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12 Answers

Glide Ratio

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Aerodynamics

I need the glide ratio for my Piper Archer 11 for Foreflights new distance to glide option. An instructor told me that most general aviation airplanes have a glide ratio of 8 to 1 but to allow for sloppy piloting and maneuvering to land I should use 7 to 1. I was talking to one of Foreflight's advisors who told me the opposite, I told him that I was using 7 to 1 and he said  to allow for sloppy piloting and maneuvering to land I should 8 to 1. Who is correct?

12 Answers



  1. Mark Kolber on Apr 28, 2017

    ForeFlight. The glide ratio the are asking for is forward:down. Let\’s use simple numbers.

    A glide ratio of 10:1 means you will travel 10 feet forward for each foot of altitude you lose in no wind with perfect piloting technique.

    Does imperfect technique and needing to maneuver mean you will do better than that, get an extra foot or two distance for each foot you descend, 11:1 or 12:1? Or does it mean you will do worse and get less distance only get 8 or 9?

    I\’d review it with your CFI. It was probably a wee bit of math challenge and getting the ratio backward. It\’s easy for some of us to do. Especially since most POH provide the information in the miles:feet format generally more helpful to human brains and we need to first convert the miles to feet?

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  2. George on Apr 28, 2017

    You say, “does imperfect technique and needing to maneuver mean you will do better than that?” No you will do worse than that. So Who is correct?

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  3. Mark Kolber on Apr 29, 2017

    I already said ForeFlight and explained why. Did you try to understand the “why” or are you just looking for the answer? If just the answer, you can stop at the first word.

    If you are looking for the “why” read the rest. It’s all about what “glide ratio” means, which is about what “ratio” means. To keep it as simple as possible, the “imperfect maneuvering pilot” will, as you said, do =worse= than best glide. Look at the best glide table in your POH. For the C172 it show something like 6 miles at 4,000 AGL. The imperfect, maneuvering pilot is not going to get those 6 miles. So his real glide range is =less= than published. Which means the circle of choice is =smaller=. Which means the ratio of =distance to altitude= is =less=.

    If you don’t understand what a ratio is, that last sentence will make no sense.

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  4. Ken White on Apr 30, 2017

    I’m gong to make one assumption here, and that assumption is that the Foreflight rep knows the actual glide ratio from the Archer II’s POH. I’m going to say that they are both your CFI and the Foreflight rep were correct in theory but that the Foreflight rep was just a little more accurate. Your CFI used a average GA glide ratio of 8:1 for his example but the Archer II’s POH shows more like a 9.5/10: 1 ratio. Both your CFI and the the Foreflight rep said to take the glide ratio you know and reduce it due to imperfect technique, which in the case of the Archer II is probably more like 8:1.

    If on the other hand the Foreflight rep said that you should take a 7:1 ratio and make it 8:1 to account for pilot error then he was incorrect.

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  5. LTCTerry on Apr 30, 2017

    A typical American training glider has a glide ratio of about 24:1. A modern German counterpart about 33:1. Higher is better. 40 or more to one for single seaters has been common for 30 years or so. An extremely expensive, highly competitive glider today can have a 60:1 glide ratio.

    The six miles from 4,000′ AGL mentioned above could be put to the test. Imagine you are on a six-mile final 4,000′ above the ground with the engine at idle. Will you make the runway?

    Six miles is 31,680′ horizontal distance. If at best glide speed this distance requires 4,000′ feet, then this is a glide ratio of 31,680:4,000. Divide both sides by 4,000 for a simplified ratio of 7.9:1. At 7°, this slope would be higher/steeper than a “standard” 3° glide slope. This ought to be as safe as any other long, straight in approach.

    Watch the end of the runway. If it looks like it’s moving down in your view, you are overshooting, so you can adjust to make the runway. If it’s moving up in your view, then you will not make it. This is no different than being on final otherwise.

    Consider trying this somewhere with a long runway and start half a mile closer. This lets you aim for the middle of the runway. If you are a little short, it’s still OK.

