RVSM approval consists of 3 distinct processes.
1) Airworthiness (equipment) approval
2) Maintenance Approval
3) Operational Approval
First you need an airworthiness approval (STC or SB
typically) for the equipment and systems installatons that will allow auto-pilot coupled flight in RVSM airspace that will provide the required accuracy. The automatic control system must keep the aircraft altitude within ±20 m (±65 ft). This typically includes the installation of Micro-Air Data Computers (MADC) and sometimes new autopilot components. An altitude alerting system is also required.
The air data system must also be analyzed and sometimes modified prior to determining the Static Source Error Correction (SSEC) for the system that will eventually be used to provide altitude correction. Some airframes have excessive "skin waviness" around the satic ports to provide reasonably accurate static pressure measurements so modifications or even new static ports may need to be installed.
SSEC is determined by flight testing and analysis. You need a minimum of 5 airplanes for "group approvals" if you want the data to be applicable to the entire serial number range of the aircraft model.
The flight test procedure is quite extensive and invloves having the aircraft fitted with a trailing cone static air reference.
A maintenance program is developed based on the required modifications. The maintenance program must include any special instructions dealing with the new equipment and the airframe surrounding the static ports. Only RVSM qualified technicians can work on the air data system of RVSM airplanes.
Once the equipment is approved, a GMU flight (GPS Monitoring Unit Flight) is performed to verify performance on each airplane.
Once the GMU results are verified, operational approval can be applied for. The approval is provided in the form of a Letter of Authorization (LOA).
The vast majority of RVSM approvals are based on OEM developed data because of the complexity and cost of development.