Pitot-Static System: Why is the pitot-static system important?

Pitot-Static System

Pitot-Static System:
Why is the Pitot-Static System Important? How does the Pitot-static System Work?

Pitot-Static Systems

In order to determine an aircraft's airspeed, Mach number, altitude, and altitude trend, a pitot-static system is a very important system of pressure-sensitive equipment. A static port, a pitot tube, and pitot-static instruments make up most pitot-static systems. Air data computers, flight data recorders, altitude encoders, cabin pressurization controls, and various airspeed switches are additional equipment that could be fitted. Pitot-static system reading errors can be exceedingly harmful because the data it provides, such altitude, has the potential to be safety-critical. Pitot-static system failure has been linked to a number of commercial airline catastrophes. The most crucial flight equipment get their information about air pressure by measuring it. The pitot-static system's job is to collect and distribute varied air pressures for the flight instruments.

Pitot Tubes and Static Vents

A pitot-static system head or pitot tube with impact and static air pressure ports, along with leak-free tubing connecting these air pressure pick-up sites to the instruments that need the air for their indications, may be used on simple aircraft. The three most popular pitot-static instruments are the altimeter, the airspeed indicator, and the vertical speed indicator.

To receive the full force of the impact air pressure as the aircraft travels forward, a pitot tube is open and faces into the airstream. To prevent moisture and debris from entering the tube and damaging the system, this air is passed via a baffled plate. Below the baffle, a drain hole is provided to let moisture out. The chamber in the assembly's shark fin is where the ram air is routed afterward. This pressurised air exits the pitot assembly through an upright tube, or riser, and travels to the airspeed indicator.

Small holes on the top and bottom surfaces of the aft section of the pitot tube are included to gather atmospheric air pressure in a static, or still, situation. The air is run out of the pitot assembly through tubes and attached to the altimeter, the airspeed indicator, and the vertical speed indicator. The static portion also includes a riser tube.

Heating elements are utilized in many pitot-static tube heads to stop icing while in flight. When ice-forming conditions are present, the pilot can use a switch in the cockpit to direct electrical current to the element. Pitot tube heaters can be linked with the pitot tube heat switch to prevent accidental battery drain when the ignition switch is turned off when the aircraft is shutting down. When in close proximity to the pitot tube, care should be taken because the heating elements make the tube too hot to touch without getting burned.

The pitot-static tube is installed on the outside of the aircraft at a point where the air is least likely to be turbulent. It is parallel to the plane's path of flight and pointing forward. The location could change. Some are on the fuselage's nose, while others might be on a wing. Even the empennage may include a few of them. The goal is to collect impact air pressure and static air pressure and direct them to the appropriate instruments. This is accomplished through a variety of designs.

The majority of aircrafts with pitot-static tubes have a backup supply of static air pressure available in case of an emergency. If it seems that the flight instruments are not giving reliable information, the pilot can use a switch in the cockpit to pick the alternative. Alternate static sources on low-flying, unpressurized aircraft could be as simple as cabin air. On pressurized aircrafts, the cabin air pressure and ambient air pressure may differ greatly. Instrument indications would be wildly off if utilized as a substitute for static air. Multiple static vent pickup sites are used in this instance. The pilot can choose which source supplies air to the instruments from any of the ones that are positioned outside the aircraft and are plumbed. The decision on which source to use on an electronic flight display is made either by the computer or by the flight crew.

A different kind of pitot-static system enables the separation of the static and pitot sources at different locations on the aircraft. In this configuration, the pitot tube is simply utilized to collect ram air pressure. Information on static air pressure is gathered using separate static vents. These are often flush to the side of the fuselage. Two or more vents could be present. It is usual to have a primary and backup source vent in addition to dedicated vents for the pilot and first officer's instruments. Additionally, two major vents might be placed on the fuselage's opposing sides and connected to the instruments by Y tubing. This is carried out to account for any fluctuations in the static air pressure at the vents brought on by the attitude of the aircraft. 

