When considering the pitot static system it is important to be aware of and understand three different pressure types.
The pitot static system consists of two primary pressure sensing elements: the pitot tube and static port. The pitot tube has a small opening at the front that measures the total pressure. In addition to the larger hole in the front of the pitot tube, there is a small hole in the back of the chamber that allows moisture to drain from the system should the aircraft enter precipitation. The pitot tube is also typically heated to prevent ice buildup.
Static ports are small holes exposed to the free undisturbed air on the side(s) of the aircraft. They measure the static (ambient) pressure. An alternate static source is provided in some aircraft to provide static pressure indications should the primary static source become blocked.
The altimeter is an instrument that measures the height of an aircraft above a given pressure level. A stack of sealed aneroid wafers comprises the main component of the altimeter – an aneroid wafer is a sealed wafer that is evacuated to a constant internal pressure. These wafers are free to expand and contract with changes to the static pressure. A large static pressure presses down on the wafers and causes them to collapse. A lower static pressure allows the wafers to expand. A mechanical linkage connects the wafer movement to the needles on the indicator face, which translates compression of the wafers into a decrease in altitude and translates an expansion of the wafers into an increase in altitude.
It is easy to maintain a consistent height above ground if the barometric pressure and temperature remain constant, but this is rarely the case. The pressure and temperature can change between takeoff and landing even on a local flight. If these changes are not taken into consideration, an aircraft may not have the required obstacle clearance.
If an aircraft is flown from a high pressure area to a low pressure area without adjusting the altimeter, a constant altitude may be indicated, but the actual height of the aircraft above the ground would be lower than the indicated value.
Adjustments to compensate for nonstandard pressure do not compensate for nonstandard temperature. Since cold air is denser than warm air, when operating in temperatures that are colder than standard, the true altitude is lower than the altimeter indication.
The vertical speed indicator (VSI) indicates whether the aircraft is climbing, descending, or in level flight. The rate of climb or descent is indicated in feet per minute (fpm). If properly calibrated, the VSI indicates zero in level flight.
The VSI contains a diaphragm with connecting linkage and gearing to the indicator pointer inside an airtight case. The inside of the diaphragm is connected directly to the static line of the pitot-static system. The area outside the diaphragm, which is inside the instrument case, is also connected to the static line but through a restricted orifice (calibrated leak).
Both the diaphragm and the case receive air from the static line at existing atmospheric pressure. When the aircraft is in level flight, the pressures inside the diaphragm and the instrument case are equal, and the pointer is at the zero indication. When the aircraft climbs or descends, the pressure inside the diaphragm changes immediately, but due to the metering action of the restricted passage, the case pressure remains higher or lower for a short time, causing the diaphragm to contract or expand. This causes a pressure differential that is indicated on the instrument needle as a climb or descent.
The airspeed indicator (ASI) is a sensitive, differential pressure gauge that measures and indicates the difference between total and static pressure. These two pressures are equal when the aircraft is parked on the ground in calm air. When the aircraft moves through the air, the pressure in the pitot line becomes greater than the pressure in the static lines. This difference in pressure is registered by the airspeed pointer on the face of the instrument, which is typically calibrated in knots.
The ASI is the one instrument that utilizes both the pitot, as well as the static system. The ASI introduces the static pressure into the instrument case while the total pressure is introduced into the diaphragm. The diaphragm expands or contracts depending on value of the total pressure and static pressure. The diaphragm is linked via a series of mechanical linkages to pointer on the instrument face.
If the pitot tube becomes blocked and its associated drain hole remains clear, ram air is no longer able to enter the pitot system. Air already in the system vents through the drain hole, and the remaining pressure drops to ambient (outside) air pressure. Under these circumstances, the ASI reading decreases to zero because the ASI senses no difference between total and static air pressure. The ASI no longer operates since dynamic pressure cannot enter the pitot tube opening. Static pressure is able to equalize on both sides since the pitot drain hole is still open. The apparent loss of airspeed is not usually instantaneous but happens quickly.
Remember that the pitot tube measures total pressure and the relationship between total pressure, dynamic pressure and static pressure is:
Total = Static + Dynamic
The pitot tube is only used by the airspeed indicator which essentially calculates an aircraft’s speed by calculating the dynamic pressure and displaying that as an airspeed to the pilot.
Dynamic = Total – Static
In the case of a completely blocked pitot tube and drain hole, the total pressure remains constant since the last reading is now trapped within the system. The airspeed indicated will now only respond to changes in static pressure as it climbs or descends. Using this information, it can now be seen that if an aircraft operates at altitudes greater than that at which the blockage occurred, the airspeed indicator will over-read due to a decrease in static pressure. The reverse is true for operating at altitudes below the altitude at which the blockage occurred
Since the static pressure is used by the altimeter, airspeed indicator and vertical speed indicator, a blockage in the static port will affect all three instruments.
If the static system becomes blocked but the pitot tube remains clear, the ASI continues to operate; however, it is inaccurate. The airspeed indicates lower than the actual airspeed when the aircraft is operated above the altitude where the static ports became blocked because the trapped static pressure is higher than normal for that altitude. When operating at a lower altitude, a faster than actual airspeed is displayed due to the relatively low static pressure trapped in the system.
A blockage of the static system also affects the altimeter and VSI. Trapped static pressure causes the altimeter to freeze at the altitude where the blockage occurred. In the case of the VSI, a blocked static system produces a continuous zero indication.
Some aircraft are equipped with an alternate static source in the flight deck. In the case of a blocked static source, opening the alternate static source introduces static pressure from the flight deck into the system. Flight deck static pressure is lower than outside static pressure. Check the aircraft AOM/POH for airspeed corrections when utilizing alternate static pressure.