Sectional chart reference
Class D
Blue dashed lines.
What you are looking at
A DASHED BLUE line = Class D, a single small field WITH a control tower. A dashed line is still a crisp class boundary, just a small, single-airport one.
Mnemonic: 'color follows the tower'. Dashed blue = D = a TOWERED field. Its ceiling sits in a dashed box.
The rule itself
No person may operate a small unmanned aircraft in Class B, Class C, or Class D airspace or within the lateral boundaries of the surface area of Class E airspace designated for an airport unless that person has prior authorization from Air Traffic Control (ATC).
Quoted in full from 14 CFR § 107.41. Authorization normally comes through LAANC.
Reading the numbers
The number in the dashed box = the ceiling, top in hundreds of feet MSL: [25] = 2,500 MSL, 40 = 4,000 MSL.
Where people get this wrong
Dashed blue (D) vs solid blue (B): both blue and both need authorization, but D is one small towered field while B is the big layered hub.
How it is tested
A dashed blue ring shows [25] in a box. What does 25 mean?
- Ceiling 2,500 ft MSL
- Floor 2,500 ft AGL
- 25,000 ft MSL
Show the answer
Ceiling 2,500 ft MSL
The boxed number is Class D's ceiling, in hundreds of feet MSL: 25 → 2,500 MSL.
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