Autopilot — Page 352, Lesson 417

I want to walk you through the autopilot, starting from the very beginning. This is a big subject, so let's build it up properly. First, the core purpose. The main reason we have an autopilot is to relieve the pilot of the physical and mental fatigue of flying the aircraft, especially during long flights. Think about that for a second — the result is that the pilot is more alert during the critical phase of landing. That's the whole point: not to replace the pilot, but to keep the pilot fresh for the moments that truly matter. There's a second major purpose. Autopilot systems enable the aircraft to fly a prescribed route accurately, and they do this because of the autopilot's ability to react quicker than a human pilot to disturbances. A disturbance could be a gust of wind pushing the nose off track. The autopilot senses that and corrects it far faster than a human ever could. Now, there are many different autopilot systems out there, offering many different modes of operation and facilities. But generally, today's modern airliner, when fitted with an autothrottle system, will have the facility to fly the aircraft automatically for almost the entire route. Let me unpack that. An autothrottle system is what controls the engine power automatically. So with that fitted, the aircraft can fly itself for nearly the whole journey. Here's how the whole thing works together. In each such system, the autopilot flies the aircraft as it responds to commands from three sources: attitude sensors, navigation systems, and pitot-static systems. Let me define each. Attitude sensors tell the autopilot the aircraft's orientation in space — pitch and roll. Navigation systems tell it where the aircraft is and where it needs to go. And pitot-static systems provide the airspeed and altitude data. So the autopilot takes commands from all of these and flies the aircraft accordingly. Power is controlled separately. It's controlled through the engine throttles, which are moved by autothrottle servos. A servo is essentially a motor that moves a control on command. These autothrottle servos respond to commands from the thrust management computer. So the thrust management computer decides how much power is needed, and the servos physically move the throttles to set that power. Now, an important limitation. As yet, the autopilot does not carry out the take-off — that has to be done by the pilot. The autopilot, though, can be engaged shortly after take-off at about 400 feet, or possibly even a lower height. So the pilot hand-flys the take-off and the initial climb, and then engages the autopilot once the aircraft is safely airborne. Let me give you some history and the two major reasons the basic autopilot was introduced. The basic autopilot has been in existence for about 50 years. It was introduced as an aid to the pilot flying the aircraft for two major reasons. The first is reduction in workload. Allowing automatic systems to fly the aircraft means that the crew not only are more rested for the more demanding phases of flight, but it also allows the pilots to concentrate on other tasks such as navigation. So instead of constantly making small corrections to keep the aircraft straight and level, the pilot can focus on where the aircraft is going. The second reason is the response time. An autopilot is much quicker than a human, and as a result it can fly an aircraft more accurately. Here's the key number: a human pilot takes approximately one-fifth of a second — that's 200 milliseconds — in detecting a deviation. That's the reaction time just to notice something is wrong, before even starting to correct it. The autopilot does that detection and correction in a fraction of that time, which is why it flies more accurately. So to tie it all together: the autopilot relieves fatigue, flies accurately because it reacts faster, takes commands from attitude sensors, navigation systems, and pitot-static systems, and works alongside the autothrottle system for power control. But the take-off remains the pilot's job, and the autopilot can be engaged from about 400 feet after take-off.