971
Federal Aviation Administration, DOT
Pt. 36, App. H
Position N represents the helicopter location
on the measured approach flight path for
which PNLTM is observed at measuring sta-
tion A, and N
r
is the corresponding position
on the reference approach flight path. The
measured and reference noise propagation
paths are AN and AN
r
, respectively, both of
which form the same angle,
q
APP
, cor-
responding to PNLTM relative to their ap-
proach flight paths.
(e)
Correction of noise at source during level
flyover. (1) For level overflight, if any com-
bination of the following three factors, air-
speed deviations from reference, rotor speed
deviations from reference, and temperature
deviations from reference, results in a noise
correlating parameter whose value deviates
from the reference value of this parameter,
then source noise adjustments must be de-
termined from the manufacturer’s data that
is approved by the FAA.
(2) Off-reference tip Mach number adjust-
ments must be based upon a sensitivity
curve of PNLTM versus advancing blade tip
Mach number, deduced from overflights per-
formed at different airspeeds surrounding
the reference airspeed. If the test aircraft is
unable to attain the reference value, then an
extrapolation of the sensitivity curve is per-
mitted if data cover at least a range of 0.03
Mach units. The advancing blade tip Mach
number must be computed using true air-
speed, onboard outside air temperature, and
rotor speed. A separate PNLTM versus ad-
vancing blade tip Mach number function
must be derived for each of the three certifi-
cation microphone locations,
i.e., centerline,
sideline left, and sideline right. Sideline left
and right are defined relative to the direc-
tion of flight for each run. PNLTM adjust-
ments are to be applied to each microphone
datum using the appropriate PNLTM func-
tion.
(f)
PNLT corrections. If the measured ambi-
ent atmospheric conditions of temperature
and relative humidity differ from those pre-
scribed as reference conditions under this ap-
pendix (77 degrees F and 70 percent, respec-
tively), corrections to the EPNL values must
be calculated from the measured data under
paragraph (a) of this section as follows:
(1)
Takeoff flight path. For the takeoff
flight path shown in Figure H1, the spectrum
of PNLTM observed at station A for the air-
craft at position L is decomposed into its in-
dividual SPL(
i) values.
(i) Step 1. A set of corrected values are
then computed as follows:
SPL(
i)
r
= SPL(
i) + C[
a(i) ¥ a(i)
o
]AL +
C
a(i)
o
(AL ¥ AL
r
) + 20 log (AL/AL
r
)
where SPL(
i) and SPL(i)
r
are the measured
and corrected sound pressure levels, respec-
tively, in the
i-th one-third octave band. The
first correction term adjusts for the effect of
change in atmospheric sound absorption
where
a(i) and a(i)
o
are the sound attenuation
coefficients for the test and reference atmos-
pheric conditions, respectively, for the
i-th
one-third octave band, and AL is the meas-
ured takeoff sound propagation path. The
conversion factor constant,
C, is 0.001 for
English System of Units and is 0.01 for Inter-
national System of Units. The second correc-
tion term adjusts for the effects of atmos-
pheric attenuation due to the difference in
the sound propagation path length where AL
r
is the Reference takeoff sound propagation
path. The third correction term, known as
the ‘‘inverse square’’ law, adjusts for the ef-
fect of the difference in the sound propaga-
tion path lengths.
(ii) Step 2. The corrected values of the
SPL(
i)
r
are then converted to reference con-
dition PNLT and a correction term cal-
culated as follows:
D
1
= PNLT ¥ PNLTM
which represents the correction to be added
algebraically to the EPNL calculated from
the measured data.
(2)
Level flyover flight path. (i) The proce-
dure described in paragraph (f)(1) of this sec-
tion for takeoff paths is also used for the
level flyover paths, with the values of SPL(
i)
r
relating to the flyover sound propagation
paths shown in Figure H2 as follows:
SPL(
i)
r
= SPL(
i) + C[
a(i) ¥ a(i)
o
]AM +
C
a(i)
o
(AM ¥ AM
r
) + 20 log (AM/AM
r
)
where the lines AM and AM
r
are the meas-
ured and reference level flyover sound propa-
gation paths, respectively.
(ii) The remainder of the procedure is the
same for the flyover condition as that pre-
scribed in the paragraph (f)(1)(ii) of this sec-
tion regarding takeoff flight path.
(3)
Approach flight path. (i) The procedure
described in paragraph (f)(1) of this section
for takeoff paths is also used for the ap-
proach paths, with the values of SPL(
i)
r
re-
lating to the approach sound propagation
paths shown in Figure H3 as follows:
SPL(
i)
r
= SPL(
i) + C[
a(i) ¥ a(i)
o
]AN +
C
a(i)
o
(AN ¥ AN
r
) + 20 log (AN/AN
r
)
where the lines AN and AN
r
are the measured
and reference approach sound propagation
paths, respectively.
(ii) The remainder of the procedure is the
same for the approach condition as that pre-
scribed in the paragraph (f)(1)(ii) of this sec-
tion regarding takeoff flight path.
(4)
Sideline microphones. (i) The procedure
prescribed in paragraph (f)(1) of this section
for takeoff paths is also used for the propa-
gation to the sideline locations, with the val-
ues of SPL(
i)
r
relating as follows to the
measured sideline sound propagation path
shown in Figure H3 as follows:
SPL(
i)
r
= SPL(
i) + C[
a(i) ¥ a(i)
o
]SX +
C
a(i)
o
(SX ¥ SX
r
) + 20 log (SX/SX
r
)
where S is the sideline measuring station
and, based upon the flight condition, the hel-
icopter positions, X and X
r
, correspond to:
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