Page 17 - Volume 10 Number 9

radials reflect magnetic courses to/from the station and, thus, are always straight lines. In contrast, GPS routes that are not pre-programmed to follow true or magnetic courses are, by default, great-circle courses between fixes (curved lines that are actually shorter than a straight line between the same points because they follow the curvature of the earth). The point being – GPS courses are generally far more stable and easier to intercept and track because they are not subject to constantly changing signal sensitivity. Most of us already know that, intellectually. But, recalling the nuances of intercepting and tracking VOR radials at various distances from the station, when GPS has unexpectedly failed you, is a skill that can only be retained through routine training and practice.
During your next training event in a simulator or the actual airplane, ask the instructor to create a scenario requiring you to tune and identify a VOR. Then choose a specific radial to intercept and track. Do this for courses both TO and FROM the station. Overfly the station to refresh your memory and skills related to the ever-increasing signal sensitivity leading to the “cone of silence” (the area overhead the station where VOR signal reception is briefly lost). Then leave an assigned radial to intercept and track a different radial and, again, practice this for both TO and FROM courses. You might be surprised how much you fumble with these very basic IFR skills which, if you are like most pilots, you’ve allowed to atrophy through disuse.
Distance Measuring Equipment (DME)
In U.S. airspace, an IFR enroute certified GPS unit can be used in lieu of enroute DME. Similarly, an IFR approach certified GPS may be used in lieu of DME required for terminal procedures. Thus, the use of true DME has fallen dramatically with the widespread usage of GPS. Yet, most turbine aircraft still have DME unit(s) installed or incorporated into integrated avionics packages.
When enroute or terminal VOR navigation becomes necessary, one must still be able to track position along the route as well. This requires the ability to utilize DME and/or triangulation via crossing radials/bearings from other navigation aids. Traditional airway intersections and fixes are all still identified and charted via one or both of those methods. Yet, pilots struggle mightily to read IFR enroute charts to determine alternate means of identifying fixes when GPS/Moving Maps fail. Such struggles often increase sharply while attempting to setup and interpret the backup avionics appropriately.
DME and GPS distances do differ slightly in that GPS is, again, the great-circle distance to the active waypoint. DME, on the other hand, is straight-line, magnetic- course, slant-range distance to the station. While DME slant-range error is greatest near and/or at high altitudes above the station, great-circle versus magnetic course error is greatest at the midpoint between given GPS
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waypoints. When using GPS in lieu of DME, we’ve become accustomed to measuring our distance TO the active waypoint. DME might identify the same fix via a specific distance TO or FROM the DME station. Arrival at a GPS waypoint will always happen at an indicated 0.0 distance, while arrival at a fix using DME measurements will almost never occur at 0.0 (unless that fix is the VOR which is co-located with the DME station, while at altitudes below 600 feet AGL [0.1NM]).
During the same practice suggested in the VOR section, incorporate DME usage into the training exercises. Observe the slant-range error by flying directly over the VOR/DME station. Track radials to specific DME distances, while flying towards and away from the station. Choose radial intercept angles that will allow interception to occur at specified DME distances.
DME Arc Procedures
DME Arcs are still an aircraft separation tool used by Air Traffic Control (ATC) in both published and non- published versions. Published DME Arcs are generally limited to the initial legs of Instrument Approach Procedures (IAPs), where they are used in lieu of traditional procedure turns. Modern IFR approached certified GPSs have such arcs built into their databases as curved flightpaths between GPS waypoints. As such, they are flown simply by keeping the CDI centered while tracking around the arc and distance is measured TO the active waypoint (usually the arc’s exit point or a step-down fix along the arc where an altitude change may be initiated). Yet, such published arcs are usually associated with VOR/LOC/ILS type approaches and, thus, can be flown without the aid of GPS, if necessary. Without GPS, traditional navigation of the arc would be required via your backup navigation equipment (VOR and DME) [Figure 2].
Non-published DME Arcs are assigned randomly by ATC, usually for the purpose of aircraft separation in times of heavy traffic load, radar outages (or non-radar environments), and/or when airspace or terrain require it. Non-published arc courses cannot be overlayed with GPS courses and must be flown using VOR crossing- radial navigation methods to determine position along the arc’s course. Those savvy in the art of modern GPS navigation might say they can fly a non-published DME Arc using GPS information alone ... and they would be correct. However, the process for properly programming and setting up such a procedure is both beyond the scope of this article and is, in fact, every bit (if not more) complicated than that required to set up and fly the arc using more traditional methods.
Flying a DME Arc requires good situational awareness and the ability to visualize your position, even with a functional GPS, and certainly so without one. These are skills that are becoming more and more scarce as the population of IFR pilots shifts from those that learned on conventional gauges to those that learned on (and have
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