Ask Captain Lim

Hi Capt Lim,

I have some questions regarding the fly-by-wire control system:

1. How does the fly-by-wire system of the Boeing 777 differs from the standard Airbus fly-by-wire system?

2. I have heard that the fly-by-wire system comes on shortly after takeoff. How long after takeoff? What altitude?

3. Why is the fly-by-wire system not put on during takeoff? It would greatly increase the safety of the takeoff.

Thank you,

Peter.

Hi Peter,

You have raised an interesting topic. Allow me to expand on it for the benefit of other readers.

1. When the Wright brothers first flew about Century ago, I believe their plane's flying controls were achieved via strings only. Today, with the advancement of technology, computers have been used to assist the human pilot in actually flying the airplane. This was born the concept of 'fly-by-wire' technology which simply means that computers on the plane, transmit the pilot inputs into electrical signals through wires to actuators that move the control surfaces. Hence the name, 'fly-by-wire'. On conventional planes after the Wright brothers, the flight-control surfaces are moved by hydraulic devices which are controlled by cables that run through the airplane.

The first flying machine to use this digital fly-by-wire concept successfully was the Lunar Module. This Module, using the concept, was able to take men from orbit to the surface of the moon in 1969 as landing a rocket on its own required a deftness and control that no human being could master. This concept was also applied on the new-generation military aircraft such as the very successful F-16 and the F-117 Stealth fighter.
Even though the military had adopted the fly-by-wire concept by the early 1980's, the commercial sector was less enthusiastic. The argument then was that, the commercial jet did not need the agility required to fly a fighter nor did they have to worry about designing for stealth. But the fact is that, fly-by-wire concept did offer lower fuel costs and smoother flights through bad weather.

Boeing continued to the chose conventional control systems for its 757 and 767 aircraft but Airbus Industries went ahead and introduced digital fly-by-wire in its A320 airplanes. It was only on the Boeing 777 that the Company finally decided to introduce the digital fly-by-wire controls. Thus, this concept which is basically the result of wanting to put a man on the moon, have today become an accepted part of modern aviation design.

Although the Boeing 777 and the Airbus 320 series and later, adopted this new concept, there are slight differences in their applications. Airbus has taken a much different philosophical approach to using computers than Boeing. The European airplane maker designed its new fly-by-wire jets with built-in protections or hard limits.

The Boeing Company, on the other hand, believes pilots should have the ultimate say, meaning that on the Boeing jets, the pilot can override onboard computers and their built-in soft limits. The issue is, should pilots or a computer have the ultimate control over a commercial jetliner as the plane approaches its design limits in an emergency? There were strong arguments by pilots on both sides of the debates. Some pilots were of the opinion that computer protection of the A320 is very good whereas other pilots support the Boeing philosophy that they must have the final say in controlling the airplane.

Both have valid arguments. In 1995, a Boeing 757 crashed into a mountain while trying to land at Cali in Columbia, killing 159 people on board. In this accident, the warning system on board had alerted the crew that they were about to crash onto the mountain. The Captain executed a climb but forgot to retract the speed brake. On an A320, Airbus points out, the protection in the computer would have retracted the speed brakes automatically. But Boeing argues that, the jet would have hit the ridge even if the speed brakes had been retracted. Airbus planes with their fly-by-wire technology and 'automatic protections' have also crashed. In fact, six of the A320s have so far been lost. One of the very first A320 jets crashed shortly after the jet entered service in 1988, raising many questions about the Airbus philosophy.

The pilots were making a low-and-slow fly-pass during an air show in Habsheim, France. They were supposed to fly by with the gear down at about 100 feet. Instead, they came in at less than 30 feet off the ground. When the plane gets below 50 feet, the computer assumes the pilots are trying to land. The plane did exactly what it was supposed to do and crash-landed onto the trees! I hope you can see the difference of this system between Boeing 777 and Airbus now.

2. You are right, the fly-by-wire system comes on after the plane lifts off from the ground. It is controlled by the weight switch. How does this principle work? Well, when there is no weight exerting on it, especially when the airplane is airborne, the switch is automatically triggered on to activate the system.

3. So when the airplane is airborne, the fly-by-wire system immediately comes on. Thus, safety is not compromised. It would instantly provide any protection that it was designed to do. On the ground, with the wheel weight-switch still compressed and fly-by-wire not activated, the flight controls still move in the conventional sense, just like any other airplanes.

Hi Lim,

Thank you for the information regarding the fly-by-wire system. I just have a comment about the A320 crash record. I think it is fair to note that, 5 out of the 6 A320 crashes were because, in the beginning, pilots did not understand the fly-by-wire system well. So there were 5 accidents due to that. Since 1993, there has only been one such crash.

I think the safety of the A320 should be judged afresh since the fly-by-wire system is now understood. I don't know if I am totally right though.

Thank You,

Peter

Hi Peter,

Since the introduction of flight deck automation, there were strings of Airbuses crashes in which misunderstanding between pilot and computer were to be blamed. The classic example, was the A320 crash at Hansheim, in France in 1988. Most pilots take time to adjust to these new concept and unless they are very current and well trained, the confusion that arise in the pilot-computer interface from the conventional, pre-automation era, could lead to undesirable consequences.
As you have rightly said, pilots today are better trained and understand the system well, so that previous accidents are not repeated. I believe that the flight deck of the Airbus 380 would have even better and safer features than the latest state-of-the-art cockpit automation we have today.

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