Engine out
Optimizing performance in a glide
Best glide airspeed, VG, is the one that guarantees the minimum descending glide angle or, equivalently, maximum glide ratio (above). Pilots are taught to immediately pitch for VG on engine failure but often lack nuanced information that could increase the likelihood of reaching a viable landing spot. We’ll discuss how VG is determined, what it is and what it isn’t, and why the optimal airspeed is rarely the value published in the POH.
What is best glide airspeed? During manufacture and certification, test pilots glide the aircraft in calm air across a wide range of true airspeeds and record the associated vertical speed. After plotting these data values, they fit a vertical speed versus airspeed curve among the points (facing page). Of course, since the aircraft descends, the vertical speed is negative, so the graph lies below the horizontal axis. The top of the curve, where vertical speed is maximized, or equivalently where sink rate is minimized, is the minimum sink airspeed VMS. This is the go-to speed should an engine failure occur over a promising landing site, and the goal is to maximize time aloft for, say, troubleshooting the problem.
If surrounding terrain is inhospitable, then the pilot should accept a higher descent rate and use VG instead to reach a more favorable area. If we connect the origin to a point on the graph above, the descent angle γ (in radians) is the angle between this line segment and the horizontal axis. The associated glide ratio is 1/γ :1. For example, gliding at VMS = 70 knots, the aircraft descent rate is 10 knots for a glide ratio of 70:10 or 7:1. But if we form the line segment from the origin that barely touches the graph, the tangent line, the glide ratio improves to 100:12 or 8.33:1. The airspeed corresponding to this point of tangency is VG, best glide airspeed.
Best glide airspeed depends on aircraft weight. By considering the forces in a glide, it can be shown that the glide angle γ is approximately equal to the reciprocal of the lift-to-drag ratio L/D. Minimizing the glide angle corresponds to using the airspeed that maximizes L/D, called max L/D speed. Because weight is not a factor in L/D, glide angle is independent of weight but the airspeed necessary to maximize L/D does depend on weight. The value for VG published in an aircraft POH typically corresponds to maximum gross weight, so a good thumb rule is to decrease VG by 5 percent for each 10-percent reduction in weight.
Using best glide airspeed probably won’t give you an optimal glide. Note that, in defining best glide airspeed, we assumed winds were calm so that ground speed is the same as true airspeed. Glider pilots learn, however, that the speed for optimal glide distance varies with winds and know it as the speed to fly VSF. If the airplane flies into a 30-knot headwind, the new point of tangency happens at 85 knots groundspeed, corresponding to a true airspeed of VSF = 115 KTAS. For a 30-knot tailwind, the point of tangency occurs at 120 knots groundspeed or VSF = 90 KTAS. A headwind always hurts glide range but flying a bit faster will help make a bad situation better. When gliding with the wind, a pilot can afford to fly a little slower but never slower than minimum sink airspeed VMS.
VG is not VY. A common misconception is that the airspeeds for best glide and for best rate of climb are the same, but this is easily dispelled by considering various aircraft manuals. For example, the Piper Saratoga (PA–32R-301) lists VY = 93 KIAS and a VG = 83 KIAS. On the other hand, the Beechcraft Bonanza (BE–33C) features a similar VY = 96 KIAS but VG = 105 KIAS. Each is a full 10 knots difference and occur on opposite sides of VY. Best glide airspeed is determined from the curve where power is assumed to be zero, but VY is the point at which excess power is maximized. Changing the engine or propeller can make VY change. For example, the Beechcraft A36 Bonanza lists a VG = 110 KIAS, but the best rate of climb for the normally aspirated and turbocharged versions, respectively, feature a best rate of climb speed as 96 KIAS and 110 KIAS.
Best glide airspeed doesn’t (necessarily) happen with full nose-up trim. I’ve never found a regulation in Part 23 that mandated a connection between elevator trim setting and attendant airspeed. Still, I read with regularity the advice to, upon engine failure, immediately set full nose-up trim. Before heeding this advice in a real emergency, go up to altitude and try it yourself to see if it’s appropriate for the rigging on your airplane. Personally, I don’t find that advice any simpler than pitching for VG on the airspeed indicator and relieving control pressure. But, if you do and the technique works for your airplane, then there is nothing wrong with using it.
Cleaner is better. Glide ratio is sensitive to aircraft configuration and published VG usually refers to a minimal drag setup: gear, flaps, and cowl flaps retracted and propeller at minimal rpm. But following the emergency checklist may not be enough to ensure that. For example, the Piper Saratoga POH lists an optimal glide ratio of 9:1. That assumes propeller has been pulled back for lowest possible rpm, an action that is not listed on the emergency engine failure checklist. That action is tucked into a paragraph within the amplified procedures—the reference no one is going to read in an emergent situation.
The airspeed that provides the best chance of reaching a hospitable area for landing is a complex topic, but a fuller understanding of VG can lead to a better outcome should the engine fail. Remember that VG is below its published value at less than gross weight, and it’s better to err on the side of faster when flying into a wind and slower when flying with a tailwind, but never below minimum sink airspeed.
