Tuesday 22 August 2017

If Your Engine Fails, Should You Fly Best Glide Or Minimum Sink

If Your Engine Fails, Should You Fly Best Glide Or Minimum Sink?

Thanks to Boldmethod for sharing...

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.

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