The minimum fit. 

To be approved for RVSM, an aircraft must carry a specific minimum equipment fit — the required functions set out in CS ACNS.E.RVSM.010 and listed in SPA.RVSM.110, the RVSM equipment requirements. There are four pillars, and you maintain all of them. First, two independent altitude-measurement systems. Each one samples free-stream static pressure through its own static source — typically cross-coupled static ports with ice protection — and converts that pressure to a pressure-altitude output, applying static-source error correction where needed. Two independent systems give redundancy and a means of cross-checking. Second, an altitude-alerting system — it warns the crew if the aircraft drifts from its selected level. Third, an automatic altitude-control system, the altitude-hold function that physically keeps the aircraft pinned to its assigned flight level far more precisely than a human could. And fourth, a secondary surveillance radar transponder with altitude reporting — Mode C or Mode S — connected to the altitude-measurement system in use for altitude control, so that the level the aircraft reports to ATC is the level it is holding. One more thing to hold in mind: CS-ACNS does not just list these functions, it certifies them to defined safety levels. CS ACNS.E.RVSM.020 sets the integrity requirement — how unlikely an undetected altimetry error must be — and CS ACNS.E.RVSM.025 sets the continuity requirement — how reliably the system must keep working. You do not demonstrate those levels yourself; a Part-21 design organisation does, at certification. But every leak check, contour check and calibration you perform exists to keep the aircraft at the integrity and continuity levels it was certified to. That is the condition you are preserving.

The error budget. 

Here is the concept that makes RVSM maintenance different from ordinary instrument work: the Altimetry System Error budget, or ASE. ASE is the difference between the pressure altitude the system displays and the pressure altitude the aircraft is actually flying. For RVSM, that error is held to a tiny budget. For a group of aircraft, the mean ASE must stay within roughly eighty feet — twenty-five metres — and the mean plus three standard deviations within two hundred feet, sixty metres, in the basic envelope, widening only modestly to two hundred and forty-five feet in the full envelope. Every defect you deal with eats into that budget. The error budget is built from static-source error, residual correction error, and avionics tolerances. The single biggest practical contributor — the one the altimetry-accuracy provisions of CS ACNS.E.RVSM.035 and its AMC concentrate on — is the static-source system and the airframe geometry around it.

Why geometry is safety-critical. 

A pressure altimeter does not measure height directly. It infers height from static air pressure. Anything that disturbs the airflow over the static port changes the pressure the system senses — and the system has no way to know it is being lied to. A dent, a wavy skin panel, a repair patch that sits slightly proud, a misaligned probe, a leaking static line: each one shifts the sensed pressure, and therefore the displayed altitude, by an amount that can blow the ASE budget. This is the mental model to carry into the rest of the course. The avionics boxes are reliable and self-checking. The vulnerable part of an RVSM aircraft is the air-data path — the holes in the skin, the lines behind them, and the shape of the metal around them. Those are maintained by hand, by you, and they are the reason RVSM has a dedicated maintenance discipline at all. In the next lesson we move from the what to the who and how — starting with the CAMO and the maintenance programme that holds all of this together.