When making the transition from highway travel to flight, a student realizes early in training that the airspeed indicator is not the same as an automobile speedometer. Calculating how soon you will get to your destination using indicated airspeed--the number to which the needle is pointing--will likely be erroneous.
Airspeed definitionsIndicated airspeed: The airspeed read directly from the airspeed indicator. |
Several different speeds are commonly used to describe flight. All can be derived from the airspeed indication, but they require consideration of other factors, including atmospheric pressure, altitude, temperature, wind direction, and wind speed. However, only two speeds are important to a pilot. One explains how the airplane performs geographically (how soon will we be there), and the second describes how the airplane performs aerodynamically (how to fly the aircraft).
Calculating groundspeed before the introduction of GPS and its ground-based predecessor, loran, was a time-honored test of pilots' navigation skills. Comparable to a car's speed along a highway, groundspeed calculation requires the measurement of the time that an aircraft takes to travel a defined distance. A distance (leg) is measured on a chart, and time to fly that distance is clocked. The formula, distance = rate x time, is then solved for rate.
Modern technology makes these measurements and calculations unnecessary. GPS receivers automatically measure distance and time, providing an instantaneous groundspeed. That groundspeed permits the unit to calculate time to destination or the next waypoint. It can estimate the time of arrival and if the GPS receives information regarding the fuel flow and fuel on board, it can predict whether there is enough fuel aboard to complete the flight. None of this requires reference to the airspeed indicator, and none of these calculations help the pilot to operate the aircraft from minute to minute.
A pilot must be aware of how the aircraft is interacting with air, and this best corresponds to indicated airspeed. The pressure caused by an aircraft's forward motion through the air is proportional to its speed through air. On some very old aircraft, a large postage-stamp square of metal was attached to the stem of a spring. The device was mounted on a wing strut, easily visible to the pilot in the open cockpit. With the airplane in motion, air hitting the square pushes it back against the force of the spring. The farther the metal square is deflected, the faster the airspeed; the deflection is measured against a calibrated scale.
More modern aircraft since have pitot-static systems, but they work on the same principle. Impact air is collected by a pitot tube and plumbed to an air pressure gauge--the airspeed indicator. There, impact air pressure is compared with static air pressure, and the difference drives the needle.
This system is more accurate than using a spring for comparison, but it requires that static (non-moving) air also be piped to the instrument. This more complex system induces several potential errors. While in cruise flight, the pitot tube is pointed squarely at incoming air. However, when the airplane slows and the angle of attack increases, the pitot tube also angles and no longer is incident to air flow. The angle will also be affected by flap position. And, it can be difficult to find a location where accurate static air pressure can be sampled in a fast-moving airplane. Airplanes typically have two static ports, one on each side of the fuselage, and they are connected in a T-fashion; the static pressure presented to the airspeed indicator is the average. This helps to correct errors induced by the aircraft not flying straight--during a slip or skid, one static port may be subjected to impact air pressure, and the other to a relative vacuum.
These faults are called position errors. A table in the pilot's operating handbook (POH) or placards provide correction values for these errors, and are used to convert indicated airspeed (on the airspeed instrument) to calibrated airspeed.
If the pitot tube or the static source is blocked, the airspeed indicator will not display correctly. Because ice is the most frequent culprit, most pitot tubes and many static ports are heated to prevent ice from forming. However, insects, covers that should have been removed before flight, and tape placed over ports while an airplane is washed also will result in erroneous airspeed indications.
For the oral exam of your practical test, it is likely that you will be asked to explain common failure modes of the airspeed indicator. If the pitot tube is blocked, air trapped in the pitot line provides constant pressure, which does not change with the aircraft's speed. This is compared with static air pressure that decreases as the airplane ascends. As the airspeed indicator can only signal the difference in pressures, it erroneously indicates increasing airspeed as the airplane climbs. This is a particularly dangerous failure, especially if the pilot has poor visual references outside the cockpit and does not monitor the attitude indicator, altimeter, and vertical speed indicator. Because airspeed appears to be increasing in the climb, the pilot's reflex action is to raise the nose. Still not seeing the expected reduction in airspeed, the pilot continues to raise the nose--and the airplane stalls with the pilot focusing on an airspeed indicator close to red line.
