Avionics Technology: Air Data Today

It has been confirmed that plating material coming loose at the air inlet can have a significant impact on displayed airspeed. One recent Airworthiness Directive issued by Transport Canada covering the Challenger 600 addresses drain bottles within the pitot system. Under heavy rain conditions the existing bottles can overfill which might result in partial or total blockage. Deficiencies in anti ice elements have caused problems with ram pressure reporting systems as high wattage heaters do provide protection not only from freezing but also rain ingress. Care should always be taken when handling an air data probe when the heating system is energized.

Some newer generation aircraft utilize air data probes that incorporate an electronic module complete with a pressure transducer and a digital output. Typically the functionality of the electronics can be validated anytime the aircraft avionics are powered. These units do allow for a reduction in plumbing but are still subject to deterioration and damage similar to their mechanical counterparts.

Static ports share in some of the anomalies that interfere with pitot systems. Surface defects on or near a sensing port can cause a disruption to airflow resulting in pressure-sensing errors. Even airframe skin deformities and ridges on paint stripes can cause a static defect. 

Leaks

Leaks are another common occurrence in a pneumatic air data system. United States Federal Aviation Regulations (FAR) provide direction for testing and return to service. FAR 91.411 stipulates altitude indicating systems be certified every 24 months and uses criteria found in FAR Part 43 Appendix E. It is important to understand the criteria found within are considered the maximum allowed and often time specific manufacturer’s tolerances are tighter. This can be true not only for the primary altimeters but also the standby devices when required.

Moisture drains are one of the most common culprits when it comes to leaks and per Appendix E; leak testing should be accomplished on air data systems anytime the system is open with the exception of moisture drains. Once again, airframe manufacturers' recommendations may require a leak test anytime system integrity is breached.  

Transponder testing will generally tie into altimeter testing and shares the 24-month certification requirement per FAR 91.413. Certification of ATC reporting equipment is a bit more involved than the altimetry system and is defined in FAR Part 43 Appendix F and Advisory Circular (AC) 43-6C was issued recently to clarify those capable of returning systems to service.

Freedom of movement

Angle of attack (AOA) has been transformed over the past decade from a system that is a nice to have indication into a prime input to flying the aircraft. Some fly-by-wire or automatic flight control systems (AFCS) use this information as primary drivers for determining flight path and detecting wind shear.

The sensors used to detect airflow relative to the aircraft fuselage most frequently appear as a small moveable airfoil or a cone protruding in the airstream. Freedom of movement should be verified on a daily basis as any binding or rough spots may have serious consequences. European Aviation Safety Agency (EASA) has recently issued an Airworthiness Directive against the AOA probes on several Airbus models.

During Airbus final assembly line flight tests, AOA data from two different aircraft were found to be inaccurate which was confirmed by flight data analysis. The results of the investigation conducted by Airbus and their vendor on the removed sensors revealed oil residue between the housing and the moveable part of the AOA vane. This was the result of incorrect removal of machining oil during the manufacturing process and at low temperatures the oil becomes viscous causing delayed and/or reduced vane movement. Multiple sensors could be simultaneously affected, providing incorrect indications of the pitch angle of the aircraft. This condition, if not corrected, could lead to erroneous AOA information and consequent delayed activation or nonactivation of the protection systems.

In short, air is essential to life and characteristics of airflow relative to an aircraft are critical to ensuring safe and reliable transportation. After all, it’s all about AIRworthiness.

Currently when not writing for AMT, Jim Sparks is the manager of aviation maintenance for a private company with a fleet including light single engine aircraft, helicopters, and several types of business jets. You can reach him at sparks-jim@sbcglobal.net.