Floating and DA

marcoseddi

marcoseddi

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marc
Hi guys ,

Yesterday I flew solo just doing pattern work and noticed I was floating a bit more, just gliding down the runway very smoothly felt like forever at like a foot or two off the runway.. With DA being about 2k last night at basically sea level runway is that the cause? Not sure if it was just something I was doing.. Always like to hear your guys comments suggestions so.. Let me have it.
 
Fly the same INDICATED airspeed and you won't float.

You should have been faster so far as you true airspeed is concerned, so likely no as if anything you would have come up too slow had you been solely relying on visual cues
 
floating has nothing to do with altitude and everything to do with flying too fast
 
Was you fuel load less, and no passengers compared to last time? What about headwinds?
 
tonycondon said:
floating has nothing to do with altitude and everything to do with flying too fast
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True, but a large change in DA could lead someone to fly the incorrect speeds, however it would be going from high to low DA that might make you float.

But if you trust the ASI and keep it in the same place every time regardless of how fast or slow it LOOKS like you are going you will eliminate that.
 
I had exactly the same problem the first few times I soloed...turns out that without the CFI onboard I had about 5-10 KIAS more on final because I didn't make any adjustments to compensate for the lower weight...
 
bnt83 said:
Was you fuel load less, and no passengers compared to last time? What about headwinds?
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You know what not so much and typically when i fly i have like 6 7 knot headwind it was like almost 3 at that point.
 
Hey marc, don't take this the wrong way, but are you sure you were landing into the wind? I was up doing landings the other day as well but with a 6kt tailwind on purpose and noticed that exact scenario. My landings were actually smoother as well with the tailwind. My instructor just wanted me to have that experience.
 
Thermals off the runway on a hot day can cause excessive float, regardless of the density altitude.
 
Yes, I agree with the the heat!

While you always fly the numbers, its easier to grease one in winter then summer for me. Especially in hot conditions back in Oklahoma. There were always some fancy foot play and just holding back and letting things settle over the toasty runway.
 
High density altitude will increase your landing distance due to the higher true airspeed (but you should fly at the same indicated airspeed). If your flare is normally, for example, 3 seconds at low DA, it should still be 3 seconds at high DA, but you'll eat up more runway in the process.
 
Sac Arrow said:
Thermals off the runway on a hot day can cause excessive float, regardless of the density altitude.
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So that explains why it took me so long to learn to land in Florida, and all this time I thought it was just me being a crappy learner and pilot.
 
I think it's already been said: higher DA means higher ground speed so if the float is exactly the same time-wise you will travel a greater distance during that interval.
 
i dont really understand how if the da is high, how is the ASI off, isnt the engine not as efficient so if it says 80 technically it should be slower if anything. Why is it that it would be going faster than indicated?
 
The air is less dense with a high DA. You have to be going faster over the ground to get the same amount of pitot/static pressure differential. Its the same reason your IAS is not the same as your TAS at 8000 feet MSL while at sea level they match.
 
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marcoseddi said:
i dont really understand how if the da is high, how is the ASI off, isnt the engine not as efficient so if it says 80 technically it should be slower if anything. Why is it that it would be going faster than indicated?
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How does an airspeed indicator work?
 
marcoseddi said:
i dont really understand how if the da is high, how is the ASI off, isnt the engine not as efficient so if it says 80 technically it should be slower if anything. Why is it that it would be going faster than indicated?
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It's got nothing to do with engine efficiency. The airspeed indicator will work even if the engine is idling, or even shut off, right? If you're not sure about that, take it from a glider pilot: it works as long as the airplane is moving forward.

Think about how the pitot tube and airspeed indicator work: basically the incoming air molecules push on a diaphragm inside the airspeed indicator. That diaphragm moves the needle on the airspeed indicator dial. It has to move a certain distance to move that needle to a given point on the dial, like your normal "over the fence" speed. It isn't a fancy inertia system with gyros and a computer, nor is it GPS-based. It has no way of knowing your speed through 3-dimensional space itself or your speed relative to the ground... it just reacts to a certain number of air particles hitting it in a given unit of time.

Less air molecules in a given unit of volume means that in order to move that diaphragm to that point, the pitot tube needs to gather more volume of air per second than it does when the air is thicker. The end result is that the true airspeed will actually be a bit higher than the indicated airspeed.
So unless you were actually floating excessively due to your indicated airspeed being higher than usual, you were probably just traveling farther in those few seconds when you were "flaring".
But you also mention a weaker headwind than usual... that would also be a significant factor, possibly more of a factor than the DA. Even when your airspeed and descent rate are exactly the same on two different days, you will cover less ground gliding into a stronger wind, and more ground into a weaker wind. The airplane is flying exactly the same path through the air, but the air itself is moving, so the whole "glide triangle" (height/horizontal distance/glide path) is being carried along in the moving air, either backwards or forwards relative to the airplane's direction.
 
