RVSM or Reduced Vertical Separation Minima was introduced to allow aircraft in certain flight level airspace to operate with a reduced physical separation of 1000 feet instead of the previous 2000 ft that was the requirement for that airspace. RVSM was made possible due to the technical improvements of the altimeter systems which made the altitude readings from the equipment more accurate.
To ensure that the aircraft can maintain the altitude accuracies required for RVSM approved aircraft, the aircraft must comply with several avionics-related technical orders and pass a Skin Waviness Inspection. This inspection ensures that the static ports and the areas around the static ports have not been damaged or distorted in any way. Any waviness, irregularities or damage in the aircraft skin around the static ports could change the airflow over the static ports in flight and will create inaccuracies in the critical altimeter readings from the static ports.
These skin mapping checks are required to be performed every 24 months or following unscheduled repairs or alterations or if visual inspections indicate any structural damage that would have an effect on the airframe contour in the areas surrounding the static ports.
RVSM Skin Waviness Inspection Procedures
The equipment that is needed to perform the Skin Waviness Inspection are a Skin Mapping Reference Bar, a Digital Indicator type of Depth Gauge, a 24 in. Flexible Steel Ruler and a Dry Erase Marker.
First, a reference grid is drawn on the aircraft skin around the upper and lower static ports. The grid is aligned with the centerline of the static ports and contains reference dimensions that will be used for the mapping. The grid extends approximately 14 to 24 inches in front of the static ports depending on the aircraft type.
The Depth Gage is placed within the rail of the Skin Mapping Reference Bar and the bar is placed on horizontally on one of the grid lines.
The Depth Gage is zeroed at one end of the bar. The Depth Gage is then moved across the bar in 1-inch increments and the depth variations from the gage are recorded, or in some cases, the measurements from the gage are sent to a computer program. The procedure is then repeated for each of the gridlines drawn on the aircraft.
The Depth Gage is then removed from the Skin Mapping Reference Bar and is used to measure the depth of any skin overlaps in the grid area. The Depth Gage base plate is placed at the upper layer of the fuselage skin and zeroed. The Gage is moved to the lower layer and the reading is recorded to measure the gap between upper and lower fuselage skin on the overlap.
The measurements obtained are used to determine the slope of the aircraft’s skin in reference to the location of the static ports. All of the data collected during the skin mapping procedure and the resulting calculations must be within the minimum and maximum allowable thickness specifications, outlined in the aircraft manufacturer’s maintenance manuals or service bulletins for each horizontal skin contour grid line, including any skin overlap that occurs in the grid area.
If the measurements are outside of the manufacturer’s specifications, then structural repairs will need to be made. Depending on the measurements obtained and condition of the aircraft skin, those repairs could range from minor surface treatments to even the surface, applying patches to restore the proper contour to the area or even replacement of the aircraft’s skin, which is a very lengthy and costly procedure.
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