Airspeed myths

I am often surprised and sometimes even alarmed by the aerodynamic misconceptions that abound in aviation.

Lately I’ve heard airspeed myths that have left me scratching my head. In one case, I witnessed an instructor explaining to his student that an aircraft flying with a headwind generates more lift because of the increased flow across the wings. Of course, this fails to be true once the aircraft lifts off the runway and becomes part of the airmass itself. Lift is generated when the engine propels the airplane wings through the air mass surrounding it. It’s especially concerning when ideas that defy accepted flight principles are perpetuated by a flight instructor, but it seems to happen with regularity.

To see where such notions originate, I reread several texts, watched some online videos, and paid closer attention when speaking with flight instructors. I realized that all these sources lack precision when discussing aircraft motion and associated speed. In conversations with pilots, few could explain all of the following speeds: indicated, calibrated, equivalent, true, and ground.

The aircraft pitot tube measures total pressure and the static port detects static pressure. Dynamic pressure is the difference between total and static pressures and an increase in that difference causes the needle on the airspeed indicator to read a higher value of indicated airspeed (IAS).

Were the pitot-static measurement system error-free, IAS would perfectly describe the way the aircraft moves with respect to the air mass containing it. Designing and locating the system elements on the aircraft prove challenging, so manufacturers publish a correction table or graph in the pilot’s operating handbook. Applying this correction to IAS results in calibrated airspeed (CAS). In the POH for my Beechcraft Bonanza, Niky, CAS is remarkably close to IAS for almost the entire airspeed spectrum. The POH for Wilbur, my Cessna 152, reports that IAS reads two knots high in cruise flight and six knots low around the stall.

At very high speeds and altitudes, the pitot system can produce a dynamic pressure reading that is erroneously high due to compressibility effects. Equivalent airspeed (EAS) corrects CAS for such errors. For many common general aviation aircraft, equivalent airspeed is not mentioned in the pilot’s operating handbook because the effects are minimal. For transport category aircraft, though, equivalent airspeed is an important one to know.

EAS, or CAS if you fly airplanes like mine, properly describes aircraft motion with reference to the air mass containing it. But sometimes the appropriate frame of reference is the space that contains those air molecules.

An aircraft that flies at a fixed calibrated airspeed will travel much farther through space in the flight levels than it does at sea level since air density decreases with altitude. True airspeed (TAS) corrects EAS for such density effects. For example, an aircraft cruising at 12,000 msl with 166 KCAS travels around 200 KTAS. If you want to get to your destination with time efficiency, select a power setting and altitude that provides a maximum TAS.

Of course, winds also play a role in flight planning, and this changes our reference system to a point on the ground. When the headwind/tailwind component is applied to TAS, the result becomes groundspeed. With calm winds, these two speeds are the same.

For the remaining discussion, let’s consider a common general aviation aircraft like a Bonanza and several flight situations for which choosing a particular airspeed is important. In each case we need to decide whether the appropriate reference system is with respect to the air mass (CAS) or a point on the ground (groundspeed).

Turbulence

The stresses that an aircraft undergoes are related to CAS. In turbulent conditions, pilots are wise to fly with a CAS at or below the gust penetration speed or, if one is not specified, maneuvering speed. The idea is that the aircraft in rough air will stall rather than exceed its structural capabilities. But rough air often comes as a surprise, so we might be vulnerable during the time it takes to reduce speed.

I generally fly my Bonanza at 65-percent power because that setting offers reasonable compromise between time efficiency and fuel efficiency. As altitude increases, the Bonanza’s CAS decreases from its maximum value at sea level. TAS, on the other hand, initially increases with altitude before falling so a pilot can reap the benefits by wisely selecting an altitude. On a standard day, it yields a maximum TAS of 161 knots at 8,000 feet msl. But choosing a higher altitude can lower stress for both the pilot and the airplane. At 12,000 feet it will cruise with 158 KTAS, just three knots shy of the maximum, but the associated 131.9 KCAS is now slightly below its maneuvering speed. That means any bumps along the way shouldn’t cause a problem. A bonus is that fuel economy continues to increase with altitude, so 12,000 is a great choice to save fuel.

Canyon turn

Whenever it’s necessary to reverse course in a confined area, such as the typical box canyon scenario, choosing the right speed is imperative, and that involves both CAS and groundspeed. In a no-wind situation, turn radius is proportional to the square of TAS so a tight turn means a low airspeed. In this case, though, the pilot should select a low CAS that maintains some margin over a stall. If the turn is made at a high altitude, the pilot has no choice but to accept the larger turn radius that results from the corresponding TAS. Any air movement can be used to your advantage by turning into the wind and thereby decreasing the footprint of the turn with a lower groundspeed.

Forced landings

After an engine failure beyond gliding distance of an airport, a pilot is challenged with guiding the airplane to a safe landing. Minimizing injuries means touching down with minimal kinetic energy, which is proportional to the square of groundspeed. The pilot should choose a slow approach CAS but one that continues to provide margin above the stall. As with the canyon turn, a forced landing into high terrain automatically means a higher kinetic energy. For example, 60 KCAS at 10,000 msl equates to 70 KTAS and that 17 percent increase in touchdown speed represents a 37 percent increase in kinetic energy—just one of the risks we assume by flying around high terrain. At high altitude, it is especially important to land into the wind, if possible, to minimize groundspeed and kinetic energy.

These are just a few scenarios for which determining which reference system applies can help a pilot select an appropriate airspeed. This reasoning is neither easy nor quick, so an emergency situation shouldn’t provide the first opportunity for such thought experiments. Achieving an energy management mindset involves not only understanding airspeed theory but the ability to put that knowledge into practice. Perhaps your next hangar-flying session is an ideal time to consider these and other scenarios to improve your own knowledge and correct airspeed myths in the pilot community.