July 25, 2019, ©. Leeham News: In our series about classical flight controls (“fly by steel wire”) and Fly-By-Wire (FBW or “fly by electrical wire”) this week we cover the difference in system infrastructure the two controls methods call for.
We will use the Boeing 737 as the classical control example and the Airbus A320 as the FBW example.
Last week we talked about the very wide flight envelope in dynamic pressure, Q, an airliner experiences and how this affects the displacement and force needed for moving the control surfaces when flying the aircraft.
A classical system like the 737 flight controls use hydraulic power to achieve this and clever aerodynamic aids to provide a manageable direct mechanical backup in pitch and roll should the hydraulics fail.
The FBW A320 relies on a functioning hydraulic system at all times. It has no mechanical flight control backup. It has a temporary “FBW reboot” backup using mechanical elevator trim for pitch control and mechanical rudder control to cater for roll via secondary yaw-roll coupling. The control mode is good enough for continued flight during the reboot but not for descent and landing. While this mode needs no electrics (not even battery, it moves the valves on the hydraulic jacks mechanically) it needs hydraulic pressure to the horizontal stabilizer trim jack and at least one of the rudder actuators.
Consequently, the A320 needs more redundancy in the hydraulic system. In addition to a dual circuit base system (Figure 1) with engine-driven pumps and an electrical pump pressurized third backup system (powered by batteries if needed), it has a fourth backup system.
The backup hydraulic system has a Ram Air Turbine (RAT) hydraulic pump adding a fourth level of redundancy with longer endurance than a battery-driven backup pump. The resulting hydraulic system is shown on the right-hand side of Figure 1.
Figure 1 also shows how the three different hydraulic systems are dividing the aircraft’s different control surfaces between them, with several redundant actuators per surface (the rudder, for instance, has three actuators each feed by its circuit).
Different to the A320, the 737 flight control system is architected so the aircraft can be flown and landed without functioning hydraulics. Hence its hydraulic system can function with a three circuit system with principally the same architecture but without the RAT pump.
Similar to the hydraulic system, the A320 electrical system has a higher redundancy level than the 737 to guarantee an uninterrupted supply of power to the FBW system. It has five levels of redundancy, whereas the 737 is fine with four levels. The 737 and A320 have dual main electrical buses which distribute AC and DC power from engine-driven generators, Figure 2 left graph. These can individually supply all systems in the aircraft.
In addition, both have an APU with a generator which can be run in the air and supply all electrical buses on the aircraft, second graph. If the two engine generators and the APU generator can’t supply power (third graph), the A320 has a generator driven by the RAT hydraulics which supplies consumers on the Essential buss with electrical power. Finally, should all generators fail, the essential bus connected systems, which includes critical parts of the FBW, are feed by the dual batteries (last graph).
The 737 has in principle the same system without the RAT based emergency generator, giving it a four-level redundancy. This is enough as the Flight Control system can work without electrical power.
In the next Corner, we look at how the classical and FBW control systems achieve their control function redundancy.