Safety Investigation Report 2018:1 Factual Information/1.6/1.6.8 Aircraft Systems/5
SAFETY INVESTIGATION REPORT MH370 (9M-MRO)
5) Flight Controls
The flight control system is an electronic fly by wire system. It is divided into two separate systems to control the aircraft in flight.
Primary Flight control system (PFCS) is a modern three axis, fly by wire system. It controls the roll, yaw and pitch commands using the ailerons, flaperons, spoilers, elevators, rudder and horizontal stabilizer. The high lift control system (HLCS) comprises of inboard and outboard trailing edge flaps, leading edge flaps and Kruger flaps. It supplies increased lift at lower speeds for take-off and landing.
The PFCS and HLCS use 3 dedicated ARINC 6299 Flight Control digital busses to transmit data signals to command the flight controls. Mechanical control is available to two spoilers and horizontal stabilizers.
The PFCS has three operational modes of command - Normal mode, Secondary mode and Direct mode. The PFCS command signals are computed by three redundant Primary Flight Computers (PFCs) in Normal and Secondary modes and directed through four Actuator Control Electronic (ACE) units. In Direct mode, the control surface command signals are computed by the ACEs without reliance on the PFCs.
The PFC also receives airspeed, altitude and inertial reference data from Airplane Information Management System (AIMS), Air Data Inertial Reference Unit (ADIRU) and Secondary Attitude and Air Data Reference unit (SAARU). The PFCs calculate the flight control commands based on control laws, augmentation and envelop protections. The digital command signals from the PFCs go to the ACEs that will change the digital signal to analogue format and send to the power control units (PCU) that will command the control surface movement.
The HLCS operates in three modes, primary, secondary and alternate. Command signals are transmitted from the flap lever to two Flap Slat Electronic Units (FSEU).
The FSEU process the flap command and control the sequence of flaps and slats operation. It also commands auto slat, load relief and asymmetry protection.
Two spoilers and the horizontal stabilizer receive mechanical control signals from pilots input.
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9 Aeronautical Radio, Incorporated (ARINC) 629 is an aeronautical standard which specifies multi- transmitter data bus protocol where up to 128 units can share the same bus.