Imagine you’re cruising high above an undercast, in smooth, clear skies. The GPS in your panel shows you making good time, with about an hour remaining to your destination. The Center frequency has been fairly quiet; you know there are a lot of other IFR airplanes out there, but everyone is settled into cruise so all you hear are the handoffs to the next sector or approach facility, or the occasional clearance for an approach into a rural airport.
Then, without warning, your GPS advises it’s lost a usable signal. The magenta line by which you’ve been navigating direct to your destination airport disappears and you have no more groundspeed or position information. Everything else seems normal—it’s not an electrical failure, at least not to the airplane’s entire system—but you no longer have GPS navigation. After a moment’s confusion, you punch your autopilot’s heading mode to stay on a general track while you start to troubleshoot. Flipping over to the status page, you find a problem you’ve never seen before: your GPS is not receiving any satellites. None. At. All.
Suddenly, the Center frequency is clogged with pilots reporting loss of GPS signal, and controllers’ replies that they don’t know what’s going on, but to stand by. The cacophony doesn’t stop, however, and within seconds it’s obvious that no one is receiving GPS—the entire Global Positioning System appears to be down. “All aircraft, stand by,” commands an authoritative voice from Center and the increasingly chatty frequency calms. “All aircraft maintain present heading and altitude, and stand by for a revised clearance.” Then, with sector controllers watching individual radar plots closely for collision avoidance, senior controllers start a rapid-fire prioritization of re-routes—first with airplanes nearing their destination and then through the rest of the aircraft aloft—clearing them to the nearest ground-based navaid and then the rest of the way via Victor airways or jet routes. Pilots of airplanes inbound to airports with GPS-only approaches are put in holds and asked for their choice of the nearest airport with a ground-based approach system. Anyone outside of Class A or B airspace is encouraged to cancel IFR if they are in VMC. Pilots requesting a new clearance on the ground or in the air will be denied until controllers sort out those already aloft, and when they are cleared it’ll be via the ground-based airways system. All around the world, any air traffic utilizing U.S.-operated GPS satellites is going through the same transition to 1980s-style instrument flight. This will be “the day the waypoints died.”
I wasn’t airborne on September 11, 2001, when the U.S. National Airspace System was shut down. I’ve spoken to many pilots who were, and most say although it was confusing why they were doing so, overall there was a very quick and orderly response, and everyone still airborne after the fourth hijacked airliner crashed got down without injury or damage.
A scenario like the opening of this article might actually make the 2001 emergency shutdown pale by comparison, because the skies nationwide were unusually clear and calm that September morning, and controllers could tell pilots to navigate themselves to the nearest airport—almost all, certainly, using GPS. If the GPS system itself went down, every pilot would have to re-route using the shrinking network of ground-based navaids, meaning more airplanes would be converging over the same locations.
Given that en route IFR GPS has been with us for a decade and a half, many pilots’ VOR skills may have diminished. In fact, a whole generation of IFR pilots may never have flown a routine departure, cross-country and approach exclusively using VOR and Victor airways. Any number of things could cause the GPS system to blink off without warning—sunspot activity (the current solar cycle peaks in May 2013), a terrorist strike, some minor but unseen technical flaw, a military or civilian decision (right or wrong) to scramble GPS in response to a national emergency, teenage hackers on some boring night in the central U.S., anything. Whatever the reason, if the GPS system goes down we’ll need to remember how to use VORs.
So with that long build-up to this month’s topic, let’s look at some of the possibly forgotten characteristics of VOR navigation—just in case you need it.
The VHF Omni-directional Range (VOR) navigation system makes use of ground-based transmitting stations that send out two unique signals. One is a rotating beacon, received only when it is pointing directly at the receiver. In other words, your VOR receiver only senses this first signal when the beam sweeps past it during each revolution. The second signal is a pulse signal that transmits every time the rotating beacon spins through magnetic north. This pulse goes in all directions (it’s “omni-directional”), so your receiver will sense this signal regardless of where you are relative to the ground transmitter.
