815 

Federal Aviation Administration, DOT 

Pt. 36, App. G 

(c) No corrections for helical tip Mach 

number variation need to be made if the pro-
peller helical tip Mach number is: 

(1) At or below 0.70 and the test helical tip 

Mach number is within 0.014 of the reference 
helical tip Mach number. 

(2) Above 0.70 and at or below 0.80 and the 

test helical tip Mach number is within 0.007 
of the reference helical tip Mach number. 

(3) Above 0.80 and the test helical tip Mach 

number is within 0.005 of the reference hel-
ical tip Mach number. For mechanical ta-
chometers, if the helical tip Mach number is 
above 0.8 and the test helical tip Mach num-
ber is within 0.008 of the reference helical tip 
Mach number. 

(d) When the test conditions are outside 

those specified, corrections must be applied 
by an approved procedure or by the following 
simplified procedure: 

(1) Measured sound levels must be cor-

rected from test day meteorological condi-
tions to reference conditions by adding an 
increment equal to 
Delta (M) = (H

T

a

—0.7 H

R

)/1000 

where H

T

is the height in feet under test con-

ditions, H

R

is the height in feet under ref-

erence conditions when the aircraft is di-
rectly over the noise measurement point 
and 

a 

is the rate of absorption for the 

test day conditions at 500 Hz as specified 
in SAE ARP 866A, entitled ‘‘Standard 

Values of Atmospheric Absorption as a 
function of Temperature and Humidity 
for use in Evaluating Aircraft Flyover 
Noise’’ as incorporated by reference 
under § 36.6. 

(2) Measured sound levels in decibels must 

be corrected for height by algebraically add-
ing an increment equal to Delta (1). When 
test day conditions are within those speci-
fied in figure G1: 

Delta (1) = 22 log (H

T

/H

R

) 

where H

T

is the height of the test aircraft 

when directly over the noise measure-
ment point and H

R

is the reference 

height. 

When test day conditions are outside those 

specified in figure G1: 

Delta (1) = 20 log (H

T

/H

R

) 

(3) Measured sound levels in decibels must 

be corrected for helical tip Mach number by 
algebraically adding an increment equal to: 

Delta (2) = k log (M

R

/M

T

) 

where M

T

and M

R

are the test and reference 

helical tip Mach numbers, respectively. 
The constant ‘‘k’’ is equal to the slope of 
the line obtained for measured values of 
the sound level in dB(A) versus helical 
tip Mach number. The value of k may be 
determined from approved data. A nomi-
nal value of k = 150 may be used when M

T

 

is smaller than M

R

. No correction may be 

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