All of us who fly for a living have had a passenger stick his (or her) head in the cockpit and say, "Hey, how fast are we going?". Just, what number does that passenger want anyway? In knots or in miles per hour? I don't always know but let's look at the options.

The most obvious choice is probably the airspeed under the needle on the airspeed indicator, or, the indicated airspeed (IAS).

The airspeed indicator is actually a pressure gauge. The scale on the face of the gauge, instead of being calibrated in pounds per square inch of pressure, or some equivalent, is calibrated in speed corresponding to a given pressure. This pressure is sensed by a tube pointing forward into the flight path. The faster the airplane goes the higher the pressure in the tube and the higher the "speed" indicated on the guage.

The tube is called a Pitot(pronounced pee-tow) tube because this method of measuring speed was invented by Henri Pitot, a French physicist.

The Pitot tubes can be a single aluminum tube protruding from the wing leading edge such as those on low performance airplanes.

When the airplane is on the ground the gauge is calibrated to read zero airspeed. However, the pressure measured by the tube is the atmospheric pressure. The airspeed system will not give an accurate indication of airspeed unless the static pressure is accounted for. For this reason there are also one or more static ports connected to the airspeed indicator. The pressure measured is then the difference between the pressure in the Pitot tube generated by the airplane's forward speed, and the static pressure. This then gives us an indication of airspeed that is as accurate as it can be measured.

In the interest of accuracy, the Pitot tubes are located in a position on the airplane so that the air coming off of the airplane does not effect the pressure they sense. The static ports are simply holes located in the side of the fuselage in an area of neutral pressure. If the air flowing past the airplane is rammed into the ports the static pressure will read higher than the actual static pressure; if the air flowing over the ports causes a suction in the tube connecting it to the airspeed indicator the static pressure will be lower than the actual static pressure. Finding just the right location for the Pitot tube(or tubes), and especially for the static ports, can be a very dificult and time consuming task during initial flight tests.

High performance airplanes usually have more than one Pitot tube mounted on the nose, one for the pilot's airspeed, one for the co-pilot's airspeed, and sometime's a third for the standby airspeed. The static ports are holes drilled into the sides of the Pitot tubes on these high performance airplanes.

The Pitot tubes installed on airplanes certified to fly in Instrument Meterological Conditions(IMC) are heated to prevent ice from plugging them up. The Pitot heat on these airplanes is on all the time while in flight and the tubes turn a bluish-brown color.

If you happen to be flying a jet your airspeed indicator will also have a moving scale that shows your IAS in Mach(Earnst Mach - 1836-1916) . The Mach number is simply your speed relative to the speed of sound and is strictly a function of the temperature of the air in which you are flying. As the altitude increases the temperature decreases; as the temperature decreases the speed of sound decreases. The speed of sound at a given altitude(temperature) is Mach one. If the indicated Mach is 78 then you are traveling at 78% of the speed of sound. The speed of sound at sea level in a standard atmosphere(temperature 59 degrees F) is approximately 660 knots; at 35,000 feet(temperature -66 degrees F) it is aproximately 575 knots.

Factors such as non-standard atmospheric conditions, instrument errors, the installation configuration, and compressibility effects, may cause a variance between the airspeed indications and the actual flight speed.

Calibrated airspeed (CAS) is the result of correcting IAS for errors in the airspeed instrument and errors due to position or location of the installation. Each aircraft type has a Pitot static system that is unique to that type. Instrument errors must be small by design and are usually negligible if the instrument is properly maintained. Position errors are most usually confined to the static system and must be small in the range of airspeeds for the most critical or common performance condition. The most critical or common performance condition in corporate and personal airplanes is cruise.

Equivalent airspeed is the result of correcting the CAS for compressibliity effects. At high flight speeds compressibility of the airflow recovered in the Pitot tube produces a pressure which is greater than if the airflow were incompressible. This causes an erroneous airspeed indication.

True airspeed is the speed of the airplane through the block of air in which it is operating. The airspeed indicator is calibrated for airspeeds corresponding to the dynamic pressure at sea level in the standard atmosphere, i.e. at sea level in the standard atmosphere the IAS and the TAS will be the same.

However, as a practical matter the airplane operates in a non-standard atmosphere all of the time. Therefore, the TAS must be calculated using the EAS and the density altitude. The density altitude is the altitude in the standard atmosphere corresponding to a particular value of air density. Density altitude is computed using the pressure altitude(PA) - the pressure in the standard atmosphere corresponding to a particular pressure - and, the temperature at that altitude.

