position to take manual control of the thrust for the landing. With the
A/THR engaged but not in the automatic landing conditions, this message
is delivered at 20 ft.
If no longitudinal mode is active on the AP/FD, the A/THR mode selected
is SPD/MACH.
When the AP and FD are not engaged, the A/THR mode selected is SPD/MACH
on condition that the RETARD mode is not already active.
The RETARD mode is kept if the AP/FD is disengaged.
As for the AP/FD modes, the A/THR modes of the FMGC which has no priority
are synchronized on the A/THR modes of the FMGC which has priority.
B. Alpha Floor Protection (Ref. Fig. 002) When the angle of attack reaches a limit value dependent on the configuration and longitudinal wind gradient, the FAC sends an order to the FMGC. This order is taken into account in the FMGC as soon as the landing gear shock absorbers are extended at takeoff and up 100 ft at landing. This causes:
-
the engagement of the A/THR function
-
the activation of the Alpha FLoor Protection of the A/THR which
commands max. thrust to the engines whatever the AP/FD mode engaged. The activation logic of the Alpha Floor Protection is not synchronized between the two FMGCs. The Alpha Floor can only be cancelled through the disengagement of the A/THR function.
C. A/THR Control Laws and Associated Reference Data The A/THR laws available are:
-fixed thrust hold (THRUST law)
-speed or MACH hold (SPD/MACH law of A/THR).
(1) THRUST law This law is selected when the A/THR operates in THRUST or RETARD mode or when the Alpha floor protection is active. The THRUST law permits to command to the two engines a fixed thrust which can be:
-
N1/EPR LIM: limit thrust (calculated by the FADECs according to the throttle control levers position) when the AP/FD mode gives a climb order
-
N1/EPR IDLE: reduced thrust when AP/FD mode gives a descent order or RETARD mode engaged
-
N1/EPR MAX : max. thrust whatever the position of the throttle control levers when the Alpha Floor Protection is active. This thrust corresponds to the thrust that would be obtained when the two throttle control levers are in TO/GA position.
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Alpha Floor Protection
Figure 002
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1
-FM N1/EPR target: thrust that the flight management section
calculates when it ensures the longitudinal guidance of the
aircraft.
(2) SPD/MACH law (Ref. Fig. 003) This law is selected when the A/THR operates in SPD/MACH mode on condition that the Alpha Floor Protection is not active. Depending on the logic for MACH control selection and selected or managed speed control (Ref. 22-10-00-AP/FD), the SPD/MACH law enables to acquire and hold:
-either the speed displayed on the FCU when the selected control of the speed is active - or the Mach on the FCU when the Mach control is selected and the
selected speed control is active - or a speed chosen by the managed speed control - or a Mach chosen by the managed speed control when the Mach control
is active.
Depending on the target speed or Mach, a precommand N1/EPR is
calculated:
-
from an aerodynamic model
-
from a simplified engine model independent from the flight
management section. A direct channel and an integrated channel permit to hold the speed or Mach and compensate the computation error of the precommand N1/EPR The VLS and VMAX limit the target speed and Mach. A load factor precommand in V/S and ALT ACQ mode permits to minimize the speed variation during dynamic maneuvers which involve climb or descent at constant speed.
(3) Elaboration of N1/EPR controlled by the A/THR (Ref. Fig. 004) To perform the automatic thrust control, the A/THR transmits to the FADECs the rate which must be commanded to the engines: N1/EPR target. The N1/EPR target is elaborated in the basic loop of the A/THR. When the A/THR is not active, the N1/EPR target which is transmitted is the recopy of the N1 corresponding to the throttle control lever in the most advanced position. When the A/THR is activated, the basic loop permits to pass smoothly from N1/EPR TLA to the N1/EPR elaborated by the THRUST or SPD/MACH control laws.
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