During the preflight check a remote pilot notices one motor arm has a hairline crack. The aircraft still hovers normally in a test. What is the correct action?

Part 107 requires the remote pilot in command to check the aircraft before flight and prohibits operating it with a discrepancy that would affect its safe operation. A hairline crack in a motor arm is exactly that: the arm carries the thrust and torque of a motor at the end of a lever, and a crack is a stress concentration that grows under cyclic loading. The seductive part of this scenario is the successful test hover, and it is worth naming why the reasoning fails. A hover is the gentlest loading the airframe ever sees — no wind loading, no acceleration, no manoeuvring, minimal duration. The crack will propagate under the conditions that follow, not under the conditions that tested it, and the failure when it comes is sudden and total: an arm separating means the loss of a motor and immediate loss of control. The rule does not ask whether the aircraft flew; it asks whether a discrepancy affecting safe operation exists.

Part 107 prohibits operating with a discrepancy affecting safe operation, and a cracked motor arm is a stress concentration that propagates under load toward sudden structural failure.

Part 107 requires a preflight check of the aircraft and control station and prohibits operating with a known discrepancy affecting safe operation. Structural cracks propagate under cyclic loading toward sudden failure.

A Hover Proves Nothing. It is the gentlest thing you will ever ask that arm to do.

Watch for options offering mitigations — shorter flights, observers, reduced payload. None of them repair a structural defect.

• •Accepting a successful test hover as proof of airworthiness
• •Substituting operational limits for a repair
• •Continuing because the mission is already scheduled