Most maintenance protects equipment. RVSM skin waviness requirements ask you to protect a shape — and that is an unfamiliar idea for organisations used to thinking in part numbers, tolerances and functional tests. A pressure altimeter does not measure height. It infers height from static air pressure sensed at a port on the fuselage skin, and it has no means of knowing when the airflow over that port has been disturbed. Change the local geometry and the system reports a height it believes to be correct.

Why the geometry is part of the design, not a detail of it

CS ACNS.E.RVSM.030 sets out the system performance the installation has to deliver. The automatic altitude control system controls the altitude within ±20 m (65 ft) about the selected altitude, when the aircraft is operated in straight and level flight under non-turbulent, non-gust conditions. The tolerance of the alert issued when the displayed altitude deviates from the selected altitude by ±60 m (±200 ft) or greater is no greater than ±15 m (±50 ft). Where an altitude select or acquire function is provided, no more than ±8 m (±25 ft) of error exists between the value selected by and displayed to the flight crew and the corresponding output to the control system.

Those are tight numbers, and the airframe is part of how they are met. AMC1 ACNS.E.RVSM.030 makes the relationship explicit: if the design and characteristics of the aircraft and its altimetry system are such that the performance requirements are not satisfied by the location and geometry of the static sources alone, then suitable Static Source Error Corrections should be applied automatically within the altimetry system.

Read that sentence as an engineer rather than a lawyer. The correction is derived for a particular surface, at a particular location, with a particular local flow field. In our experience that is the single most useful thing to explain to a technician: the aircraft is not tolerant of geometry changes near a static source because the correction table was built around the geometry that left the factory. There is no feedback loop that notices the shape has moved.

What the rules require after a repair

Two provisions of the EASA Easy Access Rules framework carry the obligation, and they say the same thing from two directions.

AMC1 ACNS.A.GEN.010(b)(3) provides that airframe geometry or skin waviness checks should be performed following repairs or alterations which have an effect on airframe surface and airflow. AMC1 ACNS.A.GEN.010(b)(2) provides that any repairs, not covered by approved maintenance documents, that may affect the integrity and accuracy of the altimeter system — for example those affecting the alignment of pitot/static probes, repairs to dents or deformation around static plates — should be subject to a design review which is acceptable to the competent authority.

AMC3 SPA.RVSM.105(b)(3), in Regulation (EU) No 965/2012, requires the operator’s continuing airworthiness procedures to establish a process to ensure the proper maintenance of airframe geometry for proper surface contours and the mitigation of altimetry system error, surface measurements or skin waviness as specified in the instructions for continued airworthiness, to ensure adherence to RVSM tolerances — and states that these checks should be performed following repairs or alterations having an effect on airframe surface and airflow.

Note the framing, because it is the point this whole series exists to correct. That is an operator-facing requirement in Part-SPA. The operator holds the RVSM approval; the operator cannot demonstrate compliance with it unless a CAMO has written the process and a Part-145 organisation can execute it. We set out the ownership question in full in why RVSM is a maintenance problem, not just an operations one.

The trigger is location and effect, not repair quality

Read the trigger wording once more: repairs or alterations which have an effect on airframe surface and airflow. Nothing there is about whether the repair is structurally sound. A doubler that sits slightly proud, a blend-out that leaves a faint wave, a panel replacement close to a static plate — each can move the sensed static pressure while being a perfectly acceptable structural repair. Structural acceptability and altimetry effect are two separate assessments, and passing one is not evidence about the other.

One distinction worth teaching properly

Engineers repeatedly conflate two numbers, and the confusion is unhelpful in both directions.

• Certification criteria live in CS ACNS.E.RVSM.035. For group aircraft, throughout the full envelope, ASEmean shall not exceed 25 m (80 ft) and ASEmean plus three standard deviations shall not exceed 60 m (200 ft). For non-group aircraft, the limits are 50 m (160 ft) in the basic envelope and 60 m (200 ft) in the full envelope.
• An operational reporting trigger lives in SPA.RVSM.115(a)(2), at an altimetry system error of ±75 m (±245 ft) for an individual aircraft.

There is no 75 m or 245 ft figure anywhere in CS ACNS.E.RVSM.035. Calling ±245 ft “the certification limit” or “the full-envelope ceiling of the ASE budget” is simply wrong, and it misleads engineers into thinking an aircraft measured at 200 ft of error is comfortably inside tolerance when, for a group aircraft, that number is already at the outer edge of what the type was certified to achieve. We take the two apart in detail in RVSM height monitoring and error reporting.

It is also worth being clear about who does the statistical work. CS-ACNS Appendix B provides that, for aircraft being manufactured, every Nth aircraft will be inspected in detail and every Mth aircraft will be flight test calibrated, where N and M are determined by the aircraft constructor and agreed to by the competent authority. Mean-plus-three-standard-deviations tracking for a group is the manufacturer’s activity, not the CAMO’s.

Competence, and where it should be recorded

AMC3 SPA.RVSM.105(c) states that additional training may be necessary for continuing airworthiness and maintenance staff to support RVSM approval, and names three subjects: aircraft geometric inspection techniques; test equipment calibration and use of that equipment; and any special instructions or procedures introduced for RVSM approval. The wording is permissive. Reading a waviness gauge, interpreting a contour measurement and applying the constructor’s tolerances are nonetheless learned skills, not common sense, and in our experience an authority reviewing an operator’s RVSM application will ask how those competencies are held. Our RVSM maintenance training for CAMO and Part-145 personnel course covers all three.

The practical output for a shop floor is a mental map of the area around each static port and probe, and a habit: inside it, ask before you cut. What happens when nobody asks is the subject of the static-port repair that costs an aircraft, and where the check should have been scheduled is covered in what the CAMO must build into the RVSM maintenance programme.

Frequently asked questions