Ask a pilot how fast the aircraft is going and you may get a question back: which speed do you mean? The cockpit of a jet can show four or five different numbers that all claim to be its speed, and at cruising altitude they can differ by hundreds of knots.

None of them is wrong. Each answers a different question, and pilots use each one for a different job. Here is what indicated airspeed, true airspeed, groundspeed and Mach number actually mean, and why a fighter at high altitude can be flying far faster than its airspeed indicator says.

Indicated airspeed: what the wing feels

The most important number in the cockpit is indicated airspeed, or IAS. It comes from the pitot-static system: a pitot tube facing into the airflow measures total pressure, a static port measures the still air pressure around the aircraft, and the difference between the two is the dynamic pressure of the air rushing past.

The airspeed indicator turns that pressure into a speed, but it has to make an assumption: it is calibrated as if the aircraft were always at sea level in a standard atmosphere. As the aircraft climbs into thinner air, the same speed produces less dynamic pressure, so the indicator reads lower than the aircraft is really going.

That sounds like a flaw, but it is exactly why pilots fly by it. A wing produces lift according to dynamic pressure, and so does the airspeed indicator. An aircraft will stall at the same indicated airspeed for a given configuration, whatever its altitude or true speed. That is why flight manuals publish stall, approach and limit speeds as indicated airspeeds, and why air traffic control gives speed instructions in IAS.

Boldmethod explains why true airspeed and indicated airspeed are different.

Calibrated, equivalent and true

Between the raw indication and the real speed sit a series of corrections, which pilots remember with the acronym ICE-T. Calibrated airspeed (CAS) is IAS corrected for instrument errors and for the position of the sensors on the airframe. Equivalent airspeed (EAS) also corrects for the compressibility of air, which matters at high speed. True airspeed (TAS) then corrects for the actual air density.

True airspeed is the aircraft's real speed through the air mass around it, and it is what pilots use for navigation and flight planning. The gap grows with altitude. As a rule of thumb, the airspeed indicator reads about 2% less than true airspeed for every 1,000 feet of altitude. An aircraft at 15,000 feet in a standard atmosphere showing 100 knots indicated is actually flying through the air at about 126 knots. Modern aircraft let an air data computer do the sums and simply display TAS.

Groundspeed: what the map sees

Neither indicated nor true airspeed tells you how fast you are crossing the ground, because the air mass itself is moving. Groundspeed is true airspeed with the wind added or subtracted. An airliner cruising at 480 knots true into a 100-knot jet stream headwind covers the ground at only 380 knots; turn around and it makes 580 knots.

Groundspeed is what decides arrival times and fuel planning, and with GPS it is easy to display. But it tells the pilot nothing about how the wing is doing. A slow light aircraft flying into a headwind almost as strong as its airspeed could show nearly zero groundspeed while flying perfectly happily.

Mach: the speed that matters up high

At high speed and high altitude, another number takes over: the Mach number, the aircraft's true airspeed as a fraction of the local speed of sound. Mach 1 is the speed of sound itself.

The catch is that the speed of sound is not fixed. It depends on air temperature. In the standard atmosphere it is about 661 knots at sea level, but in the cold air above about 36,000 feet it falls to around 573 knots. So Mach 0.85 at cruising altitude is a slower true airspeed than Mach 0.85 at sea level.

Jets fly by Mach number at altitude because the dangerous effects of high speed, shock waves forming on the wing, buffeting and loss of control, depend on how close the airflow gets to the speed of sound. Jet aircraft therefore have two speed limits, a maximum operating speed in knots and a maximum Mach number, and need both an airspeed indicator and a Machmeter.

Why jets switch from knots to Mach number at high altitude.

Why a fighter can be faster than it looks

Put it all together and you get some surprising numbers. A fighter at 40,000 feet may show only a modest indicated airspeed while its true airspeed is far higher, and with a tailwind its groundspeed higher still. Conversely, a jet flying very fast at low level will show a high indicated airspeed, because the dense air is hammering on its pitot tube, even though its Mach number may be lower than it routinely reaches at altitude.

That is why fighter pilots talk about speed in different ways depending on the situation: knots indicated for take-off, landing and manoeuvring, Mach number for high-altitude intercepts and supersonic runs. The aircraft is the same. The question being asked is different.

Captain Joe's guide to every type of airspeed pilots need to know.

And a compact overview of all five speeds in one place.

IAS, CAS, TAS, groundspeed and Mach, explained by JxJ Aviation.

Sources: Wikipedia (Indicated airspeed, True airspeed, Airspeed indicator, Mach number), Boldmethod, Captain Joe, JxJ Aviation, PilotClimb