Best Glide or Minimum Sink
IF YOUR ENGINE FAILS, SHOULD YOU FLY BEST GLIDE OR MINIMUM SINK?
When you think about power off landings, there are probably a lot of things that go through your head, like finding an airport within gliding distance, finding an off-field landing site if there aren’t any airports, and last-ditch efforts to get your engine running again before you’re out of altitude.
In 2013, there were thirteen fatal accidents related to power off landings, according to the NTSB. You’re faced with some very serious decisions during a power off landing. But after you’ve run your checklists and determined your engine isn’t coming back to life, handling a power-off landing really comes down to three simple things: aviate, navigate, and communicate.
MAXIMIZING GLIDE RANGE, OR TIME ALOFT?
The first question you need to answer in a power-off landing scenario is this: do you want to maximize the distance you can glide, or do you want to maximize the amount of time you can stay aloft?
Most often you want to maximize the distance you can glide, at least initially, as you set up for a power off landing. The airspeed you want to pitch for is best glide speed.
No matter what aircraft you fly, best glide speed is usually published in the aircraft POH, and it’s the best airspeed to start with as you’re setting up for a power off landing.
Best glide gives you the best glide angle as you drift down, which means that if you maintain best glide all the way to the ground, you’ll travel the furthest distance possible without power.
There’s something you need to keep in mind about best glide, though. Like most airspeeds in the POH, best glide is calculated at max gross weight. And as weight decreases, so does the speed that will maximize your distance. The change is minor, but if you’re trying to get the most out of your glide and you’re lighter than max gross weight, a slightly slower speed may help you out.
MAXIMIZING YOUR TIME ALOFT
If you want to stay in the air for the longest time possible, you want to fly at the minimum sink speed. Unfortunately, there’s a problem with that. The minimum sink speed is rarely published for powered aircraft. But there is a way you can figure it out: try it in your plane.
Minimum sink is always slower than best glide, because it’s the point on the power required curve where the least amount of power is required. Keep in mind, though, you’re going quite a bit slower than your best glide speed, and that can significantly impact your glide range.
Unless you have a good landing site below you, and you’re trying to maximize your time aloft to troubleshoot the engine and talk to ATC, minimum sink isn’t necessarily going to be as helpful as sticking with best glide will be.
SELECTING A LANDING SITE: AIRPORT
Once you’ve accomplished the “aviate” part of the flight by configuring the airplane, and pitching/trimming for best glide, your next step is to “navigate” and find a place to land.
When it comes to landing sites, you really have two choices. Land at an airport, or land somewhere else. Typically, you first choice is to land at an airport, if you can.
If you have GPS on board, whether it’s panel mounted or an EFB like ForeFlight, the “Nearest Airport” function gives you a quick list of nearby airports.
Once you pick an airport and go direct to it, you’ll know your distance to the runway. The next question is: can you get there? That’s where some quick mental math comes in.
Most GA airplanes, whether they’re a Cessna 172, or a Cirrus SR-22, glide about 1 1/2 miles for every 1,000′ of altitude.
So for example, if you’re 4,000′ above the ground, you’ll be able to glide about 6 nautical miles before your wheels are on the ground. You should always look at your POH maximum glide chart, but if you don’t have it handy during your next engine failure, the 1 1/2 miles per 1,000′ feet will at least get you close.
If you have ForeFlight’s new “Glide Advisor” feature, that can tell you even faster what airports you’re within gliding distance of.
So what should you do if you don’t have GPS? Pull out your sectional chart, if you have time. You probably won’t have a plotter handy to measure the distance from your position to a nearby airport, but you can always use the distance measurement at the bottom of the chart. Use your fingers to get a measurement from your position to the airport, move it down to the legend, and there you have it, the distance to your airport.
APPROACHING THE AIRPORT
As you get close to the airport, you need to plan your landing, and that’s going to start by choosing a runway. There are a few ways you can do it. If you know the ASOS frequency, you can dial it in and pick up the winds. And if you’re in a position where you’re circling over the airport at altitude, you can look at the wind sock.
But there’s one more way that might even be easier: talk to ATC.
That brings up step three of the power-off landing process: “communicate”.
If you have even a little time before reaching the airport, you should try to let ATC know you’re having an emergency. First, squawk 7700. If you’re in radar contact, that will light up ATC’s radar scopes. At that point, they’ll start tracking you and getting emergency response ready.
Also, you want to talk to ATC if you can. So what frequency should you use?
There are a couple you can start with. If you don’t know what Center or Flight service frequencies are available where you are, start with a radio call on the universal emergency guard frequency of 121.5. 121.5 is meant for aircraft in distress, and most ATC facilities monitor the frequency.
If you’re within range of ATC, they’ll hopefully hear you. Many airline and corporate jets monitor guard frequency as well, so if you can’t reach ATC, there’s a good chance you can reach a jet flying above you, and they can relay information back and forth to ATC.
And if none of that works, you can always try the universal Flight Service frequency of 122.2.
