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Two commercial airplane accidents in the past two years were linked to maintenance on air data instrumentation. Both led to hull loss. The first accident involved a large commercial airplane and was suspected to be caused by a blocked pitot probe, which measures total pressure (the pressure of the freestream airflow for a specific airplane airspeed and altitude) and provides it to the air data computer for airspeed and altitude calculation. The airplane had been parked outside and was exposed to the environment for an extended period of time without the necessary protective maintenance. An undetected insect nest or other debris was suspected of blocking the probe, which supplied erroneous airspeed indications to the flight crew. The second accident occurred after maintenance personnel installed adhesive tape over the static ports, which are mounted flush with the external skin of the airplane and measure the static pressure - the pressure of still air - to calculate airplane airspeed and altitude. The ports were covered to protect them during polishing of the lower airplane skin. After polishing was completed, the adhesive tape was not removed from the static ports, and it was not detected during subsequent inspections prior to the first flight. With the adhesive tape left installed on the static ports, the flight crew received erroneous information during flight. The false indications contributed to actions by the flight crew that led to the loss of the airplane. Both accidents illustrate the need for careful, detailed inspection when returning an airplane to service after extended downtime. Though the static ports and pitot probes are small and highly reliable parts, maintenance of these items was a critical factor in both accidents.
The manufacturer's Aircraft Maintenance Manual (AMM) provides instructions to help maintenance personnel protect and prevent unintended blockage of the pitot probes and static ports. Chapter 12 provides instructions for using a protective cover over the static port during airplane cleaning and polishing; Chapter 10 provides instructions for using protective covers during normal and prolonged parking.
	Boeing recommends the following for protecting both the pitot probe and static ports on Boeing-designed airplanes during prolonged parking: PITOT PROBE Maintenance personnel should install a protective cover with an attached red streamer. In addition, a red paper tag that reads "PITOT PROBES COVERED" must be attached to the left control wheel in the flight deck.
STATIC PORTS. Maintenance personnel should place one end of a 3-ft piece of orange barricade tape over the static port and secure the orange barricade tape with yellow vinyl adhesive tape. In addition, a red paper tag that reads "STATIC PORTS COVERED" must be attached to the left control wheel in the flight deck.
The red streamers and orange barricade tape are highly visible indicators to maintenance and flight crews during the preflight airplane walk-around that the pitot probes and the static ports are covered and not in flight-ready condition. The red paper tags serve as secondary reminders to the maintenance and flight crews that the pitot probes and static ports are covered and are therefore not airworthy. In the event that maintenance personnel fail to remove the protective cover and the barricade tape from the pitot probes and static ports, respectively, the physical reminders provide a final chance for the flight crew to correct the situation prior to flight. The red streamers, orange barricade tape, and the red paper tags compensate for the small size of the pitot probes and static ports, which are functionally critical to safe flight.
	Adjacent to the static ports on the external skin of the airplane, a fuselage exterior marking reads: STATIC PORTS. DO NOT PLUG OR DEFORM HOLES. INDICATED AREAS MUST BE SMOOTH AND CLEAN. (The marking for Douglas-designed airplanes is identical except for the words "AREA WITHIN RED LINE MUST BE CLEAN AND SMOOTH" instead of the words "INDICATED AREAS MUST BE CLEAN AND SMOOTH.")
This exterior marking is an additional reminder to maintenance personnel and the flight crew during their preflight walk-around about the safety implications of the static ports. To increase safety awareness, Boeing has developed a safety poster intended to remind maintenance personnel about the need to remove coverings from the pitot probes and static ports. The poster is available from Boeing Field Service representatives. In addition, the maintenance planning document specifies visual inspection of the static ports and pitot probes as well as the total air temperature probe and angle-of-attack vanes during a transit check prior to the next flight. (Some airplanes have a pitot-static probe instead of a pitot probe. A pitot-static probe incorporates a static port into the side of the pitot probe, allowing the combined part to measure both total pressure and static pressure.)
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