Air Speed Indicator

The airspeed indicator is constructed to measure the difference between ram (impact) air pressure from the pitot head and atmospheric pressure from the static source. The instrument is contained within a sealed case in which is mounted a diaphragm sensitive to pressure changes. The impact pressure line is connected directly to one side of the diaphragm, while the inside of the case is vented to the static source. As the aircraft accelerates or decelerates, expansion or contraction of one side of the diaphragm moves the indicator needle by means of gears and levers.

The face of the indicator may show indicated airspeed, true airspeed, or a combination of both and these values may be calibrated in miles per hour or knots or both.

The face is also colored coded and represents several important airspeeds that effect the operation of your aircraft.

White Arc - Represents the flap operation range. The lower end marks Vso or stall speed in landing configuration. The top of the arc represents Vfe the maximum flap extended speed. Operating above the Vfe will cause damage to the flap system. (The Blanik L-23 does not have flaps, however several other sailplanes do.)

Stall speed always stays the same regardless of altitude. Stall speeds are indicated air speeds. Example: Vso is the same at 1000' or 10,000'.

Green Arc - This area represents normal operations. The top of the arc is Vno or maximum structural cruising speed. In rough air, you always need to ensure your speed is within the green arc. The lower end of the green arc is Vs1 or stalling speed in a specific configuration. Normally, that is cruise configuration.
Yellow Arc - This is the caution area. Fly within the yellow arc in smooth air only. Flying in the yellow arc during turbulent conditions can cause structural damage or failure.

Red Line - This is a never exceed speed called Vne. (velocity never exceed) Airspeeds in excess of red line will cause structural damage or failure and may cause a destructive aerodynamic phenomena called "flutter." A control surface that flutters will self-destruct in a matter of seconds. Don't fly past Vne.

Maneuvering Speed - This speed is not on the airspeed indicator, however, it is a very important design speed. At or below this speed your aircraft can structurally survive a full deflection input of any flight control. What that really means is that your chances of surviving severe turbulence are excellent at or below maneuvering speed. An encounter with severe turbulence above maneuvering airspeed could cause structural damage.

Maneuvering speed for the Blanik L-23 is 81 knots. Commit it to memory!!

A recent accident of an L-33 glider suggests the design limits of the glider were exceeded during a recovery from a dive. One wing separated from the glider and the pilot was able to bail out of the glider. This accident illustrates the importance of not exceeding the design limits of the glider. Gliders are inherently strong, but as with all aircraft, they have limits. Know and respect the limits.