There are two major divisions in the pitot-static system:
- Impact air pressure due to forward motion (flight) captured in the pitot tube transferred to instruments by way of air pressure lines or tubing.
- Static or free air pressure sensed at static air ports designed to be free of motion induced pressure variations. This reference pressure is necessary because the free pressure of the atmosphere decreases as altitude increases and changes due to the current weather (barometric) pressure variations. This static or free air pressure is transferred to instruments by way of the static pressure lines or tubing.
Impact and Static Pressure Lines
The impact air pressure (air striking the glider because of its forward motion) is taken from the pitot tube, which is mounted on either the nose or the vertical stabilizer. [Figure 4-1] Pitot tubes are aligned with the relative wind. These locations minimize air disturbance or turbulence during glider flight through the air.
When a glider is in flight, the oncoming air tries to flow into the open end of the pitot tube. [Figure 4-2] Connecting a diaphragm to the back end of the pitot tube means that the air flowing in has nowhere to go. The pressure in the diaphragm rises until it is high enough to prevent any further air from entering. Increasing the airspeed of the glider causes the force exerted by the oncoming air to rise. More air is able to push its way into the diaphragm and the pressure within the diaphragm increases. The pressure inside the diaphragm to oncoming airflow increases as airspeed increases.
The static pressure (pressure of the still air) is taken from the static line that is attached to a port, or ports, mounted flush with the side of the fuselage or tube mounted on the vertical stabilizer. [Figure 4-3] Gliders using a fuselage flush mounted static source have two vents, one on each side of the fuselage. This compensates for variation of static pressure due to changes in glider attitude and air turbulence.
The openings of both the pitot tube and the static port(s) should be checked during the preflight inspection to ensure they are free from obstructions. Clogged, or partially clogged, openings should be cleaned by a certificated mechanic. Blowing into these openings is not recommended, because this could damage flight instruments.
The airspeed indicator displays the indicated airspeed (IAS) of the glider through the air. [Figure 4-4] Some airspeed indicator dials provide color-coded arcs that depict permissible airspeed ranges for different phases of flight. The upper (top) and lower (bottom) limits of the arcs correspond to airspeed limitations for specific gliders configurations which are discussed later in this chapter. These speed limitations are set by the manufacturer. Figure 4-5 shows the anatomy of the airspeed indicator and where the pitot and static pressure inlets are located.
The airspeed indicator depends on both pitot pressure and static pressure. [Figure 4-6] When pitot pressure and static pressure are the same, zero airspeed is indicated. As pitot pressure becomes progressively greater than static pressure, airspeed is indicated by the needle pointing to the speed scale. The airspeed instrument contains a diaphragm that senses differential pitot and static pressure. The diaphragm expands or contracts according to the difference of static and pitot pressures; this movement drives the needle (airspeed needle pointer) on the face of the instrument. [Figure 4-7]
The Effects of Altitude on the Airspeed Indicator
Like pressure, air density also decreases with altitude. The airspeed indicator’s diaphragm is calibrated to correctly display airspeed when the air through which the aircraft is moving is of average sea level density. Above sea level, due to the lower air density, the buildup of pressure in the diaphragm is lower, and the airspeed indicator reads artificially low. The higher the altitude above sea level, the more erroneous the airspeed indicator value. [Figure 4-8]
Types of Airspeed
There are three kinds of airspeed that the pilot should understand: IAS, calibrated airspeed (CAS), and true airspeed (TAS). [Figure 4-9]
Indicated Airspeed (IAS)
IAS is the direct instrument reading obtained from the airspeed indicator, uncorrected for variations in atmospheric density, installation error, or instrument error. Figure 4-10 shows that the IAS at which a glider stalls in steady wings-level flight does not vary with altitude. However, pilots must remember that different gliders stall at different speeds. The IAS shown in Figure 4-10 may not be the stall speed for each particular glider.
The IAS at which never exceed speed (VNE) is reached decreases with altitude. The glider flight manual (GFM) should include a table, such as the one shown in Figure 4-11, that details how VNE should be reduced with altitude. These figures vary from glider to glider; therefore, pilots should always refer to the manual specific to the glider they are flying, which should show a chart similar to the one in Figure 4-11.
Calibrated Airspeed (CAS)
CAS is IAS corrected for installation and instrument error. Although manufacturers attempt to keep airspeed errors to a minimum, it is impossible to eliminate all errors throughout the airspeed operating range. At certain airspeeds and with certain flap/spoiler settings, the installation and instrument error may be significant. The error is generally greatest at low airspeeds. In the cruising and higher airspeed ranges, IAS and CAS are approximately the same.
It is important to refer to the airspeed calibration chart to correct for possible airspeed errors because airspeed limitations, such as those found on the color-coded face of the airspeed indicator, on placards in the cockpit, or in the GFM or Pilot’s Operating Handbook (GFM/POH), are usually CAS. Some manufacturers use IAS rather than CAS to denote the airspeed limitations mentioned. The airspeed indicator should be calibrated periodically.
Dirt, dust, ice, or snow collecting at the mouth of the pitot tube may obstruct air passage and prevent correct indications. Vibrations may also destroy the sensitivity of the diaphragm.
True Airspeed (TAS)
TAS is the true speed at which the aircraft is moving through the air. The airspeed indicator is calibrated to indicate TAS only under standard atmospheric conditions at sea level (29.92 inches of mercury (“Hg) and 15 °C or 59 °F). Because air density decreases with an increase in altitude, the glider must be flown faster at higher altitudes to cause the same pressure difference between pitot impact pressure and static pressure. Therefore, for a given TAS, IAS decreases as altitude increases; for a given IAS, TAS increases with an increase in altitude.
Fortunately for the pilot, the amount by which the airspeed indicator underreads approximately cancels out the air density related changes to the glider’s flight dynamics. This means that if the IAS at the point of stall at 1,000 feet in steady wings-level flight is 40 knots, then the IAS at 20,000 feet at the point of the stall is also 40 knots, despite the fact that the stall is actually occurring at a TAS that is 14 knots higher. Therefore, the pilot needs to remember only one set of numbers that work at all altitudes. A decrease in air density with altitude also affects a glider’s flight dynamics. For example, a glider that stalls at a TAS of 40 knots in steady wings-level flight at 1,000 feet stalls at a TAS of 54 knots at 20,000 feet. Consider the inconvenience that this would cause if the airspeed indicator did actually display TAS. The pilot would need to use quick reference cards continuously to look up the stall speed, best L/D speed, and minimum sink speed for the current altitude—not a particularly convenient thing to do while trying to fly. Figure 4-10 shows how steady, wings-level flight stall speed is affected by different altitudes. Figure 4-10 is only an example, and the figures used in it do not pertain to all gliders. For specific stall speeds, always refer to the POH of the glider that is being flown.
A pilot can find TAS by two methods. The first method, which is more accurate, involves using a flight computer. In this method, the CAS is corrected for temperature and pressure variation by using the airspeed correction scale on the computer. A second method, which is a rule of thumb, can be used to compute the approximate TAS. This is done by adding to the IAS 2 percent of the IAS for each 1,000 feet of altitude.