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.  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 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.  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.  3-11  Lightning Strike Protection  Fluid Spills on Composites  0  +45  -45  0  90  0  -45  +45  0  0  +45  -45  0  90  0  -45  +45  0  Pyramid pattern matrix crack from impact  Low Energy Impact  0  +45  -45  0  90  0  -45  +45  0  0  +45  -45  0  90  0  -45  +45  0  Delaminations  Back side fiber fracture  Local fiber/matrix crushing  Medium Energy Impact  0  +45  -45  0  90  0  -45  +45  0  0  +45  -45  0  90  0  -45  +45  0  Delaminations  High Energy Impact  Through  penetration small damage  zone  Loose  fiber ends 