Understanding Airspeed

Speed is the number pilots watch most closely, yet the phrase "airspeed" hides several different quantities. This lesson explains what the airspeed indicator actually measures, the difference between indicated, calibrated, true and ground speed, how the colour arcs on the dial protect you, and why the stall is about the wing rather than the number on the dial.

What the indicator measures

An airspeed indicator is a pressure gauge. The pitot tube measures the total pressure of the air rammed into it, the static port measures the ambient (static) pressure around the aircraft, and the instrument shows the difference: dynamic pressure, the pressure created by motion. Dynamic pressure exists only when something moves through the air, and it does not care where the motion comes from. An aircraft flying at 70 knots through still air and an aircraft sitting still in a 70 knot wind experience the same dynamic pressure.

That one fact explains both the usefulness and the limitation of the instrument. It is useful because dynamic pressure is exactly what the wings and controls respond to: it is the "aerodynamic speed" that matters for lift, stalls and control forces. It is limited because dynamic pressure depends on air density. Fly into thinner air at the same true speed and the indicator shows less; slow down in dense air and it shows more. The needle measures a pressure, not a distance covered.

Four speeds, four questions

Beginners often ask "how fast am I going?" and mean four different things. Each speed answers a different question:

Some details worth pinning down:

• The correction from IAS to CAS matters most at low speeds and particular flap settings; in the cruise the two are close enough that pilots treat them as equal.
• TAS is where the altitude effect lives: as density falls with height, the aircraft must fly faster through the air to produce the same pressure difference, so for a given CAS, TAS increases as altitude increases. A jet may stall at roughly the same calibrated airspeed at sea level and at FL 380 while the true airspeed behind that number grows from about 152 KTAS to about 287 KTAS.
• Ground speed is what a GPS moving map displays, and it is where wind enters the picture. GS decreases with a headwind and increases with a tailwind.
• Units: a knot is one nautical mile per hour, and a nautical mile is 6,076.1 feet (versus 5,280 feet in a statute mile). Aviation uses knots and nautical miles so that distance, speed and time stay in one clean family: speed (knots) × time (hours) = distance (nautical miles).

The colour arcs

Aircraft weighing 12,500 pounds or less, manufactured after 1945, carry a standard colour-coded marking system on the airspeed indicator. The colours turn the dial into a quick reference for the limits that matter:

The two stall speeds anchor the bottom of the arcs: $V_{S0}$ with the aircraft in its landing configuration (flaps and gear down) and $V_{S1}$ in the specified clean-ish configuration. Because configuration changes lift and drag, the aircraft stalls at different indicated speeds in each.

Speed, drag and the stall

Speed ties directly back to the four forces. Parasite drag increases as the square of the airspeed, while induced drag varies inversely with the square of the airspeed, so total drag dips to a minimum in between and climbs steeply at both ends. That shape is why aircraft have an economical cruise speed and why dragging an approach in at very high speed costs so much distance.

The stall itself is about angle of attack, not airspeed. As angle of attack rises, lift rises with it up to a point; at the critical (stalling) angle of attack, lift diminishes rapidly. Airspeed determines how quickly you arrive at that angle, but it does not fix the angle. In practice: it is possible to stall the wing at any airspeed, at any flight attitude and at any power setting. Load factor, gross weight and flap setting all move the indicated stall speed, which is why handbook stall speeds are quoted for a particular weight and configuration.

Two practical consequences:

• Weight changes the stall speed. Heavier aircraft need more lift, so they reach the critical angle of attack at a higher speed.
• Manoeuvres change it too. In a 60 degree level bank the load factor is 2 G, and the aircraft stalls at a speed about 41 percent higher than the 1 G stall speed.

Worked example: from indicated speed to arrival time

An aircraft cruises at 120 knots indicated where the air density is 80 percent of its sea level value ($\sigma = 0.80$), with a 15 knot headwind.

1. Find the true airspeed. The pressure (and therefore the indicated speed) stays the same while the aircraft moves faster through thinner air: $TAS \approx IAS / \sqrt{\sigma}$. So $TAS = 120 / \sqrt{0.80} = 120 / 0.894 \approx 134$ knots.
2. Apply the wind. A 15 knot headwind reduces the ground speed: $GS = 134 - 15 = 119$ knots.
3. Plan with ground speed. A 240 nautical mile leg takes $240 / 119 \approx 2.0$ hours.
4. Sanity check. With no wind it would be $240 / 134 \approx 1.8$ hours. The headwind costs about 12 minutes on this leg, and flying home with the same wind gives it back.

Now the stall-speed check: if the aircraft's published stall speed at 1,100 kg is 48 knots, at 1,200 kg the speed scales with the square root of weight: $\sqrt{1{,}200/1{,}100} = \sqrt{1.091} \approx 1.044$, so the stall speed rises by about 4.4 percent to roughly 50 knots. Small loading changes, measurable effects.

A realistic scenario: the overstressed approach

You are flying a long leg toward an airport and, wanting to get down quickly, you descend at 140 knots indicated and then turn final still carrying speed with flaps extended. The plan fails twice. First, your ground speed is not your indicated speed: at altitude with a tailwind it is higher, so the distance covered during the descent is longer than the naive calculation suggested. Second, arriving fast with flaps deployed puts you near the top of the white arc while the runway is short. The correction is unglamorous: slow to a target indicated speed early, let the wind be handled as a ground-speed calculation, and arrive on the numbers the handbook publishes for the configuration you are in.

Common misconceptions

"The airspeed indicator shows how fast I am moving over the ground." It shows dynamic pressure. Over the ground you move at ground speed, which is true airspeed adjusted for wind. Flying into a 30 knot wind at the same indicated speed, you cover ground 30 knots slower.

"There is one stall speed for this aircraft." There is a stall speed for each weight and configuration, and load factor shifts it further. The number on a placard is a reference condition, not a law of nature, and the wing will stall at any airspeed if the critical angle of attack is reached.

"Flying inside the green arc is always safe." The green arc is the normal operating range in smooth air. Turbulence, loads in turns and configuration changes all move the real limits, and the yellow arc exists precisely to say "smooth air only, watch the red line".

"At altitude the airspeed indicator has caught up with reality." The opposite happens: as density falls, true airspeed runs ahead of indicated airspeed. The jet example above stalls at the same calibrated speed at FL 380 while moving nearly twice as fast through the air.

"A headwind only costs time." It costs time off the flight plan, but it also shortens the take-off ground roll: a headwind decreases the ground speed necessary to achieve flying speed, so the aircraft reaches rotation speed in less distance. Wind is a cost in the cruise and a benefit on the runway.

Recap

• The airspeed indicator compares pitot (total) pressure with static pressure to show dynamic pressure.
• IAS is what the wings feel, CAS corrects instrument and installation error, TAS corrects for density, GS corrects for wind.
• Knots and nautical miles keep planning arithmetic simple: knots × hours = nautical miles.
• Colour arcs encode configuration limits: white for flaps, green for normal, yellow for caution, red line never to exceed.
• Parasite drag grows with the square of speed while induced drag falls, so drag is high at both speed extremes.
• The stall is an angle of attack event: any airspeed, any attitude, any power, with the number shifting for weight, configuration and load factor.