Your VOR receiver compares the times between the rotating signal and the pulse signal and uses that time difference to compute the direction the receiver is from the transmitting station. When you turn the omnibearing selector (OBS) of the VOR’s display, its course deviation indicator (CDI) needle will align straight up and down when the OBS-selected course matches the direction FROM the tuned station. You may also spin the CDI to the reciprocal bearing and the needle will align vertically on the bearing that would take you TO the station. A TO/FROM indicator or flag will tell you in which hemisphere the CDI is turned, i.e., whether the selected bearing is generally TO or FROM the actual bearing you’re on at the time.
I always found one of the most challenging concepts to teach in all aviation was this: Even with the VOR tuned and the CDI centered, that does not mean you are headed directly to or from the station. All it indicates is that, for the moment, you have identified what direction you are from the ground transmitter. To drive home the point, I used to have my students dial up a VOR and center the CDI, then fly steep turns. If you’re sufficiently far from the ground transmitter (more on that in a moment), the CDI will remain centered throughout the entire turn. Instead, I’d teach them, the centered CDI tells you what direction you’d have to fly to go directly to or from the VOR, but you still need to reference the airplane’s heading indicator (or even the magnetic compass) to aim the airplane in the proper direction.
Another concept that was not too difficult to teach then, but may seem foreign to a pilot brought up on GPS, is that the VOR signal is not a fixed line across the ground (like the GPS’ magenta line), but instead a beam that radiates away from the station, getting wider the farther you are from the ground-based transmitter. At 40 miles, an airway’s width is usually four nautical miles each side of its centerline because you can be as far as four miles from the center of the airway at that point and still have the CDI needle near the center of the VOR head.
The real concept here is you can have the needle centered and be flying a precise heading to match the OBS indication in zero wind and still drift off course as you get closer to the station. Flying an airway—or any VOR course—is a matter of bracketing: gradual heading changes one way and the other to compensate for variable signal width (in addition to winds) as you get closer to a VOR station.
Think of it this way: GPS navigation follows great-circle routes that project a straight grid path onto the earth. By contrast, VOR navigation follows signals that radiate from fixed spots on the surface, flying outbound on one before continuing inbound on the next.
Tune and Identify
Another lost art (in fact, it was commonly overlooked even “back in the day,” to the occasional peril of an IFR pilot) is that of tuning and identifying the VOR signal. I used to catch pilots with this in training scenarios. The Hutchinson, Kan. (HUT), airport is only 32 nm from Wichita Mid-Continent Airport (ICT). Frequently, I would take pilots of high-performance piston singles or light twins to HUT for practice approaches, followed by a hop over to ICT.
This was in the late pre-GPS era, so we were dependent on VOR radios, a mental image of our location and progress and, if we were real lucky, a Loran navigator with little or no mapping capability. So after flying the ILS 13 into HUT, localizer frequency 110.1, we’d fly over to ICT, usually for the ILS 1R, frequency 110.3. The airports were close enough to one another that if the pilot forgot to retune his/her receiver for the Wichita approach, for part of the time on vectors for the ILS the VOR was still receiving the Hutchinson signal—so there was no VOR flag.
Only when the pilot was vectored to intercept the localizer and the CDI failed to show the expected intercept would a pilot (usually) figure out his or her mistake. Sometimes the student wouldn’t figure it out at all, and either get very confused by the discrepancy between expectation and reality that he/she got lost on the approach, or worse yet, the pilot would begin to descend toward the airport even though no course guidance or glideslope appeared on the display.
To avoid mistuning situations like this, as well as to detect transmitter and airborne receiver outages, VOR flying requires tuning the receiver, then actually listening to the Morse code signal and identifying the correct station by comparing it to what appears on the chart as that station’s identifier. This requirement isn’t limited to localizers, either—it’s a necessary safety check any time you tune a ground-based navaid.
Now, some GPS units will actually “listen” and identify the VOR signal matching the tuned frequency automatically. But even that would not protect you against the Hutchinson/Wichita scenario. So tune and identify any time you’re using the VOR or localizer for course guidance.
