Pitot Tube Replacement

Pitot tube replacement is an important aircraft mechanic job, and requires knowing the fundamentals of  Pitot-Static tubes.  The basic Pitot tube was invented by Henri Pitot in 1732.  Despite its ancient lineage, it is still useful today.  Pitot-Static tubes (also called Prandtl tubes) are used on aircraft as “speedometers.” The actual tube on an aircraft is usually around 10 inches (25 cm) long with a 1/2 inch (1 cm) diameter.  Several small holes are drilled around the outside of the tube and a center hole is drilled down the axis of the tube. The outside holes are connected to one side of a device called a pressure transducer.

Pitot Tube

Pitot tube Airspeed Probe, Airbus A380. Credit Wikipedia, David Manniaux

Pitot tube replacement

Pitot and airplane

The center hole in the tube is kept separate from the outside holes and is connected to the other side of the transducer. The transducer measures the difference in pressure in the two groups of tubes by measuring the strain in a thin element using an electronic strain gauge.  The transducer then sends a current which reflects the pressure difference. The pitot-static tube is mounted on the aircraft so that the center tube is always pointed in the direction of the air flow and the outside holes are perpendicular to the center tube.

On some airplanes, the Pitot-static tube is put on a longer boom sticking out of the nose of the plane or the wing.  The overall approach to obtaining velocity from the measured pressure difference is given below,  courtesy of the NASA Glenn Research Center.  Just bear with me, the math is not that hard!

Pitot Tube Principles

Credit NASA - Glenn Research Center

Dynamic Pressure is the difference between Static and Total Pressure

Since the outside holes are perpendicular to the direction of flow, these tubes are pressurized by the local random component of the air velocity. The pressure in these tubes is the static pressure  (ps) discussed in Bernoulli’s equation. The center tube, however, is pointed in the direction of travel and is pressurized by both the random and the ordered air velocity. The pressure in this tube is the total pressure  (pt)  in Bernoulli’s equation. The pressure transducer measures the difference in total and static pressure which is the dynamic pressure  q.

measurement~=~q~=~pt~-~ps

We solve for Velocity

With the difference in pressures measured and knowing the local value of air density from pressure and temperature measurements, we can use Bernoulli’s equation to give us the velocity. Here, and on the graphic, the Greek symbol rho is used for the air density. Bernoulli’s equation states that the static pressure plus one half the density times the velocity V^2  is equal to the total pressure.

ps~+~.5~*rho~*V^2~=~pt

 

Solving for V:

V^2~=~2(pt-ps)/rho

V~=~sqrt{2(pt - ps)/rho}

Thus,  by knowing two pressure values, the velocity can be determined in real time.

Pitot-Static Tubes and Pitot Tube replacement

Now that we know the central importance of the Pitot tube measurements, we can see how the real time measurements fit within the Pitot-static system, as shown here.

Pitot-Static system

Pitot-Static system. Credit FAA

Pitot Tube replacement

An aircraft mechanic performing a Pitot tube replacement on an airplane is described in the following video.  ( Courtesy of Sander Lampe and http://aircraftmech.com )

The importance of a functioning Pitot tube

 

Air France A330-200 F-GZCP. Credit Wikipedia, Photo by Pawel Kierzkowski

On June 1, 2009, Air France Flight 447, an  Airbus A330-200 aircraft, took off on a scheduled airline flight from Rio de Janeiro to Paris.  Two hours later, Flight 447 crashed into the Atlantic Ocean, killing all 216 passengers and 12 aircrew.   A theory,  now widely accepted,  is that ice formed on the pitot tubes, which would have caused them to freeze over,  giving inconsistent airspeed measurements, owing to the reliance on air pressure measurements to give aircraft speed.  Icing would have interfered with accurate pressure determinations.

Reports in May and July 2011 from the BEA stated that the aircraft crashed following an aerodynamic stall.  Several minutes before the crash, the pitot tubes  started to give inconsistent readings, so the autopilot disconnected. The plane rolled slightly and in response, the pilot adjusted the nose of the aircraft back. The pilot continued to pull back on the stick, producing a stall, continuing this action even while the stall warning sounded continuously for 54 seconds.  Subsequent detailed analysis of the weather conditions for the flight shows it is possible that the aircraft’s final 12 minutes could have been spent “flying through significant turbulence and thunderstorm activity for about 75 mi (121 km)”, and may have been subjected to rime icing, and possibly clear ice or graupel.

Pitot tube blockage is also suspected of having contributed to other airline crashes in the past — such as Birgenair Flight 301 in 1996 and Aeroperú Flight 603, also in 1996.  In the case of Aeroperú Flight 603,  instrument failure caused the airplane to crash at sea and all nine crew members and sixty-one passengers died. The cause of the Aeroperú Flight 603 instrument failure was an aircraft maintenance worker’s failure to remove duct tape covering the static ports necessary to provide correct instrument data to the cockpit. Although the Pitot tube itself was not covered over with tape, the static ports were covered, rendering them ineffective.

Airbus has since replaced many of its Pitot tubes on other Airbus aircraft with heated versions, hoping to avoid the Flight 447 accident scenario in the future. Although the Flight 447 crash was caused by bad weather and pilot error,  it is important for aircraft mechanics to realize the importance of  Pitot tubes for safe aircraft travel.

by Steve Adams

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