The Pitot-Static System

 


The Pitot-Static system is the name of a combined system that measures dynamic and static pressure changes during the movement of the aircraft through the air. Those pressure measurements provide critical flight data to multiple primary and secondary flight instruments such as the Altimeter, Airspeed Indicator, and the Vertical Speed Indicator (VSI).
A failure of the Pitot-Static system can reflect the pilot an unreliable critical flight data such as false airspeed and altitude settings and trends. In order to ensure a safe operation, pilots are required to be able to recognize Pitot-Static system failures and possible mitigation strategies in case of that kind of a threat.

Altimeter, Airspeed Indicator, Vertical Speed Indicator 

Pitot Tube

The pitot tube measures dynamic pressure that is being imposed on an aircraft when it is moving through the air. Pitot-Tube is interconnected with the static port and calculates the total of the Dynamic pressure and the Static pressure to provide an airspeed indication. When the pitot tube is blocked the only instrument that is being affected is the Airspeed Indicator.

Static Port

A static port is a chamber that measures the static pressure outside of the aircraft anytime. The static pressure that is being measured by the Static port is used for all three instruments, and a clogged, or a blocked static port will have an impact on the Airspeed indicator, Altimeter, and the Vertical Speed Indicator.

Pitot-Static System (PHAK 8-1)

Altimeter

The Altimeter provides the pilot an indication of the aircraft's altitude based on a static pressure measurement. Once the pilot sets the Altimeter to a specific atmospheric pressure setting on the ground and the static port will sense a pressure change, the Altimeter will indicate the airplane's altitude over the ground / over sea level.
A static port blockade will cause the Altimeter to freeze and do not indicate any altitude changes. A pilot can recognize a clogged static port when relying on the Airspeed Indicator, which will show a speed change (Due to a dynamic pressure change in the Pitot tube) and will represent to the pilot a climb or descent indication.
In order to recover and prevent false altitude indications, airplanes have a back-up static port installed in the flight deck. Once the alternate static pressure is being measured, it subjects to accuracy corrections since the static pressure in the airplane is lower than the pressure outside.

Altimeter look from inside (PHAK 8-2)

Airspeed Indicator

The Airspeed Indicator gathers its data from the entire Pitot-Static system. The airspeed which is being shown on the ASI in the total of the Dynamic and Static pressure together. An Airspeed Indicator failure could be dangerous and even fatal due to the possibility of the pilot to lose situational awareness and get the airplane into a stall. Since the ASI is gathering data both from the Pitot tube and the Static port, any malfunction with either the static port or the pitot tube will affect its normal operation.
A clogged static port impact on an ASI will cause an inaccurate airspeed reading due to the missing static pressure data.
In the case of a clogged Pitot tube, Once the main hole of the pitot tube is clogged, the airspeed reading will drop to Zero. If the entire tube is clogged, the ASI will gather its data only from the static port, and the Airspeed Indicator will start reacting as an Altimeter, showing false speed trends.
In order to avoid blockades, crewmembers use covers on the ground to avoid dirt and bugs accumulation. The anti-ice system provides heat to the pitot tubes to prevent ice buildup in flight on the Pitot tube.

Airspeed Indicator look from inside (PHAK 8-7)

An example of a clogged pitot tube that led into a fatal accident is the famous Air France Airbus A330 that has been crashed over the ocean on its way from Brazil to Paris. The pitot tubes that got frozen due to a continuous flight in "supercooled water droplets" cause the aircraft's ASI to react as an altimeter. As a result, the pilots encountered a situation of an unreliable airspeed and showed an opposite indication of acceleration when climbing and declaration when descending. The false airspeed indication has finally led to a stall and diving to the water due to the loss of the pilots' situational awareness.



Air France 447 tail that has been found after the crash

Vertical Speed Indicator

The Vertical Speed Indicator measures the differential static pressure to indicate a climb or descent trends. The VSI is connected to the Static port only and should indicate Zero in level flight (Due to constant static pressure). In case of a static port failure, the VSI will indicate Zero and will now show the pilot any altitude change trend. In order to identify the malfunction, the pilot should rely on the secondary instrument, which is the Airspeed Indicator in this case. Any pitch attitude changes will result in an increase or decrease in airspeed and will allow the pilot to identify a climb or a descent.
In order to overcome the failure, the pilot can use the alternate Static port or break the VSI glass to use the instrument direct static port, which will result in a delayed indication.


(Vertical Speed Indicator look from inside (PHAK 8-5)



The pitot-Static system is one of the critical systems in the airplane, of which a proper operation of the system will ensure an excellent situational awareness of the pilots. Also, crewmembers are expected to be familiar with the symptoms of a system's malfunction and with the required ways to overcome and bring the airplane back into safe operation.
After the Air France 447 crash report came out, several airlines developed a specialized training to the pilots that allowed them to sense and point out the failure in the simulator in order to prevent such a horrifying event from happening again.


References:

Federal Aviation Administration, & United States. Federal Aviation Administration. (2009). Pilot's Handbook of Aeronautical Knowledge. Retrieved from https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/pilot_handbook.pdf
‏

Wise, J., Rio, A., & Fedouach, M. (2011). What really happened aboard Air France 447. Popular Mechanics, 6, 35-36.‏ Retrieved from: http://www.edugeek.net/attachments/forums/general-chat/12451-why-air-france-flight-447-crashed-what-really-happened-aboard-air-france-447-popular-mechanics.pdf

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