Letters to the Editor

Understanding modern aeroplanes

Friday, March 15, 2019

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Dear Editor,

In the March 13, 2019 Clovistoon, published in the Jamaica Observer, the person standing beside an aeroplane is seen to say, “It's so sad, aircraft manufacturers need to come up with more safety features!” When you understand modern aeroplanes better you may see why this might not be necessarily a good thing.

Years ago, Airbus introduced the first of the new breed of aeroplanes with what is known as “fly by wire” controls. This was something to make the aeroplanes lighter and more responsive. Up to that time hydraulically assisted controls were what was in use, where hydraulic oil at very high pressure was pumped around the aircraft to hydraulic cylinders (smaller versions of the hydraulic cylinders on heavy earthmoving equipment like bulldozers), cables, or linkages were used to move the control surfaces to make the aeroplane go up, down or sideways.

What Airbus did was to get rid of the heavy (and long) high-pressure hydraulic oil piping, hydraulic oil tanks, pumps, etc, and replace them with much lighter high-voltage electrical wiring to either high power electric motors or small hydraulic systems at the control surfaces. Having electric systems, they then thought, why control them by computers where you can modify how the control surfaces move simply by software.

How an aeroplane flies or drops out of the sky is a function of many things such as how fast it is flying, how steeply it is trying to climb, how the aeroplane reacts if you haul back on the controls in the cockpit too hard, etc. Before the aeroplane designers write the “How to fly this aeroplane” manual they have test pilots fly it to its limits by actually trying to keep the aeroplane flying while going slower and slower until it starts to drop out of the sky, dragging the tail on the ground while taking off, bend the aeroplane until it breaks, etc. These are called the aeroplane's Control Laws. If your ordinary motorist breaks the law he might get a ticket. If you break the aeroplane's Control Laws, you might die, along with everybody else in your aeroplane.

Airbus wrote the computer code to incorporate the Control Laws into the control systems. So that there is a computer between the pilot and the control surfaces. For example, if the pilot tries to go too slowly, he will be warned by the flight computers shaking the controls in his hand before it becomes actually dangerous until he makes corrective actions. Some conditions actually allow the computers to take control of the aeroplane and guide it back into compliance with the Control Laws. This is similar to modern cars that have sensors that tell the car's computer to apply the brakes before you run into the back of the car in front of you whose driver has just jammed on his brakes.

Now, the computer code to do all of this runs into millions of lines of code and has to take into consideration all kinds of occurrences. Mistakes in the computer code, sensors not reading correctly, or unforeseen events can make the control computers do something really stupid.

I'm sure the Boeing 737 Max 8's designers tried to incorporate as many safety features into the computer code but a bad sensor or an undetected code error may have caused the crash.

More safety features? Yes, but this almost always leads to lots more lines of computer code. Be careful what you wish for. Often an experienced pilot in direct control is better than a computer receiving bad information, and, or executing bad code.

Howard Chin


Jamaica Institution of Engineers


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