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This is all great info and really clearing things up for me. Thanks guys for your time and help!

Makes sense when explained the way you guys did.
 
marc, I'm glad you asked this because this is now making me aware of something I never thought much about or at least thought about it in a different manner.

DA change at an airport is not something I would have thought would cause this. I flew the other day with a DA warning on the AWOS that had a change of about 1200' from normal elevation of 1000' to 2200' and still didn't notice any difference landing.

So if this can affect your landing speed so drastically; lets say hypothetically you are landing with a 10,000 ft DA. on a sea level runway. Are your calculations for landing length bringing the float into play? You obviously cant slow your airplane below the indicated stall speed and it sounds to me that his 2K ft DA difference made a big difference for him. So if he is landing on a runway that is 1200' Length today with a DA of 0' and then goes to do that on a DA day of 2K' is that calculating the ground roll and the float which you wont be able to control until the airplane actually sets down on the ground?

Why does all this stuff need to keep getting more and more complicated. :mad2:
 
JasonM said:
So if this can affect your landing speed so drastically; lets say hypothetically you are landing with a 10,000 ft DA. on a sea level runway. Are your calculations for landing length bringing the float into play? You obviously cant slow your airplane below the indicated stall speed and it sounds to me that his 2K ft DA difference made a big difference for him. So if he is landing on a runway that is 1200' Length today with a DA of 0' and then goes to do that on a DA day of 2K' is that calculating the ground roll and the float which you wont be able to control until the airplane actually sets down on the ground?
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The ground roll figure in the POH assumes the aircraft has already touched down--that's why it's called ground roll. On the other hand, the "Total distance over a 50' obstacle" figure assumes you're going to flare and will take into account the normal amount of "floating" that occurs after an approach at the recommended airspeed.

However, book figures aren't all that accurate. They were developed with brand new airplanes that had experienced test pilots at the controls. Add a healthy safety margin.
 
FWIW, one very common cause of excessive float is failing to close the throttle completely as you start to flare (or sooner). Many pilots like to "carry a little power" into their touchdown because they've found it easier to make a smooth landing but IMO this is a crutch that should be discarded and AFaIK it's led to many an overrun.
 
marcoseddi said:
Hi guys ,

Yesterday I flew solo just doing pattern work and noticed I was floating a bit more, just gliding down the runway very smoothly felt like forever at like a foot or two off the runway.. With DA being about 2k last night at basically sea level runway is that the cause? Not sure if it was just something I was doing.. Always like to hear your guys comments suggestions so.. Let me have it.
Click to expand...
I can't see any way it had to do with DA, except indirectly. It was hot (I assume), so there were probably lots of thermals. I've been floated (ok, not sure it's a transitive verb) by thermals most of the way down a runway and even had to go around because of them. There can also be headwind gusts that increase your indicated airspeed temporarily and cause you to float.

Best advice is to note the correct indicated airspeed (1.3 Vso adjusted for weight), fly it on final, and then if you float longer than expected anyway, have a landmark picked out along the runway that if you pass it, you will go around.
 
JasonM said:
marc, I'm glad you asked this because this is now making me aware of something I never thought much about or at least thought about it in a different manner.

DA change at an airport is not something I would have thought would cause this. I flew the other day with a DA warning on the AWOS that had a change of about 1200' from normal elevation of 1000' to 2200' and still didn't notice any difference landing.

So if this can affect your landing speed so drastically; lets say hypothetically you are landing with a 10,000 ft DA. on a sea level runway. Are your calculations for landing length bringing the float into play? You obviously cant slow your airplane below the indicated stall speed and it sounds to me that his 2K ft DA difference made a big difference for him. So if he is landing on a runway that is 1200' Length today with a DA of 0' and then goes to do that on a DA day of 2K' is that calculating the ground roll and the float which you wont be able to control until the airplane actually sets down on the ground?

Why does all this stuff need to keep getting more and more complicated. :mad2:
Click to expand...

DA changes from 1-2k will not have a major affect, but go to LXV, fly the same indicated airspeeds as normal and the landing will work out, however if you have a GPS ground speed reading to compare to the indicated airspeed the difference starts to show itself. In fact you can even see that the outside world is going by faster.

In the end the ASI measures the airspeed the same way the wing makes lift so stick to your V speeds
 
As others have mentionedm, this is not a DA thing - it is a ground effect thing. I learned this early on as a student pilot flying out of a sea level airport in New England. You will see a MUCH more pronounced ground effect on warm summer days than when the temp is 30-40F in the winter at the same airport.
 
Skylane81E said:
True, but a large change in DA could lead someone to fly the incorrect speeds, however it would be going from high to low DA that might make you float.
Click to expand...

BTDT. Not enough looking at the ASI, and too much reliance on how fast the world *should* be going by. ;)

Then a big ****-eating grin during the next landing, at how freaking slow I could get the 182 down there. ;) ;) ;)
 
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