Wednesday, September 9, 2009
Blocked Pitot Tubes
When accessible, pitot tubes and static ports should be checked in every preflight
ENSURE PROPER PITOT-STATIC READINGS. Prevent pitot tubes from malfunctioning and providing incorrect airspeed readings by clearing them of obstructions and blockages.
In addition to participating in the French government’s investigation, the NTSB began looking into two incidents in which erroneous airspeed and altitude indications were reported on Airbus A330 airplanes. An incident on June 23, 2009, involved a Northwest Airlines flight en route from Hong Kong to Tokyo. Another incident, on May 21, 2009, involved Brazil’s TAM Airlines Flight 8091, which was en route from Miami to Sao Paulo, Brazil. Initial reports indicated that the flight crew noticed an abrupt drop in outside air temperature, which was accompanied by St. Elmo’s fire and followed by the loss of the air-data reference system (ADRS), disconnection of the autopilot and autothrust, and loss of primary airspeed and altitude information. The crew used backup instruments; after about five minutes, primary data was restored. The flight landed safely in Sao Paulo.
Whenever they’re accessible, pitot tubes and static ports should be checked for obstructions as part of every preflight. Blockages may be due to moisture (water or ice), dirt, insects, debris or failing to remove protective covers. The airspeed indicator is a differential pressure gauge or electronic device that measures the difference between pitot (dynamic or impact air pressure) and static pressure. When the aircraft is stopped on the ground, the pressures are equal and the airspeed indicator shows zero airspeed. When the aircraft moves, the pressure in the pitot line becomes greater than the static pressure, and the instrument begins to register airspeed. If changes in dynamic pressure can’t be transmitted in the pitot line because of a blockage, then the airspeed indicator has only what’s trapped inside to compare with static pressure. In a descent, static pressure increases, causing the airspeed indicator to register a decrease in airspeed. In a climb, as the static pressure decreases, the trapped dynamic pressure becomes stronger in comparison to the static pressure, so the indicator shows airspeed as increasing.
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