We're starting a new chapter now — Chapter 34, the Ground Proximity Warning… — Page 451, Lesson 552

We're starting a new chapter now — Chapter 34, the Ground Proximity Warning System, or GPWS. This is one of the most important safety systems on a modern aircraft, so I want you to think of it as the aircraft's electronic watchdog that keeps us away from the ground when we shouldn't be near it. Let me give you the roadmap of what this chapter covers, because it's structured around a set of operating modes. We begin with an introduction and definitions, then we look at the types of warnings and alerts. The chapter then focuses on a specific system — the Boeing 737 Mark II EGPWS — and gives us a table of the GPWS operating modes. There are seven modes, numbered Mode 1 through Mode 7, and each one addresses a different type of hazard. Mode 1 is excessive barometric descent rate. Mode 2 is excessive terrain closure rate. Mode 3 is altitude loss after take-off or go-around. Mode 4A is unsafe terrain clearance with landing gear not down. Mode 4B is unsafe terrain clearance with flaps not in landing configuration. Mode 5 is below glide slope deviation alert. Mode 6A is below selected minimum radio altitude. Mode 6B covers altitude call-outs and bank angle alert. And Mode 7 is windshear alerting. After we go through each mode, the chapter covers the action to be taken on receiving a warning, then moves into the Enhanced Ground Proximity Warning System — the EGPWS — which is the more advanced version. We'll look at the terrain clearance floor, integrity testing, and the inhibition of EGPWS modes. Finally, there are practice questions and answers. Now, before we dive into the modes themselves, let's establish the foundation. The introduction and definitions section sets up what GPWS actually is and what it does. The key idea here is that GPWS is a terrain awareness system — it monitors the aircraft's position and flight path relative to the ground and warns the crew when a hazardous situation is developing. The definitions section will give us the precise terminology we need, and the types of warnings and alerts section distinguishes between the different levels of urgency the system can generate. Let's start with the definitions. I want you to understand the difference between a warning and an alert, because that distinction runs through the entire chapter. A warning is the more urgent of the two — it demands immediate action from the flight crew. An alert is less urgent; it's advisory, drawing your attention to a situation that may need monitoring or action but isn't immediately critical. The chapter will use these terms consistently, so get comfortable with them now. The types of warnings and alerts section expands on this. The system doesn't just have one type of output — it has different categories depending on the severity and the nature of the threat. Some are aural, meaning you hear them through the speakers or headsets. Some are visual, appearing on the instrument displays. And some are a combination of both. The key point is that the system is designed to get your attention in the most effective way possible for the specific hazard. Now, the chapter focuses on the Boeing 737 Mark II EGPWS as its example system. This is important because it gives us a concrete implementation to study. The Mark II is a specific generation of the Enhanced Ground Proximity Warning System used on Boeing 737 aircraft. When we get to the EGPWS section later in the chapter, we'll see how the enhanced version differs from the basic GPWS — the enhancement adds a terrain database and forward-looking capability, but let's not get ahead of ourselves. For now, just know that the Mark II is our reference system. The table of GPWS operating modes is the heart of the chapter. It summarizes all seven modes in one place, showing what each mode monitors, what conditions trigger it, and what warning or alert it produces. I'd encourage you to think of this table as your quick-reference guide — when you're flying and you hear a GPWS warning, you need to know instantly which mode is active and what it means. Let me give you a preview of each mode so you have the framework before we go into detail. Mode 1, excessive barometric descent rate, warns when the aircraft is descending too fast based on barometric altitude. Mode 2, excessive terrain closure rate, warns when the aircraft is approaching terrain too quickly. Mode 3, altitude loss after take-off or go-around, warns if the aircraft loses altitude after a take-off or a go-around when it should be climbing. Mode 4A and 4B both deal with unsafe terrain clearance — 4A when the landing gear isn't down, and 4B when the flaps aren't in the landing configuration. Mode 5 is the below glide slope deviation alert, which warns when the aircraft is too far below the glide slope on an approach. Mode 6A warns when the aircraft is below a selected minimum radio altitude, and Mode 6B provides altitude call-outs and a bank angle alert. Mode 7 is windshear alerting, which warns of windshear conditions. Each mode has its own section in the chapter, and we'll go through them one by one. But before we do, let me make sure you understand the overall purpose. The GPWS is a last line of defense. It's not something you fly by — it's something that catches you if you make a mistake or if conditions conspire against you. When it activates, it's telling you that you are in a dangerous proximity to the ground, and you need to react. The action to be taken on receiving a warning section is critical. When the system warns you, there's a specific response the crew is trained to execute. This isn't optional — it's a drilled procedure. We'll cover that in detail when we get there. Then we move to the EGPWS itself. The enhanced system adds capabilities beyond the basic seven modes. It uses a terrain database and a forward-looking terrain avoidance function, which means it can see terrain ahead of the aircraft, not just below it. This is a significant improvement over the basic GPWS, which only looks downward. The terrain clearance floor is a specific feature of the EGPWS. It establishes a minimum altitude envelope around the aircraft — if the aircraft would penetrate this floor, the system generates a warning. Think of it as an invisible safety bubble that the aircraft must not break through. Integrity testing is about verifying that the system is working correctly. The EGPWS performs self-tests to ensure all its functions are operational. And inhibition of EGPWS modes covers the situations where certain modes are deliberately disabled — for example, during specific phases of flight where a mode might give false warnings. So that's the full picture of what this chapter covers. We're going to work through it systematically, starting with the definitions and the types of warnings, then each mode in turn, and finally the enhanced system. Let's begin with the definitions and the warning types, because everything else builds on that foundation.