Just out of reach

Dynon, Garmin, and ForeFlight all offer glide rings that can be displayed on moving maps, primary flight displays, tablets, and iPhones to visualize emergency landing site options in the event of an engine failure. The graphical tools show pilots how far they can expect to glide following an engine failure and point out any airports in that area.

ForeFlight

ForeFlight introduced its graphical Glide Advisor in 2017 and credits it with several “saves” in which pilots successfully glided to safe airport landings after an engine failure. Glide Advisor uses manufacturer-derived glide ratios for FAA-certified aircraft but allows users to customize them based on their own airplane’s performance. Unlike some other apps which link their glide safety features to proprietary autopilots and onboard flight data computers and display them on panel-mount avionics, ForeFlight’s Glide Advisor appears on iPads and iPhones.

Glide Advisor provides continuous updates based on the airplane’s GPS-derived ground speed, rate of descent, and height above the ground and adjusts the range ring accordingly. If an airport falls out of range, the Glide Advisor’s range rings blink red as a warning sign. Glide Advisor is available to all ForeFlight subscribers regardless of their particular subscription plan, and the company says it’s a popular feature. “We’ve heard from pilots who say it has saved their lives,” said Henrik Hansen, ForeFlight’s chief technology officer. “By calculating glide range based on altitude, terrain, and wind conditions, it enhances safety and situational awareness when it’s needed most.”

Garmin

Garmin’s Smart Glide is the most sophisticated, and some installations contain a dedicated button that engages the autopilot, turns the airplane toward the nearest suitable airport, and pitches for best-glide speed all in a single touch. To put Smart Glide to the test, I made a series of idle-power glides in a 1997 Beechcraft Bonanza A36 starting from altitudes of 4,000 feet agl or higher. I’d fly straight and level in cruise until the range ring showed the Bonanza was barely within glide range to an airport. Then I’d push the Smart Glide button, pull the throttle to idle, yank the blue propeller lever to its maximum high-pitch/low-rpm setting, and let the autopilot fly GPS direct to the airport at the Bonanza’s 110-knot best-glide speed.

In all cases, the landing gear was retracted and the cowl flaps closed to maximize glide performance. Yet in all but a few cases, the airplane would have come down short of the runway and usually outside the airport fence.

The Smart Glide system makes an aural callout when it’s engaged and states the direction and distance to the airport (e.g., “Airport, five miles, two o’clock). That’s often followed, however, by an updated warning that no airports are within gliding distance. The Smart Glide system shares that distressing information both aurally and graphically with blinking warnings that give the pilot as much time as possible to make another plan.

The reasons why real-world glides don’t always match predicted performance start with the airplane manufacturer’s claims. The Beech pilot’s operating handbook, for example, says an A36 has a 10.2:1 glide ratio, and frankly, like so many other manufacturer-derived numbers, that one seems wildly optimistic. Perhaps an A36 really does glide that well in still air on a standard-temperature day.

Garmin bases its range rings on manufacturer-supplied certification numbers until Smart Glide is engaged. Then it switches to measuring the airplane’s performance and redraws the lines. The airplane I was flying is equipped with tip tanks, air conditioning, and a three-blade propeller—all of which surely increase drag and reduce glide performance—yet individual owners can’t modify the manufacturer-supplied numbers.

Smart Glide (connected to the airplane’s Garmin GFC 500 autopilot) does a great job maintaining the target 110-knot best-glide speed throughout idle-power descents. Even in continuous light chop, the autopilot adjusts the airplane’s pitch so that the target airspeed barely strays. Yet best-glide speed itself can be a misnomer because it isn’t always the optimum speed to fly for maximum range. In a strong headwind, for example, a faster-than-best-glide airspeed will reduce the amount of time the airplane is subjected to a headwind and reduce the resulting loss of glide distance. Similarly, with a tailwind, pilots should fly a slower indicated airspeed than best glide to maximize the duration of nature’s push and increase gliding distance. Garmin air data computers know and constantly update wind strength and direction—yet the company doesn’t use that information to adjust the manufacturer-supplied best-glide speed for current conditions.

Dynon

Dynon added an “Emergency Glide” feature SkyView HDX update on August 20, 2024, that engages the autopilot to fly directly to the nearest suitable airport at best-glide speed. It shows a green glide-range ring on the moving map. The Dynon glide ring is circular at all times and its shape changes to account for mountainous terrain—but Emergency Glide takes terrain elevation into account when selecting target airports.

I made a series of flights in my Dynon SkyView-equipped Van’s Aircraft RV–4 and set up each glide in the same way: an altitude of 4,000 feet or above, airport barely within the graphical glide ring. Then I’d engage Emergency Glide (by holding down the NRST button for two seconds or more) and let the airplane turn directly to the airport at idle power and the prop in its full high-pitch/low-rpm setting.

Dynon requires pilots of airplanes registered in the experimental/amateur-built category (such as the RV–4) to set their airplane’s glide ratio and rate of descent because each aircraft is likely to vary, and sometimes they vary widely.

Van’s Aircraft doesn’t provide glide ratios, so I turned to pilots and builders of similar airplanes who provided guestimates of about 9:1. My airplane’s real-world, idle-power performance was closer to 8:1 with a 950-foot-per-minute rate of descent. Those stubby, low-aspect-ratio wings just aren’t efficient gliders.

One glide-ring characteristic that became clear during the Dynon/RV–4 glide flights was that headwinds at cruise altitude are far more likely to result in successfully reaching airports than tailwinds. That seems counterintuitive but headwinds are almost certain to diminish as a gliding airplane descends to lower altitude—and that reduced headwind increases glide range. Similarly, a tailwind at cruise altitude is also likely to diminish during a descent. That decreasing push leads the glide rings to rapidly constrict—and that makes it less likely a gliding airplane will reach its target. During headwinds or tailwinds of 10 knots or more, my airplane’s glide performance was noticeably better when I adjusted the best-glide speed while hand flying instead of using autopilot.

Conclusions

The main benefit of glide rings is that they constantly give pilots this bit of unsettling news: We’re seldom within gliding distance to an airport, even at normal cruise altitudes above populated regions. In a real engine-out emergency, glide rings show pilots instantly that they must pick an off-airport landing site.

There’s no reason to waste precious time with distractions such as searching for and choosing a nearby airport, turning in its direction, changing radio frequencies, and estimating whether or not you’ll be able to glide there. You won’t, don’t bother trying.

The trickiest aspect of glide rings is that they give the appearance of omniscience when they’re really just living in the moment. They make constant calculations and update the pilot every three seconds—yet they don’t predict how much the wind is going to shift in speed or direction at lower altitudes, and they don’t show the pilot the optimal speed to fly to maximize glide range based on current conditions.

For pilots who begin a glide to an airport that appears within glide range only to see it slip away as the rings constrict during descent, the sense of betrayal is severe. After all, as an airplane flies in the direction of a suddenly out-of-reach airport, it’s likely heading toward a more developed area with fewer suitable off-field landing sites. Pilots can find out for themselves by practicing power-off glides in various wind conditions to determine whether their anticipated glide range is accurate. Pilots also can improve glide performance by moving constant-speed propellers to the high pitch/low rpm setting; pitching up momentarily to stop props from adding drag by windmilling; flying faster than the published best-glide speed in a headwind and slower with a tailwind.

Glide rings provide a great deal of potentially life-saving information at a glance. When linked to an autopilot, they reduce pilot workload and avoid potential losses of aircraft control at critical moments. Like other safety tools, however, understanding what goes into them is the key to getting the most out of them.

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