Let me define it precisely — Page 288, Lesson 478

I want to walk you through a concept that’s absolutely critical when you’re flying aircraft with advanced automation: Mode Error. This is a type of error that can catch even experienced crews off guard. Let me define it precisely. A Mode Error occurs because automatic flight controls and engine management systems can be set up in so many different modes. Because of that variety, it’s possible for the pilot to believe that the aircraft is programmed to carry out one function when it is, in fact, performing another. In other words, you think you’ve told the autopilot or autothrottle to do one thing, but it’s actually doing something different — and you don’t realize it. This is why it is vitally important for pilots of such aircraft to maintain an accurate knowledge of the aircraft status. You do that by including the mode representation — the display that tells you what mode the automation is in — as a central part of your scan. You need to check it regularly, just like you check your attitude indicator or altimeter. It’s equally important that designers ensure that mode information is prominently and centrally displayed, so it’s hard to miss. Let me give you a real-world example from 1979. A DC-10 was climbing to cruise altitude. The crew intended to program the autopilot for airspeed hold mode — meaning the autopilot would maintain a constant airspeed during the climb. But they were unaware that they had actually programmed it for vertical speed hold mode instead. In vertical speed hold, the aircraft maintains a constant rate of ascent, regardless of what happens to airspeed. So, maintaining that constant rate of ascent, the airspeed began to decay — it slowed down — to such a point that the aircraft entered the stall buffet. That’s the aerodynamic vibration just before a stall. The crew misidentified that vibration as a problem in the number 3 engine, which they subsequently shut down. The aircraft then stalled, rolled to its right, and lost 11,000 feet before recovery was achieved. That’s a dramatic example of how a simple mode error can cascade into a serious loss of control. Now, let’s move to a related issue: Manual control. Whenever the pilot is not “in the loop” — meaning the automation is flying the aircraft and the pilot is monitoring rather than actively handling the controls — he or she is often not mentally prepared to take over the controls and fly the aircraft in the event of an automatic system failure. Your brain isn’t warmed up for the task. In addition, failures are less frequent than they used to be, but they are also less predictable, and they are much quicker and more intense than before. For example, a sudden reversion to manual control can happen with very little warning and require an immediate, precise response. Finally, there’s a subtle but important point about information quality. Although the signal received by the computer may be of poor quality — maybe a sensor is giving noisy or degraded data — this information is not normally passed to the flight crew. The computer may be working with bad data, but you don’t get a warning that the data is poor. That means you could be flying with an autopilot that’s making decisions based on unreliable information, and you wouldn’t know it unless you cross-check with other instruments.