No matter how many or where the separate static vents are located, they can all be heated, together with the separate ram air pitot tube, to avoid ice. Complex, multiengine, and pressurized aircraft can have extensive pitot-static systems. Pitot and static air information might be required by additional instruments, gauges, the autopilot system, and computers. The plumbing of the pitot-static system is altered and made more difficult by the copilot's additional set of flight instruments. Additionally, the cabin pressurization unit and the autopilot system both need static pressure data. To supply distinct static air pressure manifolds, one for the pilot's flight instruments and one for the copilot's flying instruments, separate heating sources for static air pressure are taken from both sides of the aircraft. In the case of a malfunction, this is intended to guarantee that there is always one functional set of flight instruments.

Pitot-Static Instruments

The basic Pitot Static instruments are: Altimeter, Airspeed Indicator (ASI), and Rate of Climb indicator (VSI). In modern aircraft, these three instruments are part of the glass cockpit systems. Still, these instruments are required as a standby system.

Accidents due to Pitot-Static System Failure

The pitot-static system's failure has been responsible for many accidents, but two stand out since they happened in the same year (1996), and with the same kind of aircraft (the Boeing 757). Also take note that each occurred in complete darkness over open water, with no surface lights throughout the departure sequence.

The flight crew had extensive expertise in each scenario, with the captains having logged more than 20,000 hours. While attempting to maintain control, both flight crews reported inaccurate readings from the altimeter and airspeed indicators.

The static port had first been covered with tape during maintenance while the plane was being washed. The removal of the duct tape was overlooked during the preflight since it was employed rather of a more noticeable brightly coloured tape.

Despite noticing that the airspeed indicator wasn't functioning properly in the second accident, the captain chose to carry on with the takeoff. This 757 had just left maintenance after spending 20 days there like the other one had. Pitot tube coverings had been taken off two days before the flight, and black-and-yellow dauber wasp nests appeared to be blocking the system. This led to high incorrect airspeed indicators throughout the ascent, which not only confused the crew but also, and more critically, gave the autopilot false information. The autopilot system responded by raising the pitch attitude, which caused the engine to stall, engine flame out, and spin.

In the recent instant Air France Flight crashed Killing all onboard. The aircraft's pilots were unaware that the Pitot tubes were obstructed. This was only discovered following the recovery of the cockpit and flight data recorders from the water.

The pilots of the Air France Airbus may have received inconsistent information about their air speed from the aircraft's Pitot tubes, according to investigators looking into the loss of the aircraft in the Atlantic Ocean on June 1, 2009.

There are parallels between the loss of Flights 301 and 603, which together lost a total of 487 people, and the accident of Air France Airbus, Flight AF 447.

Airbus A350 Pitot Static System

A standby pitot tube and two standby static ports are exclusively used to power the Integrated Standby Instruments System on Airbus A350 aircraft (ISIS). Six additional integrated static ports (ISP), or static ports that provide ADIRS 1, 2, and 3, are present (three on each side of the fuselage). Thus, the aeroplane has eight static ports altogether. There are a total of six static ports in the A320 and A330. For the captain, the first officer, and two standby static ports, there are a total of four. ADIRS 3 and the ISIS are supplied through the standby ports. There are no distinct static ports that only supply ISIS.

There are three multi-functional Pitot probes (MFP) on the A350, and they can measure total pressure, angle of attack, and total air temperature. The three ADIRS are linked to these three MFPs. An additional Angle of Attack (AOA) sensor is located on the left side of the aircraft and is used for the Stress Alleviation Function (LAF), which raises the wing spoilers to lessen the load on the wings. It checks the wing loads when maneuvering at high angles of attack using AOA data. Although it doesn't really send data into ADIRS 1, this AOA sensor is connected to it and uses it for anti-ice functionality. In contrast to the A350's multipurpose probes, the A320/A330 has three angle of attack sensors and three pitot tubes. They are all linked to ADIRS 1, 2, and 3 and utilized to provide flight data. Pitot pressure and AOA data require separate probes because there are no MFPs
The presence of TAT probes in the A320 and A330 is the other distinction. They were two. OAT (Outside Air Temperature) probes are present in the A350. There is no need for additional TAT probes because the MFPs in the A350 calculate TAT. The TAT probes in A320/A330 can also monitor static air temperature (SAT), which is OAT. As a result, OAT probes are not needed. Additionally, both the A350 and the A320/A330 include two ice-detecting sensors.

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