Air: The Primordial Element

Sensing current atmospheric pressure has its own unique challenges. Federal guidelines for aircraft certification stipulate that static ports be located on an aircraft where they are impervious to ice formation along with other contaminants plus be free of disruptions in airflow. Maintaining this situation can be a significant challenge as repairs or modifications to aircraft skin in the area of static ports may alter the air path over the sensing opening. Something as simple as a protruding rivet can cause a static system error. In fact on some high-speed aircraft the inspection criteria for the area around static ports will list requirements for the edges of paint stripes.

It is safe to say that any surface anomalies that could produce a ripple in airflow could be a detriment to the proper operation of the static pressure system. One consideration that is often overlooked is a pressurization leak. Once the aircraft is operating at cruise speed in the upper atmosphere, a differential cabin pressure can result in a high velocity air leak, which if in the airflow path across the static port will result in an altered air-path and a resulting static error.

Reported static system discrepancies that are not replicated during ground tests may well be the result of structurally induced errors. Frequently these errors will vary with airspeed changes. In this case a detailed visual inspection of the area forward of the static sensors would be a good starting point in the troubleshooting process. Like with the pitot system, static pressure is most frequently conveyed by plumbing to the end consumers, being either instruments or an air data computer (ADC).

Electrical Issues

Sensing air temperature is most frequently accomplished electrically. Devices with air inlets that draw a sampling of air and eliminate visible moisture and compensate for the effects of compressibility will allow a metered airflow to circulate across a platinum wire element. This wire is in fact a thermistor with specific and predictable responses to changes in temperature by varying a resistance value. Like most other air data probes the temp sensor may be electrically heated. This of course can be a problem. If the heat is selected on before the aircraft has adequate forward speed, the sensor may quickly exceed its high temperature limit and cause erroneous indications from the devices connected to the sensor. Many of the air temperature sensors contain dual thermal elements. This can be a plus when troubleshooting as the resistance value of each element should be the same and have the same coefficient to temperature change. Often filling a plastic bag with ice and securing it around the probe will provide a means of checking the resistance elements while simulating conditions at higher altitudes.

Testing and Certification Procedures

U.S. Federal Air Regulations (FAR) dictate testing and certification stipulations. FAR 91.411 states that static systems shall be tested for leaks as well as altitude accuracy once every 24 months. It goes on to say that leak tests should be performed anytime the static system is opened. The exception is the removal of static system moisture drains. The procedure for conducting the tests can be found in FAR Part 43 Appendix E. Airframe manufacturers’ tests will take precedent as long as all the basic elements of the Appendix E checks are carried out.

In recent years changes in navigation performance have resulted in a means to allow more aircraft to occupy existing airspace at the same time. Reduced vertical separation minimums (RVSM) will allow aircraft flying between 29,000 and 41,000 feet to fly with 1,000 feet of vertical separation. Prior to this initiative vertical spacing was maintained at 2,000 feet.

Any aircraft having planned flights in RVSM airspace will first have to qualify to be there. Part of the qualification is to demonstrate the accuracy of the altitude indicating system along with implementing a very accurate static correction. In studies conducted by the FAA, the vast majority of static errors are the fault of disrupted airflow over the static ports. A big part of the qualification process is verifying the fuselage skins and structure forward of where ambient pressure is monitored. In some cases, this involves complex skin mapping procedures and techniques.