Later technology introduced Teflon® and plastic lines and fittings. Plastic tubing, although durable, may not possess the same heat resistance as metal lines. In the event the plastic should come in contact with some type of electronic component that produces heat, significant damage or even failure could result. Just like rigid plumbing, flexible tubes are prone to damage if proper bend radius is not properly observed.
Appropriate documentation should always be referenced prior to conducting maintenance. Correct techniques must be utilized during disassembly and re-assembly to prevent damage. In a non-pressurized aircraft, the integrity of static lines may not be a significant issue as cabin inside pressure is essentially the same as outside. A proof test of the plumbing is required per FAR Part 23.1325 and for non-pressurized aircraft, the static system should have a suction applied that will lift a column of mercury approximately one inch. This is equivalent to a reading on the altimeter of about 1,000 feet above present field elevation. Then, after one minute without additional suction, the column of Mercury should not drop more than ten percent (that is about 100 feet on the altimeter).
In an aircraft with a pressurized cabin, the testing is a bit more stringent. The static system has to be tested at a pressure differential equivalent to the cabin maximum differential pressure. An aircraft that has a differential of around eight pounds per square inch can typically maintain a sea-level cabin up to an altitude of 23,000 feet.
In this case, the static system would have its pressure adjusted to equal an altitude of 23,000. With the suction removed, leakage should be less than two percent of the testing altitude or 100 feet, whichever is greater. The aforementioned tests are part of the certification process and should not be substituted for testing procedures called out in specific aircraft maintenance manuals. They are however, a good way to validate the integrity of the plumbing.
Each airspeed indicating system must also be calibrated. United States FAR 23.1323 provides a guideline for determining system error. In this case, three percent of calibrated airspeed or up to five knots is considered acceptable. Defects in plumbing, such as those discussed earlier, can create unacceptable system errors.
Neither snow, nor rain, nor heat
Pitot systems installed on aircraft certified for Instrument Flight Rules (IFR)are required to have some provision to prevent ice formation in the sensor as well as provide moisture drainage. In some cases, aircraft flying in intense rainstorms may encounter situations where the pitot tubes ingest significant amounts of water. Often, the wattage of the pitot tube heating element may be determined not only on the ability to prevent ice build-up, but also to be able to help relieve water saturation. In all cases, caution should be exercised when operating the heating circuits as personal injury or physical probe damage can result.
A moisture-laden pitot-static system can also result if the aircraft is washed without using specifically designed protective covers for all air data inlets. These covers should be frequently inspected to verify proper fit and function as damaged covers could promote rather than prevent the flow of water into the air data system.
One result of excess water ingress in a pitot system may be that freezing temperatures might cause obstructed airflow. This would result in the airspeed indicator having a constant pressure available even though the aircraft speed may change significantly. Although most aircraft manufacturers install moisture drains in air data systems, they might not be in the exact location where moisture may accumulate under all circumstances.
An effective method of moisture removal is to disconnect all components attached to the system and cap the lines. Introduce very low pressure nitrogen, then slightly loosen the caps to allow a slight flow, with the majority of the nitrogen exiting through the pitot or static ports. By allowing a constant flow for up to 15 minutes, the majority of moisture can be evacuated.
Another common fault is air data system leakage. Should the leak occur within the cabin of a pressurized aircraft, the altitude will often appear lower than actual and the error will vary with both cabin pressure differential and aircraft altitude.
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