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1. Indicated airspeed (IAS) is what you read off the airspeed indicator as you fly. |
One of two major components necessary for an answer is true airspeed in knots (KTAS). The other is wind direction and speed. Ground speed results when the two are combined, allowing calculation of a time, speed, and distance problem for the estimated time of arrival (ETA).
True airspeed in knots (nautical miles per hour, as used in modern airplanes) is the speed of the airplane relative to the air mass it is in. It is not highly accurate, but it is as close as we can get using conventional small-airplane instrumentation. Determination starts with the indicated airspeed in knots (KIAS).
Airspeed used for aircraft control with ailerons, elevator, and rudders is read off the airspeed indicator in the upper left corner of the standard six-instrument panel display. KIAS is essential for takeoffs, landings, and stall practice, but it is inadequate for evaluating aircraft performance.
The next airspeed up the ladder of corrected values is calibrated airspeed (CAS), which equals IAS corrected for instrument and position error. Every mechanical device in use has some inaccuracy. Engineers for instrument makers and aircraft manufacturers run tests under tightly controlled conditions to determine instrument errors that cannot be designed out of the system.
Position error is the inaccuracy in IAS when the air does not enter straight into the pitot tube. With any increase in the angle between the relative wind (air entering the tube) and the axis of the tube, the greater the error. An extreme example may be experienced when operating at a high angle of attack--for example, while practicing slow flight. Position errors are determined by flight tests, and when combined with instrument error, are published in table or graphic form in the pilot's operating handbook (POH). There will be qualifying notes for flap position and, in some airplanes, power and weight. If the alternate static source--which measures static pressure in the airplane when the primary source is blocked by ice or debris--is in use, an additional table with conditions for windows, heaters, defroster, ventilators, and cabin air may have to be considered.
In high-speed airplanes (generally more than 200 KTAS), equivalent airspeed (EAS) is a factor. It can be ignored in small training airplanes, but for completeness: EAS corrects CAS for adiabatic (no heat transfer) compression flow. At 20,000 feet and higher or more than 200 kt, air is compressed in front of the pitot tube, resulting in substantially higher indicated airspeeds. But for our purposes, let's forget EAS and high-speed, high-altitude airplanes and jump directly to true airspeed.
As the Earth rotates on its axis and revolves around the sun, the Earth and atmospheric temperatures vary considerably, in turn producing pressure changes. To standardize aircraft performance calculations, including true airspeed, the world of meteorology came up with the standard atmosphere. This is nothing more than atmospheric temperatures and pressures from around the world at various altitudes averaged over a number of years. True airspeed, then, is calibrated airspeed corrected for nonstandard temperature and pressure, translating the current atmospheric conditions to the standard atmosphere.
Calculating TASThe old and the new
The TAS calculations mentioned here are referred to as the "old" true airspeed method. In the "modern" method, air compression, temperature rise, and an aircraft unique recovery coefficient from 0.6 to 1.0 are considered in addition to air temperature and pressure altitude.
As implied in the article, the faster an airplane travels, the greater the inaccuracies in TAS. At high speeds, air piles up in and in front of the pitot tube and is compressed, causing higher-than-normal airspeed indications. The airplane's temperature probe measures free air temperature or, more accurately, actual air temperature at cruising altitude. Compressed air also piles up around the temperature bulb, causing the actual air temperature to increase. (When air is compressed, temperature increases; when it expands, the temperature falls.) Friction generated by air moving over the temperature bulb creates an additional heat source detected by the thermometer bulb. Compressibility and temperature rise yield a higher-than-actual TAS under the old method.
For accurate temperature measurements in high-speed, high-altitude airplanes, the temperature probe design and installation become very important. The installation and design errors can be compensated for by a unique recovery coefficient for a specific airplane in a manner similar to the way CAS is derived from IAS through knowledge of installation and instrument errors.
