828
14 CFR Ch. I (1–1–24 Edition)
§ 171.109
Back course sector
means the course
sector on the opposite end of the run-
way from the front course sector.
Course line
means the locus of points
along the final approach course at
which the DDM is zero.
Course sector
means a sector in a hori-
zontal plane containing the course line
and limited by the loci of points near-
est to the course line at which the
DDM is 0.155.
Displacement sensitivity
means the
ratio of measured DDM to the cor-
responding lateral displacement from
the appropriate reference line.
Front course sector
means the course
sector centered on the course line in
the direction from the runway in which
a normal final approach is made.
Half course sector
means the sector in
a horizontal plane containing the
course line and limited by the loci of
points nearest to the course line, at
which the DDM is 0.0775.
Point A
means a point on the front
course in the approach direction a dis-
tance of 4 nautical miles from the
threshold.
Point A1
means a point on the front
course in the approach direction a dis-
tance of 1 statute mile from the thresh-
old.
Point A2
means a point on the front
course at the threshold.
Reference datum
means a point at a
specified height located vertically
above the intersection of the course
and the threshold.
Missed approach point
means the
point on the final approach course, not
farther from the final approach fix
than Point ‘‘A2’’, at which the ap-
proach must be abandoned, if the ap-
proach and subsequent landing cannot
be safely completed by visual ref-
erence, whether or not the aircraft has
descended to the minimum descent al-
titude.
§ 171.109 Performance requirements.
(a) The Simplified Directional Facil-
ity must perform in accordance with
the following standards and practices:
(1) The radiation from the SDF an-
tenna system must produce a com-
posite field pattern which is amplitude
modulated by a 90 Hz and a 150 Hz tone.
The radiation field pattern must
produce a course sector with the 90 Hz
tone predominating on one side of the
course and with the 150 Hz tone pre-
dominating on the opposite side.
(2) When an observer faces the SDF
from the approach end of runway, the
depth of modulation of the radio fre-
quency carrier due to the 150 Hz tone
must predominate on his right hand
and that due to the 90 Hz tone must
predominate on his left hand.
(3) All horizontal angles employed in
specifying the SDF field patterns must
originate from the center of the an-
tenna system which provides the sig-
nals used in the front course sector.
(4) The SDF must operate on odd
tenths or odd tenths plus a twentieth
MHz within the frequency band 108.1
MHz to 111.95 MHz. The frequency tol-
erance of the radio frequency carrier
must not exceed plus or minus 0.002
percent.
(5) The radiated emission from the
SDF must be horizontally polarized.
The vertically polarized component of
the radiation on the course line must
not exceed that which corresponds to
an error one-twentieth of the course
sector width when an aircraft is posi-
tioned on the course line and is in a
roll attitude of 20
°
from the horizontal.
(6) The SDF must provide signals suf-
ficient to allow satisfactory operation
of a typical aircraft installation within
the sector which extends from the cen-
ter of the SDF antenna system to dis-
tances of 18 nautical miles within a
plus or minus 10
°
sector and 10 nautical
miles within the remainder of the cov-
erage when alternative navigational fa-
cilities provide satisfactory coverage
within the intermediate approach area.
SDF signals must be receivable at the
distances specified at and above a
height of 1,000 feet above the elevation
of the threshold, or the lowest altitude
authorized for transition, whichever is
higher. Such signals must be receiv-
able, to the distances specified, up to a
surface extending outward from the
SDF antenna and inclined at 7
°
above
the horizontal.
(7) The modulation tones must be
phase-locked so that within the half
course sector, the demodulated 90 Hz
and 150 Hz wave forms pass through
zero in the same direction within 20
°
of
phase relative to the 150 Hz component,
every half cycle of the combined 90 Hz
829
Federal Aviation Administration, DOT
§ 171.109
and 150 Hz wave form. However, the
phase need not be measured within the
half course sector.
(8) The angle of convergence of the
final approach course and the extended
runway centerline must not exceed 30
°
.
The final approach course must be
aligned to intersect the extended run-
way centerline between points A1 and
the runway threshold. When an oper-
ational advantage can be achieved, a
final approach course that does not
intersect the runway or that intersects
it at a distance greater than point A1
from the threshold, may be established,
if that course lies within 500 feet lat-
erally of the extended runway center-
line at a point 3,000 feet outward from
the runway threshold. The mean course
line must be maintained within
±
10 per-
cent of the course sector width.
