25 March 2016, ©. Leeham Co: Last week we covered the natural stability of commercial aircraft and the most important movements the aircraft would go into if we had no pilot intervention.
Now we will cover how Fly-By-Wire (FBW) systems make enhanced flight control laws practical to implement. We will cover the principal build up of a FBW system with enhanced control laws and two of the most common approaches used in the market for such control laws: the Airbus and the Boeing implementations.
The discussion will focus on the essential and forgo many deeper discussions over redundancy and safety. Otherwise the subject expands into a book rather than an easy to read article and that is not what we want.
Airbus control laws
When we described the first commercial aircraft FBW, which was for the Concord, the electrical signalling controlled the movables directly with some minor modifications of the signals by a stability augmentation system. During the development of the A320, Airbus decided to use the knowledge gained from the Concorde but go one step further.
Digital computers were introduced that were allowed to change the movable controls signals in a rather major way. Instead of the pilot’s stick deflection in pitch commanding a certain movement of the pitch movables, the pilot’s intention was interpreted by the control computers and they commanded the aircraft to react with a certain pitch rate (low speed) or pitch load factor (high speed). For bank, the computers gave the pilot a roll rate. For yaw, the computers provided yaw damping and turn coordination (kicked a bit or rudder to fly cleanly without the pilot needing to do it). The yaw channel will also compensate for thrust asymmetry including if an engine goes inoperative during take-off or flight.
The gradual change from pitch rate to load factor as speed increases is the characteristic of a control law called C* (C star). This flight law is phased in after Take-Off, on the ground and during take-off the aircraft is controlled in a normal way, i.e., stick movements control movable deflection.
The feeling of flying an Airbus FBW aircraft after lift-off is as if the aircraft was in autopilot stick control mode all the time. The aircraft keeps the attitude and roll that one sets, regardless of speed or external disturbances. For landing, the computers put on a gradual pitch down during flare to simulate an aircraft that gets nose heavier as speed reduces.
All the changes in control laws are very gradual and as a pilot, one has the feeling of a normal aircraft except that one does not have to chase the aircraft in pitch and roll; it remains where one left it. It was easy to notice those that had flown a classical aircraft recently when we did the A350 media pilot test in April 2015. These pilots corrected every gust that hit the aircraft.
Having read the Airbus training manuals for the A330/A350 and trained in A320 and A330 simulators, I let the aircraft correct the disturbances. It worked perfectly and lowered my workload. I could focus on just controlling the aircraft when I wanted an attitude change.
One consequence of a C* controlled aircraft is that the attitude remains constant regardless of speed. Even when the aircraft approaches stall speed, the flight law keeps the nose steady, i.e., with low thrust from the engines, it flies the aircraft into stall. Airbus therefore let the computers stop that; they had all the data they needed (airspeed, altitude, aircraft configuration, load factor, alfa angle….) to stop the aircraft from entering dangerous flight situations.
There is a lot written about these protections modes and how they work. I’m not going to repeat that here. I was allowed to test them all (stall, overspeed, to high bank….) during the A350 flight test and they all worked smoothly and naturally (more throttle followed by pitch down for stall, pitch up for overspeed, roll back for to high bank).
It did not feel un-natural or constrained to have these protections come in and sort out any mess that one had created. If one reach these limits, there is something really wrong and one is grateful for any help.
We will now go on to describe the Boeing philosophy, which is a bit different. I fully understand the ideas behind it and would love to fly it (I would probably like it) but for those that are prone to armchair slamming of the Airbus way of implementing flight laws, let me tell you it’s the closest to the flight laws used on all modern fighters (these are unstable FBW aircraft and you need to be hard protected the Pilots from flying them into stall).
Boeing control laws
Boeing implemented computer augmented FBW about 10 years after Airbus for the 777. Boeing decided to use a modified C* flight law for pitch that included a feel for if the aircraft was trimmed in speed or not. The law is called C*U and it does lower/raise the nose if the aircraft moves away from trimmed speed. Roll is controlled with displacement for 777 and roll rate like Airbus for 787. Kicking rudder gives movable displacement for 777 and a certain yaw angle for 787. The yaw channel also does automatic turn coordination and thrust asymmetry compensation like the Airbus system.
The result is that the aircraft feels like a normally stable aircraft to fly; lose speed and the aircraft lowers the nose to regain speed. With a more conventional reaction of the aircraft in pitch to changes from steady flight Boeing decided that they would offer protection in a softer way than Airbus. Lower the speed and the aircraft will dip the nose to gain speed and you would need to pull back on the yoke to keep pitch attitude. As you pull the yoke back to keep pitch attitude, back-drive servos will increase the pitch force and make it harder and harder to keep the nose up and fly the aircraft into stall.
The philosophy is similar for the other axis. If you are bent on rolling the aircraft to 90°, you can, but you need to be strong; the yoke will fight you. This means the aircraft can be stalled and overloaded in pitch and it can get to any desired roll angle. The Boeing protection tells you that you are way outside the normal by making it very physical to get there.
I have not flown the Boeing philosophy but I can imagine that it feels natural. Whether I would prefer one of the other I have to reserve until I have tried them both in real flight. One area that I know that I will like better on the 787 is the way the throttles work. On Boeing aircraft, a set auto-throttle will command thrust changes through the cockpit throttle levers, i.e., one can see and feel how the auto-throttle is working.
On Airbus aircraft, the throttles are set at Climb position after take-off and remain there in an autopilot/FMS controlled flight until one throttles back for landing. It takes some getting used to.
Benefits of computer controlled FBW
There are several benefits of a computer controlled FBW that are independent of the flight law philosophy chosen. With the FBW computers having access to all data from the aircraft’s avionics systems (air data computer, inertial platform, distributed accelerometers, GPS…), a number of functions can be realised to lower the loads on the airframe and thereby gain weight and passenger comfort.
The computers can limit the maximum deflection speed and angle of the movables to avoid airframe overload. They can also modify the wing’s lift distribution, making it more central for flight close to the aircraft’s MTOW and then to spread it out to the drag optimal elliptical distribution as weight decreases during flight due to fuel burn off. Once again, this limits the loads on the wing which decreases weight.
The FBW can also be implemented to give the passengers gust alleviation. Accelerometers in the aircraft’s nose signal the gust to the FBW which control spoilers, ailerons and stabilator to alleviate the gust.
All FBW implementations use a multitude of channels and computers to offer a high level of redundancy. Commercial airliners also offer a mechanical back-up mode that enables the crew to restart FBW computers should a severe electrical problem put these out of order.
Such backup systems are normally made of a separate horizontal tail trim system and direct control (mechanical or electrical) of the aircraft’s rudder. Through the secondary roll effect, one can thereby control bank as well as pitch until the FBW computers are back on-line.