FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 4. Metal Structure, Welding, and Brazing | 3. Precautionary Measures | 4-36. Flutter and Vibration Precautions
9/8/98
SECTION 3. PRECAUTIONARY MEASURES
AC 43.13-1B
4-36. FLUTTER AND VIBRATION
PRECAUTIONS. To prevent the occurrence
of severe vibration or flutter of flight control
surfaces during flight, precautions must be
taken to stay within the design balance limita
tions when performing maintenance or repair.
a. Balance Changes. The importance of
retaining the proper balance and rigidity of air
craft control surfaces cannot be overempha
sized. The effect of repair or weight change on
the balance and center of gravity is proportion
ately greater on lighter surfaces than on the
older heavier designs. As a general rule, repair
the control surface in such a manner that the
weight distribution is not affected in any way,
in order to preclude the occurrence of flutter of
the control surface in flight. Under certain
conditions, counter-balance weight is added
forward of the hinge line to maintain balance.
Add or remove balance weights only when
necessary in accordance with the manufac
turer’s instructions. Flight testing must be ac
complished to ensure flutter is not a problem.
Failure to check and retain control surface bal
ance within the original or maximum allow
able value could result in a serious flight
hazard.
b. Painting and Refinishing. Special
emphasis is directed to the effect of too many
extra coats of paint on balanced control sur
faces. Mechanics must avoid adding addi
tional coats of paint in excess of what the
manufacturer originally applied. If available
consult the aircraft manufacturer’s instructions
relative to finishing and balance of control
surfaces.
c. Trapped Water or Ice. Instances of
flutter have occurred from unbalanced condi
tions caused by the collection of water or ice
within the surface. Therefore, ventilation and
drainage provisions must be checked and re
tained when maintenance is being done.
d. Trim Tab Maintenance. Loose or vi
brating trim tabs will increase wear of actuat
ing mechanisms and hinge points which may
develop into serious flutter conditions. When
this happens, primary control surfaces are
highly susceptible to wear, deformation, and
fatigue failures because of the buffeting nature
of the airflow over the tab mechanism. Trail
ing-edge play of the tab may increase, creating
an unsafe flutter condition. Careful inspection
of the tab and its mechanism should be con
ducted during overhaul and annual inspection
periods. Compared to other flight control sys
tems on the aircraft, only a minor amount of
tab-mechanism wear can be tolerated.
(1) Free play and stiffness may best be
measured by a simple static test where “up
ward” and “downward” (or “leftward” and
“rightward”) point forces are applied near the
trailing edge of the tab at the span-wise at
tachment of the actuator (so as not to twist the
tab). The control surface to which the trim tab
is attached should be locked in place. Rota
tional deflection readings are then taken near
the tab trailing edge using an appropriate
measuring device, such as a dial gauge. Sev
eral deflection readings should be taken using
loads first applied in one direction, then in the
opposite. If the tab span does not exceed
35 percent of the span of the supporting con
trol surface, the total free play at the tab trail
ing edge should not exceed 2 percent of the tab
chord. If the tab span equals or exceeds
35 percent of the span of the supporting con
trol surface, the total free play at the tab trail
ing edge should not exceed 1 percent of the
distance from the tab hinge line to the trailing
edge of the tab perpendicular to the tab hinge
line. For example, a tab that has a chord of
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