Going the Distance
I had been an instrument flight instructor for nearly three years before I flew my first airplane equipped with distance measuring equipment, DME. The technology was a godsend after learning, and teaching, to mark out cross-bearings from offset navigation beacons at 10 or 20 mile intervals along the airways, timing how long it took to fly 10 or 20 miles, with that information computing the groundspeed, and then estimating time to the next fix and time to destination based on how quickly we were progressing. I got pretty good at it—we all did back then—but suddenly having a device that automatically told me not only my ground speed but also my distance from my tuned navigation stations to the tenth of a mile made instrument flight far, far simpler. On approaches, DME was considered much more accurate for step-down fixes and identifying the missed approach point. In fact, on many approaches the minima were lower with DME thanks to increased accuracy.
Many IFR airplanes still have old DME sets installed, but most new airplanes haven’t left the factory with one for at least 15 years. Meanwhile, many older airplanes have had DMEs removed as they required (hard-to-get) service or simply to remove the weight of the box. Why? Because GPS provides an even more accurate indication of distance. In fact, pushed by airplane owners and owners groups, the FAA permits using GPS in lieu of DME information in those operations that previously required DME.
But what happens if “the waypoints die?” Then we’re all back to drawing fixes using cross-bearings on airways on the charts, timing our flight from one fix to the next, computing groundspeed and estimating our time to the next fix and to destination. Fun stuff.
With WAAS-enhanced GPS, we’re getting pretty used to every approach being a straight-in, often with some sort of vertical guidance, i.e., a glideslope. If GPS goes away, we’ll still have the glideslope for an ILS. But if you’re landing somewhere not served by an ILS system (like, most airports), you may need a refresher in three aspects of non-precision, VOR approaches.
First, without glideslope information you have two choices: mentally compute the rate of descent your groundspeed requires for you to descend from the final approach fix (FAF) to the missed approach point (MAP), remembering that groundspeed will vary as the winds change with altitude, and that in many non-precision approaches just arriving at minimum descent altitude (MDA) when reaching the MAP often will not place you in a position to continue a normal descent to touchdown. In other words, you may see the runway but not be able to land.
The alternative choice is the traditional “dive and drive” approach, descending fairly rapidly to MDA and then flying at that minimum safe altitude until you can see to land normally, or reach the MAP and “go missed.” Recall there may be a visual descent point (VDP) you must pass before you descend out of MDA, even if you have the runway or runway environment in sight.
Second, quite frequently VOR approaches are not directly aligned with the runway. This is an artifact of VOR stations being built not where they made sense for nearby airports, but because a single VOR may host approaches for several airports, or even because the VOR station was built where the government was able to buy or lease the land, which is not always in an optimal location.
If a non-precision approach is roughly aligned with the runway, then that runway’s number is included in the name of the approach—VOR Rwy 15, for instance. But if the angle between the final approach course and the runway heading is more than 30 degrees, the approach requires a circle-to-land maneuver and the approach name contained a letter instead of a runway number—VOR-B, for example, as depicted in the approach plate at left. Note that even a numbered approach may bring you in at an angle of up to 30 degrees. Look at the final approach course on the airport overhead view on the approach chart to visualize what direction out the window you’ll need to look to see the runway.
Third, and if history is a guide, the hardest item to remember, without glideslope information very often the MAP is identified by a cross-bearing from a second VOR (or, egad, an NDB), or even less accurately, merely by timing from the FAF inbound using your best estimate of your groundspeed and the time-to-MAP tables on the instrument approach chart.
With luck there will never be “the day the waypoints died.” But like any other risk-management strategy, it’s good to have a practiced backup in case a single-point failure demands it. We don’t like to think of it as such, but the GPS network on which we depend so much is a single-point failure system, because if anything happens to the satellite signal (or many of the satellites themselves) then the whole IFR navigation system goes down with it.
The further we go down the path to a GPS-primary system, the more critical a system outage becomes. That’s why you need to remember the ancient ways of the VOR aviators in the GPS-driven, glass-cockpit world in which we live.