PHAK 3-11

0 0 +45 -45 0 90 90 0 -45 +45 0 0 0 0 +45 -45 0 90 90 0 -45 +45 0 0 Pyramid pattern matrix crack from impact Low Energy Impact 0 0 +45 -45 0 90 90 0 -45 +45 0 0 0 0 +45 -45 0 90 90 0 -45 +45 0 0 Delaminations Back side fiber fracture Local fiber/matrix crushing Medium Energy Impact 0 0 +45 -45 0 90 90 0 -45 +45 0 0 0 0 +45 -45 0 90 90 0 -45 +45 0 0 Delaminations High Energy Impact Through penetration small damage zone Loose fiber ends Figure 3-16. Impact energy affects the visibility, as well as the severity, of damage in composite structures. High and medium energy impacts, while severe, are easy to detect. Low energy impacts can easily cause hidden damage. medium and high energy impacts, the damage is visible to the eye, but low energy impact is difficult to detect. [Figure 3-16] If an impact results in delaminations, crushing of the surface, or a puncture, then a repair is mandatory. While waiting for the repair, the damaged area should be covered and protected from rain. Many composite parts are composed of thin skins over a honeycomb core, creating a “sandwich” structure. While excellent for structural stiffness reasons, such a structure is an easy target for water ingress (entering), leading to further problems later. A piece of “speed tape” over the puncture is a good way to protect it from water, but it is not a structural repair. The use of a paste filler to cover up the damage, while acceptable for cosmetic purposes, is not a structural repair, either. The potential for heat damage to the resin is another disadvantage of using composites. While “too hot” depends on the particular resin system chosen, many epoxies begin to weaken over 150 °F. White paint on composites is often used to minimize this issue. For example, the bottom of a wing that is painted black facing a black asphalt ramp on a hot, sunny day can get as hot as 220 °F. The same structure, painted white, rarely exceeds 140 °F. As a result, composite aircraft often have specific recommendations on allowable paint colors. If the aircraft is repainted, these recommendations must be followed. Heat damage can also occur due to a fire. Even a quickly extinguished small brake fire can damage bottom wing skins, composite landing gear legs, or wheel pants. Also, chemical paint strippers are very harmful to composites and must not be used on them. If paint needs to be removed from composites, only mechanical methods are allowed, such as gentle grit blasting or sanding. Many expensive composite parts have been ruined by the use of paint stripper and such damage is generally not repairable. Fluid Spills on Composites Some owners are concerned about fuel, oil, or hydraulic fluid spills on composite surfaces. These are generally not a problem with modern composites using epoxy resin. Usually, if the spill does not attack the paint, it will not hurt the underlying composite. Some aircraft use fiberglass fuel tanks, for example, in which the fuel rides directly against the composite surface with no sealant being used. If the fiberglass structure is made with some of the more inexpensive types of polyester resin, there can be a problem when using auto gas with ethanol blended into the mixture. The more expensive types of polyester resin, as well as epoxy resin, can be used with auto gas, as well as 100 octane aviation gas (avgas) and jet fuel. Lightning Strike Protection Lightning strike protection is an important consideration in aircraft design. When an aircraft is hit by lightning, a very large amount of energy is delivered to the structure. Whether flying a light general aviation (GA) aircraft or a large airliner, the basic principle of lightning strike protection is the same. For any size aircraft, the energy from the strike must be spread over a large surface area to lower the amps per square inch to a harmless level. If lightning strikes an aluminum airplane, the electrical energy naturally conducts easily through the aluminum structure. The challenge is to keep the energy out of avionics, fuel systems, etc., until it can be safely conducted overboard. The outer skin of the aircraft is the path of least resistance.