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.
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