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Federal Aviation Administration, DOT 

§ 171.111 

(iii) The identification signal must 

employ the International Morse Code 
and consist of three letters. 

(iv) The identification signal must be 

transmitted at a speed corresponding 
to approximately seven words per 
minute, and must be repeated at ap-
proximately equal intervals, not less 
than six times per minute. When SDF 
transmission is not available for oper-
ational use, including periods of re-
moval of navigational components or 
during maintenance or test trans-
missions, the identification signal 
must be suppressed. 

(b) It must be shown during ground 

inspection of the design features of the 
equipment that there will not be condi-
tions that will allow unsafe operations 
because of component failure or dete-

(c) The monitor must be checked pe-

riodically during the in-service test 
evaluation period for calibration and 
stability. These tests, and ground 
checks of SDF radiation characteris-
tics must be conducted in accordance 
with the maintenance manual required 
by § 171.115(c) and must meet the stand-
ards and tolerances contained in 
§ 171.111(j). 

(d) The monitor system must provide 

a warning to the designated control 
point(s) when any of the conditions of 
§ 171.111(j) occur, within the time peri-
ods specified in that paragraph. 

(e) Flight inspection to determine 

the adequacy of the facility’s oper-
ational performance and compliance 
with applicable performance require-
ments must be conducted in accord-
ance with the ‘‘U.S. Standard Flight 
Inspection Manual.’’ Tolerances con-
tained in the U.S. Standard Flight In-
spection Manual, section 217, must be 
complied with except as stated in para-
graph (f) of this section. 

(f) Flight inspection tolerances speci-

fied in section 217 of the ‘‘U.S. Stand-
ard Flight Inspection Manual’’ must be 
complied with except as follows: 


Course sector width. 

The nominal 

course sector width must be 6


. When 

an operational advantage can be 
achieved, a nominal course sector 
width of 12


may be established. Course 

sector width must be adjusted and 
maintained within the limits of 



percent of the nominal value. 


Course alignment. 

The mean course 

line must be adjusted and maintained 
within the limits of 


10 percent of the 

nominal course sector width. 


Course structure. 

Course deviations 

due to roughness, scalloping, or bends 
must be within the following limita-


Front course. 



) Course structure 

from 18 miles from runway threshold to 
Point A must not exceed 


40 micro-




) Point A to Point A–1—linear de-

crease from not more than 


40 micro-

amperes at Point A to not more than 


20 microamperes at Point A–1; 



) Point A–1 to Missed Approach 

Point—not more than 


20 micro-




) Monitor tolerances: width 


17 per-

cent of nominal; alignment—


10 per-

cent of nominal course sector width. 


Back course. 



) Course structure 

18 miles from runway threshold to 4 
miles from runway threshold must not 


40 microamperes. Four miles 

to 1 mile from R/W must not exceed 



microamperes decreasing to not more 


20 microamperes, at a linear 




) Monitor tolerances: width—



percent of nominal; alignment—



percent of nominal course sector width. 

[Doc. No. 10116, 35 FR 12711, Aug. 11, 1970, as 
amended by Amdt. 171–9, 38 FR 28557, Oct. 15, 

§ 171.111

Ground standards and toler-


Compliance with this section must be 

shown as a condition to approval and 
must be maintained during operation 
of the SDF. 



(1) The SDF must oper-

ate on odd tenths or odd tenths plus a 
twentieth MHz within the frequency 
band 108.1 MHz to 111.95 MHz. The fre-
quency tolerance of the radio fre-
quency carrier must not exceed plus or 
minus 0.002 percent. 

(2) The modulating tones must be 90 

Hz and 150 Hz within 


2.5 percent. 

(3) The identification signal must be 

1020 Hz within 


50 Hz. 

(4) The total harmonic content of the 

90 Hz tone must not exceed 10 percent. 

(5) The total harmonic content of the 

150 Hz tone must not exceed 10 percent. 

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08:20 May 17, 2019

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