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12 Answers

Slow AS on Final

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Aerodynamics

With any amount of moderate headwind on final in a C172, the IAS tends to decrease with the same amount of power setting. Shouldn't flying into the wind yield a higher IAS and a lower GS? Why is it that I need to add more power to maintain the same airspeed when there is a headwind?

12 Answers



  1. Kris Kortokrax on Dec 02, 2014

    So, you believe that if you climb to altitude and perform steep turns at a constant power setting, that your IAS will vary as you go around in circles with a higher airspeed when flying into the wind and a lower airspeed when flying with the wind?

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  2. Drew on Dec 03, 2014

    Theoretically, yes, that’s what I think, but it doesn’t. A 1500 RPM short final with calm winds will result in a higher IAS than one with a 15 kt headwind. If IAS is just a slightly inaccurate CAS, then the airplane should be feeling the additional air against the pitot tube and show a higher reading, shouldn’t it?

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  3. Mark Kolber on Dec 03, 2014

    Theoretically, no. Reality, no. There is no “additional air.”

    If the airplane is sitting on the ground, it will read changes in IAS as the wind blows. But that’s because it is not part of the airflow. When an airplane is in the air, it moves with the airflow and IAS is the speed of the aircraft relative to the air, not the ground.

    Consider a balloon in flight. What is its airspeed? Zero, isn’t it? No matter how hard the wind blows.

    The only time one would get an IAS change in the air is when there is a gust and that’s only because of the momentary change in aircraft momentum relative to the wind.

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  4. John D Collins on Dec 03, 2014

    Drew,

    You have a fundamental misunderstanding and in some instances it could be very dangerous if you act on your misunderstanding. For a flying aircraft, the IAS is totally unaffected by the direction of a steady wind. GS is affected, but not IAS. Imagine you are flying over an overcast and therefore don’t have a visual GS reference. You can not determine the direction of the wind by IAS indications or visual cues. Regardless of the speed and direction of the wind, for the same power setting, same conditions (altitude, temperature, configuration, attitude ..) the IAS will be the same.

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  5. Drew on Dec 03, 2014

    Okay. But my question still stands: why does a 1500 RPM on final yield a slower IAS with a 20 kt headwind than when the wind is calm? Or put another way, why do I need to add more power to maintain the same IAS when there is a headwind? Does the propeller being a fixed-pitch have anything to with it?

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  6. John D Collins on Dec 03, 2014

    Drew,

    The assertion in your question is flat out wrong. If you are a pilot, get some help from a local flight instructor. However, you give me the distinct impression you are not a pilot and just a troll trying to stir up controversy.

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  7. Steve Butler on Dec 03, 2014

    In the book “Stick and Rudder”, it describes a weather system as a big bowl of soup. If you float a cracker (your airplane) in the middle of that bowl of soup, then pick up the bowl and move it, the cracker will remain in the middle (0 airspeed). You would have to take your spoon and move the cracker across the bowl to generate the equivalent of airspeed. If you turn final at the same distance from the threshold as you would on a no wind day, that headwind is moving the bowl of soup away from the runway at the speed on the wind. If you normally make the approach at 1500 rpm and maintain your normal pitch, you will land short of the runway at a distance corresponding to the movement of the soup bowl. What you are doing is pitching the nose up to try and stretch the glide. By doing so, you are decreasing the IAS. If you were already making the approach at your best glide speed, this will steepen your approach rather than stretching it which is why you add power in addition to pitching up.

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  8. Mark Kolber on Dec 03, 2014

    Drew, I don’t think you are a troll. I think you have a common confusion.

    why does a 1500 RPM on final yield a slower IAS with a 20 kt headwind than when the wind is calm?

    It doesn’t. This is simply not the case. OTOH:

    Or put another way, why do I need to add more power to maintain the same IAS when there is a headwind?

    That’s actually a completely different question.

    The headwind – airflow relative to the ground/runway – means your groundspeed is 20 kts lower for the same IAS. In that situation, you need more power to decrease your descent rate while maintaining the same IAS in order to make the runway. Without getting into a useless pitch/power debate, in this situation the need for additional power is tied to the need to reduce your descent rate because of the lower groundspeed.

    IOW, if you keep the 1500 RPM and the same pitch attitude, you will have the the same IAS and the same decent rate in calm winds and in a steady 20 kt headwind. Only problem is the same descent rate will leave you short of he runway because you will get down too soon.

    If that’s where your error is I can understand it. If you are a pilot, you really need to sit down and discuss this one-on-one with an instructor. Thinking that changes in steady-state wind speed affects airspeed is, as John said, a potentially dangerous misconception.

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  9. Drew on Dec 03, 2014

    John, I apologize if I sounded so stupid that came off as if I was a troll. I can see that my tone in the previous post could have been interpreted as rather demanding, and I did not mean to sound that way. I respect your expertise and appreciate your willingness to help learning pilots like me.

    Steve and Mark, that answered my question. I see my error, and I understand now. Thank you.

    It looks like my knowledge on wind and AS is way off. I’ll ask my instructor to go in depth on airspeeds, again.

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  10. Brad on Dec 06, 2014

    “With any amount of moderate headwind on final in a C172, the IAS tends to decrease with the same amount of power setting.”

    Simply using this statement, you are correct. I know I will get some push back, but stay with me for a minute. If you fly a standard VASI it is a 3 degree angle. If you add a headwind, you will get low on the glide slope. Your IAS and decent rate will not change, but your flight path will shallow. This is shown in the example of a headwind equal to your airspeed. You will see the same IAS but a zero ground speed. The airplane will land like a helicopter straight down! So if you buy this, now we only have a moderate headwind and you don’t want to be low on your 5 mile final. You actually have to shallow out your rate of decent as shown on your vertical speed indicator to maintain the same decent angle. To do this you raise the nose a little. If you do this without adding power, you will be flying the approach at a lower IAS! The above statement is true!

    I agree that later posts show a misunderstanding of movement in air, but the initial post was in fact correct.

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  11. Bradley Donaldson on Dec 06, 2014

    Wait a minute,

    “With any amount of moderate headwind on final in a C172, the IAS tends to decrease with the same amount of power setting.”

    Simply using this statement, you are correct. I know I will get some push back, but stay with me for a minute. If you fly a standard VASI it is a 3 degree angle. If you add a headwind, you will get low on the glide slope. Your IAS and decent rate will not change, but your flight path will shallow. This is shown in the example of a headwind equal to your airspeed. You will see the same IAS but a zero ground speed. The airplane will land like a helicopter straight down! So if you buy this, now we only have a moderate headwind and you don’t want to be low on your 5 mile final. You actually have to shallow out your rate of decent as shown on your vertical speed indicator to maintain the same decent angle. To do this you raise the nose a little. If you do this without adding power, you will be flying the approach at a lower IAS! The above statement is true!

    I agree that later posts show a misunderstanding of movement in air, but the initial post was in fact correct.

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  12. Kris Kortokrax on Dec 21, 2014

    A little pushback.

    “Your IAS and decent rate will not change, but your flight path will shallow. ”

    With the headwind and no intervention by the pilot, the IAS will remain the same, the descent rate will remain the same, but the flight path will steepen (not shallow).

    So far, the headwind has NOT caused any change in the IAS. The airplane would continue to maintain the IAS all the way to the ground, in the presence of the headwind. The touchdown point would, however, be short of the intended touchdown point.

    The pilot responds incorrectly by raising the nose without changing the power. THIS causes the decrease in the IAS.

    The headwind has still not caused any change in the IAS. It is the faulty inputs by the pilot which caused the decrease in IAS.

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