I thought I had covered the whole subject of the controller/pilot interface pretty thoroughly. In the January 2008 issue ("The Controller Failed to Inform..."), I used two accidents to illustrate the perils of depending on the controller to keep out of weather and away from high terrain. In the July issue ("Saved by the Controller") I presented the other side of the story, describing two of the many cases where the controller did in fact save a pilot from almost certain harm. Then last month I shared advice from a book by John Stewart on how pilots can do their part in "Working With Controllers."
A seven-page letter I received from Craig Drake, a general aviation pilot who works as a controller at Denver Center, convinced me that there was a lot more on this subject pilots need to consider. Because Craig is both a pilot and a controller, he has a unique perspective on some of the topics I had covered. It turns out that the whole issue of why a controller may not warn a pilot about bad weather or high terrain ahead is much more complicated than I ever imagined. It just so happened that I was on my way to Denver to provide Error Prevention Training to Lockheed Martin Space Systems and United Launch Alliance employees the following week, and I arranged an appointment for a tour of Denver Center so I could see for myself what Craig was talking about.
Like many pilots, I assumed that controllers have a terrain display similar to what is available on even some of the most inexpensive GPS units. It seemed like it should be simple for a controller to see an airplane heading towards high terrain and issue a low altitude safety alert, and it was hard to understand how a controller would not issue an alert when appropriate, or could even vector an airplane towards higher terrain. As Craig says, "The FAA says it's our job, training manuals say that we will do it, and perhaps tragically, some pilots may depend on it in low visibility conditions."
Craig explained to me that it is not at all that simple. He showed me his display for the sector he often works west of Denver. The display looked to me like a puzzle maker had gone wild and tried to fit as many oddly shaped pieces of different sizes as possible into one area. Each piece, which is actually referred to as a polygon, has an associated altitude called the Minimum IFR Altitude (MIA) that provides 2,000 feet of terrain or obstruction clearance in mountainous terrain from the highest obstacle out to four nautical miles from its boundaries.
The terrain within each polygon may include mountain peaks and valleys that vary in elevation by many thousands of feet, yet the radar display doesn't show any mountains or valleys, "just large areas encompassed by high altitudes that the majority of airplanes we give VFR flight following to will never reach," and all airplanes departing from or approaching an airport in a polygon will of necessity be below the MIA. For this reason, the low altitude alerting system, which is set to alert the controller when an airplane is below the floor of the MIA for that polygon, is automatically inhibited for VFR airplanes and for anyone close to their departure point or destination. Otherwise the alarms would be going off all the time.
For example, the MIA in the vicinity of the Continental Divide is 16,000 feet, with higher areas around Mt. Evans and the Rocky Mountain National Park. Yet Craig says that every VFR day he watches numerous airplanes crossing that area at 11,000 feet or less, 5,000 feet or more lower than the MIA. The only way Craig could be sure they are going to make it would be to use a Sectional Chart located above his console to try to plot the moving airplane's location and then determine the terrain elevation at that specific location. Obviously a controller who is busy working numerous airplanes will not have the time to do that unless it is an emergency situation.
Thus the controller is in a tough position. Most of the mountain airports and VFR airplanes requesting advisories in that area are between 4,000 and 8,000 feet below the published MIA. If the controller issues a terrain advisory to every airplane arriving, departing or transiting the area below the MIA, pilots will get annoyed and will quickly disregard those advisories. On the other hand, if an airplane that did not receive an advisory crashes, then everyone wants to know why the controller did not warn the pilot.
Craig said that many controllers try to strike a balance by routinely advising VFR traffic of the MIA in that area. Some pilots seem to understand what is going on, but others get confused and reply that they can't fly that high or that they don't have oxygen. Occasionally pilots just cancel flight following. Even worse, if a pilot who is tracking down a valley in poor visibility receives a safety alert because he is below the MIA, he may turn towards the higher terrain on either side when he would have been safer proceeding straight ahead, and the controller would be blamed for turning the airplane into the terrain.