The four conversions: the Part 107 questions that are procedures, not facts

Candidates do not fail these because they lack a fact. They fail because nobody gave them a repeatable procedure. Each of the four turns a number on a chart or in a report into a number you can act on, and each has a single step where most of the errors happen.

1. Charted ceiling to height above the ground

You are given a boxed or ringed altitude on the sectional, and the field elevation from the airport data block.

What it means. Both are MSL. Airspace ceilings, floors and terrain are charted in feet above sea level. Your 400-foot limit is above the ground.

The operation. Charted altitude minus field elevation. A Class D box reading 42 over a field at 1,716 feet is 4,200 − 1,716 = 2,484 feet AGL of Class D above you.

The trap. Reading the boxed figure as a height above the field. Bring both numbers to one datum before you compare either to anything.

2. The structure inspection envelope

You are given an obstruction annotated with a bold figure and a second figure in parentheses.

What it means. The bold figure is the top of the obstruction in MSL. The figure in parentheses is the structure’s own height in AGL.

The operation. Structure height plus 400. A tower marked 1036 (453) supports 453 + 400 = 853 feet AGL — and only within a 400-foot radius of it (14 CFR 107.51(b)).

The trap. Adding 400 to the MSL figure. And both conditions bind: inside 400 feet laterally and no more than 400 feet above the top. Fail the lateral one — say, 500 feet from the tower — and the plain 400-foot limit returns.

3. Cloud base, then the legal ceiling

You are given surface temperature and dew point, in degrees Fahrenheit.

What it means. As air rises, temperature and dew point converge at about 4.4 °F per 1,000 feet, and cloud forms where they meet. This is the FAA’s own method, in the Pilot’s Handbook of Aeronautical Knowledge, chapter 12, and it is stated in Fahrenheit. A Celsius version circulates widely; it is not in the handbook.

The operation. (Temperature − dew point) ÷ 4.4 × 1,000 = cloud base in feet AGL. Then subtract 500 for the required clearance below cloud (14 CFR 107.51(d)). At 82 and 60 °F: 22 ÷ 4.4 = 5, so 5,000 feet AGL.

The trap. Stopping at the cloud base, or subtracting 500 and stopping there. Two limits apply at once and the lower governs. A base at 700 feet leaves you 200, not 400. A base at 1,500 feet leaves you 1,000 by the cloud rule, but the 400-foot limit is lower, so 400 governs.

4. Runway number to heading to reciprocal to pattern side

You are given a runway number, and a radio call naming a leg and a direction of turns.

What it means. A runway number is its magnetic heading with the final zero dropped. Downwind is flown on the reciprocal.

The operation. Number × 10 = heading. Add 180 = downwind track. In left traffic the runway lies off the pilot’s left wing, so the aircraft is on the opposite side. “Midfield left downwind, Runway 13” → heading 130 → downwind 310 → runway to the pilot’s left, which on a 310 track is south-west → the aircraft is north-east of the runway.

The trap. Answering from the chart’s overhead view instead of the pilot’s. The call describes where the pilot is looking from, not where you are standing.

---

Every figure above is worked from the cited FAA source. The same four procedures, with the traps, are on our decoders page, and the bank drills each one — because a procedure is what survives when the figure in front of you is one you have never seen.

The decoders page, including the four conversions →