Aerodynamic Warnings — Page 451, Lesson 549

We're starting a new chapter now — Aerodynamic Warnings. This is all about the systems that protect the aircraft from getting into an aerodynamic condition it can't recover from. Let's begin with the core concept that everything else hangs on: the critical angle of attack. The critical angle of attack is the angle at which the airflow over the wing separates and the wing stalls. Now, here's the key point I want you to lock in: this stall condition occurs at a particular angle, not at a particular speed. That's why the critical angle of attack is also called the stalling angle. Because it's an angle, its value is tied directly to the design of the aerofoil section adopted for the wings of that particular aircraft type. So the value varies from aircraft to aircraft. Typically, it's between 12° and 18° for straight wings, but it can be as high as 30° or 40° for swept or delta wings. So don't think of stall as a speed problem — think of it as an angle problem. Now, to detect that angle, we need sensors. There are two types of angle of attack probes in current use: the conical slotted probe and the vane detector. Let's look at the conical slotted probe first, because it's a clever piece of mechanics. The conical probe extends through the aircraft skin, perpendicular to the airflow. It's attached to a paddle inside the transmitter housing, and both the probe and the paddle are free to rotate. Here's how it works: the probe has two sets of slots that allow pressure variations — caused by changes in airstream direction — to be transmitted through separate air passages to opposite sides of a paddle chamber. So when the pressure acting on one side of the paddle is greater than the pressure on the other side, the paddle and probe rotate until the pressures are equal. The probe thus positions itself to determine the angle of attack of the aircraft. It's a self-balancing pressure sensor. The vane detector is simpler in concept: it's a counter-balanced aerodynamic vane that positions the rotor of a synchro. The vane aligns itself with the airflow, and that mechanical position is converted into an electrical signal by the synchro. Both types are protected against ice formation by a heater — that's essential, because ice on the probe would give false readings. Now, where are these mounted? The probes or vane detectors are mounted on both sides of the fuselage, usually forward of the wing line. The reason for mounting on both sides is to compensate for side-slip and yaw. If the aircraft is yawing, the airflow isn't straight down the fuselage, so having sensors on both sides lets the system average out that effect. So where does all this information go? The angle of attack probes send information to three places: the stall warning system, the ADC — that's the Air Data Computer — and, if fitted, the Flight Envelope Protection Systems and Angle of Attack Indicators. Now, those Angle of Attack Indicators may be fitted in addition to the stall warning system. They give the pilot a direct readout of the angle of attack, which is useful in low-speed, high-angle situations. Let's move on to the Configuration Warning systems. Some aircraft incorporate these for take-off and landing. The landing configuration warning would give an audible warning if certain throttles are retarded without the landing gear locked down. There may also be warnings for flaps. Now, note this: this would not be required on aircraft fitted with GPWS — the Ground Proximity Warning System — which is discussed in the next chapter. Then there's the Take-off Configuration Warning, or TOCW. As the name suggests, it lets the pilot know, by means of an audible warning, that the aircraft is not in the correct configuration for take-off. Different aircraft may have different parameters for activation, but typically the TOCW would sound if the throttles are advanced with any of these conditions: - Flaps not in take-off position - Slats not in take-off position - Stabilizer trim outside take-off range - Spoilers or speedbrakes deployed - Remotely operated flight control locks not disengaged - External doors or hatches not locked closed - Parking brake applied So the TOCW is a final safety net — it checks that the aircraft is properly configured before you commit to the take-off roll. If any of those items is wrong, you get an audible warning the moment you advance the throttles. That's the core of aerodynamic warnings: detecting the critical angle of attack, sensing it with probes, and warning the pilot about configuration errors before take-off and landing.