Aerodynamic Warnings — Page 451, Lesson 549

Let’s start with the core idea of this chapter: aerodynamic warnings. These are the systems that tell you, before it’s too late, that the wing is about to stop producing lift. And the key to understanding all of this is the concept of the critical angle of attack. Here’s the crucial distinction I want you to lock in right now. A stall does not happen at a particular speed. It happens at a particular angle. That angle is the critical angle of attack, and because it’s tied to an angle rather than a speed, we also call it the stalling angle. The value of that angle depends entirely on the design of the aerofoil section used for the wings of that particular aircraft type. So it varies from aircraft to aircraft. Typically, for straight wings, it’s between 12° and 18°. But for swept or delta wings, it can be as high as 30° or even 40°. So the geometry of the wing dictates where the stall happens. Now, to detect that angle, we use angle of attack probes. There are two types in current use: the conical slotted probe and the vane detector. Let me walk you through each. The conical slotted probe is shown in Figure 33.6. It extends through the aircraft skin, perpendicular to the airflow. The probe is attached to a paddle inside the transmitter housing, and both the probe and the paddle are free to rotate. Here’s the clever part: the probe has two sets of slots. These slots 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 on one side of the paddle is greater than the pressure on the other side, the paddle and the probe rotate until the pressures are equal. The probe essentially positions itself to match the airflow, and that position tells us the angle of attack of the aircraft. The vane detector, shown in Figure 33.7, works differently. It’s a counter-balanced aerodynamic vane. That vane positions the rotor of a synchro. A synchro is an electrical device that transmits angular position—so as the vane aligns with the airflow, it rotates the synchro rotor, and that rotation is converted into an electrical signal representing the angle of attack. Both types are protected against ice formation by a heater. That’s a critical detail—ice would ruin the aerodynamic shape and give you false readings, so they’re always heated. Now, where are these mounted? The probes or vane detectors are mounted on both sides of the fuselage, usually forward of the wing line. Why both sides? To compensate for sideslip and yaw. If the aircraft is slipping sideways, one probe would see a different airflow than the other, so having both sides lets the system average or correct for that. And where does the 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, about those Angle of Attack Indicators. They may be fitted in addition to the stall warning system. Figure 33.8 shows a simple schematic layout of the installation, with the angle of attack probe feeding the system. So this is a cockpit display that shows you the current angle of attack, complementing the automatic stall warning. Let’s move on to the Configuration Warning. Some aircraft incorporate configuration warning systems for take-off and landing. Let me explain the landing one first. The landing configuration would give an audible warning if certain throttles are retarded without the landing gear locked down. So if you pull the power back to land but the gear isn’t down and locked, you get a warning. There may also be warnings for flaps. Now, the text notes that this would not be required on aircraft fitted with GPWS—that’s 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 moment you push the throttles forward for take-off, the system checks all of these. If any one of them is wrong, you get an audible warning telling you the aircraft isn’t ready to fly. Let me pull this together for you. The whole chapter is about warnings that protect you from aerodynamic and configuration errors. The angle of attack probes measure the critical parameter—the angle of attack—and feed the stall warning system, the ADC, and possibly envelope protection and indicators. The configuration warnings, on the other hand, check that the aircraft is set up correctly for take-off and landing. Both are audible warnings designed to catch a mistake before it becomes a problem. Take a look at Figure 33.6 and Figure 33.7 to see the two probe types, and Figure 33.8 for the angle of attack indicator installation.