The reverse occurs with static port blockage. The airspeed indicator shows progressively slower than correct airspeed as the airplane climbs above the altitude where the blockage occurred. Pitot air pressure decreases in a constant-airspeed climb, but without the proper opposing comparison to decreasing static pressure, the airspeed indication is erroneously low. While less likely to have the bad consequences of a pitot failure, it certainly is disorienting to see no change in indicated airspeed as the nose is lowered and power is adjusted.
Indicated airspeed (IAS) is the speed that best reflects aerodynamic performance. Stall speed, maximum cruise speed, best rate of climb, best angle of climb, approach speed, glide speed, and flap and gear operating limitations all are expressed in indicated airspeed. For these practical purposes, corrections to calibrated airspeed are small and usually can be disregarded, unless the pilot plans to use the value for further computations.
| If the pitot tube is blocked, airspeed will not be displayed correctly. Insects or ice can block the pitot tube--or forgetting to remove the cover during your preflight. |
An important measurement of performance is true airspeed (TAS), which provides the answer to how fast an aircraft is moving through the air. However, it requires further calculations and consideration of an important factor, air density.
Compare throwing a ball in air with an attempt to throw the same ball under water at the same speed. The ball thrown in water experiences greater impact pressure than a ball thrown in air. In the same way, pitot air pressure is dependent not only on speed, but also the density (thickness) of the air. The calculation of air density requires knowledge of air temperature and atmospheric pressure. Hot air and air under low pressure are less dense, which is why hot air balloons ascend. During the climb, outside air density decreases until an equilibrium is reached, and the balloon is at maximum altitude.
The formula for TAS is complex, and it is rarely included in aviation textbooks. It will not be appended here either, as pilots no longer manually calculate or graphically plot TAS computations. We depend on an electronic calculator, a GPS receiver's computer, or a true airspeed indicator. Whichever of these tools are used, the calculation requires inputs of outside air temperature, pressure altitude (which is indicated altitude corrected to 29.92 mm Hg), and calibrated airspeed. (At true airspeeds much in excess of 200 knots, additional factors must be considered.)
So now we have true airspeed, and can compare this with the speed predicted in the POH and touted in airplane advertisements.
However, TAS tells us little about groundspeed, and that is because wind speed and direction--collectively called the wind vector--cannot be measured directly while aloft. (A vector is mathematically defined by both direction and speed.)
Assume you are swimming across a river at night and soon cannot see either shore. Presumably you know how fast you can swim, and if you carry a compass you know the direction in which you are swimming. However, this is insufficient information to predict where you will land along the opposite shore. And, with no outside reference you cannot calculate how fast the river is moving.
Determining the wind vector, the aircraft vector (TAS and aircraft heading) is known. The ground vector (ground speed and direction or course) must be derived from a map and compass, or from GPS or other ground navigation aids. Then a wind triangle can be constructed; the third side of the triangle is the wind vector.
Wind influences groundspeed and aircraft heading. Easiest to visualize are direct headwind and direct tailwind. The entire amount of wind speed is added to or subtracted from TAS to provide groundspeed.
However, most of the time aircraft fly at some angle to the wind. It is then customary to resolve the wind vector into two vectors: the headwind vector directly affecting ground speed, and a crosswind vector resulting in drift to the right or left of course. To offset the crosswind component, aircraft heading must be crabbed into the wind. This means the nose of the airplane is no longer pointed where the plane is going. The difference between the heading of the aircraft and the course along the ground is called the wind correction angle.
It would seem a direct (90-degree) crosswind should not influence an aircraft's groundspeed. However, groundspeed in that situation is always less than TAS. Because of the airplane's wind correction angle, the entire TAS is not directed toward the destination.
Calculations of the wind triangle and resolving wind vectors by trigonometric formulas or graphically have become a lost art. Some private pilots are taught to use an E6B flight computer, but most prefer an electronic calculator. The FAA allows either device to be used during knowledge tests, and the test computer itself includes a wind triangle calculator.
During flight all the needed information is readily available on a GPS receiver, and pilots can become complacent. A warning is appropriate. The information a GPS provides is as of that instant; it cannot predict the future. Changes in wind, airspeed, and groundspeed occur in every flight. A pilot who does not appropriately respond to changing speeds may not be able to complete the flight that was so carefully planned.
Dr. Ian Blair Fries is a CFI, senior aviation medical examiner, and ATP, and holds a Lear 35 type rating. He serves on the AOPA Air Safety Foundation Board of Visitors and is cochairman of the AOPA Board of Medical Advisors.
Want to know more?
Links to additional resources about the topics discussed in this article are available at AOPA Flight Training Online.
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