Tailwind question

Discussion in 'Pilot Training' started by Hippike, Dec 1, 2017.

  1. Hippike

    Hippike Pre-takeoff checklist

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    Since started training, I’ve been sharing my new knowledge with hubby, and every now and then he asks me questions that catch me off guard.

    I was telling him about why we land into the wind, etc. He says what if you are flying in a C-172 at approx. 90 kts and suddenly experience a 90 kts tailwind. What happens (if power setting remains unchanged)? Would the wings still generate lift to keep the plane flying? Would relative wind disappear when faster wind (or wind at same speed as airspeed) is blowing from behind?

    Can someone please enlighten me?
     
  2. jimhorner

    jimhorner Pre-takeoff checklist

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    That’s a good question. What you are asking about is called wind shear, and it can be a dangerous thing close to the ground. While in the steady state, an airplane is moving relative to its enclosing airmass, and wind has no effect on airspeed, an abrupt change of wind speed or direction will have an effect on the airspeed of the plane. In the extreme case you ask about, the airspeed of the plane would drop to 0. If this occurs at high enough altitude, you would have time to recover. Near the ground it can be disastrous.

    It would be extremely unusual to see a 90 kt windshear close to the ground (might happen at the flight levels), but low level windshear does occur, and you will need to be prepared for it, know how to recognize it, and know how to deal with it. Gusty conditions can cause a similar effect; that’s one reason why it’s recommended to land with a little more airspeed (typically add 1/2 the reported gust speed) when landing in gusty conditions.

    Some references for you. Your instructor can recommend others:

    [edit] Removed the links since they didn’t seem to work. Use google to find:

    FAA-P-8740-40, Wind Shear

    FAR/AIM Section 7.1.23, Windshear PIREPs

    AC 00-54, Pilot Windshear Guide



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    Last edited: Dec 1, 2017
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  3. Hang 4

    Hang 4 Pre-takeoff checklist

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    So strictly as a thought exercise, since an instantaneous 90mph tailwind is practically impossible. The plane would "stop flying", but only sort of. You would still have thrust, the nose would drop and the plane would start "flying" again. It would basically stall, then quickly recover. As long as you were at decent altitude, it would be a non-event.
     
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  4. Clark1961

    Clark1961 Touchdown! Greaser!

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    Wind shear is one of those interesting topics...in the situation you describe the wing would stop flying until the aircraft accelerated relative to the new, sudden tailwind. From a practical sense it is extremely unlikely for a strong tailwind to suddenly appear. Microbursts can develop some strong winds with rapid onset so we try to avoid flying near them. Wind shear and microbursts have been known to swat aircraft large and small from the skies.

    I'm sure other folks have better contributions on the topic. There have been some "interesting" reports and discussions on the forum over the years.
     
  5. Skip Miller

    Skip Miller En-Route

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    Hence the old adage “altitude is your friend!”
     
  6. FastEddieB

    FastEddieB Final Approach

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    But that would be true whether you were landing into or with the wind.

    I suspect a breakdown in communication. Barring that a potential “Stock and Rudder Moment”. For student, or instructor, or possibly both.

    Regardless, the real reason we land into the wind is to decrease the groundspeed, and hence the energy carried, on landing.
     
  7. jimhorner

    jimhorner Pre-takeoff checklist

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    True, but you answered a question which was not the question that the OP asked. Her question was not why we land into the wind, it was what would happen if there were an instantaneous change in the wind speed in which the plane is flying. That is wind shear.




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  8. SkyDog58

    SkyDog58 Touchdown! Greaser!

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    That’s why I am very careful when penetrating the eye wall in a 172.
     
  9. Clark1961

    Clark1961 Touchdown! Greaser!

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    I'd suggest being very careful penetrating anything in a 172.
     
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  10. Bradley W

    Bradley W Ejection Handle Pulled

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    As a passenger Orlando has many 55 kt windshear events. Do watch flying into KSFB.
     
  11. tawood

    tawood Pattern Altitude

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    I would agree with everything but the “non event” part. Lol
     
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  12. FastEddieB

    FastEddieB Final Approach

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    If I misunderstood, I apologize.

    I think I was misled from her premise that she was explaining to her instructor why we land into the wind. And at least the implication that her instructor was expanding on that with examples concerning the effects of shear.

    Certainly, her “Would relative wind disappear when faster wind (or wind at same speed as airspeed) is blowing from behind?” set off alarms. .”Wind” doesn’t “blow from behind”. But if she’s talking about the effects of positive and negative shear, neither is directly related to landing direction.
     
  13. jimhorner

    jimhorner Pre-takeoff checklist

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    She was explaining to her _husband_, (not instructor) why we land into the wind, and her _husband_ (not instructor) posed the hypothetical question of what would happen if the airplane, flying at 90kts in calm wind conditions, suddenly experienced a tailwind of 90 kts. The question posed by her husband, at least as related by the OP, was not limited to during landing or related to landing direction. It was a question triggered by her explanation to her husband why it is desirable to land into the wind.

