Basic for flight
By Jim Sparks
Air is one of the basics needed to support life. In fact for most of us, it is also the basis for our profession. After all, without air, there would be no aircraft. Going back to the first days of A&P school, we learn that when there is adequate airflow over an airfoil, the result is lift. How does a pilot know when the aircraft is capable of sustaining flight? It is not by coupling a speedometer to a wheel on the landing gear. It is of course by comparing the ram air pressure generated by aircraft forward movement to relative atmospheric pressure. As lift increases, flight is achieved and height is measured by sensing the air pressure of the atmosphere. Obviously, at higher altitudes, there is less air pressing down, resulting in a lower air pressure. The pilot will utilize the airspeed indicator and altimeter to determine appropriate aircraft speed and relative position in the atmosphere. A dedicated system is needed to supply the various pressures to the components used to calculate and display this information to the flight crew. Plumbing, consisting of either rigid or flexible tubes, is routed strategically to bring the specific air pressures to the necessary equipment.
What could possibly be complicated with a system such as this? Many Airworthiness Authorities around the world have adopted a policy of verifying the integrity and accuracy of altitude indicating systems every 24 months. In the United States, this requirement is listed in FAR 91 section 411. Many aircraft today are complying with Reduced Vertical Separation Minimums (RVSM), which will allow aircraft to maintain 1,000 feet vertical separation rather than 2,000 feet in certain areas. In this situation, errors resulting in the static system have to be closely monitored.
Proper tube installation is critical
Part 23 (Airworthiness Standards), of the Federal Aviation Regulations (FAR), lists numerous situations that should be avoided to insure integrity of the system. The design and installation of a static system should provide for positive moisture drainage. Proper tube installation is critical. First, the material used should be suitable for the application — many aircraft utilize rigid aluminum tubing. This of course should always be inspected for the obvious chafing or dents. Anytime a dent is observed, cracks should be expected. In all cases, this should be checked with a magnifying glass. As tube bending results in a reduction in material thickness, all bend radius should be considered as potential for leakage. Another factor to consider when replacing a rigid line is the type of material. Some alloys have different degrees of porosity, which means small holes may open anytime the metal is stretched (once again bending is an issue).
Corrosion is another concern with metal lines and in accordance with FAR 23.1325, corrosion protection should always be applied. Also, certain adhesives may react with certain alloys that can create a situation where corrosion occurs when a replacement tube needs to be manufactured or an identification decal needs to be applied to an existing line. Flexible lines are also widely used in air data systems and just like in their rigid system counterparts, numerous types of plumbing are used. Older aircraft often used synthetic rubber lines. Components of this type were often resilient to damage and provided ease of removal and installation of attaching equipment.
One drawback to the use of synthetic rubber is that it tends to break down with age. Most aircraft using these hoses have a mandatory time change or life limit.
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