Moving maps may be what sell the top-of-the-line handheld GPS units, but one of the niftiest features common to all GPS units is the nearest feature that points you at the closest airports as quickly as you can push the buttons. Its a good one, for sure, but doesnt relieve you of your responsibility to constantly update your engine-out plan.
The moment of power loss is very, very busy. I have been there. After a rapid diagnosis of the problem and trimming for best glide, the next item is Where am I going? If youre flying IFR, Center may be of assistance – or maybe not, as the occupants of an MU2 found out in 1993 on their way into an Iowa barnyard.
The push of a button (or several buttons, depending on the brand) on a GPS can call up nearby airports, reporting heading, distance, longest runway and even frequencies. The unit may generate 20 nearby airports, allowing you to pick the one that best meets the emergency at hand.
The old-fashioned way involves hitting the sectional, finding the course line, quickly sizing up distances on the map and then guessing what heading puts you there.
Cross-country students are usually loaded to the max staying on the dead reckoning course. Its not until I pull the throttle that it becomes apparent whether the student has really prepared for the cross country. The well-prepared student makes the initial turn as he or she trims for best glide, knowing ahead of time which way to turn. Thats followed by engine diagnosis, radio communication and, when everything works the way it should, a successful approach into a nearby airport.
Loran changed all that, but GPS has cemented the change in the minds of most pilots.
With a single button push, the panel-mount Northstar M3 in my Seneca gives heading, distance and runway information on one line for each airport. Turning a single knob moves the selections from the very nearest to the next nearest, then to the third nearest and so on.
Of course, when it comes to displaying nearest airport information, not all GPS units are created equal.
The Lowrance 100 is a very useful handheld, but a laggard in regard to this critical function. It is limited by the small screen, but the logic is also not optimal for finding an airport in an emergency. Pressing two buttons simultaneously gives the airport screen, with identifier, distance and heading to nine nearby airports. Obtaining further information, however, is several buttons away. If you want to look at whats available at another airport, you have to back out of the menus to go back to other potential airports on the list.
When the heat is on, it may be difficult to go through the exercise of pushing eight buttons just to find out if the runway is appropriate, only to have to start over if your first selection was an unlighted 1,800-foot grass strip.
The larger screen Garmin GPS 195 is set up a bit more conveniently. The nearest function yields a display of the 10 nearest airports with heading, distance and longest runway length. Although a 3,000-foot strip may be an ambiguous quantity without further button pushing, you can pretty much assume that something 8,000 or 10,000 feet long is pretty useful.
Another popular handheld, the Apollo Precedus, lies between the Garmin and the Lowrance. It takes one button pushed twice to render a screen with five airports, each with a distance and bearing. Four more strokes gives a screen showing the selected airports runway lengths, surfaces, orientation and lighting. Going back to the list involves two taps on the GO TO/NRST button.
The consequences of a poor choice, or a not quite prompt choice, may cost the bacon, but its also the pilots job to know whether the closest airport is also the one the airplane is most likely to make. Calculating that involves knowing how far you can glide, how long you can glide and what the winds are doing to your flight path.
How Much Glide?
To figure out how far you can glide, approach the problem in reverse by calculating how much of your altitude will not be available for getting to the airport. Its pretty uncomfortable to arrive in the pattern at less than 1,000 feet agl, and your options are definitely limited. In addition, GPS airport datum may be quite distant from the spot where you need to arrive to set up a landing, so you really need to get to the airport area with at least 1,500 feet between you and hostile terrain.
If the runway is short, you will need to align into the wind. If its long you might be able to accept a tailwind. But clearly you need a margin beyond I can get there. When youre calculating your best gliding distance, start by lopping 1,500 feet off the height you think you have for gliding.
A word about the GPS datum. The 1983 North American Datum and the World Geodetic (WGS 84) datum differ from each other only very slightly – not enough to matter to pilots, anyway. What matters more is the location of the point on the airport. Its not commonly on a runway and, in some cases, may be a mile or more away. Thats one reason why go to can only get you close; you really do need the last 1,500 feet for maneuvering.
The range you can obtain depends primarily on your aircraft and the prevailing wind.
In the case of a Mooney 201, the glide ratio according to book is 10:1. This means that it will glide 10,000 feet for every 1,000 vertical feet lost in still wind conditions, with propeller windmilling, gear up, flaps up, cowl flaps closed and in coordinated flight.
Stalling the prop changes the numbers a bit. Slowing the aircraft to just about wing stall speed will generally cause the prop to stop, resulting in a superior glide ratio. In the Mooney 201, the maneuver costs about 1,000 vertical feet but improves the glide ratio to about 12:1. That means it would be to your advantage any time you had more than 6,000 feet of VMC altitude to play with.
