823 

Federal Aviation Administration, DOT 

Pt. 36, App. H 

corrected to reference conditions may be de-
rived from FAA approved manufacturer’s 
data. 

(ii) The sound propagation paths to the 

microphone from the aircraft position cor-
responding to PNLTM must be determined 
for both the test and reference profiles. The 
SPL values in the spectrum of PNLTM must 
then be corrected for the effects of— 

(A) Change in atmospheric sound absorp-

tion; 

(B) Atmospheric sound absorption on the 

linear difference between the two sound path 
lengths; and 

(C) Inverse square law on the difference in 

sound propagation path length. The cor-
rected values of SPL must then be converted 
to a reference condition PNLTM value from 
which PNLTM must be subtracted. The re-
sulting difference represents the correction 
which must be added algebraically to the 
EPNL calculated from the measured data. 

(iii) As observed at the noise measuring 

station, the measured PNLTM distance is 
different from the reference PNLTM distance 
and therefore the ratio must be calculated 
and used to determine a noise duration cor-

rection factor. Effective perceived noise 
level, EPNL, is determined by the algebraic 
sum of the maximum tone corrected per-
ceived noise level (PNLTM) and the duration 
correction factor. 

(iv) For aircraft flyover, alternative source 

noise corrections require FAA approval and 
must be determined and adjusted to account 
for noise level changes caused by the dif-
ferences between measured test conditions 
and reference conditions. 

(b) 

Takeoff profiles. 

(1) Figure H1 illustrates 

a typical takeoff profile, including reference 
conditions. 

(i) The reference takeoff flight path is de-

scribed in section H36.3(c). 

(ii) The test parameters are functions of 

the helicopter’s performance and weight and 
the atmospheric conditions of temperature, 
pressure, wind velocity and direction. 

(2) For the actual takeoff, the helicopter 

approaches position C in level flight at 65 
feet (20 meters) above ground level at the 
flight track noise measuring station and at 
either V

y

±

5 knots or the lowest approved 

speed for the climb after takeoff, whichever 
speed is greater. 

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