Control Laws — Page 420, Lesson 520

Let's pick up with the high speed protection circuits. These are the first line of defense against the aircraft exceeding its design speed limits. The design speed limits here are Vd and Md. Vd is the design diving speed, and Md is the design diving Mach number. These are the absolute structural limits of the aircraft. The high speed protection circuits prevent the aircraft from ever reaching Vd or Md. Here's how it works. When the aircraft exceeds VMO or MMO — that's the maximum operating speed and maximum operating Mach number — the protection circuit adds a positive nose-up g demand to the pilot's stick demand. That nose-up demand is proportional to the amount of speed overshoot beyond VMO or MMO. So the faster you go past the limit, the stronger the nose-up correction. This is a beautiful design feature. It means a pilot can push the stick forward and enter a steep dive rapidly, safe in the knowledge that the high speed protection will prevent the aircraft from exceeding the design speed limits. The pilot doesn't have to worry about overstressing the airframe in a dive. Now, what are the inputs and outputs here? The inputs to the unit are airspeed and Mach number, which come from the air data computer. The output is applied to the elevators. So the air data computer senses the speed, the protection circuit computes the correction, and the elevators physically raise the nose. Next, let's look at pitch attitude protection. This is only available in fly-by-wire aeroplanes. It enhances the high angle of attack protection and the high speed protection we just discussed. The circuit reduces the pitch demand of the stick when the aircraft reaches pre-defined maximum pitch attitude values. Those values are 30 degrees nose-up and 15 degrees nose-down. So if the pilot pulls back hard and the nose rises to 30 degrees, the circuit starts reducing the pitch demand. The input here is the pitch angle from the attitude gyros, and the output is applied to the elevators, just like the high speed protection. Now let's move to bank angle protection. On a commercial aircraft, the bank angle does not normally exceed 30 degrees. But in certain circumstances, higher bank angles might be required. Bank angle protection, again only available in fly-by-wire aeroplanes, allows the pilot to achieve any roll manoeuvre efficiently and prevents the aircraft entering into an uncontrollable state. Let me give you the specific limits for an Airbus aircraft. In the normal flight envelope, the bank angle limit is 67 degrees. When high angle of attack is triggered, the limit drops to 45 degrees. And when high speed protection is triggered, the limit drops further to 40 degrees. Here's an interesting detail. After a roll manoeuvre, if the pilot releases the stick, the aircraft would return to a bank angle of 33 degrees. So the aircraft doesn't return to wings level — it returns to a 33-degree bank. The bank angle limit is achieved by reducing the roll rate demand progressively as the bank angle increases. So as you approach the limit, the aircraft rolls more and more slowly. Finally, let's look at load factor protection. A commercial aircraft is designed to withstand a maximum load factor, beyond which structural damage is likely to occur. In conventional aircraft with no protection, the pilot has to assess the instantaneous g load and could overstress the aircraft in an urgent situation. Load factor protection, available in fly-by-wire aeroplanes, is provided by sensing the g load on the aircraft with accelerometers. The g load limiter protects the aircraft against overstress by maintaining it within its structural limitations, while 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. So to tie it all together: we have four protection circuits — high speed, pitch attitude, bank angle, and load factor. Each one senses a different parameter, computes a correction, and applies it to the appropriate control surface. Together they keep the aircraft within its structural and aerodynamic limits while giving the pilot full authority to manoeuvre aggressively when needed.