A launch site sits at a ground elevation of 1,200 feet MSL. Class E airspace in that area begins at 1,700 feet MSL. What is the highest AGL altitude the aircraft can reach before entering Class E?

This item tests the conversion between MSL and AGL, which is the arithmetic remote pilots get wrong most often because charts and airspace floors are published in MSL while Part 107's altitude limit is expressed in AGL. Mean sea level is measured from a fixed datum; above ground level is measured from the surface directly beneath the aircraft. To find how much room exists between the launch surface and an airspace floor, subtract the ground elevation from the floor: 1,700 MSL minus 1,200 MSL leaves 500 feet of vertical space. Note the practical overlay: §107.51 already caps most operations at 400 feet AGL, so the airspace floor is not the binding limit here — the regulation is.

Subtracting the 1,200-foot ground elevation from the 1,700-foot MSL floor of Class E leaves 500 feet of altitude above the surface, so 500 feet AGL is the highest the aircraft can climb before entering Class E. The aircraft would be at 1,700 feet MSL at that point, exactly at the floor.

Airspace floors and ceilings on sectional charts are published in MSL. Part 107's 400-foot limit is AGL. Converting between them requires the ground elevation at the point of operation, which is read from the chart's terrain contours or field elevation.

Floor Minus Ground equals Room. MSL is measured from the sea, AGL from your shoes. Subtract where you stand from where the airspace starts.

The 400-foot answer will almost always be offered on these conversions. Solve the arithmetic first, then decide whether 400 is the real operational limit — it usually is in practice, but it is rarely the answer to the question asked.

• •Answering with the 400-foot regulatory limit instead of the airspace floor
• •Treating a published MSL floor as though it were AGL
• •Adding rather than subtracting the field elevation