Best Glide Speed versus Minimum Sink Speed – Cessna 172 – Establishing Each?

October 12, 2008

For those pilots who are familiar with the Cessna 172, here's the questions:

How does one best establish these two speeds? What I am wanting to hear, is there a tip to establish them, independent of Gross Weight? And in what situations would you use each?

Are these always the same as the ones in the POH? What about headwinds? Tailwinds?

And what about if a Check Ride Examiner pulls the throttle to idle – what speed do you choose?

Hi I hope to clarify this for you. The best glide speed will give you the most range, NOT the most time in the air. The minimum sink speed will give you the most time in the air, but not the best range.

Best glide speed will be higher than Minimum sink speed.

During an engine failiure you should usually maintain the best glide speed especially if you are over water or rough terrain and are trying to make it to a safe and level spot, some distance away. Best glide speed will be your most important value there. You should only really use minimum sink speed if distance is not an issue, but you are trying to buy some time in the air, perhaps to have a few more goes at starting the engine. But really that speed is nnot of much use.

No the speeds are not always the same as in the POH. Usually the POH of a light aircraft such as the C172 will give you all the speeds at Maximum Gross Weight. Both speeds should decrease very slightly, but it really is a negligable difference, especially considering that there is no "perfect pilot" who can maintain speed right on the spot all the way. There will always be slight fluctuations and so a few knots really won't make much of a difference, you are just much better off maintaining the published speed. Also an actual engine failiure is something you have to see and experience to believe!!! You wouldnt want to get into any calculations about speed and weight in this critical time, especially if it will only result in a 1 or 2 knot difference. For this reason the POH is published as it is.

Ur next question is about winds. Always remember that a published speed is what you should always fly with. Don't confuse the air speed and the ground speed! Planes fly in the air and it's their relative speed to the air that matters. Lets say for example your best glide speed is 75 kts and you have a 25kt headwind. You should still ignore the wind and maintain 75kt as that is your relative speed to the airflow. Your ground speed will be 50kts and therefore your range will also be decreased. Best thing to do could be to turn around and use the wind as a tail wind which will give you a ground speed of 100kts and thus double your glidig range!!

And ur last question about the checkride. If the examiner pulls the power, he will expect that you maintain the best glide speed as published in the POH. Don't confuse yourself by trying to calculate a new speed according to the aircrafts current weight.

The minimum sink speed really isn't that important and practical.

So to summarise: Best glide speed and minimum sink speeds do change with weight but the difference is so small that its just beter left for the Physics class. Also remember that the engineers and test pilots have thought of everything, so if you don't see something in the POH, it's probably not important and you shouldn't try and resolve it as it will just make you life more complicated

I hope this helps and sorry I can't provide you with any numbers. I trained for my ATPL on Pipers. Good luck!!!

Comments

7 Responses to “Best Glide Speed versus Minimum Sink Speed – Cessna 172 – Establishing Each?”

  1. FutureCaptain on October 12th, 2008 10:35 am

    Hi I hope to clarify this for you. The best glide speed will give you the most range, NOT the most time in the air. The minimum sink speed will give you the most time in the air, but not the best range.

    Best glide speed will be higher than Minimum sink speed.

    During an engine failiure you should usually maintain the best glide speed especially if you are over water or rough terrain and are trying to make it to a safe and level spot, some distance away. Best glide speed will be your most important value there. You should only really use minimum sink speed if distance is not an issue, but you are trying to buy some time in the air, perhaps to have a few more goes at starting the engine. But really that speed is nnot of much use.

    No the speeds are not always the same as in the POH. Usually the POH of a light aircraft such as the C172 will give you all the speeds at Maximum Gross Weight. Both speeds should decrease very slightly, but it really is a negligable difference, especially considering that there is no "perfect pilot" who can maintain speed right on the spot all the way. There will always be slight fluctuations and so a few knots really won't make much of a difference, you are just much better off maintaining the published speed. Also an actual engine failiure is something you have to see and experience to believe!!! You wouldnt want to get into any calculations about speed and weight in this critical time, especially if it will only result in a 1 or 2 knot difference. For this reason the POH is published as it is.

    Ur next question is about winds. Always remember that a published speed is what you should always fly with. Don't confuse the air speed and the ground speed! Planes fly in the air and it's their relative speed to the air that matters. Lets say for example your best glide speed is 75 kts and you have a 25kt headwind. You should still ignore the wind and maintain 75kt as that is your relative speed to the airflow. Your ground speed will be 50kts and therefore your range will also be decreased. Best thing to do could be to turn around and use the wind as a tail wind which will give you a ground speed of 100kts and thus double your glidig range!!