    While a 90kt wind shear is extremely unlikely, sudden changes of wind velocity (wind shear) do happen, and, inertia being what it is, the airplane will see this as an abrupt change in indicated airspeed. If, on descent to land, for example, one flies from an altitude in which there is a headwind to an altitude in which the headwind is reduced or even is replaced with a tailwind (can happen with temperature inversions), then the airspeed will drop. If the airspeed drops too much when near the ground, bad things can happen.

    The same thing can happen at altitude. Wind shear can cause sudden altitude changes as the relative airspeed is abruptly changed causing sudden changes in the amount of lift the wings are producing.

    Good for her for posting her question; it will lead to a deeper understanding on her part.


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    Last edited: Dec 1, 2017
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  14. Clark1961

    Clark1961 Touchdown! Greaser!

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    I've found that at altitude wind shear is only mildly interesting when on a dark night over empty plains (read no farm lights). Yer flying along fat dumb and happy thinking you could log this IMC if ya wanted then all of the sudden you've got 30 degrees bank and are 20 degrees off course. Maybe your head bounced off the overhead or side of the aircraft. Later ya think, damn I was 30 miles or better downwind of Pikes Peak...

    The daylight VMC events are nadda. You can read them in the clouds mostly. Never found true clear air crap but have thought about it and how to avoid.

    And there is the true IMC crap that ya can't read and ya just hope isn't an embedded...like the gulf coast stuff where ya fly into a thousand foot thick cloud layer and start getting your teeth shaken out...and the weather display has been showing pop-ups the whole time you've been flying into the region...
     
  15. Ryanb

    Ryanb Final Approach PoA Supporter

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    Is KMCO a better choice to fly into?
     
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  16. mscard88

    mscard88 Touchdown! Greaser!

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    For what? MCO is primarily airline. Nothing wrong with SFB. Depending what part of the Orlando area you're visiting, there's also ORL right in Orlando. Disney World? KSM.

    Never heard of that claim of many wind shear events. Where do people get this stuff?
     
    Last edited: Dec 1, 2017
  17. jimhorner

    jimhorner Pre-takeoff checklist

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    Funny, I would define a wind having the same direction as my vector of flight as a tailwind or “wind blowing from behind”. It’s coming from behind me if I am facing in the same direction as my flight velocity vector. On the other hand, a headwind, or a wind whose vector direction is opposing my flight vector, could be considered as wind, “blowing from in front”. Couldn’t it?

    If I’m flying East, and the wind is out of the West, why wouldn’t the wind be blowing from behind me?

    What am I missing here?


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  18. SkyDog58

    SkyDog58 Touchdown! Greaser!

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    As a passenger you would be very unlikely to know that.
     
  19. FastEddieB

    FastEddieB Final Approach

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    Sure.

    But the mental image often clung to is that the wind is “blowing” on the airplane, “pushing” it to go faster.

    It’s common enough - see the dreaded downwind turn - that I’m alert to phrasing that implies it.

    You and I may have internalized that once in the air, there is no wind acting on the plane* - the plane is simply flying in a mass of air that is itself moving. With no “blowing” or “pushing” involved.

    Again, if I misread the phrasing, my bad.

    And weird I read “instructor” where it clearly says “hubby”.


    *Barring gusts, of course, which I guess was the main thing being asked about in the OP.
     
  20. Zeldman

    Zeldman Final Approach

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    Maybe during hurricane season?

    I went from student pilot to multi commercial at SFB and thought it was one of the lightest wind airports I have ever been to. MCO had those long runways and I actually got to do a touch and go there very late one night in a C-152. If I am remembering right ORL had the Sun Bank building that could get in the way....
     
  21. mscard88

    mscard88 Touchdown! Greaser!

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    Sure, maybe during hurricane season, just like anywhere else a hurricane would be. I recall one of those bomber squadron restaurants at ORL, but it's been a long time since I've been there.
     
  22. Hippike

    Hippike Pre-takeoff checklist

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    - You are spot on Jimhorner!

    - he does this all the time. I learn something new and once I share with husband my newfound knowledge he always comes back with these weird questions. But it's good because they make me think deeper and help me realize whether I actually know the stuff or just parroting back what I read. If I'm not certain, I can always come to POA! :p

    Thank you guys!
     
  23. James331

    James331 Touchdown! Greaser!

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    LLWS.

    You will have a religious experience
     
  24. mscard88

    mscard88 Touchdown! Greaser!

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    Is your hubby a pilot also? Whether he is or isn't, I agree with you. Good for you to dig deeper by his questioning.
     