The glide ratio with the prop stopped does not appear in the POH, as it is not an approved emergency procedure. The only way to figure out what it is for your airplane is to actually try it. Remember that this test creates a real in-flight emergency. Vacuum instruments will become unavailable, your alternator will be off-line and you may have difficulty restarting the engine at the end of the test. A bona fide deadstick landing becomes a real possibility.
Wind in Your Face
Another key component of glide range is the effect of the wind. Think in terms of minutes of exposure to the wind. Consider the M20J at max gross weight, trimmed to best glide airspeed. The Mooney will be coming down at about 900 fpm, a figure that is not in the POH because it depends on the aircrafts gross weight. You can calculate it for max gross weight from the POH by dividing glide range at a given altitude by glide speed to get time of descent, then dividing that given altitude by the time of descent.
To determine the descent rate at less than max weight, establish best glide airspeed at your usual gross weight, and adjust the power and prop until the GPS tells that you are making just the book distance for each 1,000 feet lost. This prop/power setting represents windmill drag for that particular gross weight. In most airplanes, the vertical descent rate will be somewhere around 1,000 fpm.
In the Mooney M20J, a glide from 12,000 feet agl will result in about 13 minutes of exposure to the wind. A nautical mile is about 6,300 feet, so a 10:1 ratio yields about 19 nautical miles of still wind glide. Lets say youre shooting for a runway 15 miles away, with a 30-knot quartering headwind 60 degrees off the true course. That presents a 15-knot headwind. In 13 minutes, this will cost you 3.3 miles. That theoretical 19-mile glide has now become 15.7 miles.
If you remember to leave some altitude for maneuvering once you get to the field, figure that you have only 10,500 feet of the 12,000 feet available. That gives 11.6 minutes of wind exposure. The 15-knot headwind costs 2.9 miles, for a useable glide of only 16.1 miles.
Now consider the same problem in a Cessna 172 with an 8:1 glide ratio and a best glide speed of 73 knots. That same 12,000 feet of altitude gives 14.9 nm of still wind glide range. Allocating 1,500 vertical feet to maneuvering, youd still have 13.3 miles of still wind glide.
The descent rate at gross will be near 980 fpm and about 10.7 minutes will be spent getting there. The same 15-knot headwind component costs 2.7 miles, reducing the glide range to 10.6 miles. Options are rather more limited in this aircraft. If the headwind is 30 knots, youll lose 5.4 miles, leaving only 7.9 miles.
A direct crosswind also diminishes your glide range. At 15 knots, the geometry of crosswind vectors reduces your range 1.8 percent in the Mooney; at 30 knots the decrement is 7 percent. In the Cessna 172, the reductions are 2 percent and 8.4 percent respectively.
What can we conclude from this exercise? As a rule of thumb for general aviation singles, a 15-knot headwind will cost about 20 percent of the glide range and a 30-knot headwind will cost about 40 percent of the glide range. A tailwind will extend glide range accordingly, though you must remember that the tailwind boost falls off near the ground. But, in order to have significant maneuvering room at the airport dont plan to use more than 70 percent of the apparent still wind glide range with a 15-knot headwind. Flying into the teeth of a 30-knot headwind using more than 58 percent of the apparent still wind glide will be very uncomfortable.
And, with the pressure on, anything near the last 10 percent of your calculated actual glide range is not a good bet. Even if you actually know the best glide speed for your gross weight, your hands will be full staying within 5 knots of that speed.
What about when trouble strikes four miles off the end of a runway? With a 10:1 glide ratio and no wind, youll need to be 2,110 feet agl. Most of the time youll have something of a headwind if youre inbound, and in the Mooney you need to be at 2,500 feet if youre flying into a 15-knot headwind.
Four miles off the end of the runway is a bad idea with no power, and this exercise should also explain the merits of keeping airport traffic patterns tight.
Although the GPS can tell you where the nearest airport is, figuring out if you can make it means you have to know what the wind is doing when the moment of truth arrives and you have to make the heading decision. You should always have a current estimate in hand, as anyone following his flights progress will.
The GPS, of course, gives very accurate figures for ground speed and course, while true airspeed and heading are simple to determine in the cockpit. Good GPS units, particularly handhelds, have built-in E6B calculators that will figure the winds aloft and the headwind component for you.
Once youre reasonably sure how far youll be able to go, consider what lies on a direct line between you and the salvation of the runway. The Mooneys angle of glide at 10:1 is 5.7 degrees in straight and level flight, and thats not likely to fit any instrument approach requirements. If your problem happens in IMC, you can only hope that you break out with enough altitude to maneuver to a runway, taxiway or spot of flat dirt.
