Aircraft speed

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Speed combines two factors, the distance travelled in a certain amount of time. In aviation speed is most often expressed in knots (kt). One knot is one nautical mile per hour. In aircraft the speed is "measured" with a pitot tube, the result is not the speed of the aircraft, it is the speed of the air flowing around the aircraft, the airspeed.

In older planes, notably German WW II fighter planes, the airspeed is indicated in kilometres per hour (km/h), which is still used in present-day European glider planes. The conversion factor is 1.852, i.e. you can roughly divide a reading in km/h by two in order to get the value in knot.

If the speed is indicated in knot, sometimes a 'K' is put before the acronym, so KEAS stands for 'equivalent airspeed in knot'.

For (near-)supersonic planes the speed can be expressed in Mach.

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

Airspeed is usually measured with a #Pitot tube at the front of the aircraft. The IAS can be the 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 aerodynamical 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.

At sea level, a IAS of 400 knot roughly corresponds to 400 knot TAS. 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

Modern equipment can most often can indicate the CAS. For navigation the CAS is the first step to calculate the GS.

Equivalent airspeed

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. The EAS is the calculated result from the ram pressure (measured by the #Pitot tube) and the static pressure (measured by the altimeter).

Mach number

  • The Mach number (M) is the speed of the aircraft divided by the speed of sound (at that altitude). It is a calculated number without a unit.

The aircraft's behaviour at Mach 1 at sea level is about the same as the behaviour 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)

V speeds

For the complete V speed "definitions" list please visit Wikipedia. Here a small abstract. Note that V speed definitions can depend of local Flight rules. Most V speeds depend on the aircraft configuration (how much it weights etc.) so must be calculated forehand and must be included in the flight-plan. V speeds are used to compare aircraft performance and will be mentioned in the aircraft flight manual (AFM).

  • M speeds are expressed in Mach.
V1 Take-off decision speed & Critical engine failure recognition speed.

During take-off the speed at which the aircraft safely can take-off even when one (of more) engine fails ("eats a bird"). The co-pilot (FO) will call out V1 during take-off, the pilot will check if all engines are running and decides to continue or abort take-off.

V2 Take-off safety speed.
V3 Flap retraction speed.
VA Design manoeuvring speed. Above this speed it is a bad idea to make sudden manoeuvres.
VC Design cruising speed, also known as the optimum cruise speed, is the most efficient speed in terms of distance, speed and fuel usage.
VD Max dive speed (for certification only)
VFE Maximum flap extended speed.
VLE Maximum landing gear extended speed.
VLO Maximum landing gear operating speed.
VMO Maximum operating limit speed.
VNE Never exceed speed.
VNO Maximum structural cruising speed or maximum speed for normal operations.
VR Nose-wheel take off speed.

The speed at which the nose-wheel leaves (should leave) the ground. As the speed increases the yokes will be pulled at Vr. It is also the speed at which the aircraft still can be stopped if there is a critical failure. The co-pilot (FO) will call out "rotate" during take off. VR is very similar to VROT and VREF.

VRef Landing reference speed or threshold crossing speed.
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 Stalling speed in a specified configuration
VX Best angle of climb speed
VXSE Best angle of climb speed, one engine out
VY Best rate of climb speed
VYSE Best rate of climb speed, one engine out
  • Not knowing the (complete list of) V speeds has caused dramatic accidents. It has occurred that the pilot and co-pilot were not aware of the minimal speed of an aircraft during landing with one engine damaged causing loss of control just before touch-down (the pilot gave full throttle hoping to gain speed expecting to get back control causing the left-over engine push the aircraft to one side).

Pitot tube

The pitot tube is the tool to measure the airspeed. It is a tube directed forwards, exposed to the airstream. The air is being pushed inwards (rammed) by the motion of the aircraft and the (ram) pressure is measured. The measured pressure is corrected indicating the airspeed. The ram pressure is also called the dynamic pressure opposite the static pressure that us used to indicate altitude. Bigger aircraft have two pitot tubes and the indicator displays the average of the two. However, most often only one pitot tube is used to control the autopilot, even when the indicator is connected with two.

The pitot tube can be blocked easy, once blocked, or worse, partially blocked the IAS will have no relation with the speed of the aircraft. This situation is enhanced if the pitot tube controlling the autopilot is blocked.

Ice is a known cause of blockage of the pitot tube hence there are pitot heaters that should prevent forming of ice. Another known cause of blockage are insects. Blockage of pitot tubes is a known cause of some very dramatic accidents and every pilot should learn how to deal with strange behaving speed indicators and autopilots.

Additional information

External links