Using an electronic flight computer, a comparison can be made for different speeds and altitudes. A typical training aircraft at 110 KCAS in a standard atmosphere at 8,000 feet trues at 124 KTAS in both the old and new methods. At 200 KCAS at 8,000 feet, the airplane trues at 225 (old method) and 222 (new). At 400 KCAS, the calculation yields 450 "old" TAS and 20 kt lower in the new method.
If that venerable trainer could make it to 26,000 feet, it would true at 167 in the old system and 164 in the new one. The 400 KCAS airplane trues at 608 old and 74 lower in the new for 534 KTAS.
Certainly student pilots do not need this knowledge for FAA testing purposes or for day-to-day flying as private (or, for that matter, commercial) pilots in small airplanes, but it is fun to think about--and besides, one day you may occupy the left seat in the cockpit of a Boeing 777 or other high-speed aircraft. |
Thermodynamics tells us that only two measurements (temperature and pressure in this case) are necessary to define the state of a pure substance--and for our purposes the air is pure. At sea level with standard temperature and pressure, TAS will equal CAS. As an airplane climbs in a standard atmosphere, the TAS will increase to maximum around 6,000 to 9,000 feet for a normally aspirated engine at a maximum safe continuous power. These are good numbers to keep in the back of the mind for flight planning. (The optimum altitude for a given airplane can be determined by studying the performance charts in the POH, but remember, this is pressure altitude, not indicated altitude.)
During preflight planning, a pilot can estimate TAS at the planned altitude by referencing the performance tables in the pilot's operating handbook with estimated pressure altitude and air temperature. Depending on the airplane's manufacturer, calculating density altitude with an E6B or electronic flight computer may be necessary before using the POH. The information in performance tables and graphs is not all equal; different aircraft manufacturers may present the data differently.
While en route, actual temperature and pressure can be used. Read air temperature on the outside air temperature gauge at cruise altitude in degrees Celsius and pressure altitude on the altimeter with 29.92 in the altimeter setting window (don't forget to reset the altimeter to the correct setting). Place the two opposite each other in the computer TAS window. Read TAS on the outside scale opposite CAS. The airspeed indicator in many airplanes incorporates a mechanical TAS calculator on the dial face. (It is simply an E6B with only the TAS window and the outside and inside scales from the calculator side.) Position temperature and pressure altitude opposite each other; TAS will appear under the airspeed pointer.
For a better understanding of aircraft performance, it is instructive to calculate TAS in the climb while maintaining a constant climb airspeed. This verifies the increase in TAS with altitude, as will a study of cruise performance tables or graphs in the POH. One popular training airplane's TAS increases by 10 kt from sea level to 8,500 feet at 75-percent power with the correct engine rpm and properly leaned mixture.
Note the preflight and in-flight values and how they compare. Are they comparable to the flight plan value? The FAA expects the filed TAS to be corrected if in error by more than 10 kt or 5 percent, whichever is greater--the break point is 200 kt. This is especially true in an IFR flight plan. Keeping a record of airspeed and other performance parameters such as fuel consumption for a given airplane can forewarn a decrease in performance or a need for maintenance.
A quick computer scan of the private pilot knowledge test shows no questions on how TAS is determined--this is truly amazing! The FAA has neglected this important subject. Of 754 questions in the test question bank, 20 refer to TAS; 17 give TAS and ask questions on cross-country flight planning. Nowhere is the applicant asked to compute TAS from pressure altitude and calibrated airspeed. Another quick check of the commercial and instrument knowledge tests reveals that only seven questions on each test mention TAS, and none asks it be calculated from CAS and pressure altitude.
But you may think, I don't need all this useless information! I have GPS in my airplane! Well, yes, but that GPS and other equipment can quit. Then what do you do? The batteryless E6B, flight log, dead reckoning, and pilotage will always point the way home. What's most important, we can quantify the answer to that vital question: "Are we there yet?"
Dick Branick is a 6,400-hour airline transport pilot with more than 4,100 hours of dual given as a single- and multiengine instrument airplane instructor and ground instructor with advanced and instrument ratings. He is a turbine test engineer on contract to NASA.
Want to know more?
Links to additional resources about the topics discussed in this article are available at AOPA Flight Training Online.
