Ground Proximity Warning System — Page 461, Lesson 561

I want to walk you through Mode 7, the windshear alerting mode of the Ground Proximity Warning System, and then the critical actions you take when any warning sounds. Let's start with Mode 7. This mode gives you both visual and aural windshear warnings. The system decides to warn you when several parameters together indicate you're entering an area of windshear. Those parameters are ground speed, airspeed, barometric height, rate of descent, and radio altitude. So it's not one single reading — it's the combination of these that tells the system you're at the initial conditions of entering windshear. Now, here's the key limitation. Just like the terrain threat display we discussed, Mode 7 has no scanning beam looking ahead. It cannot detect windshear ahead of you and avoid it entirely. Instead, the benefit comes from timing. The system lets you initiate the windshear go-around procedure earlier than you otherwise would, which gives the aircraft a greater probability of avoiding an accident. And one very important point: Mode 7 warnings take priority over all other modes. If Mode 7 fires, it overrides everything else. Now let's move to what you do when you receive any warning. The response to all alerts or warnings should be positive and immediate. Establishing the cause of GPWS activation should take second place. Your first job is to react, not to diagnose. There's a real human factor issue here. Repeated experience of unwanted alerts or warnings may reduce your confidence in the system. That's why flight crews should report ALL alerts and warnings to the operator. This ensures appropriate analysis and remedial action can be taken. There's a GPWS operation reporting form specifically for this purpose. So what's the immediate response? 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 — for example, if you must 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 note the order: react first, diagnose second. Now, there are only three circumstances when you may discontinue the climb to that altitude. 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. Finally, let's look at the Enhanced Ground Proximity Warning System, or EGPWS, also called the Terrain Awareness System, TAWS. The fundamental point is that GPWS does not look ahead. Any Mode 2 warning when you're flying towards high ground will be dependent upon the steepness of the terrain. So a sheer cliff ahead will not generate a Mode 2 warning — and any subsequent warning would come too late to be useful. That's precisely the gap the enhanced system is designed to address. Let me pause here