Pitot Static System
Aircraft depend for a great deal on measuring air pressure for indication of airspeed, altitude and vertical speed. These instruments are called the pressure instruments and they operate by sensing changes in the pressure of the air surrounding the aircraft.
We will investigate the concept of aircraft speeds: indicated airspeed, true airspeed and how it is used in navigation and keeping the airplane flying without stalling.
The loss of lift and stall symptoms will be discussed as are the best rate- and angle-of-climbs speeds typically used when the aircraft needs to clear obstacles.
Best glide speed is discussed together with the forced landing procedure as these two are intimately related. The use of flaps and influence of aircraft weight and wind is shown.
Stall speeds and spins are also in this section as they can be the result of an airspeed which got too low.
Static pressure
Air pressure is measured by two sensors: the static port and pitot tube. On some aircraft these are integrated in one probe and others have more than one static port to equalize pressure differences around the fuselage.
The air exerts a pressure equally in all directions at any point in the atmosphere, this is called static pressure. This pressure results from the weight of the air molecules above that point and it decreases with a gain in altitude.
The name implies that there is no involvement with the movement of the aircraft through the air, the measurement is done with a port (hole) facing at a right angle (90°) to the airflow, usually at the fuselage but sometimes included in the pitot tube.
Dynamic pressure
As air has mass (from the molecules) and air in motion has dynamic (kinetic) energy which is converted to pressure the moment a body tries to slow it down or stop it. This is called dynamic pressure. Usually measured in a tube (pitot) and it includes static pressure at that point too.
Dynamic pressure is mathematically expressed as: 1/2 ρ V2, as we know from aerodynamics. It shows clearly that dynamic pressure relies on two things: air density (Rho) and aircraft speed (V). Density is depends on a number of factors: altitude, humidity, temperature and ambient air pressure. These factors will result in a number of different speeds: IAS and TAS; see aircraft speeds for more detail.
Pitot pressure
Total pressure as measured in a pitot tube consists of static and dynamic pressure. The airspeed indicator uses connections to both pitot and static lines measuring static and total pressure to indicate dynamic pressure (as airspeed) on the instrument (ASI).
Pitot Static system
In the pitot static system three instruments are connected to the static port(s) and the pitot tube via tubing, in some cases there is an alternate static port provided in the cockpit.
These three pressure instruments are:
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Image to the right shows how the components are connected to the system.
Pitot tube
To be able to measure the dynamic pressure the pitot tube must face forward in the undisturbed airflow. The movement of the aircraft creates a pressure over the static pressure and this is fed through tubing (pitot line) to the airspeed indicator. Some pitot tube are fitted with a connection for static pressure, these are called pitot head.
It is common practice that the pitot tube is heated to protect against ice blockage.
Static port
Usually taken at the fuselage and even more common at both sides to equalize pressure when the aircraft is flown out of balance or the nature of airflow around the aircraft (turbulence). Some light or ultralight aircraft have no outside static port but take static pressure inside the cocpkit where it is slight lower than outside the aircraft. Less accuracy is the result and the opening or closing of windows will influence the static pressure.
Alternate static port
Other aircraft (unpressurized) have an alternate static port to be used when the outside ports fail or the static line is blocked for some reason. Usable in unpressurized cockpits and the indication will have an error as described above.
Operational aspects
During preflight the pitot tube cover must be removed and the pitot tube and all static ports carefully checked for blockage of any kind (especially after washing or painting of the aircraft and insects have been known to nest inside pitot tubes). Do not blow into them and if heater is installed, turn it on briefly and touch carefully with your hand to see if it will heat up. On the center zero ammeter this will be indicated as a discharge from the battery.
During pre-start checks, inspect the instruments for obvious errors. The ASI should read zero (or indicate the headwind), the VSI must also read zero and the altimeter turned on the aerodrome elevation. When obtaining the local QNH make sure that the altimeter is within +30 and -45 feet of aerodrome elevation or else it will need calibration.
During the initial take-off roll check for airspeed alive (ASI) and increasing and shortly after climbing an increase in altitude and an indication of rate of climb on the VSI.
In flight if the pitot tube is blocked the ASI will be affected by reacting as an altimeter. Any blockage of the static port will trap the air in the static line and all three will be affected. The VSI will indicate zero and remain there and the altimeter will not indicate a change in altitude when climbing or descending.
