Control Laws — Page 420, Lesson 518

I want to walk you through flight envelope protection — this is where fly-by-wire really earns its keep. Let me start by defining what we mean by the flight envelope. It's the full range of speeds, angles, and loadings within which the aircraft is certified to operate safely. Now, on a fly-by-wire aircraft, the system takes envelope protection to the extreme. Its whole aim is to ensure the aircraft remains inside that normal flight envelope in every phase of flight — takeoff, climb, cruise, descent, approach, landing, all of it. The system steps in to prevent the envelope being violated during extreme situations. Let me name the ones the book lists: windshear, very high turbulence, mid-air collision avoidance, GPWS or TCAS activation, and mismanagement by the crew. Let me expand those acronyms for you. GPWS is the Ground Proximity Warning System — it warns you the aircraft is getting too close to terrain. TCAS is the Traffic Collision Avoidance System — it detects other aircraft and gives you resolution advisories to avoid a collision. And "mismanagement by the crew" simply means a pilot error that would push the aircraft outside its limits. Now, why does the system exist? What's its purpose? Three things. First, it gives full authority to the pilot so they can consistently achieve the best possible aircraft performance in those extreme conditions. Second, it reduces the risks of overcontrolling and overstressing the aircraft. And third, it provides the pilot with an easy, instinctive, and immediate procedure to achieve the best possible performance when required. So the philosophy is: in an emergency, you don't want the pilot to have to think about delicate control inputs — you want the aircraft to protect itself while the pilot flies. Let me now look at what the protection actually covers. The system provides protection 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. Angle of attack is the angle between the wing's chord line and the relative airflow — it's the key to lift and stall. Speed is self-explanatory, but here it means protecting against both overspeed and underspeed. Pitch attitude is the nose-up or nose-down angle of the aircraft relative to the horizon. Bank angle is the angle of the wings relative to level — how steeply you're turning. And load factor is the ratio of the lift being generated to the aircraft's weight — it's the g-force you feel, and it's what stresses the structure. Now let me focus on the high angle of attack protection, because that's where the book goes into detail. The protection enables the pilot to execute a rapid pull-up manoeuvre in an emergency — say, a mid-air collision avoidance situation — at maximum angle of attack, which we call alpha max, without overcontrolling the aircraft. The technique is beautifully simple: the pilot just "snatches the stick fully back." That's the phrase the book uses. The aircraft will go to alpha max and hold there, and the pilot doesn't have to worry about pulling too hard and stalling or overstressing. But here's the important part — if the aircraft exceeds the normal flight envelope for any reason, the pilot is immediately made aware of the situation by two cues. First, the pitch auto-trim stop. That's a mechanism that stops the automatic trimming of the stabiliser, so the pilot feels a physical resistance. Second, the aft pressure required on the stick to keep the flight path. So the stick gets heavy — you feel you're holding against something. These are the tactile warnings that you're at the edge of the envelope. Now, one critical point: high angle of attack protection is an aerodynamic protection, but thrust is required to maintain the flight path. The aerodynamics alone won't keep you climbing — you need engine power. 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 expand TOGA — it stands for Take-Off / Go-Around thrust, which is maximum thrust. And the "floor" is that specific angle of attack value at which the auto-thrust kicks in with TOGA power. So the sequence is: the aircraft approaches alpha max, and before it gets there, it hits the floor, and the auto-thrust automatically spools up to TOGA. Finally, let me look at the circuit itself. The input to the circuit is the angle of attack. That's the sensed parameter. And the output is applied to two places: the elevators and the auto-thrust. So the angle of attack signal drives the elevator deflection to control the pitch, and it also drives the auto-thrust to manage the power. That's the closed loop — angle of attack in, elevator and thrust out. Let me pull this together with a diagram so you can see the signal flow. So the whole picture is: fly-by-wire gives the pilot full authority, but the envelope protection acts as a safety net. In an emergency, the pilot can yank the stick fully back, the aircraft goes to alpha max, the auto-thrust provides TOGA at the floor, and the pilot feels the auto-trim stop and the aft stick pressure as confirmation they're at the limit. That's the essence of high angle of attack protection.