During the service life of an aircraft, occasions may arise when something out of the ordinary care and use of an aircraft could possibly affect its airworthiness. When these situations are encountered, special inspection procedures, also called conditional inspections, are followed to determine if damage to the aircraft structure has occurred. The procedures outlined are general in nature and are intended to acquaint the aviation mechanic with the areas to be inspected. As such, they are not all inclusive. When performing any of these special inspections, always follow the detailed procedures in the aircraft maintenance manual. In situations where the manual does not adequately address the situation, seek advice from other maintenance technicians who are highly experienced with them. The following paragraphs describe some typical types of special inspections.
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Hard or Overweight Landing Inspection
The structural stress induced by a landing depends not only upon the gross weight at the time, but also upon the severity of impact. The hard landing inspection is for hard landings at or below the maximum design landing limits. An overweight landing inspection must be performed when an airplane lands at a weight above the maximum design landing weight. However, because of the difficulty in estimating vertical velocity at the time of contact, it is hard to judge whether or not a landing has been sufficiently severe to cause structural damage. For this reason, a special inspection is performed after a landing is made at a weight known to exceed the design landing weight or after a rough landing, even though the latter may have occurred when the aircraft did not exceed the design landing weight.
Wrinkled wing skin is the most easily detected sign of an excessive load having been imposed during a landing. Another indication easily detected is fuel leakage along riveted seams. Other possible locations of damage are spar webs, bulkheads, nacelle skin and attachments, firewall skin, and wing and fuselage stringers. If none of these areas show adverse effects, it is reasonable to assume that no serious damage has occurred. If damage is detected, a more extensive inspection and alignment check may be necessary.
Severe Turbulence Inspection/Over “G”
When an aircraft encounters a gust condition, the airload on the wings exceeds the normal wingload supporting the aircraft weight. The gust tends to accelerate the aircraft while its inertia acts to resist this change. If the combination of gust velocity and airspeed is too severe, the induced stress can cause structural damage.
A special inspection is performed after a flight through severe turbulence. Emphasis is placed upon inspecting the upper and lower wing surfaces for excessive buckles or wrinkles with permanent set. Where wrinkles have occurred, remove a few rivets and examine the rivet shanks to determine if the rivets have sheared or were highly loaded in shear.
Through the inspection doors and other accessible openings, inspect all spar webs from the fuselage to the tip. Check for buckling, wrinkles, and sheared attachments. Inspect for buckling in the area around the nacelles and in the nacelle skin, particularly at the wing leading edge. Check for fuel leaks. Any sizeable fuel leak is an indication that an area may have received overloads that have broken the sealant and opened the seams.
If the landing gear was lowered during a period of severe turbulence, inspect the surrounding surfaces carefully for loose rivets, cracks, or buckling. The interior of the wheel well may give further indications of excessive gust conditions. Inspect the top and bottom fuselage skin. An excessive bending moment may have left wrinkles of a diagonal nature in these areas.
Inspect the surface of the empennage for wrinkles, buckling, or sheared attachments. Also, inspect the area of attachment of the empennage to the fuselage. These inspections cover the critical areas. If excessive damage is noted in any of the areas mentioned, the inspection must be continued until all damage is detected.
Lightning Strike
Although lightning strikes to aircraft are extremely rare, if a strike has occurred, the aircraft is carefully inspected to determine the extent of any damage that might have occurred. When lightning strikes an aircraft, the electrical current must be conducted through the structure and be allowed to discharge or dissipate at controlled locations. These controlled locations are primarily the aircraft’s static discharge wicks, or on more sophisticated aircraft, null field dischargers. When surges of high-voltage electricity pass through good electrical conductors, such as aluminum or steel, damage is likely to be minimal or nonexistent. When surges of high-voltage electricity pass through non-metallic structures, such as a fiberglass radome, engine cowl or fairing, glass or plastic window, or a composite structure that does not have built-in electrical bonding, burning and more serious damage to the structure could occur. Visual inspection of the structure is required. Look for evidence of degradation, burning, or erosion of the composite resin at all affected structures, electrical bonding straps, static discharge wicks, and null field dischargers.
Bird Strike
When the aircraft is hit by birds during flight, the external areas of the airplane are inspected in the general area of the bird strike. If the initial inspection shows structural damage, then the internal structure of the airplane must be inspected as well. Also, inspect the hydraulic, pneumatic, and any other systems in the area of the bird strike.
Fire Damage
Inspection of aircraft structures that have been subjected to fire or intense heat can be relatively simple if visible damage is present. Visible damage requires repair or replacement. If there is no visible damage, the structural integrity of an aircraft may still have been compromised. Since most structural metallic components of an aircraft have undergone some sort of heat treatment process during manufacture, an exposure to high heat not encountered during normal operations could severely degrade the design strength of the structure. The strength and airworthiness of an aluminum structure that passes a visual inspection, but is still suspect, can be further determined by use of a conductivity tester. This is a device that uses eddy current. Since strength of metals is related to hardness, possible damage to steel structures might be determined by use of a hardness tester, such as a Rockwell C hardness tester. [Figure]
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| Rockwell C Hardness Tester |
Flood Damage
Like aircraft damaged by fire, aircraft damaged by water can range from minor to severe. This depends on the level of the flood water, whether it was fresh or salt water, and the elapsed time between the flood occurrence and when repairs were initiated. Any parts that were totally submerged are completely disassembled, thoroughly cleaned, dried, and treated with a corrosion inhibitor. Many parts might have to be replaced, particularly interior carpeting, seats, side panels, and instruments. Since water serves as an electrolyte that promotes corrosion, all traces of water and salt must be removed before the aircraft can again be considered airworthy.
Seaplanes
Because they operate in an environment that accelerates corrosion, seaplanes must be carefully inspected for corrosion and conditions that promote corrosion. Inspect bilge areas for waste hydraulic fluids, water, dirt, drill chips, and other debris. Additionally, since seaplanes often encounter excessive stress from the pounding of rough water at high speeds, inspect for loose rivets and other fasteners; stretched, bent or cracked skins; damage to the float attach fitting; and general wear and tear on the entire structure.
Aerial Application Aircraft
Two primary factors that make inspecting these aircraft different from other aircraft are the corrosive nature of some of the chemicals used and the typical flight profile. Damaging effects of corrosion may be detected in a much shorter period of time than normal use aircraft. Chemicals may soften the fabric or loosen the fabric tapes of fabric-covered aircraft. Metal aircraft may need to have the paint stripped, cleaned, and repainted and corrosion treated annually. Leading edges of wings and other areas may require protective coatings or tapes. Hardware may require more frequent replacement.
During peak use, these aircraft may fly up to 50 cycles (takeoffs and landings) or more in a day, most likely from an unimproved or grass runway. This can greatly accelerate the failure of normal fatigue items. Landing gear and related items require frequent inspections. Because these aircraft operate almost continuously at very low altitudes, air filters tend to become obstructed more rapidly.