British Young Pilots · Aircraft General Knowledge · Module AGK-24.2
Your car has a speedometer. An aeroplane has an ASI, the airspeed indicator, and it does the same job with one twist. A little tube on the wing, the pitot tube, points straight into the oncoming air, so the ASI measures how fast the air is rushing past the aeroplane, not how fast the ground is sliding by underneath.
Think of a swimmer in a river: speed through the water is one thing, progress along the bank is another. Aeroplanes measure the swimming kind. The dial reads in knots, the pilot's unit of speed.
Source: Pooleys Vol 4 (10th ed.) p.328.
The airspeed indicator displays indicated airspeed (IAS), and what actually drives it is dynamic pressure: the pressure of the air ramming into the pitot tube. Dynamic pressure is total (pitot) pressure minus static pressure, and the instrument does that subtraction mechanically. Total pressure is fed to one side of a diaphragm (capsule) and static pressure from the static line to the other side. The diaphragm settles at a position set by the difference, which is dynamic pressure, ½ρV², and a gearing system moves the pointer to match.
Source: Pooleys Vol 4 (10th ed.) p.328.
Hold your hand flat out of a car window (someone else driving, please). Slow: a gentle push. Fast: a hard push. The ASI feels exactly that push.
The pitot tube catches the oncoming air and feeds it to one side of a squashy capsule inside the instrument. The other side feels only still air from a tiny hole called the static vent. The faster you fly, the harder the capsule is squeezed, and a lever and gears swing the needle further round the dial. Speed up, needle up.
FOUNDATIONSClass action: hold a flat hand up and "fly" it; the instructor blows harder and harder, and everyone's needle finger creeps round an imaginary dial.
Source: Pooleys Vol 4 (10th ed.) p.328.
A pressure gauge can only be graduated in speed if you fix the air density. The ASI assumes density stays constant at its mean sea level value (it does not), which lets the scale be marked in units of speed, usually knots. The price is that the reading is exactly right only on an ISA standard day at mean sea level (15 °C, pressure altitude 0 ft). As airspeed rises, dynamic pressure rises while static pressure stays the same, so the diaphragm moves and the pointer reads higher.
Some dials also carry a second scale in miles per hour alongside the knots. Know which units your aeroplane uses before you fly it.
Source: Pooleys Vol 4 (10th ed.) p.328–329.
Coloured arcs run round the numbers, and they work like traffic lights:
A rhyme to keep them: white is for flaps, green means go, yellow means gently, red means no.
FOUNDATIONSClass action: the instructor calls a situation (cruising on a calm day, landing with flaps down, bumpy air), the class shouts the colour.
Source: Pooleys Vol 4 (10th ed.) p.330.
To assist the pilot, modern ASIs mark speed ranges and specific speeds in a conventional colour code. Reading from slow to fast:
The two stall speeds carry their own codes: VS0 with landing gear down, flaps lowered and power off (the bottom of the white arc); VS1 with landing gear up (if retractable), flaps up and power off (the bottom of the green arc).
STRETCHThe white arc starts at the stall speed at maximum all-up weight. On a day when the aeroplane is much lighter, would the real stalling speed be above or below the start of the arc? Why paint the heaviest case?
Screen colours are theme tokens: on the real instrument the solid inner arc is white, the dashed arc is green, the bright arc is yellow and the radial line is red. The dial speeds are illustrative, not any real aeroplane's. Source: Pooleys Vol 4 (10th ed.) p.329–330.
Two more things a pilot checks. First, the units: most dials read in knots, but some also carry a second ring of numbers in miles per hour, so you must know which ring you are reading.
Second, the shape. In a modern glass cockpit there is no round dial at all. The airspeed shows as a moving strip, the speed tape, down the left-hand side of the screen, painted with the very same white, green, yellow and red bands. Same rules, new outfit.
Source: Pooleys Vol 4 (10th ed.) p.329 and p.331.
Some ASIs carry blue radial lines for other important speeds, for example the best single-engine speed on a light twin. Every marking on an ASI is an indicated airspeed, never TAS, and wherever weight affects a limit speed (the stall speeds, for instance) the value painted is for the maximum all-up weight (AUW).
On an EFIS the airspeed appears as a moving speed tape, always down the left-hand side of the screen, because the ASI always sits in the top-left corner of the Standard T layout; the glass panel keeps that standardisation with a conventional one. The tape uses the same colour coding as the round dial: coloured bands for the speed ranges and limits.
Source: Pooleys Vol 4 (10th ed.) p.330–331.
What the needle really measures. Its position is set by ½ρV², the dynamic pressure, so the ASI is a differential pressure gauge. Graduating it in knots needs a fixed density, and the maker uses the ISA mean sea level value (15 °C, pressure altitude 0 ft). Away from those conditions the needle still shows the dynamic pressure the airframe is feeling, expressed as the speed that would produce it at sea level: that is precisely what indicated airspeed means, and why the reading is exact only on an ISA standard day at MSL.
Every limit is an IAS limit. The arcs and radial lines are all indicated speeds, never true airspeeds, and the weight-dependent ones are painted for maximum all-up weight. How IAS relates to true airspeed is a later module.
Source: Pooleys Vol 4 (10th ed.) p.328 and p.330.
Tap a card to flip it.
Tap a name, then tap the box its pointer line leads to. Screen colours are the theme's: on the real dial the solid inner arc is white, the dashed arc is green, the bright arc is yellow and the short radial line is red. Speeds shown are illustrative, not any real aeroplane's.