Autopilot — Page 352, Lesson 417

I want to walk you through the autopilot chapter, and I'll start with the big picture before we get into the hardware. The main purpose of the autopilot is to relieve the pilot of the physical and mental fatigue of flying the aircraft, especially during long flights. That's the headline. And there's a knock-on benefit: because the pilot isn't worn out from hand-flying for hours, they're more alert during the critical phase of landing. So think of the autopilot as a fatigue-management tool as much as a flying tool. There's a second purpose, and it's about accuracy. Autopilot systems enable the aircraft to fly a prescribed route accurately, because the autopilot can react quicker than a human pilot to disturbances. A disturbance might be a gust of wind pushing the nose off heading — the autopilot senses it and corrects faster than you could. Now, the systems themselves. Many different autopilot systems exist, 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. The autopilot flies the aircraft as it responds to commands from three sources: attitude sensors, navigation systems, and pitot-static systems. Attitude sensors tell it the aircraft's pitch and roll attitude. Navigation systems tell it where it should be going. Pitot-static systems give it airspeed and altitude data. Power is controlled through the engine throttles, which are moved by autothrottle servos responding to commands from the thrust management computer. So the autopilot handles the flight path, and the autothrottle handles the power — they work together. One important limitation to note: as yet, the autopilot does not carry out the take-off, which 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 take-off roll and initial climb are yours; the autopilot takes over once you're safely airborne. Now let's look at the basic autopilot itself. It's been in existence for about 50 years, and 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 the crew are not only more rested for the more demanding phases of flight, but it also allows the pilots to concentrate on other tasks such as navigation. So it's not just about rest — it frees up your attention for the broader job of managing the flight. The second reason is response time. An autopilot is much quicker than a human, and as a result it can fly an aircraft more accurately. Here's the number to remember: a human pilot takes approximately one-fifth of a second — that's 200 milliseconds — to detect and respond. The autopilot beats that, which is why it holds a route more precisely than you could by hand. That's the foundation of the autopilot: why it exists, what it responds to, and the two reasons it was introduced.