GroundProximityWarningSystem — Page 82, Lesson 66

I want to walk you through the Ground Proximity Warning System—what to do when it goes off, and how the Enhanced version fixes a key limitation of the basic system. Let's start with the action to be taken on receiving a warning. The response to all alerts or warnings should be positive and immediate. Establishing the cause of the GPWS activation should take second place. In other words, your first job is to react, not to diagnose. There is a risk that repeated experience of unwanted alerts or warnings may reduce confidence in the system. To manage that, there is a GPWS operation reporting form. Its purpose is to ensure that appropriate analysis and remedial action can be taken. So if you get a nuisance warning, you document it—don't just ignore it and let your trust erode. Now, the immediate response to a warning is very specific. You must level the wings and initiate a maximum gradient climb. That climb should be maintained until the aircraft attains the minimum safe altitude. Once established in the climb, every effort shall be made to determine the cause of the warning and to verify the aircraft's position. There are only three circumstances where a climb to this minimum safe altitude may be discontinued. First, when the cause of the warning has been positively identified and the warning has ceased. Second, when the aircraft is operated by day in clear visual conditions. Third, when 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. So unless one of those three conditions is met, you keep climbing. Now let's move to the Enhanced Ground Proximity Warning System, or EGPWS, also called the Terrain Awareness System, or TAWS. Basic GPWS has a fundamental limitation. It does not "look ahead." Any Mode 2 warning when flight is towards high ground will be dependent upon the steepness of the terrain. Here's the critical example: a sheer cliff ahead will not generate a Mode 2 warning. And any subsequent warning due to rising ground beyond the cliff will be delayed until the aircraft is over that ground. So basic GPWS can't see a vertical wall in front of you—it only reacts to what's directly below or rising immediately beneath you. This limitation has been overcome by Enhanced GPWS with the Terrain Threat Display. This system uses essentially an electronic map of the world that gives ground elevation, combined with information from the aircraft's navigational system—be that INS, GPS, or any combination. Given the location of the aircraft, its course, and its height—either from the Air Data Computer or derived from GPS—a display can be created showing the locations of terrain that could threaten the safety of the aircraft. Using this system, EGPWS can warn of approaching high terrain even when that terrain is not in close enough proximity to initiate a Mode 2 warning. The terrain threat display and warning will be initiated in sufficient time to comfortably avoid any threat of flight into terrain—approximately 40 to 60 seconds before predicted impact. The display uses colour coding. Terrain is shown in yellow and red, and the display indicates terrain not only below the aircraft but also ahead of its flight path. At a certain time before predicted impact, the warning will issue a "Caution Terrain" or "Terrain Ahead" message. If the system database has obstacle data, the alert may be "Obstacle Ahead." At this stage, the threat terrain will turn solid yellow. If the situation is allowed to deteriorate so as to close further with the high ground, the second message—"Whoop Whoop Pull Up"—will sound, and the most threatening terrain will turn solid red. This will happen at sufficient spacing to avoid impact with the terrain, but this time using more positive control movements. So you get two stages: first a caution with yellow terrain, then a full warning with red terrain and the "Whoop Whoop Pull Up" command. That's the core of how EGPWS gives you forward-looking terrain awareness and a structured, time-phased warning system to keep you clear of the ground.