Aircraft Speed

In aviation, V-speeds are standard terms used to define airspeeds important or useful to the operation of all aircraft. These speeds are derived from data obtained by aircraft designers and manufacturers during flight testing for aircraft type-certification testing. Using them is considered a best practice to maximize aviation safety, aircraft performance or both.

The actual speeds represented by these designators are specific to a particular model of aircraft. They are expressed by the aircraft’s indicated airspeed (and not by, for example, the ground speed), so that pilots may use them directly, without having to apply correction factors, as aircraft instruments also show indicated airspeed.

In general aviation aircraft, the most commonly used and most safety-critical airspeeds are displayed as color-coded arcs and lines located on the face of an aircraft’s airspeed indicator. The lower ends of the green arc and the white arc are the stalling speed with wing flaps retracted, and stalling speed with wing flaps fully extended, respectively. These are the stalling speeds for the aircraft at its maximum weight. The yellow range is the range in which the aircraft may be operated in smooth air, and then only with caution to avoid abrupt control movement, and the red line is the VNE, the never exceed speed.

Expressing speed

Ground speed

Ground speed (GS) is the horizontal speed in which the aircraft moves relative to a fixed point on the ground. One needs to know the GS in order to see how long a flight from A to B actually takes. Nowadays GS can be directly measured using a GPS system, and some aircraft equipped with such a system have a GS indicator. The GS can be calculated from TAS by correcting it for the prevailing wind at altitude or by measuring the time between passing two points on the ground radio beacons with a known distance, but in Flightgear you can always cheat and get it from the property browser under velocities/groundspeed-kt.

GS is the velocity in the horizontal direction of the aircraft. I.e. in a steep dive, the aircraft can move very fast, but because the motion is chiefly vertical, the ground-speed can be very small at the same time. This is where the GS differs from the ground-speed of a car.

True airspeed

True airspeed (TAS) is the speed in which the aircraft moves relative to the surrounding air. The difference between TAS and GS is that the air itself may move with respect to the ground (that’s wind), and dependent on course relative to the wind direction a discrepancy between TAS and GS is induced. TAS can’t really be measured directly but needs to be calculated, unless standing still on the ground where the TAS can be “seen” with the windbag.

Knowing TAS during flight is surprisingly useless - for navigation, ground speed is needed, and aerodynamic limits do not depend on TAS but rather IAS. The chief value of TAS is as a measure of aircraft performance and in pre-flight planning before the wind effect is taken into account.

The TAS can be calculated from CAS, air temperature and pressure altitude and is the second step to calculate the GS from IAS for navigation.

Often TAS and GS are assumed (confused) to be the same, they are not.

Indicated airspeed

Indicated airspeed (IAS) is the number displayed on the airspeed indicator. The Indicated Airspeed is determined with Total Pressure (measured with a #Pitot tube) and Static Pressure. Because of measurement faults IAS usually has a failure. Without this failure you get CAS. The IAS is not the TAS since the pressure differs greatly with altitude (more specific the density of the air). The higher the altitude the lower the IAS while flying the same TAS.

In spite of this dependence on altitude, IAS is a very useful quantity in flight. Many aerodynamic properties, for example drag, lift, the stress on the airframe, stall speed or the forces on control surfaces depend on the dynamic pressure generated by the airstream, not on the actual aircraft speed. The stall speed of an aircraft at sea level is very different from the stall speed (in TAS) at 30.000 ft - but they correspond to the same IAS reading.

Per definition CAS = TAS in standard ISA conditions and sea level. At 80.000 feet (the cruising altitude of a SR-71), the IAS of 400 knot corresponds to a TAS in excess of 1600 knot (..that corresponds with about Mach 3 at that altitude).

Calibrated airspeed

Calibrated airspeed (CAS) is calculated from IAS and correcting it for measurement errors. Modern equipment can most often can indicate the CAS. For navigation the CAS is the first step to calculate the GS.

Equivalent airspeed Equivalent airspeed (EAS) takes into account another correction (above #Calibrated airspeed, this time having to do with air properties rather than sensor errors. EAS at low altitude and low airspeeds is very close to CAS, but CAS incorporates compressibility effects, EAS assumes no compressibility. At high altitude, the compressibility of air changes, so even CAS becomes more and more unreliable. For the SR-71 Blackbird with a ceiling of 85.000 feet, the CAS becomes very unreliable and the plane has to be flown based on a EAS. For more conventional aircraft, EAS is not used. Thus, EAS is what a perfect dynamic pressure sensor would show when properly calibrated for the air compressibility at the current altitude.

