Air Speed Indicator and Colour Markings + V Speeds
AIR SPEED INDICATORS
An aircraft, stationary on the ground, is subject to normal atmospheric or static pressure which acts equally on all parts of the aircraft structure. In flight the aircraft experiences an additional pressure on its leading surfaces due to a build up of the air through which the aircraft is travelling. This additional pressure due to the aircraft’s forward motion is known as dynamic pressure and is dependent upon the forward speed of the aircraft and the density of the air.
Pitot Static Instruments
The pitot static family is made up of all the instruments that obtain their information from either a pitot or static source of air.
Pitot tube air is sometimes called ram air because it is the air forced through a tube.
Static air pressure is the air pressure measured from a relatively non moving or static source located on or in the ac.
The 3 pitot static instruments are ASI, Altimeter and VSI
Working of ASI
The instrument uses Ram Air (Dynamic + Static pressure) from the pitot tube as well as static air (Static Pressure).
Ram Air pushes the diaphragm inside the instrument, which then expands or contracts accordingly
Static Air inside the case modifies the diaphragm expansion
Pitot Pressure is fed to the capsule
Static pressure is fed to the case
Expansion of capsule = Pitot – Static = Dynamic Pressure ∝ Air speed
Pitot Pressure = Static Pressure + Dynamic Pressure
Dynamic Pressure = ½ρV2 , ρ = density and V = TAS
The difference between the Pitot and the Static pressures is equal to Dynamic pressure (½ρV2). The air speed indicator measures this pressure difference (Dynamic pressure) and provides a display indication graduated in units of speed. The diaphragm is subjected to the two opposing pressures. However, the static pressure component of the Pitot pressure is balanced by the static pressure on the other side of the diaphragm so that any diaphragm movement is determined solely by the dynamic, or Pitot excess, pressure. Movement of the diaphragm is transmitted through a mechanical linkage to a pointer on the face of the ASI where the Pitot excess pressure (Pt – P) is indicated in terms of speed.
Simple Block Diagram of ASI
These two pressures will be equal when the ac is parked on the ground in calm air
When the ac moves through the air, the pressure on the pitot line becomes greater than the pressure in the static line
This difference in the pressure is registered by the ASI and read of in the instrument as speed.
Errors of ASI
There are four sources of error:
- Instrument error.
- Pressure error.
- Compressibility error.
- Density error.
Instrument Error. Instrument error is caused by manufacturing tolerances in the construction of the instrument. The error is determined during calibration and any necessary correction is combined with that for pressure error.
Pressure Error. Pressure error results from disturbances in the static pressure around the aircraft due to movement through the air. Depending upon aircraft type, the error may be influenced by:
- The position of the pressure head, Pitot head or static vent.
- The angle of attack of the aircraft.
- The speed of the aircraft.
- The configuration of the aircraft (i.e., ‘clean’/ flaps/gear/airbrakes/etc.).
- The presence of sideslip.
This error tends to vary primarily with speed and angle of attack, since it is difficult to find a location on an ac that will always measure static pressure correctly at all the possible variables of speed and angle of attack and attitudes. Position error is usually greatest at low speeds and high angle of attack ( Landing and Take Off phases) and smallest in cruise. To correct for instrument and position error a calibration chart is available in cockpit for each ac. After correction it is called Calibrated Air Speed (CAS). IAS + Pressure/Instrument Error Correction (PIEC) = CAS
Compressibility Error. The calibration formulae contain a factor which is a function of the compressibility of the air. At higher speeds this factor becomes significant. When flying faster than 300 knots (TAS), the air being rammed in the pitot tube gets compressed. However the calibration formulae use standard mean sea level values and an error is introduced at any altitude where the actual values differ from those used in calibration. At altitude, the less dense air is more easily compressed than the denser air at sea level, resulting in a greater dynamic pressure which causes the ASI to over-read. In addition compressibility increases with increase of speed, therefore compressibility error varies both with speed and altitude. Application of the compressibility error correction (CEC) to CAS produces equivalent air speed (EAS).
CAS + Compressibility Error Correction (CEC) = EAS
Nature of Compressibility Error
- Compressibility Error always causes the ASI to Over Read
- Compressibility Error Correction is always negative.
