Airspeed Indicator

The airspeed indicator is the only pitot-static instrument that uses both air pressures. Inside the case a sealed diaphragm receives ram pressure from the pitot tube — the sum of dynamic and static pressure — while the case surrounding it receives static pressure. The instrument mechanically subtracts one from the other, leaving dynamic pressure, which is a direct function of airspeed and air density, and drives that value to the pointer as indicated airspeed. Because it works on a pressure difference, anything that corrupts either the pitot or static source corrupts the reading, which is why the ASI is taught together with the rest of the pitot-static system in the FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25), Chapter 8, Flight Instruments.

Pilots distinguish several kinds of airspeed. Indicated airspeed (IAS) is what the instrument shows. Calibrated airspeed (CAS) is IAS corrected for installation and instrument error, which is largest at low speeds and high angles of attack. True airspeed (TAS) is CAS corrected for the actual air density — pressure altitude and temperature — and is always higher than IAS at altitude because thinner air produces less dynamic pressure for the same true speed. Groundspeed is TAS adjusted for wind. Understanding that the ASI reads dynamic pressure, not true speed, is what lets a pilot make sense of why a familiar 100-knot indication corresponds to a much faster true speed on a hot, high day.

The most practical feature of the ASI is its color-coded markings, which turn abstract certificated V-speeds into an at-a-glance display for that specific airplane. The white arc is the flap operating range: its lower end is Vs0, the stalling speed in the landing configuration (gear and flaps down), and its upper end is Vfe, the maximum speed with flaps extended. The green arc is the normal operating range: its lower end is Vs1, the stalling speed in a specified clean configuration, and its upper end is Vno, the maximum structural cruising speed. The yellow arc is the caution range, from Vno up to the red line, and is structurally safe only in smooth air — the aircraft should not be flown in the yellow in turbulence. The red line marks Vne, the never-exceed speed, which must not be exceeded under any circumstances. Some multi-engine airplanes add a red radial line at Vmc (minimum control speed) and a blue line at Vyse (best single-engine rate of climb).

Because the markings are certificated for a particular weight and configuration, they are guidance rather than the whole story. Stalling speed rises with weight, load factor, and bank angle, so the bottom of the white and green arcs represents the speed at maximum gross weight in unaccelerated flight — an aircraft in a steep turn stalls well above the marked figure. Maneuvering speed, Va, is deliberately not marked on the ASI because it changes with weight, decreasing as weight decreases, and must be read from a placard or the aircraft flight manual. A pilot who treats the arcs as fixed truths rather than certificated reference points can be caught out in an accelerated stall or by flying too fast in turbulence, which is why the relationship between the ASI markings and the underlying V-speeds is core airmanship.

For flight schools, the airspeed indicator is the instrument students refer to more than any other in the first hundred hours, and the color arcs are one of the earliest ground-school lessons that map directly to the airplane in front of them. Instructors use the arcs to make V-speeds concrete: rather than memorizing a list of abbreviations, a student can see that the top of the white arc is where flaps must be stowed and that the yellow arc is off-limits in turbulence. Teaching the difference between indicated and true airspeed early prevents the common misconception that the airplane is somehow slower at altitude, and it sets up the density-altitude and cross-country planning lessons that follow.

Operationally, the ASI markings are aircraft-specific, so a school running a mixed fleet must ensure students transitioning between types re-learn the arcs for each airplane — a Vfe or Vno that is safe in one trainer may be an overspeed in another. Because the ASI depends on a clear pitot and static source, its markings are only meaningful when the system is unobstructed, tying the instrument back to preflight discipline and to the crosswind and load-factor decisions pilots make on every approach.

Aviatize's Training Management module carries type-specific reference data for each aircraft in a fleet, so lesson plans and grading for stalls, slow flight, and configuration changes reference the correct arcs and V-speeds for the airplane the student is actually flying rather than a generic set. Instructors can grade whether a student flew the right speeds at the right moments as an observable behavior instead of a subjective impression.

For multi-type operations, Ground Training & Checking keeps the airspeed-limitation briefings and knowledge checks consistent as students move between trainers, so the differences in Vfe, Vno, and Vne between aircraft are taught deliberately rather than discovered in the air.