Commercial aircrafts are frequently inspected visually. There is a primary inspection called "walk around check" both in the line and the base, transit checks, daily check, weekly check, A check, B check etc…
Line is the more about the quick checks while aircraft is in the park or in between flights.
Base is mostly the hanger, where hard checks, repairs take place.
A qualified engineer walk around the aircraft and inspect structural items visually. Engine is also checked during the walk around checks. There engineer watches for oil leaks, structural damages in the engine, inlet of the engine for composite parts fixtures to the structure, fan/first stages of the compressor, last stages of the turbine for deterioration or damages , or any unusual changes of the engine.
In this example shows how, engineer/Pilot carry on a walk around inspection of a Boeing 777 aircraft.
- Pack inlet and exhaust
- Negative pressure relief vents
- Wing illumination light
- Static ports
- Positive pressure relief valves
- Forward outflow valve
- Crew oxygen thermal discharge disc
- L pitot probe
- L ice detector probe
- TAT probe
- L AOA probe