Outline of automatic flight system
prepared and edited by Marco Gröning
**** Nonsense may follow ****
1. Outline of the system
The A300-600 AFS is designed to assist pilots to secure safe flight by realizing the optimum flight situation throughout all the phases from takeoff to landing.
The aircraft is thoroughly controlled by servo system. The command signals are calculated by FAC, FCC and TCC and sent to the system so as to carry out flight. The flight condi.tions are indicated on FMA and ECAM
The computer system as each component of FAC, FCC and TCC operates under self-monitor capabilities and has FAIL PASSIVE function which detects the malfunctions of the system and isolates the fault unit before it affects aircraft maneuver
These computers have dual operation system with FAIL OPERATIVE function which enable to switch a fault current system into standby system having operated in parallel with another power supply.
The functions of AFS are described as follows mainly in approaching and landing phase. And these functions are in accordance with the specification of China Airlines A300-600 at the time of the accident.
FAC has the following functions.
(1) YAW DAMPER function
- - DUTCHROLL DUMPING
- This function activates when the IRS detects a yaw rate.
- - TURN COORDINATION
- This function activates when the EFCU detects a roll control wheel deflection above a predetermined threshold.
- - ASSISTANCE TO THE AUTOPILOT
- This function provides an assistance to the AP in case of engine failure to counter the induced lateral acceleration during dynamic phase of the recovery.
(2) PITCH TRIM function
This function sends a pitch-command to THS and has the following subfunctions.
- ELECTORIC TRIM
The load on the control column is alleviated by this function which balances the pitch axis moving the THS. The neutral position of the elevator is on the chord line of THS. The electric trim is available when in manually controlled flight or an AP is in CWS.
- AUTO TRIM
This function is activated by AP engagement. It has the same function as the electric trim and in addition, prevents the aircraft from bumping when an AP is disengaged. It is active when the AP is engaged in CMD and when the AP is engaged in CWS and pitch trim control switch is not operated.
- MACH/Vc TRIM
This function improves the longitudinal static aircraft stability by varying the THS position as a function of Mach number or Vc. MACH TRIM is active in clean configuration (SLATS/FLAPS 0/0) and abo~e MACH 0.7. Maximum authority is 0.7 pitch-up. Vc TRIM is activated in all configurations but only above 200Kts. Maximum authority is 1.0ø pitch-up.
- ALPHA TRIM
This function restrains pitch-up produced in high Mach numbers, as well as high AOA at low speed. It is active when no AP is engaged and no AIRBREAK is extended. Maximum authority is 1.5ø pitch-down. It is also active in low speed in 15/0, 15/15 and 15/20 confi~uration. Maximum authority is 4.0ø pitch-down.
- Override function
All these trim functions can be overridden by MANUAL PITCH TRIM CONTROL WHEEL. Movement of this wheel disconnects both pitch trim systems and pilots can override the THS.
(3) Flight Envelope Protection Function
Alpha Floor Protection Function
This function protects aircraft against stall with a maximum thrust automatically selected by TRP, when an excessive AOA is detected. When the alpha floor function is activated, "THR L" is displayed on FMA and the thrust levers move forward by the speed of 8 deg/sec.
TCC assures the following functions.
- Continuously computes thrust limit corresponding to the mode selected on TRP.
- Output signals to acquire and maintain the thrust limit or a target thrust.
- Output signals to acquire and maintain the SPD/MACH.
Retract the THR to the idle position (RETARD MODE).
(2) Connection with engine control
A single electric actuator actuates the THR levers through two coupling units. Each THR lever position is sensed by 3 resolvers which transmits the command signal to the corresponding FADEC and feed back to the TCC. The pilot can control the engine thrust manually overriding the auto thrust for each engine by applying a light load on the throttle lever
4. AP/FDsystems (FCC)
- The different modes are selected through push button located on the FCU.
- The FD provides information to the pilot to allow manual guidance of the A/C.
- When an AP is engaged in CWS, the AP maintains the pitch attitude and the bank an at the time of engagement. The CWS function allows the pilot to manually char these reference values by applying a load on control column or wheels. (More load reqired in a greater Mach number.)
- When one or two AP are engaged in CMD, the aircraft is controlled automatically according to the selected mode.
- Each AP can be engaged by setting the corresponding AP levers to ON. It disconnected intentionally when the lever is set manually to OFF, or when one of the. disconnect push buttons on the control wheels is pressed.
- The two AP (FCC) can not be engaged simultaneously in CMD, except in LAND or ( AROUND node.
- By setting an AP levers to ON when another AP was engaged, the previously engaged AP will be disconnected, when the dual operation is not allowed.
(2) AP/FD Mode is activated by FCU in the following modes.
(Examples of each mode are referred to a ~h of figure 2.)
- BASIC Mode
This mode maintains V/S and HDG.
With the power ON or another mode disengaged, it stabilizes the attitude around center of gravity and maintains the current heading. It keeps V/S as the V/S mode. It a keeps the heading at the time when the bank becomes 5 deg or less as HDG mode.
- ALT Mode
This mode maintains the level-off altitude, SPD/MACH mode is engaged on ATS. I not engaged in LAND track phase.
