Control Laws — Page 420, Lesson 518

I want to walk you through the concept of flight envelope protection in fly-by-wire aircraft. This is a big deal because it fundamentally changes how a pilot interacts with the aircraft, especially in extreme situations. First, let's define what we mean by the "flight envelope." Think of it as the safe operating boundaries of the aircraft—the limits of speed, angle of attack, load factor, and so on, within which the aircraft is designed to operate safely. In a conventional aircraft, the pilot can accidentally exceed these limits. In a fly-by-wire aircraft, the system is designed to prevent that from happening. The whole idea is taken to the extreme in fly-by-wire aircraft. The system's aim is to ensure the aircraft remains within the normal flight envelope in all phases of flight. It prevents the envelope from being violated during extreme situations. Let me list those situations for you: windshear, very high turbulence, mid-air collision avoidance, GPWS or TCAS activation, and mismanagement by the crew. Now, GPWS stands for Ground Proximity Warning System, and TCAS stands for Traffic Collision Avoidance System. These are safety systems that alert the crew to potential dangers. The point is, in these high-stress, high-workload moments, the flight envelope protection system is there to keep the aircraft safe. So, what is the purpose of this protection? There are three key purposes. First, it gives full authority to the pilot to consistently achieve the best possible aircraft performance in those extreme conditions. Second, it reduces the risks of overcontrolling or overstressing the aircraft. Third, it provides the pilot with an easy, instinctive, and immediate procedure to achieve the best possible performance when required. Let me unpack that. "Full authority" means the pilot has complete control to command what they need. The system doesn't limit the pilot's inputs in a way that would prevent them from getting the performance they need. But it does protect the aircraft from being overstressed—that is, from being subjected to forces beyond its structural limits. And the procedure is "instinctive"—the pilot doesn't need to think about complex steps; the system handles the protection automatically. Now, let's look at what parameters the system protects. The protection operates in all phases of flight to prevent the aircraft exceeding the limits for these parameters: angle of attack, speed, pitch attitude, bank angle, and load factor. Let me define each of these. Angle of attack is the angle between the wing's chord line and the relative airflow. Speed is the aircraft's airspeed. Pitch attitude is the angle of the aircraft's nose relative to the horizon. Bank angle is the angle of the wings relative to the horizon during a turn. Load factor is the ratio of the lift being generated to the aircraft's weight—it's a measure of the stress on the aircraft, often expressed in "g" units. Now, let's focus on the high angle of attack protection, which is a critical part of this system. The protection enables the pilot to execute a rapid pull-up manoeuvre in an emergency situation—such as a mid-air collision avoidance situation—at maximum angle of attack, which we call alpha max, without overcontrolling the aircraft. The technique is remarkably simple. The pilot just "snatches the stick fully back." That's it. The system then manages the angle of attack to keep it at the maximum safe value, alpha max, without exceeding it. Here's the key part: if the aircraft exceeds the normal flight envelope for any reason, the pilot is immediately made aware of the situation. How? Through two cues. First, the pitch auto-trim stop—this is a mechanism that stops the automatic trimming of the stabilizer, giving the pilot a physical indication. Second, the aft pressure required on the stick to keep the flight path—the pilot feels increased resistance, telling them they're at the limit. Now, here's an important nuance. The high angle of attack protection is an aerodynamic protection. But thrust is required to maintain the flight path. So, the auto-thrust function would automatically provide TOGA thrust when the aircraft reaches a certain value, called the "floor," before it gets to alpha max. Let me explain TOGA. It stands for Take-Off/Go-Around, and it's the maximum thrust setting. The "floor" is a specific angle of attack value—a threshold that triggers the auto-thrust to kick in with TOGA power. This happens before the aircraft reaches alpha max, so the thrust is there to support the flight path as the angle of attack approaches its limit. Finally, let's look at the input and output of this circuit. The input to the circuit is the angle of attack. The output is applied to two things: the elevators and the auto-thrust. The elevators are the control surfaces that control pitch, and the auto-thrust controls the engine thrust. So, the system takes the angle of attack signal, processes it, and commands both the elevators and the thrust to maintain the aircraft within the envelope. Let me bring this together. In a fly-by-wire aircraft, the flight envelope protection is a safety net. It allows the pilot to make aggressive, instinctive inputs—like snatching the stick fully back—without fear of overstressing the aircraft or exceeding aerodynamic limits. The system monitors angle of attack, speed, pitch attitude, bank angle, and load factor, and it intervenes automatically to keep the aircraft safe. The high angle of attack protection specifically manages the pitch and thrust to keep the aircraft at alpha max, with the auto-thrust providing TOGA power at the floor value to maintain the flight path. This is the foundation of how fly-by-wire aircraft handle extreme situations. The pilot has full authority, but the system ensures the aircraft never leaves its safe operating envelope.