  25. jimhorner

    jimhorner Pre-takeoff checklist

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    Okay, I see what you’re saying, but, especially because this is the Pilot Training forum, I think is very important to be as clear and precise as possible with the language. Yes, in a steady state moving mass of air, i.e. “wind”, the _airspeed_ of the plane is not affected by the wind. However, it’s not true to say that the wind is not pushing on the plane. It most certainly is. A 90 knot tailwind will increase the _ground_ speed of the plane by 90 knots. The plane is being pushed relative to the ground by the wind. A 90 knot headwind will “push” the plane relative to the ground with a 90 knot effect. But you’re right, a steady state wind has no effect on the airspeed. I agree that the “downwind turn” is a myth. Airspeed is the critical factor in how a plane flies. Ground speed matters, though, when flight planning and when landing and taking off in that it affects runway distance.

    The problem some people have, however, is understanding that because of inertia of the plane, it can’t react instantaneously to changes in wind velocity. They get the idea of wind not affecting airspeed in the steady state, but it _does_ in gusty conditions. Airspeed changes briefly in response to wind velocity changes and then returns to normal in the steady state.

    Kudos to the OP for asking the original question.


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    Last edited: Dec 1, 2017
  26. jimhorner

    jimhorner Pre-takeoff checklist

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    Glad to help. I think that what your husband is doing is a great thing. The FAA defines several levels of learning:

    Rote–memorization
    Understanding—perceiving and learning
    Application—achieving skill to apply and perform
    Correlation—associating learning to previously learned

    Your husband’s questions are helping to move you higher up the chain from the Rote, parroting back level. That’s a very good thing.



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  27. jimhorner

    jimhorner Pre-takeoff checklist

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    If he is a pilot, sounds like he’d make a great instructor. I’ve found that the best teachers are those who, by clever questioning, really make the student think and push the student to a deeper understanding.


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  28. FastEddieB

    FastEddieB Final Approach

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    Well, I would say it certainly is not.

    The only way I can reconcile our different views is that we must be using “push” to mean different things.

    I use it in the vernacular, to provide a force. That does not happen to an aircraft with a tailwind. I honestly think the use of that word causes a lot of confusion.

    But I again apologize for my sloppy reading of the OP, and this derail. Maybe if we want to continue this dialog, it might be best to start a new thread.
     
  29. jimhorner

    jimhorner Pre-takeoff checklist

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    Well, I assert that it most definitely is providing a force. Try this thought experiment. You are flying airplane which cruises at 90kts. The distance between your origination airport and destination airport is 180 nautical miles. Take three cases (neglect taxi time, takeoff and climb times for this exercise):

    1. You have a 90 kt headwind. How long will it take you to reach the destination airport?
    2. You have no wind. How long will it take you to reach the destination airport?
    3. You have a 90 kt tailwind. How long will it take you to reach the destination airport?

    In case 1 and 2, the wind is providing a force which is either adding to the thrust of the airplane to move you to your destination airport more quickly or preventing you from getting there at all. Its force most definitely IS pushing your plane, just not relative to the air. It is, however, providing a force which pushes you relative to the ground, and that force is real. Think of it this way, the total force moving you from point A to point B is the vector sum of the thrust of the plane and the force of the wind. The wind doesn't increase the AIRSPEED, but the force is real and increases or decreases the GROUNDSPEED. A physicist or engineer would certainly agree with me that the wind is providing a force.

    I think you are using "push" to mean push relative to the air. And you're right, a steady state wind isn't pushing you relative to the air. I am using "push" in the sense that the wind is providing a force to the overall system where the frame of reference is the ground. It is definitely "pushing" you relative to the ground with a force. The wind energy input to the system is thermal and provided by the sun; the plane's energy input originates chemically in the gasoline and is then converted to heat and then mechanical via the engine (of course, since gasoline is a fossil fuel, its original energy input was also solar, just eons ago). It all depends upon one's choice of reference frame. You are choosing your reference frame as the air mass, I am choosing to use the broader frame of reference as the ground.

    Another way to look at it is if you land at the destination airport with the landing direction the same as the route of flight with the same wind conditions. What will your ground speed be at touchdown if you must touchdown at 90kts indicated in each of the three cases? If the wind isn't pushing you, what is causing the difference in touchdown speeds?
     
    Last edited: Dec 1, 2017
  30. FastEddieB

    FastEddieB Final Approach

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    Can we start another thread to avoid a continued hijack of this one?
     
  31. coloradobluesky

    coloradobluesky En-Route

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    Constant wind speed does not affect the way the plane flies. Accelerating or decelerating wind speed DOES. This is what wind shear is. Once the airplane adjusts to the new wind, it goes back to steady state (if the wind stays steady).

    Also, there is a frame of reference problem here. When we say the plane isnt affected by wind speed, that is within one frame of reference (the planes). If you take it from the frame of reference of a fixed point on earth, then the wind DOES affect the airplanes GROUND SPEED.
     
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  32. Dr. O

    Dr. O Pattern Altitude

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    There is a local pilot (now moved away) who has quite a reputation for penetration in an airplane.
    Now true, it is a Cherokee 6 with autopilot not a Skyhawk - if that makes a difference.