The aerobatic box is an imaginary cube in the sky where competition flights take place. On the ground there are markers the competitor can see – corners, middles and center “tic marks” – made of everything from white vinyl or painted plywood to permanent markers cast in concrete. Often IAC chapters will have a marked box available to their members for practice and coaching sessions too.
IAC's box graphic.
Thanks to IAC Chapter 26 for this description!
The square box marked on the ground is 1,000 meters, or 3,281 feet, on each side. At a contest the judges sit between 500’ and 800’ from the box edge. From left to right is the “x-axis”, with the “y-axis” directly across the box in front of the judges line. There is a buffer area on the edges of each side of 164’, and it is at the corners of this unmarked buffer line that the boundary judges sit and penalties assessed for flying outside the box plus buffer. For most contests, there will also be a 500’ “crowd line” on all sides where non-participants must remain behind and a FAA-“waivered” area of various dimensions, but often a one-statue mile radius, surrounding the box area permitting aerobatic maneuvers outside of Federal Aviation Regulations (FARs).
In the air, the box height, or altitude, above ground level (AGL) varies by competition category, with more experienced pilots flying closer to the ground. For Primary and Sportsman, the bottom of the box is 1,500’ AGL and the top is 3,500’ AGL. For Intermediate, the bottom lowers to 1,200’ AGL, with the top remaining at 3,500’. Advanced has the bottom set at 200m (656’) AGL and the top at 1100m (3,609’) and Unlimited can fly as low as 100m (328’) AGL and as high as 1000m (3,280’). These are the “power” limits, and there are different limits for glider competition. (Meters are used to align with international competition standards for Advanced and Unlimited.)
For all categories, the lower limit is strictly enforced for safety. Competitors flying too low in their category will be penalized, the low figures given hefty penalties, or the entire flight program being zeroed. It is best to give oneself a margin of error, rather than risk being thought too low. Perception can be a factor here, with larger planes appearing lower than smaller planes, i.e. a Great Lakes vs. a single-seat Pitts.
During contest flights, to familiarize the judges with the different lower limit sight pictures, a competitor will be asked to fly the “low lines” at the beginning of each category. This involves the pilot either flying front, back and center, or center and center on the x- and y-axis at the lowest altitude allowed. Traditionally the pilot will wag their wings as they fly over each box marker, so the judges get a feel for the edges and center of the box since they can’t see the actual markers on the ground. The upper limits are not flown.
For the first-time competitor, flying in a marked contest box can be a challenge. Not only must the aerobatic figures be flown correctly, they also must line up with the specified x- or y-axis. The box markers can be difficult to see sometimes, hidden by bushes or even missing due to surface features below. Other times, the box itself is not aligned to the runway or a local road, and there can be a tendency for the competitor to line up to these prominent features instead of the markers. Staying aligned with the correct axis is a skill that takes time to develop as the pilot needs to use outside references beyond the markers themselves.