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Figure 4
safety. Here are some suggestions you can utilize as you follow the manufacturer’s recommended checklists for aircraft preflight.
A good preflight starts as you approach the aircraft. Looking at the “big picture,” take note of any abnormalities. Is the airplane sit- ting evenly on the ramp? Is there any obvious damage, e.g., from a ground vehicle that may have contacted the airplane? Are there leaks or puddles of f luid under the aircraft? A flat tire?
change in stiffness, for example, due to delamination, will be indicated by a flattening tone from the coin tap.
Areas that carry high load should also be carefully inspected for cracks. Cracks usually emanate away from a stress point. They may initially present as small hairline blemishes and propagate and enlarge over time. The Pilatus PC-12 flap drive arm picture in Figure 3 caused the flaps to fail as the pilot deployed them on ap- proach to landing.
Likewise, having a basic un- derstanding of aircraft hardware – nuts, bolts, rod ends, etc. – is
When inspecting aero structures, a basic understanding of how loads are carried through the airframe is helpful. Attach points for vertical and horizontal stabilizers, spar lines and landing gear attach points are some of the areas of the airframe that experience the highest stress. Most metal aircraft employ a monocoque construction. In this type of construction, the skin is an integral component to the strength of the structure (just as an egg derives strength from its shell). Any sign of deformation in the skin or of popped or loose rivets in a metal aircraft structure is cause to suspect an overstressed member and to conduct a deep investigation. For example, look at Figure 1. Would you want to fly an aircraft missing rivets on its wing spar line? While in this case the rivets are completely sheared, observing cracked paint around a rivet or black oxide lines emanating from a rivet may indicate that the rivet is loose and strength is compromised.
Composite structures present a visual inspection chal- lenge because they may be compromised without showing any external signs of damage. The only sign something is amiss in the empennage is a slight wrinkly in the paint in Figure 2. A “coin tap” test can indicate the health of a composite structure. By tapping the surface with a coin and listening for the resulting sound we can detect dam- age to the underlying composite structure. A localized
helpful. Generally speaking, aircraft employ hardware that utilizes some safety retention mechanism such as lock washers, lock nuts, cotter pins and safety wire. Some- times, mechanics will use torque seal paint to mark the alignment of the nut with respect to the bolt after proper torquing. This makes it easier to detect any loosening of the hardware over time. Inspect safety wire for tension and proper installation. The safety wire should be installed in a direction that prevents the nut from loosening and is sometimes installed incorrectly. Never underestimate the importance of fasteners and treat even one missing fastener as requiring attention prior to flight. Just three missing cowl screws caused the engine cowl on a Citation to depart the aircraft in flight (Figure 4).
Corrosion, which is the oxidation of metal, may lead to significant loss of structural strength. Water from precipita- tion or from washing the aircraft, exposure to moisture in the atmosphere and exposure to deicing f luid can inf lict insidious damage on the airframe. There are many kinds of corrosion, such as surface corrosion (observable as pitting of the surface), filiform corrosion (which appears as “worms” under paint), pitting corrosion (which shows as white or gray powder deposit), and others. Corrosion will normally manifest as visible imperfections on the surface. Figure 5 shows a trim tab rod end on a Hawker. A trace of red rust
Figure 5
July 2022 / TWIN & TURBINE • 11