Mach number

The Mach number (M) is the speed of the aircraft divided by the speed of sound (at that temperature). It is usually calculated, but can also be directly determined with Impact and Static pressure. Mach has no dimension. The aircraft’s behavior at Mach 1 at sea level is about the same as the behavior of the aircraft at an altitude of 60000 feet. A Mach number below 1 means that the plane moves subsonic. A Mach number above 1 indicates supersonic flight. The Mach number is critical because a number of phenomena take place just around Mach 1 (transonic speed), for example a sudden increase in drag induced by shock-wave generation (sonic-boom). Aircraft that are not designed to fly supersonic will break up at Mach 1. The shape of the aircraft can cause parts of the aircraft being at or above Mach 1 while the fuselage is subsonic. Flying near Mach 1 can be quite dangerous, for most fast (but subsonic) aircraft Mach 0.83 is the limit. High flying aircraft, like passenger aircraft, can reach that limit easy while descending.

The speed of sound changes with the compressibility (and hence temperature) of air, the Mach number is dependent on altitude (as the air temperature drops at higher altitudes). This implies that Mach 2 at sea level corresponds to a faster TAS than Mach 2 at 30.000 ft. The precise relation between TAS, Mach number and altitude is a complicated formulae and depends in essence on the local weather pattern determining the pressure and temperature gradients in the atmosphere. The Mach number is measured/calculated from the same information as the EAS (Pitot tube and altimeter)

Regulatory V-speeds

  • V1 The speed beyond which the takeoff should no longer be aborted.

V1 is the critical engine failure recognition speed or takeoff decision speed. It is the speed above which the takeoff will continue even if an engine fails or another problem occurs, such as a blown tire. The speed will vary among aircraft types and varies according to factors such as aircraft weight, runway length, wing flap setting, engine thrust used and runway surface contamination, thus it must be determined by the pilot before takeoff. Aborting a takeoff after V1 is strongly discouraged because the aircraft will by definition not be able to stop before the end of the runway, thus suffering a “runway overrun”.

V1 is defined differently in different jurisdictions:

The US Federal Aviation Administration defines it as: “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 takeoff, 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.”

Transport Canada defines it as: “Critical engine failure recognition speed” and adds: “This definition is not restrictive. An operator may adopt any other definition outlined in the aircraft flight manual (AFM) of TC type-approved aircraft as long as such definition does not compromise operational safety of the aircraft.”

  • V2 Takeoff safety speed. The speed at which the aircraft may safely be climbed with one engine inoperative.

  • V2min Minimum takeoff safety speed.

  • V3 Flap retraction speed.

  • V4 Steady initial climb speed. The all engines operating take-off climb speed used to the point where acceleration to flap retraction speed is initiated. Should be attained by a gross height of 400 ft (120 m).

  • VA Design maneuvering speed. This is the speed above which it is unwise to make full application of any single flight control (or “pull to the stops”) as it may generate a force greater than the aircraft’s structural limitations.

  • Vat Indicated airspeed at threshold, which is usually equal to the stall speed VS0 multiplied by 1.3 or stall speed VS1g multiplied by 1.23 in the landing configuration at the maximum certificated landing mass, though some manufacturers apply different criteria. If both VS0 and VS1g are available, the higher resulting Vat shall be applied. Also called “approach speed”.

  • VB Design speed for maximum gust intensity.

  • VC Design cruise speed, used to show compliance with gust intensity loading.

  • Vcef See V1; generally used in documentation of military aircraft performance. Denotes “critical engine failure” speed as the speed during takeoff where the same distance would be required to either continue the takeoff or abort to a stop.

  • VD Design diving speed, the highest speed planned to be achieved in testing.

  • VDF Demonstrated flight diving speed, the highest actual speed achieved in testing.

  • VEF The speed at which the critical engine is assumed to fail during takeoff.

  • VF Designed flap speed.

  • VFC Maximum speed for stability characteristics.

  • VFE Maximum flap extended speed.

  • VFTO Final takeoff speed.

  • VH Maximum speed in level flight at maximum continuous power.

  • VLE Maximum landing gear extended speed. This is the maximum speed at which a retractable gear aircraft should be flown with the landing gear extended.

  • VLO Maximum landing gear operating speed. This is the maximum speed at which the landing gear on a retractable gear aircraft should be extended or retracted.

  • VLOF Lift-off speed.

