When repainting aircrafts special care has to be
taken with respect to the flight control surfaces, i.e.
rudder, elevator, and aileron. For several aircraft
types a new balancing of these parts is required.
This task can be supported by measuring the paint
thickness of these parts before stripping and after
the new painting. If the parts are of aluminum or
other metal this can be done by traditional nondestructive
paint measurement instruments based
on ultrasound or eddy current. Such instruments,
however, fail if the parts are of carbon fiber reinforced
plastic (CFRP or composite). Up to now
only the paintborer was available for paint thickness
measurements on CFRP. This is a destructive
method with a time consuming work.
Since recently, the paint thickness gauge FSC1
from FI Test- und Messtechnik GmbH is available.
This instrument uses a microwave based test method
and can be used for paint thickness measurements
also on CFRP with and without metal mesh.
The latter is often used for lightning protection of
CFRP parts of aircrafts.
This application note describes the use of the FSC1
in the repainting process of the CFRP flight control
surfaces of a Boeing 737-800 owned by TUIfly and
the subsequent balancing of these parts.
2. The paint thickness gauge FSC1
The paint thickness gauge FSC1/6 consists of two
modules: the measurement module and the display
module, see figure 1. The instantaneous measurement
area is about 2 cm2. The measurement time is
about 2 seconds. The substrates may be isotropic or
anisotropic and may have medium or high electrical
The readout is independent of the dielectric constant
(permittivity) of the isolating layer. In case of
layered paint material the complete thickness is
measured. In addition to the factory set calibration
a calibration by the user is recommended using the
CFRP substrate material in use and plastic calibration
Figure 1: Paint thickness gauge FSC1 consisting of the measuring
module and the display module.
Figure 2: Brief function test after calibration
Figure 3: Measuring the paint thickness on the left elevator
3. Paint thickness measurements on the
Boeing 737 flight control surfaces
The objective of this task is to determine the difference
of the paint thickness distributions on the
flight control surfaces before the stripping and after
the new painting. The calibration of the system was
performed using the demounted and stripped trim
tab of the left elevator, i.e. on a dark location in fig.
2. Three plastic calibration foils of known thicknesses
up to 400 μm were used. The complete calibration
procedure lasted about 4 minutes. The calibration
values were stored in the instrument.
Figure 2 also shows a brief function test on a nearby
location on the tab with a thin residual paint
Then the paint thickness of the elevator, rudder,
and aileron were measured at points which were
distributed on top and below these parts with distances
of about 20 cm between the points, see fig.3.
These points were defined by structural details and
by help of a measuring tape. The pure measurement
time lasts about 4 seconds per measurement.
Three values were taken at each point and averaged.
The paint thicknesses varied between 90μm
and 135μm. They were displayed directly and also
stored in the instrument.
After stripping and repainting, these measurements
were repeated at the same points using the calibration
values which were generated before the first
measurement series. The measurement values were
stored again. Then for each point the thickness
difference between first and second series was
calculated and then used for the balancing.
4. Balancing the flight control surfaces
There are two methods to balance flight controls on
Boeing 737. Balancing by static balance jig procedure
or by calculation. For balancing on jig removal
of elevator, aileron and rudder is necessary. For
balancing by calculation no removal of Flight Control
Surfaces is necessary. Balancing by calculation
means measuring and calculation. Difference between
the weight of the material removed and the
weight of the material added on flight controls
gives the new moment calculated per aircraft manufacturer
´s Structural Repair Manual. The necessary
data for this calculation needs to be as accurate
as possible. Results are directly depending on
the precision and the handling of the measurement
tool used. Using FSC1 on our aircraft reduced man
hours and increased measuring precision definitely.
Using the paint thickness gauge FSC1 in the repainting
process of composite flight control surfaces
considerably reduced the effort if their balancing
is necessary and when compared with paintborer
measurements. This was shown on the repainting
of a Boeing 737 and certainly holds for a lot of
other aircraft types.
Authors: Matthias Gnaas, TUIfly GmbH, Langenhagen/Hannover, Germany and
Johann H. Hinken, FI Test- und Messtechnik GmbH, Magdeburg, Germany
Prof. Dr.-Ing. Johann Hinken
FI Test- und Messtechnik GmbH
Tel.: +49-(0) 391-503894-31
Mobil : +49-(0) 171-2053208
Fax: +49-(0) 391-503894-39
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FI Test- und Messtechnik GmbH