Control Laws — Page 420, Lesson 520

I want to walk you through the protection laws that fly-by-wire aircraft use to keep the aeroplane inside its safe operating envelope. We've already covered the basics of control laws, so now we're looking at the specific protective functions — high speed, pitch attitude, bank angle, and load factor. Let's start with high speed protection. The purpose here is to prevent the aircraft from ever reaching Vd and Md — that's the design diving speed and design diving Mach number, the absolute structural limits. The circuit does this by adding a positive nose-up g demand to the pilot's stick demand. So if you push the stick forward to enter a steep dive, the protection automatically adds a nose-up command on top of yours. That added demand is proportional to the amount of speed overshoot beyond VMO and MMO — VMO being maximum operating speed and MMO maximum operating Mach number. The more you overshoot those limits, the stronger the nose-up correction. The beauty of this is that a pilot can push the stick hard forward, safe in the knowledge the protection will stop the aircraft exceeding the design limits. The inputs to this unit are airspeed and Mach number from the air data computer, and the output is applied to the elevators. Next, pitch attitude protection. This one is only available in fly-by-wire aeroplanes. It enhances both the high angle of attack protection and the high speed protection we just covered. The circuit reduces the pitch demand of the stick when the aircraft reaches pre-defined maximum pitch attitude values — and those values are 30° nose-up and 15° nose-down. So no matter how hard you pull or push, the aircraft won't pitch beyond those attitudes. The input here is the pitch angle from the attitude gyros, and again the output goes to the elevators. Now bank angle protection. On a commercial aircraft, bank angle doesn't normally exceed 30°. But in certain circumstances you might need more. This protection, again only in fly-by-wire aeroplanes, lets the pilot achieve any roll manoeuvre efficiently while preventing the aircraft from entering an uncontrollable state. Let me give you the Airbus limits as the example. In the normal flight envelope, the bank angle limit is 67°. When high angle of attack is triggered, it drops to 45°. And when high speed protection is triggered, it drops further to 40°. There's also a very interesting behaviour after a roll manoeuvre — if the pilot releases the stick, the aircraft will return to a bank angle of 33°. The bank angle limit itself is achieved by reducing the roll rate demand progressively as the bank angle increases. So the closer you get to the limit, the less roll rate the system allows. Finally, load factor protection. A commercial aircraft is designed to withstand a maximum load factor, beyond which structural damage is likely. In a conventional aircraft with no protection, the pilot has to assess the instantaneous g load themselves — and in an urgent situation, they could overstress the aircraft. Load factor protection, available in fly-by-wire aeroplanes, senses the g load using accelerometers. The g load limiter protects the aircraft against overstress by keeping it within its structural limitations, while still allowing the pilot to react immediately to an evasive manoeuvre. And importantly, the load factor protection is linked to the high angle of attack protection — they work together as part of the same protective scheme. So to tie it together: high speed protection uses air data to stop you exceeding VMO/MMO, pitch attitude protection caps you at 30° up and 15° down, bank angle protection limits roll with those Airbus values of 67°, 45°, and 40°, and load factor protection uses accelerometers to keep g within structural limits. Each one takes its input from a specific sensor — air data computer, attitude gyros, accelerometers — and applies its output to the elevators or the roll control. That's the complete protection suite.