- Compressibility Error is high at higher altitude due to lesser density of air.
Density Error. Dynamic pressure varies with air speed and the density of the air. Standard mean sea level air density is used for calibration purposes. Thus, for any other condition of air density, the ASI will be in error. As altitude increases, density decreases and IAS, and thus EAS, will become progressively lower than True air speed (TAS). The necessary correction can be calculated from the formula:
EAS (Ve) = V √ (ρ / ρ0)
Where, V = TAS
ρ = The air density at the height of the aircraft
ρ0 = The air density at mean sea level.
EAS + Density Error Correction = TAS
Nature of Density Error
- Density error generally causes the ASI to under Read
- Density Error correction is generally positive
Summary. The relationship between the various air speeds and the associated errors can be summarized as follows:
When Climbing with constant CAS then TAS increases
When descending with a constant CAS TAS decreases.
Cruising at constant altitude if temperature increases then TAS increases and Vice- Versa.
AIRSPEED COLOR MARKINGS AND V SPEEDS
The white arc on the airspeed indicator designates the flap operating range. The green arc shows the normal operating range, and the yellow (caution) arc signifies the smooth air cruising range. A red line usually indicates the Vne (never exceed) speed. Pilots should never use the caution range during turbulent atmospheric conditions. The aircraft manual defines additional V speeds not shown on the airspeed indicator.
The beginning of the White Arc is the power off Stalling Speed with gear and full flaps extended, also known as Vs0. The Vs0 (Velocity Stall 0) represents the Stalling Speed of the aircraft configured for landing. (i.e. Gear Down and Flaps Down) An easy way to remember this is to think of the Velocity (V) of Stall (s) with everything hanging Out (0) or Vs0.
Vs and Vs1
The beginning of the Green Arc is the power off Stalling Speed with the Gear and Flaps retracted. Vs is the Velocity (V) of the Stall (s), or minimum steady flight speed for which the aircraft is still controllable. As a memory aid, Vs1 is the Velocity (V) of the Stall (s) with everything Inside (1 looks like the letter i for inside). This is the Stall speed or minimum steady flight speed for which the aircraft is still controllable in a specific configuration.
The lower ends of the Green Arc and the White Arc depict the stalling speed with wing flaps retracted (Vs1), and stalling speed with wing flaps fully extended (Vs0), respectively. These Vs (Velocity of Stall) speeds are the stalling speeds for the aircraft at its maximum weight.
The Top of the White Arc depicts the Maximum Flap Extended Speed. This is referred to as Vfe for Velocity (V) with Flaps (f) Extended (e). This represents the maximum airspeed at which you may extend the flaps, or fly with them extended. The flaps may not be used above this range (White Arc) or possible structural damage may occur to the aircraft.
The Green Arc
The Green Arc on the Airspeed Indicator depicts the normal operating airspeed range. As we have learned, Vs is the Velocity (V) of the Stall (s) and the Vs or Vs1 speed is denoted by the beginning of the Green Arc. At the top end of the Green Arc, is the Vno.
As the Green Arc is the Normal Operating Range, the top of the green arc is the Velocity (V) of Normal (n) Operations (o) or Vno. This is the maximum structural cruising speed. Operation of the Aircraft at the Vno speed, and lower, is within the certified range for operations within gusts. The aircraft is certified to withstand substantial wind gusts without experiencing structural damage. Operations above Vno move into the Yellow Arc on the Airspeed Indicator. Do not exceed Vno, except in Smooth Air, and only with caution.
The Yellow Arc
Beyond the Green Arc, we see the Yellow Arc. The speed range marked by the Yellow Arc is the Caution Speed Range. The Airspeed range indicated by the Yellow Arc is for Smooth Air Only.
Operations above Vno (Top of the Green Arc) will bring you into the Caution Range of the Yellow Arc. Flight Operations in the Yellow speed range are to be conducted in Smooth Air only.
