(b)
The initial turn to intercept the localizer beam may use the RCWS or HDG SEL cruise modes.
(7)
Single Channel Approach Localizer Capture
(a)
Localizer capture occurs when the adaptive capture detector or the lateral beam sensor (LBS) trips. The adaptive capture detector uses beam error, beam rate, and PSC heading error signals to determine the capture point. This point varies with intercept angle and airspeed of the airplane as it approaches the localizer beam. For example, if the approach to the beam is at a shallow intercept angle with a correspondingly slow crossbeam rate, the capture phase is initiated closer to beam center than if a larger intercept angle with a relatively faster crossbeam rate is detected. Adaptive capture enables smooth localizer captures with minimum overshoot. Localizer mode engagement very close to beam center is monitored by a lateral beam sensor which trips at a beam error threshold of 60 mv or 0.8 degrees of beam deviation. The LBS logic discrete bypasses the adaptive capture requirements so that the capture phase can be initiated independent of crossbeam rate and PSC heading information. The capture phase is initiated 3 seconds after the adaptive capture detector or LBS trips. The 3-second delay eliminates the effects of nuisance detector tripping.
(b)
At localizer capture the airplane bank rate limit is increased from 4 deg/sec to 7 deg/sec. Fixed bank angle limits of +30 degrees are provided, the LOC mode is latched and LOC ENG is indicated on the FMA's (black on green), HDG SEL mode trips off and RCWS command is inhibited. The digital roll cruise control commands are switched out and the roll analog approach signals are enabled. Beam displacement and PSC heading error signals are coupled in to the bank command limiter. Cross track path damping is provided by complementary filtering localizer beam rate and roll attitude compensated lateral acceleration to obtain a pseudo cross-runway rate.
(c)
During cross winds, a crab angle with respect to the inbound runway heading is necessary. In this situation, a beam standoff error from the intercept limiter will result to cancel the steady state PSC heading error. The airplane will not track beam center but rather a parallel offset path. This condition is taken care of in the Localizer On Course (LOC O/C) phase.
(8)
Single Channel Approach (LOC O/C)
(a)
Localizer On Course mode latches when the detected beam error is less than 0.8 degree, beam rate is less than 0.045 deg/sec, and bank angle attitude is less than 6 degrees. Beam tracking is accomplished during the on course phase using beam error, PSC heading error, roll attitude, and cross-runway rate.
(b)
At LOC O/C, the beam integrator is unclamped allowing integration of radio beam errors. This allows control of the airplane to beam center with no beam standoff.
(c)
During the LOC O/C phase, bank angle limits reduce to + 8 degrees and bank angle rate limits are reduced to 4 deg/sec over 1 second. The crossbeam rate limit is also reduced.
(9)
Single Channel Approach on Course (AOC)
(a)
AOC mode occurs when LOC O/C, G/S ENG, and valid radio altitude is less than 1500 feet. When this mode latches, the PSC heading error is removed and a lagged roll integrator signal is coupled in. The lagged roll integrator signal augments cross-runway rate for path damping and is also used for crosswind compensation. A proportional lateral accelerometer signal is coupled in at AOC to provide compensation for asymmetrical thrust changes. Also at AOC, the beam integrator is reset momentarily during dual channel autolands to start the integrators in each channel with the same initial condition.
(b)
The localizer beam anomaly detector is enabled at AOC and 23 seconds to provide protection against the effects of localizer beam overflights and hardovers. When the detector senses beam error and beam rate in excess of 0.30 degree and 0.13 deg/sec respectively, the beam error input is shut off. The rate input is filtered to eliminate detector nuisance trips due to beam noise. The detector output logic along with the LOC superflag will automatically disengage the autopilot in the event the beam is disabled for a period greater than 10 seconds.
(10)
Single Channel G/S Capture and Track
(a)
Prior to capture of the G/S beam, the beam integrator synchronizes to store the signal necessary to cancel the beam error command to the valve amplifier. This is done so at G/S capture, the initial command to the valve amplifier is zero. As the airplane nears the beam and the vertical beam sensor trips, the integrator is unclamped and allowed to integrate the beam error. The sum of displacement beam error and beam integral error signal produces the G/S capture command.
(b) 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：737 AMM 飞机维护手册 AUTO FLIGHT 自动飞行 1(69)