The ground speed is the speed of the airplane over, or relative to, the ground. As we have already seen the airplane is flying through a block of air at a given true airspeed. However, the block of air in which the airplane is operating is also moving, due to the speed and direction of the wind. To get the ground speed we must add, or subtract, the headwind, or tailwind, component of the speed of this block of air to/from the TAS.

It is important to note that since the TAS is a measure of the airplane's speed through the block of air in which it is operating, that the TAS is the same regardless of which way or how fast this block of air is moving.

To visualize true airspeed and ground speed think of a fish swiming in a tank. The fish is swimming in a block of water the size of the tank. If the fish represents an airplane and the water in the tank represents the block of air in which the airplane is flying then the speed of the fish through the water in the tank represents the true airspeed. Now, if we put the fish tank in the back of a pickup truck and drive down the road at, say, 60 mph the speed of the truck represents the wind speed. If we then add, or subtract, the "wind speed" component to the "true airspeed" of the fish in the tank we will get the ground speed.

If the fish happens to be swimming in the tank in exactly the same direction the the truck is traveling then the ground speed is the speed of the fish plus the speed of the truck. If the fish is swimming exactly opposite the direction the truck is traveling then the ground speed is the speed of the fish minus the speed of the truck. Otherwise, the "ground speed" is the speed of the fish plus or minus some fraction of the "wind speed" depending on the angle between the direction of travel of the fish in the tank and the direction of travel of the truck.

After all of this measuring, correcting, and calculating the speed that is the most useful to us, then, is the ground speed. The ground speed is what determines how long it will take to get to our destination, and what time we will get there. You're answer to that passenger should then be something like this, "Our speed over the ground right now is 520 knots, or 600 miles per hour. This will put us in (destination) in (ETE) at (time) local time."

The jet pilots reading this will probably recall that the V₁ threshold speed during takeoff was, a few years back, and often times even today, defined as the "Take-off decision speed". Instructors usually told us that this was the speed at which we must make a decision either to take-off or stop on the remaining runway after an engine failure. The Take-off Field Length (TOFL) which is the longest of:

  1. 115% of the normal two-engine take-off distance
  2. the accelerate-stop distance, or
  3. the accelerate-go distance

taken from the TOFL charts in the Airplane Flight Manual (AFM) will allow for this choice since the TOFL is the most restrictive (the longest) of these three distances. It should be noted that it is unlikely that any two of these speeds will match for any given set of conditions.

Let's take a look at the definition of V₁ provided by some light and medium-jet AFM's:

"V₁ - Take-off Decision Speed - The maximum speed below which the pilot must initiate the first action (brake application) to discontinue a takoff. Above V1 the takeoff must be continued. Varies with weight, temperature, altitude, runway gradient, and operation of Anti-skid and Anti-Ice."
  Raytheon BeechJet (now the Hawker 400) AFM

"V₁ - Takeoff Decision Speed - The speed at which the distance to continue the takeoff to 35 feet or the distance to stop will not exceed the scheduled takeoff distance provided the brakes are applied at V₁."
  Bombardier Lear 60 AFM

"V₁ - Critical Engine Failure Speed. - The speed at which, due to and engine failure, the pilot is assumed to elect to stop or continue the takeoff. If engine failure occurs at V₁ the distance to continue the takeoff to 35 feet above the runway surface will be equal to the distance to bring the airplane to a full stop. V₁ must not be less than the critical minimum control speed (Vmcg) or greater than the rotation speed (Vr)."
  Bombardier AFM for the Lear 35A/36A

"V₁ - Engine failure speed. Speed at which one engine is assumed to become suddenly inoperative during takeoff, after which the takeoff must be continued. This speed is always greater than Vmcg (100 kts)."
  Dassault, Falcon 10 AFM

"V₁: Takeoff Decision Speed. The distance to continue the takeoff to 35 feet will not exceed the scheduled takeoff field length if recognition occurred at V₁ (accelerate-go). The distance to bring the airplane to a full stop (accelerate stop) will not exceed the scheduled takeoff field length provided that the brakes are applied at V₁."
  Cessna, AFM for the Citation 650

Three out of the five definitions start off with "Take-off Decision Speed." The remaining two call V₁ the "...engine failure speed."

The Hawker 400 definition specifies "...the pilot must initiate the first action (brake application)..." prior to V₁ to "...discontinue a takeoff." How can V₁ be the "...decision speed" if the pilot must "...initiate the first action..." before "(The) Take-off Decision Speed (V₁)."?

The manual for the Lear 60 states that the "...distance to continue the takeoff to 35 feet or...to stop..." will no exceed the ...scheduled takeoff distance (presumably the TOFL)...(if) the brakes are applied at V₁." This is not the same definition provided for their own 35/36A. The definition provided for the 35/36A seems to include the definition of "Balanced Field Length." (A Balanced Field Length, or BFL, is when the Accelerate-Stop and Accelerate-Go distances are the same.)

