Pitot-static instruments were discussed earlier in this chapter; they include the airspeed and/or Mach indicator, the altimeter, and the vertical speed indi-cator (VSI). The pitot-static system encompasses the pitot tube, static port, and associated plumbing that must be connected to the pitot-static instru-ments to make them work. Pitot pressure is ram air pressure picked up by a small open-ended pitot tube This tube is about a quarter of an inch in diam-eter and is mounted facing the oncoming air stream. Ram air entering the tube produces a pres-sure that increases with the speed of the aircraft through the air. [Figure 11-52]
Static pressure is the pressure of the still or ambient air surrounding the aircraft. It is used to measure the
Figure 11-52.The pitot tube is mounted on the outside of the aircraft where it can pick up air flowing past the aircraft. As the speed of the air increases, the pressure inside the closed pitot system also increases.
altitude and as a reference for measuring airspeed.
Ambient air pressure is taken from a static port at a location on the aircraft where flight tests have deter-mined there is a minimum disturbance to the air-flow. [Figure 11-53]
Figure 11 -53. Static ports are typically mounted flush on the exterior of the aircraft.
Only airspeed indicators are connected into the pitot system. The center port of the indicator is con-nected to the pitot tube, which usually is installed in the wing of a single-engine aircraft, outside of the propeller slipstream, or on the fuselage of a multi-engine aircraft. The pitot head is mounted in such a way that it points directly into the airflow In larger aircraft, a machmeter may also installed in the pitot-static system. [Figure 11-54]
Figure 11-54. Large jet transport airplanes normally contain multiple pitot static systems.
To prevent ice inside the pitot head, many aircraft are equipped with electric pitot tube heaters. The heater may be checked on a preflight inspection by turning it on for a few seconds, and then turning it off, which will leave the pitot tube warm to the touch. However, these heaters produce so much heat that they should not be operated for extended periods of time until takeoff, when adequate airflow will prevent overheating. In flight, a pilot can verify the operation of the heater by checking the ammeter as the switch is turned on. [Figure 11-55]
Some aircraft have a combination pitot-static head, in which the pitot pressure is taken from the open end of the tube, and static pressure from holes or slots around the head, back from the open end.
However, most modern aircraft have flush static ports on the sides of the fuselage. Many aircraft have static ports on both sides of the fuselage so side slipping will not cause an inaccurate static pressure. It is extremely important that the static holes are never obstructed, and also important that there is no distortion in the aircraft skin around the
Figure 11-55. A heating element is used to prevent the for-mation of ice in a pitot tube.
holes. Irregularities on the aircraft surface can cause turbulent airflow, which can produce an inaccurate static pressure. Water traps usually are installed in all of the static lines.
These traps
Figure 11-56. Some aircraft are equipped with a blade probe with a pitot tube in front and a static port in the bottom.
Provisions must be made to keep water and ice out of the pitot system. If water should collect in the pitot line, the airspeed indicator may oscillate as the water moves back and forth in the line. This oscil-lation can result in an inaccurate reading. If ice should form inside the pitot head, the pitot pressure inside the system will be trapped. As the aircraft descends, the static pressure increases, which can cause the airspeed indicator to show a false decrease in airspeed. To prevent accumulation of water and ice in the system, the plumbing that con-nects the pitot tube to the airspeed indicator is run as directly as possible. There usually is a T-fitting or sump at a low point in the line to collect any accu-mulated moisture. [Figure 11-57]
Most aircraft have an alternate static source that a pilot can select in the event the normal ports become plugged with ice. The alternate source valve is located on or under the instrument panel. In unpressurized aircraft, the alternate air is sampled from behind the instrument panel. On pressurized aircraft, the alternate air comes from some portion of the fuselage outside of the pressure vessel.
[Figure 11-58]
Figure 11-57. Static pressure is provided for the airspeed indicator(s), altimeter, and vertical speed indicator. Some aircraft may also have static pressure routed to a cabin altimeter, standby altimeter or a separate altitude encoder.
Figure 11-58. The alternate air valve is located where the pilot can select an alternate static air source if the primary source becomes plugged.
STATIC SYSTEM TESTING
FAR 91.411 requires that the static system be checked every 24 calendar months on aircraft that are flown under instrument flight rules (IFR). This check also is required any time the system is opened for service or repair, or may be initiated if a pilot reports a problem. For example, if an instrument attached to a static system were to become discon-nected inside a pressurized cabin, the static pres-sure would be higher than the correct ambient,
should be drained on each routine maintenance inspection. [Figure 11-56]
FAR 43, Appendix E requires the following:
1. The static system must be free of entrapped mois ture and restrictions.
2. The leak check to be within established toler ances.
3. The static port heater, if installed, must be
operative.
4. The technician must ensure that no alterations or deformations of the airframe surface have
been made that would affect the relationship between air pressure in the static pressure sys tem and true ambient static air pressure for any flight condition.
The procedure is outlined in Section B of this chapter.