(9) The nominal displacement sensi-
tivity within the half course sector
must be 50 microamperes/degree. The
nominal course sector width must be
6
°
. When an operational advantage can
be achieved, a nominal displacement
sensitivity of 25 microamperes/degree
may be established, with a nominal
course sector width of 12
°
with propor-
tional displacement sensitivity. The
lateral displacement sensitivity must
be adjusted and maintained within the
limits of plus or minus 17 percent of
the nominal value.
(10) The off-course (clearance) signal
must increase at a substantially linear
rate with respect to the angular dis-
placement from the course line up to
an angle on either side of the course
line where 175 microamperes of deflec-
tion is obtained. From that angle to
±
10
°
, the off-course deflection must not
be less than 175 microamperes. From
±
10
°
to
±
35
°
the off-course deflection
must not be less than 150 micro-
amperes. With the course adjusted to
cause any of several monitor alarm
conditions, the aforementioned values
of 175 microamperes in the sector 10
°
each side of course and 150 micro-
amperes in the sector
±
10
°
to
±
35
°
may
be reduced to 160 microamperes and 135
microamperes, respectively. These con-
ditions must be met at a distance of 18
nautical miles from the SDF antenna
within the sector 10
°
each side of
course line and 10 nautical miles from
the SDF antenna within the sector
±
10
°
to
±
35
°
each side of course line.
(11) The SDF may provide a ground-
to-air radiotelephone communication
channel to be operated simultaneously
with the navigation and identification
signals, if that operation does not
interfere with the basic function. If a
channel is provided, it must conform
with the following standards:
(i) The channel must be on the same
radio frequency carrier or carriers as
used for the SDF function, and the ra-
diation must be horizontally polarized.
Where two carriers are modulated with
speech, the relative phases of the mod-
ulations on the two carriers must avoid
the occurrence of nulls within the cov-
erage of the SDF.
(ii) On centerline, the peak modula-
tion depth of the carrier or carriers due
to the radiotelephone communications
must not exceed 50 percent but must be
adjusted so that the ratio of peak mod-
ulation depth due to the radio-
telephone communications to that due
to the identification signal is approxi-
mately 9:1.
(iii) The audio frequency characteris-
tics of the radiotelephone channel
must be flat to within 3 db relative to
the level at 1,000 Hz over the range
from 300 Hz to 3,000 Hz.
(12)(i) The SDF must provide for the
simultaneous transmission of an iden-
tification signal, specific to the runway
and approach direction, on the same
radio frequency carrier or carriers as
used for the SDF function. The trans-
mission of the identification signal
must not interfere in any way with the
basic SDF function.
(ii) The identification signal must be
produced by Class A2 modulation of the
radio frequency carrier or carriers
using a modulation tone of 1020 Hz
within
±
50 Hz. The depth of modulation
must be between the limits of 5 and 15
percent except that, where a radio-
telephone communication channel is
provided, the depth of modulation must
be adjusted so that the ratio of peak
modulation depth due to radio-
telephone communications to that due
to the identification signal modulation
is approximately 9:1. The emissions
carrying the identification signal must
be horizontally polarized.
830
14 CFR Ch. I (1–1–24 Edition)
§ 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-
rioration.
(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:
(1)
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
±
17
percent of the nominal value.
(2)
Course alignment.
The mean course
line must be adjusted and maintained
within the limits of
±
10 percent of the
nominal course sector width.
(3)
Course structure.
Course deviations
due to roughness, scalloping, or bends
must be within the following limita-
tions:
(i)
Front course.
(
a
) Course structure
from 18 miles from runway threshold to
Point A must not exceed
±
40 micro-
amperes;
(
b
) 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;
(
c
) Point A–1 to Missed Approach
Point—not more than
±
20 micro-
amperes;
(
d
) Monitor tolerances: width
±
17 per-
cent of nominal; alignment—
±
10 per-
cent of nominal course sector width.
(ii)
Back course.
(
a
) Course structure
18 miles from runway threshold to 4
miles from runway threshold must not
exceed
±
40 microamperes. Four miles
to 1 mile from R/W must not exceed
±
40
microamperes decreasing to not more
than
±
20 microamperes, at a linear
rate.
(
b
) Monitor tolerances: width—
±
17
percent of nominal; alignment—
±
10
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,
1973]
§ 171.111 Ground standards and toler-
ances.
Compliance with this section must be
shown as a condition to approval and
must be maintained during operation
of the SDF.
(a)
Frequency.
(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.