In an odd sense, it may not even matter if youve left 1,500 feet agl for a maneuvering reserve if the ceiling is 1,000 feet, because you cant use it. But remember, if a circle to land approach exists, the area around the runway ends is protected for 1.3 miles out. In that case, circling down around what you hope is the airport center in a one-mile radius turn may well keep you in airspace that is clear of towers, tall buildings and other unpleasantries.
Using the GPS to its fullest capabilities can help in an emergency, yes. But it can also raise some serious questions about the kinds of risks you take as a pilot. Give full thought to your glide range and the possible outcomes of engine failure before, for example, you fly VFR on top in a piston single over inhospitable terrain.
Some time ago I suffered progressive turbo boost loss while cruising at 18,000 feet in the clag. I circled down to Nashua, N.H., which was reporting 600-2, and I can tell you I was nervous as a cat when I reached 1,000 feet. That experience was instrumental in my changing my daytime personal minimums to 800-foot ceilings with two miles visibility below.
Knowledge of the gliding range can also affect route planning. Consider a short cross-country trip from Bedford, Mass., to Bangor, Maine. The 150 nm trip along coastal New England can be made in a Mooney in instrument conditions without ever being out of gliding range of a lighted landing strip. It requires a cruising altitude of about 13,000 feet if there is no wind.
Of course, there are many trips where its not possible to guarantee that youll be within gliding distance of an airport, and there are many parts of the country where an engine failure will almost certainly result in a non-reuseable aircraft. Woods and mountains, sure, but also sprawling suburbs and large expanses of water can ensure the airplane never flies again.
Off-airport landings, then, become another situation where the GPS can help.
We arent talking about optimal outcomes here. Perfection is the enemy; instead attempt merely to avoid injury. Small roads are generally a terrible place to land. Though the NTSB reports wont contain any of the damage free episodes, the database is packed with accounts of didnt see the wires until it was too late or left wheel went off pavement and wing tip struck road sign.
In fact you can rarely see power lines or other roadside obstructions until youre committed to land. If the option exists, a field is much better. Theres a whole folk school in the Midwest as to wheels up in beans, down in corn.
Larger roads have more room for airplanes, but they also have more cars competing for that room. It also isnt very fair to drivers to suddenly introduce a 70-knot wing spar into the interior of their car.
However, a handheld GPS gives more choices, particularly around your home field. I grew up in Chicago and now live a comfortable general aviation distance away. It turns out that my GPS was full of local spots where an emergency landing would be survivable.
Anybody flying general aviation in a large city knows that the controllers like to keep you below the edges of the Class B, even when youre on an IFR flight plan. That means no glide margins.
By employing the ability to program user waypoints into the GPS, Ive got about 70 safe havens in there: an urban greenway, a golf course, a local racetrack, a local high school athletic field, a stretch of six-lane road that has no utility wires and the old NAS Glenview, just to name a few.
If you put your mind to it, you can usually come up with a string of options, one of which might make your landing survivable one day. In my GPS they have names such as CHImdw and CHIrt59, so that I could rapidly flip through on waypoint search. Theres no way to reach an airport if you have an engine failure at 1,500 agl if youre not already entering the pattern, and itll be less than two minutes until the crunch.
If your knowledge of your position and your airplane make it clear an accident-free landing is out of the question, consider the option of employing the best duration glide. This is obtained generally by trimming all the way back. The range of glide will be less than optimal, but the sink rate will decrease to about 600 fpm.
For the Antonov AN2, a huge radial engine single, the emergency procedure for engine out consists simply of securing the systems, trimming full back, wings level and landing at about 40 mph.
This means that a descent from 12,000 feet agl will take about 20 minutes rather than 12. It permits more time to diagnose and possibly solve the engine problem, give a detailed position report to air traffic control, and brief passengers on how to deal with the landing and egress. Over water, it allows time to don life vests, organize the dry box and get the raft into position to get it out of the plane first.
GO TO/NRST is a terrific GPS option. However, it does not relieve you from taking the time to plan your options in detail.
From higher altitudes, knowing the actual winds aloft and their approximate impact on your glide range, as well as having worked out your own risk-taking profile, is essential to your success. Personal minimums need to be consciously decided as they have direct consequences.
From lower altitudes, detailed local knowledge and an almost guerrilla-like approach to local terrain features, as well as the foresight to make the information readily available in the database, are a great help.
But nothing can substitute for position awareness and the recognition that best glide is likely to give you less than 10 miles. If the GPS says the airport is farther than that, start looking for a good farm field.
Also With This Article
Click here to view “The Aerodynamics of Best Glide.”
-by Bruce Chien
Bruce Chien is a CFII-MEI and AME and owns a Seneca.