833
Federal Aviation Administration, DOT
Pt. 36, App. J
the number of level flights made with a tail-
wind component over the noise measurement
station:
(1) In level flight and in cruise configura-
tion;
(2) At a height of 492 feet
±
50 feet (150
±
15
meters) above the ground level at the noise
measuring station; and
(3) Within
±
10 degrees from the zenith.
(c) Each flyover noise test must be con-
ducted:
(1) At the reference airspeed specified in
section J36.3(c) of this appendix, with such
airspeed adjusted as necessary to produce
the same advancing blade tip Mach number
as associated with the reference conditions;
(i) Advancing blade tip Mach number (M
AT
)
is defined as the ratio of the arithmetic sum
of blade tip rotational speed (V
R
) and the
helicopter true air speed (V
T
) over the speed
of sound (c) at 77 degrees Fahrenheit (1135.6
ft/sec or 346.13 m/sec) such that M
AT
= (V
R
+
V
T
)/c; and
(ii) The airspeed shall not vary from the
adjusted reference airspeed by more than
±
3
knots (
±
5 km/hr) or an equivalent FAA-ap-
proved variation from the reference advanc-
ing blade tip Mach number. The adjusted ref-
erence airspeed shall be maintained through-
out the measured portion of the flyover.
(2) At rotor speed stabilized at the power
on maximum normal operating rotor RPM
(
±
1 percent); and
(3) With the power stabilized during the pe-
riod when the measured helicopter noise
level is within 10 dB of the maximum A-
weighted sound level (L
AMAX
).
(d) The helicopter test weight for each fly-
over test must be within plus 5 percent or
minus 10 percent of the maximum takeoff
weight for which certification under this
part is requested.
(e) The requirements of paragraph (b)(2) of
this section notwithstanding, flyovers at an
FAA-approved lower height may be used and
the results adjusted to the reference meas-
urement point by an FAA-approved method
if the ambient noise in the test area, meas-
ured in accordance with the requirements
prescribed in section J36.109 of this appendix,
is found to be within 15 dB(A) of the max-
imum A-weighted helicopter noise level
(L
AMAX
) measured at the noise measurement
station in accordance with section J36.109 of
this appendix.
Section J36.107 [Reserved]
Section J36.109 Measurement of helicopter noise
received on the ground.
(a)
General.
(1) The helicopter noise meas-
ured under this appendix for noise certifi-
cation purposes must be obtained with FAA-
approved acoustical equipment and measure-
ment practices.
(2) Paragraph (b) of this section identifies
and prescribes the specifications for the
noise evaluation measurements required
under this appendix. Paragraphs (c) and (d)
of this section prescribe the required acous-
tical equipment specifications. Paragraphs
(e) and (f) of this section prescribe the cali-
bration and measurement procedures re-
quired under this appendix.
(b)
Noise unit definition.
(1) The value of
sound exposure level (SEL, or as denoted by
symbol, L
AE
), is defined as the level, in deci-
bels, of the time integral of squared ‘A’-
weighted sound pressure (P
A
) over a given
time period or event, with reference to the
square of the standard reference sound pres-
sure (P
O
) of 20 micropascals and a reference
duration of one second.
(2) This unit is defined by the expression:
L
Log
T
P t
P
dt dB
AE
A
t
t
=
∫
10
1
10
0
0
2
1
2
( )
Where T
O
is the reference integration time of
one second and (t
2
-t
1
) is the integration
time interval.
(3) The integral equation of paragraph
(b)(2) of this section can also be expressed as:
L
Log
T
dt dB
AE
L
t
t
t
=
∫
10
1
10
10
0
0 1
1
2
.
( )
A
Where L
A
(t) is the time varying A-weighted
sound level.
(4) The integration time (t
2
-t
1
) in practice
shall not be less than the time interval dur-
ing which L
A
(t) first rises to within 10 dB(A)
of its maximum value (L
AMAX
) and last falls
below 10 dB(A) of its maximum value.
(5) The SEL may be approximated by the
following expression:
L
AE
= L
AMAX
+ <delta>A
where <delta>A is the duration allowance
given by:
<delta>A = 10 log
10
(T)
where T = (t
2
-t
1
)/2 and L
AMAX
is defined as the
maximum level, in decibels, of the A-
weighted sound pressure (slow response)
with reference to the square of the stand-
ard reference sound pressure (P
0
).
(c)
Measurement system.
The acoustical
measurement system must consist of FAA-
approved equipment equivalent to the fol-
lowing:
(1) A microphone system with frequency
response that is compatible with the meas-
urement and analysis system accuracy pre-
scribed in paragraph (d) of this section;
(2) Tripods or similar microphone mount-
ings that minimize interference with the
sound energy being measured;
(3) Recording and reproducing equipment
with characteristics, frequency response, and
dynamic range that are compatible with the
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