MIL-DTL-9490E
error allowance decreasing proportional to the distance from the VOR station. Average tracking
error shall be measured over a 5 minute period between 50 and 10 miles from the station or
averaged over the nominal aircraft flight time between the same distance limits, whichever time is
shorter.
3.1.2.8.1.2 Capture tracking. Overshoot shall not exceed 6,300 feet beyond the desired
ground track line in a no-wind condition for captures 120 miles or more from the station with
intercept angles up to 30 degrees. The required 0.3 damping ratio shall be exhibited for continuous
tracking between 120 miles and 20 miles from the station.
3.1.2.8.1.3 Overstation. The VOR/TACAN mode shall include automatic means for
maintaining the aircraft within ±1 degree of aircraft heading or ground track existing at the
inbound edge of the VOR zone of confusion (ZOC). During overflight of the ZOC, adjustment of
the preset course heading or its equivalent shall cause the roll AFCS to maneuver the aircraft to
capture the appropriate outbound radial upon exiting from the ZOC. The VOR/TACAN capture
maneuvering limits may be reinstated during overstation operation.
3.1.2.9 Automatic instrument low approach system. The approach mode of the AFCS shall
respond to localizer signals for lateral guidance and glide slope signals for vertical guidance. The
system shall be designed to automatically steer the aircraft to a minimum decision height of
100 feet during ICAO Category II weather minimums. The system shall provide timely warning to
permit the pilot to complete the landing if runway visual contact is established or to safely execute
a go-around following any single failure or combination of failures not shown to be extremely
remote as defined in 6.6. The system shall comply with the tracking requirements of
3.1.2.9.1 through 3.1.2.9.3 for probable combinations of headwinds to 25 knots, tailwinds to 10
knots, and crosswinds to 15 knots) with the probability of occurrence of such winds and associated
turbulence and wind shears as specified in 3.1.3.7.3.
3.1.2.9.1 Localizer mode. The AFCS shall cause the aircraft to maneuver to acquire the
localizer beam. Heading or roll rate and attitude commands shall be limited to provide a smooth
capture and subsequent tracking of the localizer beam. Overshoot shall not exceed 0.5 degree (37.5
μa) radial error from localizer beam center for captures with initial intercept angles of 45 degrees
at 8 miles from runway threshold and increasing linearly to 60 degrees at 18 miles from runway
threshold in a no wind condition. During localizer capture, the system shall exhibit a damping ratio
of at least 0.1 within the noted capture ranges, including the effects of system nonlinearities. The
system shall be considered to be tracking whenever the following conditions are satisfied: localizer
beam error is 1 degree (75 μa) or less, localizer beam rate is 0.025 deg/sec (2 μa/sec) or less, and
roll attitude is 5 degrees or less. During beam tracking, the system shall exhibit a damping ratio of
0.2 or greater at a distance of 40,000 feet from the localizer transmitter. The AFCS shall maintain
the aircraft 2σ position within 0.33 degree (25 μa) of localizer beam center whenever the aircraft is
between (1) 40,000 feet horizontal distance from the localizer transmitter, and (2) the point where
100 feet above the ground is reached; these criteria shall be based on a Category II localizer ground
installation and 10,000 foot runway is defined by ICAO Annex 10.
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