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For the Cessna 172 Skyhawk maximum glide is attained at a speed of 65 KIAS. If you're flying well above this speed, let's say 90 KIAS, and suffer an engine failure, what would be the best practice: immediately slow down in order to achieve 65 KIAS as soon as possible, or slow down slowly?

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    $\begingroup$ Is waiting to slow down in a C172 without any thrust too long for you? $\endgroup$ – Hugo Woesthuis Nov 27 '16 at 15:16
  • $\begingroup$ There may be some C172 models that list best glide speed as 60 KIAS, but that is not a speed that I am familiar with. The last two models that I flew—C172M and C172R, listed 80 mph (70 KIAS) and 65 KIAS, respectively. In the interest of accuracy and safety, if 60 KIAS is in fact a valid speed, you should specify the model. $\endgroup$ – J Walters Nov 27 '16 at 16:16
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    $\begingroup$ When you pull up to drop your speed, you are trading speed for altitude, something you'll appreciate as time goes on. $\endgroup$ – Ron Beyer Nov 27 '16 at 17:19
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    $\begingroup$ It isn't necessarily a bad question, because aggressive acceleration might rob energy that might otherwise go to extending the glide. $\endgroup$ – J Walters Nov 27 '16 at 20:00
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    $\begingroup$ If you were able to transfer all the excess kinetic energy into altitude, you could theoretically gain 2200ft. However, if you pull up aggressively to quickly increase your altitude, you will lose a lot of energy due to the increased drag. Gently pulling up and climbing while the airspeed is bleeding of is the most efficient way to preserve the energy. When approaching 65 knots, stop your climb (this can be a relatively quick pitch down) and maintain the max glide speed. $\endgroup$ – DeltaLima Nov 28 '16 at 8:44
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According to the amplified emergency procedures of the Cessna 172 Model N POH (emphasis added):

After an engine failure in flight, the best glide speed shown in Figure 3-1 should be established as quickly as possible.

The same POH lists 65 KIAS as the best-glide speed.

Time spent away from best glide means less glide range. If increasing time, not glide range, is your goal then the minimum sink rate speed would be what you would want to fly. In an emergency, time and glide range give you more options.

The POH and physics agree: yes, establish best-glide speed immediately after an engine failure.

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    $\begingroup$ You can also use that excess air speed to increase your altitude slightly before setting up for best-glide attitude. It could all help in an emergency. $\endgroup$ – Adam Nov 28 '16 at 6:21
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    $\begingroup$ @Ads I think this is key. The question is, should Iget to that speed as quickly as possible? If it means killing speed using e.g. drag, that lost energy is probably worse than getting to glide speed slowly. If however the emergency still allows enough control to use altitude to slow down, than that energy is stored as potential and is reusable. In this case, it is better to do it as quickly as possible. Anyone agree? $\endgroup$ – MrBrushy Nov 28 '16 at 13:27
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    $\begingroup$ How about - hold your altitude until at best glide speed? $\endgroup$ – Mike Brass Jul 18 '18 at 0:46

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