spaschal on DSK3GDR082PROD with CFR

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 station A, and Nr is the corresponding position
on the reference approach flight path. The
measured and reference noise propagation
paths are AN and ANr, respectively, both of
which form the same angle, qAPP, corresponding to PNLTM relative to their approach flight paths.
(e) Correction of noise at source during level
flyover. (1) For level overflight, if any combination of the following three factors, airspeed 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 determined from the manufacturer-s data that
is approved by the FAA.
(2) Off-reference tip Mach number adjustments must be based upon a sensitivity
curve of PNLTM versus advancing blade tip
Mach number, deduced from overflights performed 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 permitted if data cover at least a range of 0.03
Mach units. The advancing blade tip Mach
number must be computed using true airspeed, onboard outside air temperature, and
rotor speed. A separate PNLTM versus advancing blade tip Mach number function
must be derived for each of the three certification microphone locations, i.e., centerline,
sideline left, and sideline right. Sideline left
and right are defined relative to the direction of flight for each run. PNLTM adjustments are to be applied to each microphone
datum using the appropriate PNLTM function.
(f) PNLT corrections. If the measured ambient atmospheric conditions of temperature
and relative humidity differ from those prescribed as reference conditions under this appendix (77 degrees F and 70 percent, respectively), 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 aircraft at position L is decomposed into its individual SPL(i) values.
(i) Step 1. A set of corrected values are
then computed as follows:
SPL(i)r = SPL(i) + C[a(i) Section a(i)o]AL + Ca(i)o
(AL Section ALr) + 20 log (AL/ALr)
where SPL(i) and SPL(i)r are the measured
and corrected sound pressure levels, respectively, 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 atmospheric conditions, respectively, for the i-th
one-third octave band, and AL is the measured takeoff sound propagation path. The
conversion factor constant, C, is 0.001 for
English System of Units and is 0.01 for International System of Units. The second correction term adjusts for the effects of atmospheric attenuation due to the difference in
the sound propagation path length where ALr
is the Reference takeoff sound propagation
path. The third correction term, known as
the  - inverse square -  law, adjusts for the effect of the difference in the sound propagation path lengths.
(ii) Step 2. The corrected values of the
SPL(i)r are then converted to reference condition PNLT and a correction term calculated as follows:
D1 = PNLT Section PNLTM
which represents the correction to be added
algebraically to the EPNL calculated from
the measured data.
(2) Level flyover flight path. (i) The procedure described in paragraph (f)(1) of this section 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) Section a(i)o]AM + Ca(i)o
(AM Section AMr) + 20 log (AM/AMr)
where the lines AM and AMr are the measured and reference level flyover sound propagation paths, respectively.
(ii) The remainder of the procedure is the
same for the flyover condition as that prescribed in the paragraph (f)(1)(ii) of this section 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 approach paths, with the values of SPL(i)r relating to the approach sound propagation
paths shown in Figure H3 as follows:
SPL(i)r = SPL(i) + C[a(i) Section a(i)o]AN + Ca(i)o
(AN Section ANr) + 20 log (AN/ANr)
where the lines AN and ANr 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 prescribed in the paragraph (f)(1)(ii) of this section 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 propagation to the sideline locations, with the values 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) Section a(i)o]SX + Ca(i)o
(SX Section SXr) + 20 log (SX/SXr)
where S is the sideline measuring station
and, based upon the flight condition, the helicopter positions, X and Xr, correspond to:

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