Section 107.15 says no person may operate a civil small unmanned aircraft system unless it is in a condition for safe operation. Prior to each flight, the remote pilot in command must check the system to determine whether it is in that condition. No person may continue the flight when he or she knows or has reason to know that the system is no longer in a condition for safe operation.
The rule
The check is the remote pilot's, and it happens before each flight. The regulation does not send the aircraft to a certificated mechanic for an annual inspection, and it does not say a current registration makes an unsafe aircraft legal to fly. The remote pilot who finds a defect and launches anyway is the person §107.15(a) addresses. The remote pilot who discovers the defect after launch is the person paragraph (b) addresses: land, do not continue.
AC 107-2A paragraphs 7.2.2 and 7.2.3.1 put the same idea in maintenance language. When preflight discovers a need for unscheduled repair, including after a mishap, flight operations should not occur until the issue is corrected. If the small unmanned aircraft or a component cannot be returned to a condition for safe operation, replace the aircraft or the component. A crack that "might be fine for one circuit" is still an unsatisfactory condition. Whether the part was already due for a scheduled replacement does not decide it. The remote pilot in command is the person who makes the no-go call. The longer preflight list in §107.49, including weather and the control link, is on preflight familiarization.
How to apply it
Inspect what can fail in the air: propellers, motors, arms, the battery and its connector, the control station, and any payload that could shift. A hairline crack in a propeller is enough. The correct action is to replace the propeller before flight. Powering on only proves the electronics woke up. Flying gently still flies the cracked blade. If you have a spare that is in a safe condition, install it and inspect the installation. If you cannot return that propeller to a safe specification, do not fly that part. Use another aircraft that is in a condition for safe operation, or end the launch.
The same standard applies after a hard landing. If you know or have reason to know the system is no longer safe, paragraph (b) says the flight does not continue. Taxiing it home on a shaky motor to save the walk is continuing flight. Set it down. Then repair or replace before the next launch. Record the work in whatever system you use so the next preflight is not a guess. Ordinary Part 107 recordkeeping is recommended practice and is covered with small UAS maintenance. It is a different duty from the Category 4 records in §107.140.
A cracked blade on the first flight of the day
The propeller looks almost right. A hairline runs from the hub toward the tip. The aircraft is registered, the battery is full, and the client is on site. Section 107.15 still requires a condition for safe operation. You replace the propeller. If the spare is also nicked and you cannot bring either blade back to a safe specification, you do not fly a short circuit "to see whether the vibration is acceptable." You replace the component or the aircraft with one that is safe, or you do not fly.
Halfway through a later flight a motor starts to growl and the aircraft yaws under power. You have reason to know the system is no longer in a condition for safe operation. You discontinue the flight. The inspection and repair after you are on the ground are unscheduled maintenance. They do not retroactively authorize the minutes you would have kept flying.
The trap
The rule item offers an annual inspection by a certificated mechanic as the §107.15 requirement, and it offers registration as permission to fly an unsafe aircraft. The section requires a safe condition and a check by the remote pilot before each flight. The crack item offers "ignore it if the drone still powers on" and "fly gently." Both leave the cracked propeller on the aircraft. Replace it before flight.
The advisory-circular item offers a short test circuit, or a theory that the damage matters only when the propeller was already due for scheduled replacement. Paragraphs 7.2.2 and 7.2.3.1 say not to operate until the condition is corrected, and to replace the aircraft or the component when it cannot be made safe. Section 107.15(a) is the regulation under that guidance. A test hop is operation.
Take the no-go items on the free practice test. The course pairs §107.15 with the preflight list so a cracked propeller is a stop, not a technique.