GETTING READY FOR A RUNWAY TOUCHDOWN
As you approach the airport, if you have enough altitude, you want to circle down over top of the airport. That keeps you close to the runway, and lets you set up for a normal landing.
At about 1,000′ AGL, enter downwind for the runway, and keep your pattern tight, because you only have one shot to make the runway.
You also need to pick an aiming point for touchdown, so you know when to turn your base leg.
A good way to pick an aiming point is to visually split the runway into thirds, and aim for the point where the first and second thirds meet. That will help you make sure you don’t end up short of the runway, but that you still have plenty of room to stop.
As you’re abeam your aiming point, turn base. At this point, you also want to start flying your normal pattern speeds. As long as the runway is assured, add partial flaps as well.
As you turn final, judge how you’re looking for a glide path to the runway. Keep in mind that you’ll be higher than a normal 3-degree glide path, but you’re also descending much faster because you don’t have power.
At this point, you want to aim for your touchdown point, one-third down the runway.
If the point is moving down in the windshield, it means you’re high, and it’s probably time to add more flaps or slip to lose altitude. But you want to keep in mind that you need to be absolutely sure you’ll make the runway before you add flaps.
And if your aim point is moving up in the windshield, it means you’re getting low on glide path, and you shouldn’t add any more flaps until you’re sure you’ll make the pavement.
As you cross the threshold, you need to focus your attention on a safe touchdown. You’re still aiming for the touchdown point, but if you’re high and fast, it’s better to land a few hundred feet beyond the touchdown point, than it is to force the airplane on to the landing spot.
SELECTING A LANDING SITE: OFF AIRPORT
If you can’t glide to an airport, you need to pick the next best thing. And most of the time, you have quite a few options.
When you’re preparing for a power-off landing, there are two things you need to consider to make your landing survivable.
First, you need to keep the cockpit and cabin as intact as possible by using dispensable parts of the plane, like the wings, landing gear and bottom of the fuselage to slow you down during landing.
And second, you need to prevent your body from hitting the inside of the cockpit during touchdown, by making sure your seat belt is tight.
Most GA airplanes are designed to protect you at up to 9 Gs of forward acceleration.
Look at these examples: if you’re flying at 50 MPH, the required stopping distance at a 9 G deceleration is about 9.4 feet.
And if you’re flying at 100 MPH, the required stopping distance at a 9 G deceleration is about 37.6 feet.
Think about that for a minute: 37 feet isn’t a lot of required stopping distance in a survivable crash. In fact, it’s just a little bit longer than the
FINDING A PLACE TO LAND
When you’re looking for a place to land, two common places you’re going to look are fields, and roads. Unfortunately, both of them come with their fair share of risks. But if you pick a good spot, chances are very high that you’ll walk away from the landing.
CONTROLLING ATTITUDE AND SINK RATE
When you’re landing off-airport, the most critical mistake you can make is not controlling your aircraft attitude and sink rate. Fortunately, you can control both of them all the way to touchdown.
No matter where you are, if you land in a nose-low or level attitude, you risk sticking the nose into the ground. That’s going to flip your plane, or make you stop very quickly, which can easily cause more than just a few bumps and bruises.
Steep bank angles before landing are just as bad. When you’re in a steep bank, your stall speed is significantly higher. And on top of that, if you strike your wing to the ground, your plane will cartwheel, making the landing much less survivable than a straight-ahead deceleration.
No matter where you’re landing, you want to set yourself up for a straight ahead, nose-high landing into the wind, so you can land at a slow groundspeed, and use the airplane to protect yourself.
AIRCRAFT CONFIGURATION FOR OFF-AIRPORT LANDINGS
What should you do with flaps when you’re landing off-field? Flaps let you fly at slower speeds before stalling, which is obviously a good thing. But they also significantly decrease glide distance.
You need to be very careful about adding flaps too early in your setup for an off-field landing. Otherwise, your best laid plans will go out the window, and you’ll end up landing somewhere you really don’t want to be, as opposed to somewhere that’s a decent landing spot.
Landing gear position is another thing you need to be thinking about.
There’s no hard and fast rule on using them, but if you have retractible gear, you have a few more choices to make.
If you’re touching down on something soft, like a plowed field, landing with your gear down means there’s a reasonable chance your gear will dig in to the dirt and flip your plane. But if you’re landing on a hard surface, putting your gear down helps cushion your touchdown, as well as decelerate your plane all the way to a complete stop.
TOUCHING DOWN OFF-AIRPORT
As you touch down, remember the words of the legendary pilot Bob Hoover: “If you’re faced with a forced landing, fly the thing as far into the crash as possible.”
After you’ve come to a complete stop, there are two things you want do to quickly. If you have time, activate your ELT so search and rescue can find you. And second, get your passengers and yourself out of the plane.
Executing a good power-out landing comes down to flying the plane all the way to a complete stop, picking the best landing spot you can, and letting ATC know about your situation. Do all three, and you’ll have a landing you can walk away from.