GroundProximityWarningSystem — Page 82, Lesson 64

I want to walk you through the Ground Proximity Warning System — GPWS for short. This is a vital safety system on modern aircraft, and I want to make sure you understand exactly what it does, how it's built, and what each of its warnings means. Let's start with the aim of the system. The GPWS is designed to give visual and audible warning signals to a pilot when the aircraft's proximity to the terrain poses a potential threat to its safety. Now, I want to be clear: it is not a foolproof means of preventing a collision with the earth's surface. No system is perfect. But GPWS does enhance flight safety, and it can prevent those accidents which could result from crew errors or distraction, malfunction or misinterpretation of navigational equipment, or even inappropriate ATC instructions. Let's look at the three elements that make up a GPWS, which you can see in Figure 34.1. The system has inputs, outputs, and a central processing unit. The central processing unit is the brain — it will also indicate a computer failure and any failures of the six input signals. The system operates between 50 feet and 2,450 feet of actual height above the surface, and it automatically selects the correct mode of operation. Now, what are those inputs? The system takes signals from the radio altimeter, the machmeter, the air data computer, glide path deviation, undercarriage position, flaps position, and the navigation system. Those are the inputs feeding into the central processing unit. On the output side, you have a master indicator, visual warnings, and audio warnings. Let me define two key terms you need to know. An alert is a caution generated by the EGPWS equipment — that's Enhanced Ground Proximity Warning System, the more advanced version. A warning is a command generated by the EGPWS equipment. So an alert is a caution, a warning is a command — there's a difference in urgency. Now, there are two types of warnings or alerts you need to understand. A genuine warning or alert is when the equipment provides a warning in accordance with its technical specification — it's doing exactly what it's supposed to do because there's a real threat. A nuisance warning or alert is when the equipment provides an alert in accordance with its technical specification, but the pilot is flying an accepted safe procedure. So the system is working correctly, but the pilot knows they're safe — it's a nuisance, not a false alarm. Let me walk you through the GPWS operating modes. There's a table in your materials that lists them, and I want you to know each one by its mode number, what triggers it, and what alert and warning you'll hear. Mode 1 is excessive descent rate. The alert you'll hear is 'Sink Rate'. The warning, if it escalates, is 'Whoop Whoop Pull Up'. Mode 2 is excessive terrain closure rate. The alert is 'Terrain Terrain'. The warning is again 'Whoop Whoop Pull Up'. Mode 3 is altitude loss after take-off or go-around. The alert is 'Don't Sink'. There's no separate warning listed for this mode in the table — the alert itself is the key callout. Mode 4A is proximity to terrain with the gear not locked down — that is, the landing gear is unsafe. The alert is 'Too Low Gear'. The warning is 'Too Low Terrain'. Mode 4B is proximity to terrain with the flaps not in a landing position. The alert is 'Too Low Flaps'. The warning is 'Too Low Terrain'. There's a note here I want to highlight: although some manufacturers of GPWS equipment may show in their literature 'Too Low Terrain' to be an alert, the view of the CAA — the Civil Aviation Authority — is that the response to this should be as for a warning. So treat 'Too Low Terrain' as a command, not just a caution. Mode 5 is descent below glide slope. The alert is 'Glide slope'. Mode 6 is descent below minimums. This has two sub-modes. Mode 6A gives the alert 'Minimums'. Mode 6B gives the alert 'Bank angle'. Mode 7 is windshear warning. The alert is 'Windshear'. So to summarise: you have seven modes, each with specific alert and warning callouts. The system operates between 50 and 2,450 feet above the surface, takes inputs from multiple aircraft systems, and gives you both visual and audible warnings to keep you safe from terrain.