Flight instruments have evolved remarkably from yaw string to modern pitot static systems. A state-of-the-art pitot static system provides the aircraft computers with the most vital set of data: airspeed, altitude, and vertical speed.
It is a pilot’s nightmare to lose the aircraft’s pitot-static system in mid-flight as it endangers the safety of all onboard. Here is a complete guide on how a typical pitot static system works and how bad pitot static system errors could affect a flight. More importantly, we have tailored an equation to troubleshoot and identify all failure modes.
A piece of tape bringing an airliner down
A maintenance worker applied tape on the static port(s) to clean the airplane exterior. Upon finishing his work, he forgot to remove the tape and the aircraft took off with a blocked static port(s).
Pilots on Aeroperú Flight 603 were facing erroneous indications on the airspeed indicator, altimeter, and vertical speed indicator. The aircraft had lost altitude unknowingly to the pilots and crashed into the sea killing all 70 people onboard.
A wasp net killing 189 people
Birgenair Flight 301 stalled and crashed into the Atlantic ocean followed by a stall and a spin. A wasp net on the captain’s pitot tube gave rise to erroneous airspeed indications that ultimately ended up crashing the aircraft and killing 189 souls onboard.
Pitot static system and possible errors
A typical pitot static system comprises two pitot tubes, two static ports (another one may be available as the alternate static source), and the related instruments: an airspeed indicator, an altimeter, and a vertical speed indicator.
This is one of the simplest yet vital systems available in the aircraft. Unlikely the other systems, troubleshooting is fairly easy. The entire system deals with air and a blockage in the air passages is the most common error. With that said, it is time to master how a typical pitot-static system failure could occur and tactics to pinpoint the error.
A skim through the pitot-static system
In pitot static systems,
The airspeed indicator is the only instrument that receives inputs from the pitot tube.
All three pitot-static instruments receive inputs from the static ports, while vertical speed indicators and altimeters rely only on static ports.
In airspeed indicators,
The pressure inside the instrument case is the static pressure from the static ports.
Ram air pressure (dynamic + static pressure) from the pitot tube is routed directly into the diaphragm.
Movement of the diaphragm occurs due to the dynamic component of the air pressure.
A pitot system taps ram air from the free stream airflow and sends the air into the airspeed indicator. Within the airspeed indicator, a diaphragm obtains the dynamic pressure with the aid of ram air pressure from the pitot tube and static air pressure from the static port. A blocked pitot tube can be further divided into three categories depending on the location of the blockage.
Main intake blockage
A blockage that occurs at the main air intake is the most peculiar type of blockage found in pitot tubes. This prevents the entrance of ram air into the tube allowing only static air that enters via the drain hole.
As the needle of the airspeed indicator is driven by the dynamic pressure (which is the pressure difference between the pitot pressure and static pressure), the unavailability of dynamic pressure results in zero indication.
Drain hole blockage
When the blockage occurs at the drain hole, the airspeed indicator continues to function without any errors. As the drain hole is unable to cater to its intended function- draining water trapped within the pitot tube- inaccurate indications may be shown to the pilots.
Both main intake and drain hole blockage
When this dual blockage occurs, the pressure inside the blocked pitot tube is unchanged. In level flight, there will be an indication on the dial but the pointer remains frozen regardless of the aircraft’s actual airspeed.
A change in the airspeed indication is observed only when the aircraft changes its altitude. In this instance, the airspeed indicator acts like an altimeter. Here is how it happens.
When the aircraft climbs: With increasing altitude, static pressure that acts against the diaphragm reduces, hence, the diaphragm expands indicating an increment of airspeed.
When the aircraft descends: With decreasing altitude, static pressure that acts against the diaphragm increases, hence, the diaphragm contracts indicating a reduction of airspeed.
What takes you out of the trouble in pitot system errors?
Turning on pitot heat will address all three errors as pitot probes are susceptible to ice formation as they are exposed to the free stream airflow. As shown in image 01, one heating element is placed close to the intake and another one close to the drain hole. A heated pitot tube can keep the two holes clear and pilots out of trouble.
When the pitot tube is blocked by something else, like a wasp net in the Birgenair crash or when the aircraft departed with pitot tube covers installed, pitot heat will longer be a savior.
A blockage on a static port will take out the static source from all three instruments killing airspeed indication, altitude indication, and rate of climb. Time to see how each pitot-static instrument behaves with the loss of its static source.
Nothing is amiss until the aircraft changes its altitude.
When the aircraft climbs- airspeed will be decreased.
When the aircraft descends- airspeed will be increased.
Altitude remains frozen even if the aircraft descends or climbs.
Vertical Speed Indicator
Vertical speed remains at zero as the diaphragm does not move.
What takes you out of the trouble in static system errors?
An alternate static source is located within the aircraft cabin (in an unpressurized area) to be used when the external static port blockage occurs. Pilots switch the static source with a toggle switch located in the cockpit. With that done, pilots have to correct the airspeed against a calibration chart provided in the flight manual.
Pitot static system controls some of the most vital flight instruments aboard an aircraft and finding yourself in flight with the system malfunctioning is something straight out of a nightmare for a lot of pilots.
Pitot static system blockage can show altimeter, airspeed and vertical indicator errors, however it all depends where the blockages occur, so it is important to know how to counter issues like that. Luckily, with this article you should be all set for situations like that.
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Jet pilot @NASA
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