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How would I calculate my glide distance?

If I am at 8000 feet would I multiply 1.5 by 8 because the ratio is 1.5 NM per thousand ft?

Which would be 12 NM

Also where did a ratio of 9:5 come from?

I may have the numbers mixed up but I’m just trying to get an understanding on this topic because essentially I’m just trying to figure out how much distance in NM I would have if my engine cuts out at that altitude.

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    $\begingroup$ Your math is correct, but I don’t think your glide ratio is - that’s a horrible ratio for any training aircraft. (Where did 9:5 come from? How should we know, you typed it, where did you get it from?!) Anyway, what are you flying? You will get some helpful and accurate answers if you provide a little more detail. P.S. I’m editing your question because you are not asking how to calculate the ratio. $\endgroup$
    – Michael Hall
    Jul 13 at 14:17
  • $\begingroup$ Not only where 9:5 comes from, but where 1.5 comes from. The space shuttle was 4.5:1 when landing, which is basically a brick. $\endgroup$
    – tedder42
    Jul 13 at 16:03
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    $\begingroup$ 1.5 nautical miles per 1000 feet is a glide ratio of 9.1:1, which is not far off from a typical trainer aircraft (a C152 is 9.7:1, for instance). $\endgroup$
    – Chris
    Jul 13 at 16:04
  • $\begingroup$ Chris- aha. I read it as 1500ft per 1000ft. Maybe that 9.1:1 shows they meant 9.5:1, not 9:5. $\endgroup$
    – tedder42
    Jul 13 at 16:19
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    $\begingroup$ @TypeIA Certainly you may lose some altitude from the startle factor. But you would have to fail the airspeed control pretty dramatically to lose 33% of your glide distance after that. Having too big a fudge factor is also a problem- it might convince you to ignore an airport or nice field that is in fact in easy glide distance in favor of a less suitable landing spot. $\endgroup$
    – Chris
    Jul 13 at 18:24

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To calculate your glide distance, you need to know the glide ratio at best glide of your aircraft. Here is an example from a Cessna 152:

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Which shows that for every 1000 feet of altitude AGL a Cessna 152 should be able to glide 1.6 nautical miles. 1.6 nautical miles is 9722 feet, so this corresponds to a glide ratio of about 9.7 to 1. But since we usually think of altitudes in feet and ground distances in nautical miles, the figure 1.6 nautical miles per 1000 feet is probably more useful.

If your glide ratio is 1.5 nautical miles per 1000 feet, your math is correct. But that is just a common rule of thumb and may not be the correct ratio for your aircraft.

I don't know where you got 9:5. At a guess it is supposed to be 9:1 (which corresponds to around 1.5 nautical miles per 1000 feet).

Keep in mind that these are not calculations you will be relying on much in the cockpit. In a real engine-out situation there will be winds which will extend your glide (for a tailwind) or shorten it (for a headwind). The true best glide airspeed also depends on the wind speed- with a headwind you will want to be on a slightly faster airspeed and slightly slower for a tailwind.

In the cockpit, visual references will tell you how far you can glide- a spot that appears to be moving down in the windscreen is one that you will glide over at your current glide ratio, while one that is moving up in the windscreen is one that you will land short of. The spot you are gliding to is the spot that doesn't move.

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