- LVL/CH Mode
This mode is to change an altitude.
This mode is engaged except flap position 40ø when the FCU selects a different altit frnm the current altitude
- PROFILE Mode
FMS controls the vertical navigation and the thrust when AITHR push button is armed
- HED/SEL Mode
This mode is to change the current heading.
The inner knob of HDG/SEL allows the current heading to change and the outer knob allows the maximum bank angle to set.
- NAV Mode
The control of horizontal navi~ation is conducted by FMS.
- VOR Mode
This mode allows an aircraft to capture and maintain VC)R course.
This mode has three phases "Arming", "Capture" and "Track".
This mode is disengaged with GA, NAV or LAND mode selected.
- LOC Mode
This mode is to capture and maintain the LOC course.
It has three phases, "Arming", "Capture" and "Track".
- TAKE OFF Mode
This mode is to curry out the longitudinal and lateral mode operation at time of take off.
This mode is engaged by Go lever operation.
- LAND mode
This mode captures and maintains an ILS beam (LOC beam and GLIDE beam) then guides the aircraft to align the runway axis and to flare.
- Engagement conditions
LAND mode is engaged by pushing the LAND push button:
- Radio altimeter height is greater than 400ft.
- An ILS frequency and a runway course are selected on the ILS control panel.
LAND mode is disengaged by one of the following operations.
- Selection of GO AROUND mode. (all phases)
- Pressing a second time LAND push button. (except for LAND TRACK phase)
- Selecting other modes. (LOC CAPTURE, LOC TRACK, GS CAPTURE or GS TRACK phase)
- GO AROUND mode
This mode allows to perform a go-around with a longitudinal and lateral guidance of the A/C. The longitudinal mode is SRS which allows to acquire and maintain VTG+10kt. The lateral mode maintains the wings at level. In addition, it engages THR mode in ATS.
GO AROUND mode is engaged by triggering GO LEVERS provided SLATS/FLAPS is extended to at least 15ø.
GO AROUND mode is disengaged by selecting other modes except for LAND mode. If a longitudinal mode is selected, HDG mode engages as lateral mode. If a lateral mode is selected, SRS remains active as longitudinal mode.
5. Overriding by control column
When the AP is engaged in one of the modes of except LAND Trcck mode or GO AROUND mode, the AP disconnects when the pilot input to the control wheel over 15 kgf. On the other hand, when in LAND Track mode or GO AROUND mode, if the pilot applies specific force to the control wheel, he can override elevator control by AP. For these two modes, the AP do not automatically disconnect, regardless of the force the pilot applies to the control wheel. For example, when AP is engaged in GO AROUND mode if the pilot override the autopilot by applying nose down motion to the elevator, THS moves to nose up direction in order to comply with the obJectives of the current GO AROUND mode. According to FCOM, this override function was installed in order to protect the pilot against AP abnormal behavior. As described in the above, when the pilot move the control wheel and if this control input is against the elevator order of the AP, the AP will move the THS so as to maintain the aircraft on the scheduled flight path, a risk of out of trim is real and may lead to a hazardous situation in LAND and GO AROUND mode only, which is given as a caution in FCOM. (See Appendix 2-2 and 2-4)
6. Warning system
- Stall Warning
Stall warning activates when speed drops to about 1.12 Vs. AOA at the time is about 8.5ø in clean configuration, and about l Sø on other occasions. The stick shaker operates and warning (cricket) sounds.
GPWS provides pilot with aural warning and warning lights, when the current flight path flown is in proximity to terrain or when the aircraft is making approach to land without appropriate landing configurations.
- Landing capability change
Should there be a malfunction of instruments used for approach, the category of landing is lowered. When GA mode is engaged during an ILS approach in manual operation, the landing capability is lost. And at five seconds after the occasion has been detected, a warning sounds to indicate a decrease of landing capability.(See Figure of Appendix 1)
7. An example of operation and annunciation of LAND mode
- One AP in CMD with PROFILE and NAV mode is engaged. On the ILS control panel, ILS frequency and runway course is selected.
- When LAND push button is pressed, LAND mode arms and CAT2 illuminates on FMAs. CAT 3 illuminates when the both APs are engaged in CMD.
- LOC CAPTURE engages near the interception point and LOC* illuminates on FMAs..
- LOC TRACK engages after the aircraft is stabilized on the LOCALIZER, and LOC illuminates on FMAs.
- GS CAPTURE engages when GS deviation is smaller than 2/3 dots and G/S* illuminates on FMAs. And ATS is engaged in SPD mode by A/THR function, and SPD illuminates on FMAs.
- GS TRACK engages when the aircraft has been stabilized on the GLIDE SLOPE for at least 10sec, and G/S illuminates on FMAs.
- LAND TRACK engages at 400ft radio height, provided LOC TRACK and GS TRACK has been activated for more than 10sec. LAND illuminates on FMAs.
- FLARE phase initiates at about 50ft radio height, and FLARE illuminates on FMAs.
- At about 30ft radio height, RETARD of THR is initiated in ATS, and RTAR illuminates on FMAs. (See Fig 2 of Appendix 1)
Peter B. Ladkin, 1999-02-08
Last modification on 1999-06-15
by Michael Blume