Code of Federal Regulations - Title 14

920

14 CFR Ch. I (1–1–14 Edition)

Pt. 36, App. A

A36.2.1.1 This section prescribes the condi-

tions under which noise certification must
be conducted and the measurement proce-
dures that must be used.

OTE

: Many noise certifications involve

only minor changes to the airplane type de-
sign. The resulting changes in noise can
often be established reliably without resort-
ing to a complete test as outlined in this ap-
pendix. For this reason, the FAA permits the
use of approved equivalent procedures. There
are also equivalent procedures that may be
used in full certification tests, in the inter-
est of reducing costs and providing reliable
results. Guidance material on the use of
equivalent procedures in the noise certifi-
cation of subsonic jet and propeller-driven
large airplanes is provided in the current ad-
visory circular for this part.

A36.2.2

Test environment.

A36.2.2.1 Locations for measuring noise

from an airplane in flight must be sur-
rounded by relatively flat terrain having no
excessive sound absorption characteristics
such as might be caused by thick, matted, or
tall grass, shrubs, or wooded areas. No ob-
structions that significantly influence the
sound field from the airplane must exist
within a conical space above the point on the
ground vertically below the microphone, the
cone being defined by an axis normal to the
ground and by a half-angle 80

from this axis.

OTE

: Those people carrying out the meas-

urements could themselves constitute such
obstruction.

A36.2.2.2 The tests must be carried out

under the following atmospheric conditions.

(a) No precipitation;
(b) Ambient air temperature not above 95

F (35

C) and not below 14

F (¥10

C), and

relative humidity not above 95% and not
below 20% over the whole noise path between
a point 33 ft (10 m) above the ground and the
airplane;

OTE

: Care should be taken to ensure that

the noise measuring, airplane flight path
tracking, and meteorological instrumenta-
tion are also operated within their specific
environmental limitations.

perature over the whole noise path between
a point 33 ft (10 m) above the ground and the
airplane such that the sound attenuation in
the one-third octave band centered on 8 kHz
will not be more than 12 dB/100 m unless:

(1) The dew point and dry bulb tempera-

tures are measured with a device which is ac-
curate to

0.9

F (

0.5

C) and used to obtain

relative humidity; in addition layered sec-
tions of the atmosphere are used as described
in section A36.2.2.3 to compute equivalent
weighted sound attenuations in each one-
third octave band; or

(2) The peak noy values at the time of

PNLT, after adjustment to reference condi-

tions, occur at frequencies less than or equal
to 400 Hz.;

(d) If the atmospheric absorption coeffi-

cients vary over the PNLTM sound propaga-
tion path by more than

1.6 dB/1000 ft (

0.5

dB/100m) in the 3150Hz one-third octave band
from the value of the absorption coefficient
derived from the meteorological measure-
ment obtained at 33 ft (10 m) above the sur-
face, ‘‘layered’’ sections of the atmosphere
must be used as described in section A36.2.2.3
to compute equivalent weighted sound at-
tenuations in each one-third octave band;
the FAA will determine whether a sufficient
number of layered sections have been used.
For each measurement, where multiple
layering is not required, equivalent sound at-
tenuations in each one-third octave band
must be determined by averaging the atmos-
pheric absorption coefficients for each such
band at 33 ft (10 m) above ground level, and
at the flight level of the airplane at the time
of PNLTM, for each measurement;

(e) Average wind velocity 33 ft (10 m) above

ground may not exceed 12 knots and the
crosswind velocity for the airplane may not
exceed 7 knots. The average wind velocity
must be determined using a 30-second aver-
aging period spanning the 10 dB-down time
interval. Maximum wind velocity 33 ft (10 m)
above ground is not to exceed 15 knots and
the crosswind velocity is not to exceed 10
knots during the 10 dB-down time interval;

(f) No anomalous meteorological or wind

conditions that would significantly affect
the measured noise levels when the noise is
recorded at the measuring points specified
by the FAA; and

(g) Meteorological measurements must be

obtained within 30 minutes of each noise test
measurement; meteorological data must be
interpolated to actual times of each noise
measurement.

A36.2.2.3 When a multiple layering calcula-

tion is required by section A36.2.2.2(c) or
A36.2.2.2(d) the atmosphere between the air-
plane and 33 ft (10 m) above the ground must
be divided into layers of equal depth. The
depth of the layers must be set to not more
than the depth of the narrowest layer across
which the variation in the atmospheric ab-
sorption coefficient of the 3150 Hz one-third
octave band is not greater than

1.6 dB/1000

ft (

0.5 dB/100m), with a minimum layer

depth of 100 ft (30 m). This requirement must
be met for the propagation path at PNLTM.
The mean of the values of the atmospheric
absorption coefficients at the top and bot-
tom of each layer may be used to charac-
terize the absorption properties of each
layer.

A36.2.2.4 The airport control tower or an-

other facility must be aproved by the FAA
for use as the central location at which
measurements of atmospheric parameters

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