Are You Sure You Know How Fast Your Plane is Going?

The answer is harder than you think…

By the staff at MySkyForce

It’s an easy mistake to make. You’re sitting in the cockpit with six round dials in front of you, and your instructor asks how fast you are going. One of the dials has small letters that say AIR SPEED. It looks a lot like the speedometer in your car, and the needle is pointing to 90. “Ninety,” you say. “We are going 90.” But thinking it might be a trick question, you quickly add, “KNOTS. Our airspeed is 90 KNOTS.”

Good try. You remembered the KNOTS thing. But the answer to how fast you are going in an airplane is so much more complex than that.

It’s such an easy question to answer in your car. The wheels on a car have to rotate for the car to move, and since the speedometer is mechanically connected to the wheels or to a rotating component of the car’s drivetrain, whenever the wheel rotates the movement is recorded on the speedometer.

Your airplane can not have the same connection to the atmosphere that the car has to the road. The airspeed indicator relies on air molecules collected by the pitot tube on the outside of the airplane and from the static air sources. Temperature, air pressure, wind and other factors conspire to make the question, “How fast are we going?” a bit complicated for pilots. At any given time, the question could have five right answers. It is important to reference and understand which airspeed you are using.

Indicated Airspeed (IAS)

This is the easy one. It is the speed read from the airspeed indicator. It is referenced in knots and is noted as IAS.

Here is how your airspeed indicator gives you the information: The static port, which is an opening on the exterior of the plane protected from the airflow, is connected directly to the case of the airspeed indicator.

Inside the case of the airspeed indicator is a diaphragm which is connected through a line direct to the pitot tube. The pitot tube on the exterior of the plane faces the airflow which is the source of dynamic air pressure.

As the plane begins to move, ram air pressure is routed through the line from the pitot tube into the diaphragm causing it to expand. The ambient pressure in the case limits the expansion of the diaphragm as air pressure is increased and helps it to contract when the pressure decreases. A simple mechanical linkage translates the movement of the diaphragm to the needle on the face of the airspeed indicator and displays the speed in knots. All you have to do is read the right dial.

Calibrated Airspeed (CAS)

Calibrated airspeed is Indicated Airspeed corrected for installation and instrument error. At certain times, usually at slow airspeeds with certain flap settings, there can be an error of several knots due to disruptions in the airflow to the pitot tube. The error usually disappears at higher speeds, so indicated and calibrated airspeed can be nearly identical at higher speeds. The aircraft’s AFM/POH (Aircraft Flight Manual/Pilots Operating Handbook) contains an airspeed calibration chart. Using this chart, a pilot can determine his CAS by using the IAS and applying a correction. Still not too hard, is it?

True Airspeed (TAS)

True airspeed is calibrated airspeed corrected for non-standard temperature and pressure. It stands to reason if you know the IAS, the temperature at your cruising altitude, and the pressure altitude; you can determine the TAS.

Prior to your flight, you can use the AFM/POH charts and tables to estimate the TAS for your flight. Once airborne, many airspeed indicators provide a method for determining TAS; and glass cockpit displays automatically provide the information. The E-6B (mechanical flight computer) can also be used to calculate TAS, but it is not the most convenient tool for the job. But it is available if electronics fail, and the FAA recommends that all pilots know how to use the E-6B. (Instructions and exercises to help you use the E-6B are available in the MySkyForce app.)

Ground Speed (GS)

Ground speed is the actual speed at which the airplane moves across the surface of the earth. To determine ground speed, we adjust the true airspeed to account for wind conditions. Ground speed is of critical importance, since it plays a major role in determining the range of the airplane and the length of time it will take to make it to your destination.

It is possible your true airspeed and your ground speed could be the same, but it is unlikely due to wind. A headwind will reduce your ground speed and a tail wind will increase it. (Flying into a 10-knot headwind with a true airspeed 100 knots will reduce your ground speed to 90 knots.)

Can you answer this check ride question?

Ground speed can also be affected by the airplane’s attitude. There is a classic check ride question that asks, “How can a plane have a true airspeed of 500 knots and a ground speed of zero on a day with no wind?” Think about it. An Air Force or Navy pilot could be vertical in a fighter at 500 knots and have a ground speed of zero. While this is not relevant in the real world, it serves to show that factors other than wind can influence ground speed.

Equivalent Airspeed (EAS)

You probably don’t need to worry about this one yet, but you never know when you could be on a quiz show and the subject is really fast jet aircraft. Jet aircraft operating at or near the speed of sound need to account for the compressibility of the air as it enters the pitot tube. Equivalent airspeed takes this compressibility into account and expresses it as a Mach number which relates the speed of the airplane to the speed of sound.

So, the Correct Answer is . . .

The next time your CFI asks how fast you are going, tell him … it depends.

Pilot Training at Mach Speed

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