The Red Line at the top of the Yellow Arc is the Velocity (V) that you Never (n) Exceed (e). This is the Red Line of the Airspeed Indicator, and the Vne is the Maximum Speed the Aircraft should ever be operated in Smooth Air. Remember, the Yellow Arc is for Smooth Air Only. You should not exceed the Green Arc speed range unless the Air is Smooth and without gusts. Exceeding the Vne Airspeed can cause uncontrollable and destructive flutter, and cause serious or catastrophic failure of structural components on the aircraft. Aircraft designers include a slight safety margin, but do not risk or rely on this slim margin. The Vne is the Velocity (V) you Never (n) Exceed (e).
There are other important V-Speeds, but they are not shown on the Airspeed Indicator Flight Instrument. The Pilot will need to be familiar with these other speeds, but they can’t simply look at the Airspeed Dial to determine these other V-Speeds.
Manoeuvring Speed is found well below Vno. Manoeuvring Speed may be remembered as Velocity (V) of Acceleration (a) or Va. The pilot should not make full or abrupt control movements above this speed. In turbulence, you should always be at, or below, the Manoeuvring Speed (Va). The only way to ensure you will not damage the aircraft with full or abrupt control movement is to fly at or below this speed.
Retractable Gear Aircraft
Most student pilots will learn to fly on airplanes with fixed landing gear. However, if you fly an aircraft with Retractable Landing Gear, you will need to be aware of two more important V-Speeds. These are Vlo and Vle.
Vlo is the Maximum Velocity (V) for Landing (l) gear Operation (o). Do not extend or retract the landing gear above this airspeed. When the landing gear is in transition, it is more vulnerable to damage from the effects of speed. However, once the landing gear is fully extended and locked, it may sustain higher airspeeds.
Vle is the Maximum Velocity (V) of Landing (l) gear Extended (e). Do not exceed this speed with the landing gear extended.
Vx and Vy
Two easily confused Airspeeds are Vx and Vy. The student pilot must have these important airspeeds committed to memory very early in their flight instruction. These airspeeds will be demonstrated and explained. They are Best Rate of Climb (Vy) and Best Angle of Climb (Vx).
Best Rate of Climb (Vy)
After takeoff, the aircraft should normally be configured for the Best Rate of Climb. This will provide the best climb for the maximum gain in altitude in the shortest time possible. You will get to your selected cruising altitude in the shortest time possible. Altitude is your friend, and particularly after takeoff, you want to gain the maximum height above the ground in the least time possible. Vy provides you with the Best Rate of Climb.
Best Angle of Climb (Vx)
Occasionally, it may be necessary to gain the maximum altitude possible over the shortest distance on the ground. To achieve this, the pilot would use the Best Angle of Climb or Vx. This would be applicable if you needed to clear an obstacle or obstruction on the ground shortly after takeoff. The pilot would configure the aircraft for the Best Angle of Climb to gain the maximum altitude possible before reaching the obstacle (i.e. Tree) located beyond the runway.
Vx is slower than Vy. This makes sense, as Vx will have a slower forward speed. The slower forward speed of the airplane will provide more opportunity for altitude gain before reaching the obstacle to be cleared. An easy way to remember Vx vs. Vy, is to ask yourself which letter has more angles? The letter X has more angles than the letter Y. As such, you will always remember Vx is the Best Angle of Climb, and Vy is the Best Rate of Climb.
BLOCKED OR LEAKING PRESSURE SYSTEMS
Pitot. If the Pitot tube is blocked e.g. by ice, the ASI will not be affected till there is no acceleration or deceleration. ASI will not detect any of it in level flight. In climb ASI will Over Read and Under Read in descend.
Static If the static tube is blocked, in level flight there will be no effect. The ASI will over-read during descend to lower altitudes and under-read during climb to higher altitudes than that at which the blockage occurred.
- A leak in the Pitot tube causes the ASI to under-read.
- A leak in the static tube, where the pressure outside the pipe is lower than static (i.e., most unpressurised aircraft), will cause the ASI to over-read. Where the outside air is higher than static (i.e., in a pressurized cabin) the ASI will under-read.
Effects The under-reading or over-reading of an ASI is potentially dangerous. The former may cause problems in adverse landing conditions (e.g., in a strong cross-wind), and the latter condition may result in an aircraft stall at a higher indicated airspeed than that specified for the aircraft.