LMML 14/32: I will try to give you a short rundown on most of the required equipment for RVSM operations. 2 independent altitude measurement systems, each with a mean system error of less than 80 ft in normal operation, cross-coupled static sources, automatic altitude reporting, automatic altitude control with an accuracy of + or - 65 feet, 1 altitude alerting system, reference signals for autopilot and alerting systems, static source error correction (if needed), 1 altitude reporting transponder, 1 automatic altitude control system which is referenced to pressure altitude, and several other items of equipment, such as auto throttle control, that may vary by operator and aircraft type.
The process begins by measuring the fuselage around the pitot probes for "waviness." A tool is installed, and the skin is measured. These figures are submitted to the aircraft manufacturer which runs the numbers to calculate an average flatness for the area. If the skin is not flat to within certain limits, the skin around the pitot probes will need to be shimmed until the criteria is met. Then, in the case of some 747's for example, new pitot probes with a nickel-plated 10" strut will replace the older model 4" strut. The area around the pitot probes is defined by a striped line, inside which no unaccounted for dents are allowed, and any dents that do occur inside that area will need to be reported and repaired. It is very likely that the altimeters will need to be replaced with much more accurate units. Some of the newer altimeters will incorporate other features required for RVSM operation, such as altitude reporting, or a selectable mode that gives the altimeter reading in metric units. The entire pitot-static system must be carefully calibrated, and the air data computers and autopilot system must meet greater accuracy requirements.
Then other systems, such as the transponder altitude reporting feature must be verified to be within certain accuracy limits. The purpose of all these requirements is to guarantee, as much as possible, that the aircraft is actually at its assigned altitude. Obviously, that is a critical requirement in airspace with a separation of 1000 ft vertically.
If you check the picture at Click for large version
Photo © Jonathan Icasas
you will clearly see the markings around the pitot area on this 747.
This information is excerpted from the 747 maintenance manuals, chapter 22 for autopilot, and 34 for the air data systems. Also, a number of supplement pages that accompany the basic manuals. Not all operators will need to add or change their equipment, depending on how their particular equipment was purchased. I believe the numbers are accurate, but of course, are revised continually. Hope this helps, regards,