    And ur last question about the checkride. If the examiner pulls the power, he will expect that you maintain the best glide speed as published in the POH. Don't confuse yourself by trying to calculate a new speed according to the aircrafts current weight.

    The minimum sink speed really isn't that important and practical.

    So to summarise: Best glide speed and minimum sink speeds do change with weight but the difference is so small that its just beter left for the Physics class. Also remember that the engineers and test pilots have thought of everything, so if you don't see something in the POH, it's probably not important and you shouldn't try and resolve it as it will just make you life more complicated

    I hope this helps and sorry I can't provide you with any numbers. I trained for my ATPL on Pipers. Good luck!!!
    References :

  2. herkco on October 12th, 2008 10:46 am

    Well, if I remember correctlly…best glide is for distance covered per altitude lost, … ie make it to the runway.
    MinSink is time related. You have made it overhead the runway/field and now you want events to happen as slowly as reasonable…so you get it all done without rushing.
    Discuss this with your CFI.
    References :

  3. LC on October 12th, 2008 11:05 am

    Headwinds and tailwinds are irrelevant because the speeds on your airspeed indicator show how fast the air is moving over the wings, not your groundspeed.

    On a checkride, you would establish the best glide speed. This will allow you the maximum amount of time to make a decision. However, once you have made a decision, you may not need to maintain best glide.
    References :

  4. Jason on October 12th, 2008 11:53 am

    Here's a tip to establishing best glide speed in a 172: Simply trip it all the way back. This is the lowest drag configuration for the aircraft and will establish max glide within a knot or two. you can fine-tune it from there if necessary.

    The checkride reality is that they want to see you fly your airplane first and deal with the emergency second. Establish your glide speed, pick your landing point, and ONLY THEN start working on a restart. It doesnt do you any good to come up 50 feet short of a decent landing spot because you were too busy cranking the engine over and not doing a checklist, however if you have a field made and manage to get the engine restarted 1 second before touchdown that still counts. Think about it that way.

    In terms of headwinds and tailwinds versus max glide range, a headwind will necessitate a slightly higher airspeed and a tailwind will mean a slower speed (closer to minimum sink speed). The difference between the two speeds will probably be negligible and on a checkride will probably not count for much. Just make sure you pick your point and try to land into the wind. Once you are established on your point and into the wind set yourself up for a normal approach and landing just as you would on any runway.
    References :

  5. Derek on October 12th, 2008 12:02 pm

    FutureCa…gave a very good very detailed answer. I trained and took a check ride on a Piper but now fly mostly 172s. By the way my examiner did pull my throtle to idle during my check ride. He had me less than 400 ft of the ground before he brought me out of total fear that he actually wanted me to land in the field I was heading for. He did not twitch at all as I was sweating bullets trying to figure out if I will make the field. Probably a good poker player. After he congratulated me for passing he said "now you can go and learn how to fly" …. I have been trying to follow his advise ever since…. 8 years now.

    Good Luck!
    References :

  6. calnickel on October 12th, 2008 12:25 pm

    The only way is to fly the plane and experiment.

    First, your gross weight will affect all of these speeds. Your airpeeds will be reduced by the square root of the ratio of your actual gross weight divided by the max gross weight.

    At max gross, the best glide speed in the POH is right.

    For best penetration (gliding into wind) add 1/4 wind speed to glide speed, this should get you close.

    For gliding with the wind, minimum sink speed will get you further than best glide. The minimum sink speed is at the top of the power curve (same as Vy) but with the engine windmilling it is much slower than Vy and slower than best glide.

    On a calm day do some experimenting and see which speed gives you the best gliding time (use a stopwatch and go by 1000s of feet, don't use the VSI).

    On a checkride, just go for best glide speed regardless of winds. He/she might not be thinking about the other stuff. They want to see that you know the POH and fly the aircraft.
    References :
    http://www.av8n.com/how/htm/power.html#toc135

  7. Wingless Pilot on October 12th, 2008 1:04 pm

    I don't think there can be an answer thats any better than what "graVT NME" has given. Good answer "graVT NME", YahooAnswers community, take note, I think we already have a Best Answer here.

    Welcome to Y-Answers by the way "graVT NME".
    References :

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