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.
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.
Location, location, location
The location and attachments of pitot and static system sensing devices is every bit as critical as the condition of the plumbing. Pitot tubes are mounted on the aircraft in such a way that they are always sensing uninterrupted airflow relative to fuselage at a specific angle. In fact, on Transport Category Aircraft where independent air data systems are required for pilot and co-pilot, the location of air data probes on the fuselage have to be such that a single bird strike will not take out both systems.
RVSM and static pressure sensing
Reduced Vertical Separation Minimums (RVSM) has played an important role in the maintenance and inspection of static pressure sensing. Aircraft that wish to take advantage of the most frequently used altitudes in the high-density tracks over the North Atlantic have to have a very accurate altitude indicating system. Part of the process is to check the condition of the aircraft fuselage forward of the static ports. A ripple in the skin, a rivet protruding into the airstream, or even a build-up of paint at the leading edge of the static atmospheric pressure sensor can cause a discrepancy in the airflow over the static port and introduce a source error. This error will often vary with changes in airspeed or aircraft deck angle. In the event of a reported discrepancy from the crew regarding indicated altitude, one of the first questions asked should be if aircraft speed or attitude has any affect on the reported problem. Part of the qualification for the aircraft to be RVSM compliant is a specific Maintenance and Inspection Program, which may require personnel to attend special training programs. Depending on aircraft registration and mode of operation (FAR 91,121,135), not just any A&P can sign off maintenance on the static system. Also, any damage to the fuselage forward of the static ports may negate RVSM compliance. Airframe manufacturers frequently will locate static ports on the forward fuselage just after the removable nose compartment. An inappropriately secured nose compartment could conceivably lead to significant static source error. In the event testing with a pitot static test set cannot reveal any sign of problem, static source error should be considered and should involve detailed inspection of the area forward of the static ports.
In many aircraft, static sensing ports are installed on both sides of the fuselage. This provides a redundancy if one port becomes plugged but also provides a truer static sense in the event of a prolonged yaw condition.
Another regulation that comes to mind during the discussion of Altitude Indicating Systems is FAR 43 Appendix E. This is an Altimeter System Test and is divided into two parts. The second part deals strictly with the altimeter and is often accomplished in an approved shop, with the altimeter installed on a test bench.
The first part of the test involves the aircraft system and should include a comprehensive visual inspection to make sure the system is free of moisture and that the plumbing is not restricted or damaged. Next, the system is pressurized and leakage is monitored. Another requirement is a visual inspection of the airframe surface in the area around the static source that could introduce an error. Finally if the static port has a heater installed, a functional test is performed to verify proper operation. FAR 91.411 will even refer back to redoing the system leak test anytime a component or line is disconnected — except for the opening of water drains or activation of an alternate static source. Strangely enough, on many aircraft, the leading cause of Air Data leaks are malfunctioning moisture drains.
It is a good maintenance practice to leak check the system anytime anything is opened.
With troubleshooting pressure sensing systems, a good flight crew debriefing can really be a time saver. When does the problem occur? Are cabin pressure or aircraft attitude a factor? Also ask about flying conditions such as rain and if the aircraft was recently washed.
Pitot Static Systems —Simple? Maybe, but there sure is a lot to consider.