Airspeed Indicator Markings
Aircraft weighing 12,500 pounds or less, manufactured after 1945 and certificated by the Federal Aviation Administration (FAA), are required to have airspeed indicators that conform to a standard color-coded marking system. [Figure 4-4] This system enables the pilot to determine, at a glance, certain airspeed limitations that are important to the safe operation of the aircraft. For example, if during the execution of a maneuver, the pilot notes that the airspeed needle is in the yellow arc and is rapidly approaching the red line, immediate corrective action to reduce the airspeed should be taken. It is essential that the pilot use smooth control pressure at high airspeeds to avoid severe stressors upon the glider structure.
The following is a description of what the standard color code markings are on an airspeed indicator and how they correspond to maneuvers and airspeeds.
- The white arc—flap operating range
- The lower limit of the white arc—stalling speed with the wing flaps and landing gear in the landing position.
- The upper limit of the white arc—maximum flaps extended speed. This is the highest airspeed at which the pilot should extend full flaps. If flaps are operated at higher airspeeds, severe strain or structural failure could result.
- The lower limit of the green arc—stalling speed with the wing flaps and landing gear retracted.
- The upper limit of the green arc—maximum structural cruising speed. This is the maximum speed for normal operations.
- The yellow arc—caution range. The pilot should avoid this area unless in smooth air.
- The red line—never-exceed speed. This is the maximum speed at which the glider can be operated in smooth air. This speed should never be exceeded intentionally.
Other Airspeed Limitations
There are other important airspeed limitations not marked on the face of the airspeed indicator. These speeds are generally found on placards [Figure 4-12] in the view of the pilot and in the GFM/POH.
- Maneuvering speed (Va)—maximum speed at which the limit load can be imposed (either by gusts or full defection of the control surfaces for one cycle) without causing structural damage. If rough air or severe turbulence is encountered during flight, the airspeed should be reduced to maneuvering speed or less to minimize the stress on the glider structure. Maneuvering speed is not marked on the airspeed indicator. For gliders, if there is a rough airspeed (Vb) limitation published the pilot should be below that speed for maximum gust intensity.
- Landing gear operating speed—maximum speed for extending or retracting the landing gear if using glider equipped with retractable landing gear.
- Minimum sink speed—important when thermalling.
- Best glide speed—airspeed that results in the least amount of altitude loss over a given distance, not considering the effects of wind.
- Maximum aerotow or ground launch speed— maximum airspeed that the glider may safely be towed without causing structural damage.
The altimeter measures the static air pressure of the surrounding air mass. A flexible plastic tube connects the altimeters static pressure inlet to the static port holes located on the side of the glider. [Figure 4-13] If set to the proper local pressure, the altimeter needles and dial indicate heights above mean sea level (MSL). [Figures 4-14 and 4-17] Glider pilots must be fully aware of the ground elevations on the flight route or area in order to make flight decisions concerning soaring or landing options.
Atmospheric Pressure and Altitude
Atmospheric pressure is caused by the weight of the column of air above a given location. At sea level, the overlying column of air exerts a force equivalent to 14.7 pounds per square inch, 1013.2 mb, or 29.92 inches of mercury. The higher the altitude is, the shorter the overlying column of air is and the lower the weight of that column is. Therefore, atmospheric pressure decreases with altitude. At 18,000 feet, atmospheric pressure is approximately half that at sea level. [Figure 4-16]
Principles of Operation
The pressure altimeter is simply an aneroid barometer that measures the pressure of the atmosphere at the level at which the altimeter is located and presents an altitude indication in feet. The altimeter uses static pressure as its source of operation. Air is denser at the surface of the earth than aloft; as altitude increases, atmospheric pressure decreases. This difference in pressure at various levels causes the altimeter to indicate changes in altitude. Figures 4-15 and 4-17 illustrate how the altimeter functions. The presentation of altitude varies considerably between different types of altimeters. Some have one pointer while others have more.
The dial of a typical altimeter is graduated with numerals arranged clockwise from 0 to 9 inclusive, as shown in Figure 4-14. Movement of the aneroid element is transmitted through a gear train to the three hands, which sweep the calibrated dial to indicate altitude. The shortest hand indicates altitude in tens of thousands of feet; the intermediate hand in thousands of feet; and the longest hand in hundreds of feet, subdivided into 20-foot increments.
The altitude indicated on the altimeter is correct only if the sea level barometric pressure is standard (29.92 “Hg), the sea level free air temperature is standard (+15 °C or 59 °F), and the pressure and temperature decreases at a standard rate with an increase in altitude. Since atmospheric pressure continually changes, a means is provided to adjust the altimeter to compensate for nonstandard conditions. This is accomplished through a system by which the altimeter setting (local station barometric pressure reduced to sea level) is set to a barometric scale located on the face of the altimeter. Only after the altimeter is set properly will it indicate the correct altitude.