| Home | Privacy | Contact |

composite structure, impact on the surface

| First | Previous | Next | Last |

Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




In a composite structure, a low energy impact, such as a
bump or a tool drop, may not leave any visible sign of the
impact on the surface. Underneath the impact site there may
be extensive delaminations, spreading in a cone-shaped area
from the impact location. The damage on the backside of
the structure can be significant and extensive, but it may be
hidden from view. Anytime one has reason to think there
may have been an impact, even a minor one, it is best to
get an inspector familiar with composites to examine the
structure to determine underlying damage. The appearance
of "whitish" areas in a fiberglass structure is a good tip-off
that delaminations of fiber fracture has occurred.

A medium energy impact (perhaps the car backing into the
structure) results in local crushing of the surface, which
should be visible to the eye. The damaged area is larger
than the visible crushed area, and will need to be repaired.
A high energy impact, such as a bird strike or hail while in
flight, results in a puncture and a severely damaged structure.
In medium and high energy impacts, the damage is visible
to the eye, but low energy impact is difficult to detect.
[Figure 2-16]

Impact energy affects the visibility
Figure 2-16. 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.

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
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 airplanes often have specific recommendations
on allowable paint colors. If the airplane is repainted, these
recommendations must be followed. Heat damage can also
occur due to a .re. Even a quickly extinguished small brake
.re 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
doesn't attack the paint, it won't 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.