Preflight Procedures
From walk-around to runway
A preflight inspection is a structured comparison of the aircraft against its POH and its own normal condition. You are looking for three things: airworthiness (safety, security, obvious damage), servicing (fuel quantity and grade, oil, tires, brake fluid), and configuration (gust locks removed, controls free and correct, fuel selector on Both/proper tank, trim set).
Engine run-up and systems checks
During the run-up you verify mags, carburetor heat, engine gauges, suction/vacuum, ammeter, and the idle setting. Typical POH limits are a maximum drop of about 150 RPM on one magneto and no more than 50 RPM difference between them; any roughness, or an engine that quits on one mag, means no takeoff. Leaning during the mag check on a high-density-altitude day can clean up fouling - but never lean past the POH procedure just to make the drop pass. Carburetor ice can form at outside air temperatures well above freezing on a clear, humid day, so carb heat application and the resulting RPM drop (then rise) is diagnostic, not decorative.
Taxi and departure briefing
Taxi at a speed that lets you stop within the visible area ahead; use the checklist, keep the sterile cockpit discipline (required for 121/135, smart for everyone), read back all runway-holding and crossing instructions, and brief the departure: runway, initial heading, emergency plan if the engine fails on the roll or at 200 feet, and the first frequency.
Worked example
Mag check: left mag 75 RPM drop, right mag 125 RPM drop, total difference 50 RPM. Both drops are within a typical 150/50 limit - acceptable. Change the numbers to a 160 RPM drop or any roughness and the answer flips to reject the aircraft. Always answer with the POH limit, not a memorized number, if the question provides one.