Aero-Tips!

A good pilot is always learning -- how many times have you heard this old standard throughout your flying career? There is no truer statement in all of flying (well, with the possible exception of "there are no old, bold pilots.")

Aero-News has called upon the expertise of Thomas P. Turner, master CFI and all-around-good-guy, to bring our readers -- and us -- daily tips to improve our skills as aviators. Some of them, you may have heard before... but for each of us, there will also be something we might never have considered before, or something that didn't "stick" the way it should have the first time we memorized it for the practical test.

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Aero-Tips 08.14.06

"My plane's faster than your plane." Who hasn't at least thought it when comparing an owned, borrowed, rented or fantasized aircraft to another? But sometimes it's obvious that even experienced pilots don't always understand the different kinds of speed.

Aircraft speed is usually expressed in either knots or miles per hour. One knot is one nautical mile (6000 feet) per hour, or about 1.15 miles per hour. Vocabulary tip: Saying "knots per hour" is redundant. "Knots" will do.

Here are the kinds of speed, and what they describe:

• True airspeed (TAS). This is the speed the aircraft is "truly" traveling through the air. TAS corrects for all instrument error and environmental factors. If there was no wind at all, TAS would be the speed you cover the ground. TAS is the correct expression of an airplane's capability.
• Indicated airspeed (IAS). As the name implies, this is the speed read from your airspeed indicator (ASI) without any corrections applied. Since the ASI measures impact air pressure, if the airplane maintains a constant TAS its IAS will decrease as altitude (and air pressure) decreases. Similarly, if IAS remains constant, the higher you go the higher your TAS... you have to fly through space more rapidly in thin air to get the same impact air pressure to show a constant IAS.
• Calibrated airspeed (CAS). CAS is IAS corrected for installation error and indicator error. Flight in some configurations and/or some angles of attack may result in IAS error. CAS is an engineer's way of compensating for this... from the pilot's seat, those errors are built into manufacturer's recommendations for reference IAS. CAS more correctly describes the amount of air flowing over the airplane's wings.
• • Example: Generally liftoff in a light multiengine airplane is done at VMCA, the minimum controllable airspeed with one engine inoperative, plus five knots. This gives the pilot a margin above VMCA should the critical engine fail right at liftoff. In the Beech 58TC Baron I flew for several years, VMCA is 81 knots. Because of instrument error in the 58TC 86 knots IAS (VMCA +5) is 81 knots indicated. Lifting off right at the "red radial" VMCA speed IAS, as recommended in the Pilot's Operating Handbook, provided the five-knot safety margin in CAS.
• Ground speed (GS). GS is TAS corrected for wind. For a given TAS, GS increases with a tailwind, and decreases with a headwind. GS is usually what you read from a GPS. Since GS is entirely dependent on non-aircraft factors (the wind), it is not an honest value for comparing one aircraft's performance to another's. Brag about seeing 200 knots on your Skylane's GPS with a tailwind if you like, but it's still slower through the air than the Columbia 400 flying into the wind in the other direction.

Aero-tip of the day: Understand the different "kinds" of airspeed, and apply them correctly.

FMI: Aero-Tips