Ground Proximity Warning System — Page 461, Lesson 561

I want to walk you through Mode 7, the windshear alerting mode, and then the crucial part — what you actually do when any GPWS warning fires. Let's start with Mode 7. Mode 7 is the windshear alerting mode. It gives you both visual and aural warnings. The system looks at several parameters together — ground speed, airspeed, barometric height, rate of descent, and radio altitude. When those parameters indicate the initial conditions of entering an area of windshear, the warnings trigger. Now here's the key limitation, and I want you to really understand this. Just like the terrain threat display we discussed, Mode 7 has no scanning beam looking ahead. It cannot detect windshear ahead of the aircraft to avoid the condition entirely. The benefit comes from something else — it allows you, the pilot, to initiate the windshear go-around procedure earlier. That earlier initiation gives the aircraft a greater probability of avoiding an accident. And one critical priority rule: Mode 7 warnings take priority over all other modes. That means if Mode 7 fires, it overrides everything else the GPWS is telling you. Now let's move to the response procedure, because this is where professional discipline matters. The response to all alerts or warnings should be positive and immediate. Establishing the cause of GPWS activation should take second place. Do not stop to diagnose first — act first. There's a real human factor issue here. Repeated experience of unwanted alerts or warnings may reduce your confidence in the system. That's a documented risk. So the rule is: flight crews should report ALL alerts and warnings to the operator, ensuring appropriate analysis and remedial action can be taken. There's a GPWS operation reporting form specifically for this purpose. Now the immediate response itself. When a warning fires, you must level the wings and initiate a maximum gradient climb. You maintain that climb until the aircraft attains the minimum safe altitude for that part of the route being operated. Modification of this climb is permissible only in exceptional circumstances — such as the necessity to follow a curved path for azimuth terrain avoidance. Once you're established in the climb, every effort shall be made to determine the cause of the warning and to verify the aircraft's position. But here's the critical part — the only circumstances when a climb to this altitude may be discontinued are three specific conditions. First, the cause of the warning has been positively identified and the warning has ceased. Second, the aircraft is operated by day in clear visual conditions. Third, it is immediately obvious to the commander that the aircraft is not in a dangerous situation with regard to terrain, aircraft configuration, or the present manoeuvre of the aircraft. Now let's transition to the Enhanced Ground Proximity Warning System — EGPWS — and the Terrain Awareness System, TAWS. Here's the fundamental limitation I want you to grasp. GPWS does not "look ahead." Any Mode 2 warning when flight is towards high ground will be dependent upon the steepness of the terrain. So a sheer cliff ahead will not generate a Mode 2 warning — because the system only reacts to what's directly below and immediately ahead based on your current descent rate, not what's coming up over the horizon. That's the gap EGPWS and TAWS are designed to address. Let me pause here — do you want me to continue into how EGPWS and TAWS overcome that look-ahead limitation?