Pitot Static Instruments

    Pitot static instruments are instruments that measure ram and static air inputs and display their results on an altimeter, airspeed indicator, and vertical speed indicator.  They're most commonly found in general aviation aircraft; due to their design, a pilot must become familiar with the common errors associated with each of the instruments, and where they can be found in the system.

    First off, the pitot-static system relies on the accurate input of the static port and pitot tube.  The pitot tube measures ram air pressure and faces forward on the aircraft, while the static port measures static air pressure and is mounted on the side of the aircraft's fuselage.  The altimeter has static air pressure from the static port enter the instrument, and a calibrated aneroid wafer expands or contracts depending on the ambient air pressure inside the instrument.  The aneroid wafer is mechanically linked so that it can be read and calibrated easily on an analog instrument.  When the static port becomes blocked, the pressure inside the instrument becomes sealed and the altimeter will become frozen in place.  The airspeed indicator has ram air pressure and static pressure input into the instrument.  The ram air pressure is directed into a diaphragm, which expands and contracts based on the magnitude of ram air pressure. Additionally, the static air pressure is directed into the casing surrounding the diaphragm giving it a medium to accurately measure airspeed.  The diaphragm is then mechanically linked to measure airspeed on an analog instrument. If the static port was blocked, the indicated airspeed would have inherent errors because the air in the casing would have a different density than the air entering the diaphragm.  If just the pitot tube became blocked, then there would be no ram air pressure for the diaphragm to expand and would read 0.  The vertical speed indicator has static pressure directed into a diaphragm, which expands and contracts based on the magnitude of the static air pressure.  The vertical speed indicator also has a calibrated leak inside the casing which allows the instrument to measure the differential pressure.  Due to this design, there is a 6-9 second lag whenever an aircraft's pitch attitude is changed.  It is then mechanically linked to measure the rate of climb on an analog instrument.  If the static port became blocked, there would be no static pressure for the diaphragm to measure and would read 0 (PHAK, 2016).

     In general aviation aircraft, the most common way for the pitot tube and static port to become blocked is when the aircraft accumulates ice.  To combat this, aircraft manufacturers added a heating element to the pitot tube to help melt the ice.  If the static port became iced over, the pilot would activate an alternate static port, which is located inside the cabin of the aircraft, and would measure the ambient static air pressure inside the aircraft.  This also means that the instruments using static pressure would be more inaccurate.  The altimeter would read a slightly higher altitude than normal, the airspeed indicator would read a greater airspeed than the actual airspeed, and the vertical speed indicator would momentarily show a climb, then stabilize.  One obvious way a pilot could avoid failures of these systems is to avoid flight into known icing conditions.  However, insects can also block the openings of these instruments, so it is important to check the integrity of the instruments before the flight.

 

                                                Figure 8-1 Pitot Static System (PHAK, 2016)

                                                     Figure 8-2 Altimeter (PHAK, 2016)

                                               Figure 8-7 Airspeed Indicator (PHAK, 2016)

 
                                        Figure 8-5 Vertical Speed Indicator (PHAK, 2016)

                                                                                                                                                                                                                           References:

Federal Aviation Administration. (2016). 8: Flight Instruments. In Pilot's handbook of aeronautical knowledge (PHAK) (pp. 2-11). Washington, D.C.: U.S. Department of Transportation, Federal Aviation Administration, Flight Standards Service.