Reading these definitions one could conclude that perhaps V₁ is a "...decision speed to..." continue the takeoff but that the decision to stop on the runway must be made "...prior to..." V₁. However, two of the definitions say, "Above V₁ the takeoff must be continued." while the others say that the pilot has a choice to "...bring the airplane to a full stop..." at V₁.

In all of the definitions the reason for a "...decision..." is based on an engine failure only.

To help clear this up read this discussion of V(sub)1(/sub) from "When the Engine Goes 'Bang'", by Patrick B. Veillette, Ph.D., Business & Commercial Aviation, July 2006:

There is a great deal of misunderstanding regarding the definition and use of the V₁ speed. Years of inconsistant terminology in reference to V₁ has probably contributed to this confusion.

Before 1978, FAR Part 1A defined V₁ as the critical engine failure speed. In 1978, the definition was changed to takeoff decision speed, and V_ef was established as the critical engine failure speed. After a series of serious accidents involving high-speed RTO's, the aerospace industry formed a work group that examined this misconception and recommended several changes to the definition of V₁. As a matter of fact, FAA Advisory Circular 120-62 (Sep 9, 1994), Takeoff Safety Training Aid*, cautions against using V₁ as decision speed.(emphasis mine)

In February 1998, the FAA adopted new regulations that again redefined V₁ and rervised some of the takeoff performance certification standards for transport-category aircraft. FAR Part 1 now defines V₁ thusly:

V₁ means the maximum speed in the takeoff at which the pilot must take the first action (e.g., apply brakes, reduce thrust, deploy speed brakes) to stop the airplane within the accelerate-stop distance. V₁ also means the minimum speed in the takoff, following a failure of the critical engine at Vef, at which the pilot can continue the takeoff and achieve the required height above the takeoff surface within the takeoff distance.

In fact, the first half of this new definition is codified in the FAR's that deal with aircraft certification. Paragraph 25.107 of the FAR's says that V₁ is determined from the pilot's application of the first retarding means during accelerate-stop.

To put this definition into a simpler context, think of V₁ as the "brakes on" speed. If you aren't into the braking procedure by V₁, do not attempt an RTO unless you have reason to conclude the airplane is unsafe or unable to fly. Obviously the "go/no-go decision must have been made by the time the aircraft reaches V₁.

There are many situations likely to provoke an RTO, but it should be clearly understood that V₁ refers to engine failure only. Unfortunately this simple fact has not always been conveyed to pilots, and many pilots regard V₁ as the go/no-go decision speed for any recognized anomoly during the takeoff roll, regardless of other factors.

United Airlines has produced a performance training aid that further reinforces these important concepts to it's crewmwmbers. It states, "The most important concept, common to all rejected takeoffs, is that V₁ is the brakes-on speed, not the decision speed. The decision to abort must be made, and the abort initiated, at or before V₁, to ensure the liklihood of stopping on the remaining runway."(emphasis mine)

Recognizing that V₁ is the speed by which maximum braking is applied, United specifies, "The V₁ callout must be made by the PNF such that any decision to abort can be made, and the stop initiated by, and not after V₁." Since the airplane is accelerating at three to five knots per second as it approaches V₁, the accepted technique is to initiate the callout as the airspeed needle approaches within five knots of V₁.. This allows time for the call to be made, heard and acted upon by V₁.

With that said, don't forget that the performance conditions for determining V₁ may not necessarily account for all variables possibly affecting a takeoff, such as runway surface friction and failures other than a critical powerplant.

V₁ therefore, is indeed a decision speed but only for continuing the takeoff. Above V₁ a rejected take off (RTO) is not an option. V₁ cannot be the decision speed for a rejected takeoff if it is to be defined as the "...the maximum speed at which the pilot must take the first action (e.g., apply brakes, reduce thrust, deploy speed brakes) to stop the airplane...".

Another V-speed V_ef, or the critical engine failure speed, has been established which represents the decision speed for an RTO. Remember that by definition the reason for a rejected takeoff is always assumed to be an engine failure although the majority of RTO's are statistically not for an engine failure. The FAA defines V_ef in FAR Part 1 as, "...the speed at which the critical engine is assumed to fail during takeoff."

Indeed if we look at the requirements to achieve the published Accelerate-Stop-Distance in the manufacturers AFM for one of the airplanes cited above the second requirement is:

  (b) The pilot recognizes the necessity to stop because of an engine failure or other reasons just prior to V₁ (emphasis mine).

Think about it, V₁ may not be what you thought it was.

* If you would like a PDF copy of the Takeoff Safety Training Aid, CLICK HERE