  • VMC Minimum control speed. The minimum speed at which the aircraft is still controllable with the critical engine inoperative. Like the stall speed, there are several important variables that are used in this determination. Refer to the minimum control speed article for a thorough explanation. VMC is sometimes further refined into more discrete V-speeds e.g. VMCA,VMCG.

  • VMCA Minimum control speed air. The minimum speed that the aircraft is still controllable with the critical engine inoperative while the aircraft is airborne. VMCA is sometimes simply referred to as VMC.

  • VMCG Minimum control speed ground. The minimum speed that the aircraft is still controllable with the critical engine inoperative while the aircraft is on the ground.

  • VMCL Minimum control speed in the landing configuration with one engine inoperative.

  • VMO Maximum operating limit speed. Exceeding VMO may trigger an overspeed alarm.

  • VMU Minimum unstick speed.

  • VNE Never exceed speed.

  • VNO Maximum structural cruising speed or maximum speed for normal operations.

  • VO Maximum operating maneuvering speed.

  • VR Rotation speed. The speed at which the pilot begins to apply control inputs to cause the aircraft nose to pitch up, after which it will leave the ground.

  • Vrot Used instead of VR (in discussions of the takeoff performance of military aircraft) to denote rotation speed in conjunction with the term Vref (refusal speed).

  • VRef Landing reference speed or threshold crossing speed.

(In discussions of the takeoff performance of military aircraft, the term Vref stands for refusal speed. Refusal speed is the maximum speed during takeoff from which the air vehicle can stop within the available remaining runway length for a specified altitude, weight, and configuration.) Incorrectly, or as an abbreviation, some documentation refers to Vref and/or Vrot speeds as “Vr.”

  • VS Stall speed or minimum steady flight speed for which the aircraft is still controllable.

  • VS0 Stall speed or minimum flight speed in landing configuration.

  • VS1 Stall speed or minimum steady flight speed for which the aircraft is still controllable in a specific configuration.

  • VSR Reference stall speed.

  • VSR0 Reference stall speed in landing configuration.

  • VSR1 Reference stall speed in a specific configuration.

  • VSW Speed at which the stall warning will occur.

  • VTOSS Category A rotorcraft takeoff safety speed.

  • VX Speed that will allow for best angle of climb.

  • VY Speed that will allow for the best rate of climb.

Other V-speeds

  • VBE Best endurance speed – the speed that gives the greatest airborne time for fuel consumed.

  • VBG Best power-off glide speed – the speed that provides maximum lift-to-drag ratio and thus the greatest gliding distance available.

  • VBR Best range speed – the speed that gives the greatest range for fuel consumed – often identical to Vmd.

  • VFS Final segment of a departure with one powerplant failed.

  • Vimd Minimum drag

  • Vimp Minimum power

  • VLLO Maximum landing light operating speed – for aircraft with retractable landing lights.

  • Vmbe Maximum brake energy speed

  • Vmd Minimum drag (per lift) – often identical to VBR. (alternatively same as Vimd)

  • Vmin Minimum speed for instrument flight (IFR) for helicopters

  • Vmp Minimum power

  • Vms Minimum sink speed at median wing loading - the speed at which the minimum descent rate is obtained. In modern gliders, Vms and Vmc have evolved to the same value.

  • Vp Aquaplaning speed

  • VPD Maximum speed at which whole-aircraft parachute deployment has been demonstrated

  • Vra Rough air speed (turbulence penetration speed).

  • VSL Stall speed in a specific configuration

  • Vs1g Stall speed at 1g load factor

  • Vsse Safe single-engine speed

  • Vt Threshold speed

  • VTD Touchdown speed

  • VTGT Target speed

  • VTO Take-off speed. (see also VLOF)

  • Vtocs Take-off climbout speed (helicopters)

  • Vtos Minimum speed for a positive rate of climb with one engine inoperative

  • Vtmax Max threshold speed

  • Vwo Maximum window or canopy open operating speed

  • VXSE Best angle of climb speed with a single operating engine in a light, twin-engine aircraft – the speed that provides the most altitude gain per unit of horizontal distance following an engine failure, while maintaining a small bank angle that should be presented with the engine-out climb performance data.

  • VYSE Best rate of climb speed with a single operating engine in a light, twin-engine aircraft – the speed that provides the most altitude gain per unit of time following an engine failure, while maintaining a small bank angle that should be presented with the engine-out climb performance data.

  • VZRC Zero rate of climb speed in a twin-engine aircraft