    What speed should you fly? “Best glide” covers the greatest distance for the altitude lost. The published speed assumes still air. In soaring, the rule of thumb is to add half the head wind or subtract half the tailwind (but never below minimum sink speed).

    If best glide is 60kts, what happens if the wind is 10kts? With a ten-knot head wind, the ground speed becomes 50kts. So, multiply the 7.9 glide ratio by 50/60 to get 6.6:1, a pretty significant loss. And, with a tailwind 7.9 times 70/60 gives you 9.2:1 as the new effective glide ratio.

    The “move up/move down” idea also works for any field you’ve picked for an off field landing.

    I just did something similar on a German renewal checkride several weeks ago from cruise down to about 20′ over over a plowed field. ASEL simulated engine failures are so easy to manage when you’re a glider pilot. Though to be honest, I wouldn’t take someone down to 20′ over a farm. German flight training does include a lot of flying off grass. I would have been comfortable landing in the field, though the owner of the airplane probably wouldn’t have liked it.

    On a slightly related side note, on the same checkride, I did a GCA/PAR into the the base I work on. The GCA equipment was put in place in 1947 and was used 1948-49 to guide returning crews from the Berlin airlift in zero-zero weather. Super cool to have my own micro connection to some amazing aircrews!

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  6. Mark Kolber on Apr 30, 2017

    Ken, do not assume FF knows the glide ratio. It doesn’t. It only knows the glide ratio and glide speed the pilot gives it. The 8:1 is just an illustration of how to calculate yours.

    If you haven’t seen the feature, what FF does is take the glide speed and ratio =you= give it and, using GPS-derived GS and terrain database, paint a glide range bubble around you. In no wind with flat terrain, the bubble would be a circle. Using terrain and calculated winds, it is usually a non-uniform oval.

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  7. Ken White on Apr 30, 2017

    I understand the feature, I was talking about the Foreflight advisor George said he was talking to in his original post. My understanding is that person is the one who told him to use 8:1.

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  8. George on May 02, 2017

    Does the FAA care if you have a knee replacement? George, if this is a new question, this is not the right place to post it. Please use the Ask a question tab and enter your new question. I will delete this entry tomorrow.

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  9. George on May 02, 2017

    Thank you everyone for the glide comments. George

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  10. George on Jun 20, 2017

    I cannot locate the “Start/Taxi/Takeoff Fuel” for my PA28 181 Archer II. Can anyone help? Thank you. George

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  11. Ticen on Jul 17, 2017

    I would gravitate toward the lower ratio. In an emergency situation, nailing best glide speed quickly and consistently may be a high mark to hit. Plan for the worst, but expect the best. If you do manage to keep the 8:1 vs 7:1, you will be a few hundred feet higher at that point of no return. Forward slips and S turns can always get you down in a hurry – but being lower than expected – no remedy for that!

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  12. Christopher Ian on Aug 05, 2017

    A huge part of this topic is missing from here.
    Glide ratio should be in every POHandbook. Along with that, Best-pronounced, longest- glide speed MUST BE KNOWN. Along with that, current aircraft weight and aircraft gross weight MUST be known. Anything less is guess work.
    An airplane Best -Longest-Glide is achieved at a specific speed. Your aircraft will glide the same distance at gross or minimum weight if you maintain the proper speed for your current weight.
    The POHandbook gives best glide speed at Max Gross Weight.
    For each 10% less than gross, reduce speed by 5% to arrive st Best Glide speed for your current Aircraft weight. 2,400 gross (C172) Best glide is 56. 10% below gross=2,160, best glide =53.2. Yes, it makes a difference.
    My CFI once asked me the best glide speed, I answered 56, as per the book, she replied, Ok, but what is best glide speed at other than gross weight. She didn’t know the answer. I learned this formula adjustment from a Cessna flight training book.
    Making rough estimates of 7:1, 8:1 is lazy work. And inaccurate.
    Test it. Figure your weight precisely. Go up at 6 am before winds pick up. Use a tracking app with gps for accurate distance & altitude or fly between parallel roads a few miles apart recording time and altitude with at least one or two pilots passengers with you.

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