Flight Warning Systems — Page 434, Lesson 538

I want to walk you through the Flight Warning Systems chapter now. This is the start of a whole new topic, so let's build it from the ground up. The chapter opens with a fundamental principle: an audible warning is mandatory if the pilot is required to assume control. Think about that. If the aircraft needs the pilot to take over and act, the crew must be told by sound, not just by a light. That sound can take different forms depending on the aircraft type — it could be a tone, a synthetic voice message, or a combination of both. Now, here's a critical point for multiple alerts. When several warnings go off at once, they are prioritized in a strict order: Stall, then Windshear, then GPWS, then ACAS. Let me spell those out. Stall is the aerodynamic loss of lift. Windshear is a sudden change in wind speed or direction. GPWS is the Ground Proximity Warning System, which warns you about terrain. ACAS is the Airborne Collision Avoidance System, which warns about other aircraft. That priority order matters because the most immediately life-threatening condition gets your attention first. Let me now give you the specific aural warnings on Boeing aircraft. There are four distinct sounds. A bell accompanies fire messages. A siren accompanies warnings on cabin altitude, configuration, and overspeed. A wailer accompanies autopilot disconnect. And synthetic voice messages are used for ground proximity, windshear, and airborne collision avoidance. Now Airbus. The sounds are different. Continuous repetitive chimes indicate red warnings. A cavalry charge — that's the distinctive rising sound — indicates autopilot disconnect. A cricket sound is the stall warning. And synthetic voice is used for GPWS and TCAS warnings. TCAS is the Traffic Collision Avoidance System, the Airbus name for the collision avoidance function. Beyond warnings, there are cautions. Beepers with various tones, or chimes, or musical chords are used to caution the crew about potential threats to safety. So the distinction here is that warnings demand immediate action, while cautions alert you to something that could become a threat. Now let's talk about the sensory channel. There's a vibratory mode on the controls used to indicate stall approach. That vibration demands immediate action to avert loss of control. In some aircraft, a stick-pusher provides guidance to prevent further deterioration of the situation that demanded the vibratory warning. So the stick-pusher physically moves the control column forward to recover from the stall. To rationalize all these warning systems, a Master Warning Indicator light is often provided near the centre of scan — that's the central area of the instrument panel where your eyes naturally rest. In older systems, the crew member would then refer to a Master Centralized Warnings Panel, abbreviated CWP, where warnings were assembled in a rational order and annotated. In modern Electronic Flight Instrumentation Systems, abbreviated EFIS, most alerts and warnings appear on appropriate electronic screens together with associated aural messages and master warning lights. That figure shows the cockpit displays and warnings of an Airbus A320 — you can see how the electronic screens and warning lights come together. Now let's move to the Flight Warning System itself, abbreviated FWS. This is the system that generates alerts and warnings for three specific situations. First, engine and airframe systems malfunctions. Second, aerodynamic limits exceeded. Third, presence of external hazards. Engine and airframe systems malfunctions are dealt with in detail in the Engines and Systems sections of the course, so we won't go deep here. But the FWS is what flags those problems to you. Aerodynamic limits — if these are exceeded, the FWS provides three alerts to the crew: Altitude Alerting, Overspeed Warning, and Stall Warning. These are dealt with in the next chapter, so we'll come back to them. Finally, external hazard warning. The external hazards that constitute a threat to aircraft safety are proximity to terrain and proximity to other aircraft. These hazards can be avoided by the use of two systems: the Ground Proximity Warning System and the Airborne Collision Avoidance System. Those are dealt with in Chapters 34 and 35. So the big picture here is that the FWS is the central nervous system that tells you about problems with your own aircraft, problems with your flight envelope, and problems with the world around you. That's the foundation we'll build on.