Outer Wing Surfaces and Tail Section
Generally, the AFM/POH specifies a sequence for the pilot to inspect the aircraft which may sequence from the cabin entry access opening and then in a counterclockwise direction until the aircraft has been completely inspected. Besides the AFM/POH preflight assessment, the pilot must also develop awareness for potential areas of concern, such as signs of deterioration or distortion of the structure, whether metal or composite, as well as loose or missing rivets or screws.
Besides all items specified in the AFM/POH that must be inspected, the pilot should also develop an awareness for critical areas, such as spar lines, wing, horizontal, and vertical attach points including wing struts and landing gear attachment areas. The airplane skin should be inspected in these areas as load-related stresses are concentrated along spar lines and attach points. Spar lines are lateral rivet lines that extend from one side of the wing to the other, horizontal stabilizer, or vertical stabilizer. Pilots should pay close attention to spar lines looking for distortion, ripples, bubbles, dents, creases, or waves as any structural deformity may be an indication of internal damage or failure. Inspect around rivet heads looking for cracked paint or a black-oxide film that forms when a rivet works free in its hole. [Figure 2-7]
Figure 2-7. Example of rivet heads where black oxide film has formed due to the rivet becoming loose in its hole.
Additional areas that should be scrutinized are the leading edges of the wing, horizontal and vertical stabilizer. These areas may be impact damaged by rocks, ice, birds, and or hangar rash incidents—dents and dings may render the structure unairworthy. Some leading edge surfaces have aerodynamic devices, such as stall fences, slots, or vortex generators, and deicing equipment, such as weeping wings and boots. If these items exist on the airplane which the pilot intends to fly, knowledge of an acceptable level of proper condition must be gained so that an adequate preflight inspection may take place.
On metal airplanes, wingtips, fairings, and non-structural covers may be fabricated out of thin fiberglass or plastic. These items are frequently affected by cracks radiating from screw holes or concentrated radiuses. Often, if any of these items are cracked, it is practice to “stop-drill” the crack to prevent crack progression. [Figure 2-8] Extra care should be exercised to ensure that these devices are in good condition without cracks that may render them unairworthy. Cracks that have continued beyond a stop drilled location or any new adjacent cracks that have formed may lead to in-flight failure.
Figure 2-8. Cracks radiating from screw holes that have been stop-drilled to prevent crack progression.
Inspecting composite airplanes can be more challenging as the airplanes generally have no rivets or screws to aid the pilot in identifying spar lines and wing attach points; however, delamination of spar to skin or other structural problems may be identified by bubbles, fine hair-line cracks, or changes in sound when gently tapping on the structure with a fingertip. Anything out of place should be addressed by discussing the issue with a properly rated aircraft mechanic.