set of flight conditions.
FROM THE PILOT’S SEAT
From where you sit, the most important airspeed to consider is the indicated airspeed. All performance numbers are stated in knots (or miles per hour) as they’re indicated on the airspeed indicator. This makes sense when you think about it — the indicated airspeed is a function of how quickly air molecules ram into the pitot tube, itself indicative of the flow rate of air across wings and controls. If generating lift requires a certain flow rate, it’s indicated airspeed that tells you whether you’ve achieved that rate. If flaps, landing gear or airframe limitations require you remain below a certain rate of air flow, you’ll know it by reading the airspeed indicator. Keep your speed below your indicated flap or landing gear extension speeds to avoid overstressing them; stay below the indicated Never Exceed (“red line“) speed to keep from damaging the airframe. If true airspeed differs (and it almost always does; the higher you fly, the bigger difference between true and indicated air speed). What true airspeed measures is how fast you move through the air mass — not (directly) how much air is flowing around the airplane.
Use indicated airspeed regardless of density altitude. It’s still a function of how much air is flowing around the airplane — you’ll need a higher true airspeed to get the air flow in hot or high-altitude air, but the pitot / static system takes the guesswork out of it.
Insider’s Note: That’s one reason it takes more runway to take off and land at high density altitudes — the airplane has to be moving faster to get the same air flow it would at lower altitudes. While the indicated airspeeds remain the same, the aircraft requires more acceleration time for takeoff and will be moving faster when it does. At landing, more inertia must be overcome to bring the airplane to a stop.
Another important speed to the flying pilot is the groundspeed. Winds may accelerate you along your path or impede your progress — it’s the groundspeed that tells you how long it’ll take to get to your destination … and if the fuel on board will get you there. To compute ground speed (without a device like GPS, LORAN, or Distance Measuring Equipment [DME]) you need to know true airspeed and to derive that you need indicated airspeed.
ARCS AND MARKINGS
The airspeed indicator has a variety of markings to tell you limiting and (in some cases) performance airspeeds:
- A “white arc” from the (indicated) stalling airspeed with full flaps, level wings and idle power, to the maximum flap operating speed.
- A “green arc,” delineating the flaps-up, idle power, wings level stalling speed to the maximum normal operating speed — usually a little above high-speed cruising speed.
- The “yellow arc,” or caution range, picking up at the “top” of the green arc and going to
- the “red line,” or “never exceed” speed.
Some retractable-gear airplanes have marking for gear speed, while most multiengine airplanes depict the best climb speed on one engine with a “blue radial.” Turbine-powered airplanes don’t have a “yellow arc” (“red line” is at the top of the green arc), and high-altitude jets may have a moveable “barber pole” that shows the “red line” speed as a function of Mach number.
WHAT THEY DON’T TELL YOU
You won’t see on airspeed indicators: stall speeds with various amounts of engine power, or varying angle of bank; performance speeds like Vx (best angle of climb) or Vy (best rate of climb), or limitation airspeeds for partial flap extension. You also won’t see the “Maneuvering Speed” (Va) or “Turbulent Air Penetration Speed” (Vb), because they may vary significantly with airplane weight. Airspeed indicators aren’t usually marked for approach speeds or velocities for maximum airspeed or maximum endurance. Unfortunately, if you tried to mark everything on the airspeed indicator, it’d be so cluttered you couldn’t read any of it.
ANGLE OF ATTACK
As it turns out, performance speeds (as opposed to limitation speeds like flap speed, gear extension, or “never exceed“) are really a function of angle of attack, and indicated airspeeds only provide an approximation of performance speeds under ideal circumstances. In most cases, flying with the indicated airspeeds from the Pilots Operating Handbook is close enough to net the desired performance. If you’re flying aerobatics or out of ultra-short airports you’ll find best performance comes from flying by reference to angle of attack, but for whatever reason “AoA” meters have not made great gains in lightplanes despite being common in jets and Naval aircraft … and readily available for experimental aircraft.
BOTTOM LINE: There’s no such thing as “true groundspeed.” An airplane (in level flight, anyway) theoretically will not be overstressed if you keep the indicated airspeed below limits. Flying — even recreational flying — is a precision-based activity, and it’s important that we know, use and understand the terms that define that precision.
