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Do airplanes tend to float when landing in cold weather?

snopercod

Well Known Member
A few days ago I made two landings in 0 deg. C weather and, at my usual indicated airspeed, the plane didn't want to quit flying. On one one particularly cringeworthy landing, I bounced a number of times. We all know the air is more dense in cold weather and thus provides more lift at a given speed, but isn't the Pitot-static system supposed to compensate for the density of the air? When I got home, I closely reviewing the video and noticed that my AoA system was giving me a more accurate indication of lift than my Airspeed Indicator. Is this a real phenomenon or was I just imagining things?
 
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For a given throttle position, colder (denser) air means the engine can make more power. You may be carrying slightly more power than normal which results in the float.
 
I thought it had to do with the air being more dense and providing more lift... I'm sure thats not the science way to say it but I notice the same thing.

Couple that with the STOL Kit on my 172 and I float quite a bit in the winter, took me a bit to get used to.
 
On a cold day the dynamic pressure, which determines how much lift is produced, will be the same as on a warm day with the same indicated speed. On a cold day the true airspeed will be lower at a given indicated. But if you are using indicated (which you are) then temps won't make a difference. At a constant TRUE airspeed you would have more lift but even then, you would have more drag too.

The difference is the engine. I am sure your idle is higher at colder temps, causing you to float. Floating happens when you don't decelerate, either because you are too fast so you aren't on the back side of the power curve (i.e. down near the slow flight speed) or you have too much thrust. It sounds like you are in the right speed range, so you must have too much thrust.
 
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Yes they can float but only if the lake isn't frozen. :D

First thing that popped into my tiny brain when I read the title...

But more engine performance at colder Temps makes sense too. As you know it takes very little throttle to decrease sink rate.
 
Just a thought... as opposed to bouncing down the runway... if you go around you get to try again :)
 
thanks

Thanks for the opinions, guys. Yes, I was holding in a little power so that could have been a factor. What about ground effect? Could the cushion effect be greater in cold weather?
 
Scott Black gets an A+. That is one of the most beneficial aspects of our airspeed indicators showing indicated speed. If we always use the same speed on the airspeed indicator no matter the environment, our approach will be equivalently safe. Granted our ground speed will vary. This can be a real gotcha when a pilot used to flying at sea level flies into a high-elevation airport. The pilot's instincts will tell him he is coming in too fast to land even though the airspeed indicator is at the same value. There have been many stall spin accidents in the patterns at mountain airports when a flatlander pilot tries to slow to a speed he is used to sensing rather than nailing the same indicated approach speed used back home. Trust the airspeed indicator despite the trees passing by much faster (or slower) than you are used to. Same exact thing applies to hotter or colder ambient temps. Trust the instrument.

Jim
 
theory vs real world

I think we all understand the theory: A pitot tube in free air will compensate for the changes in air density and give us a (supposedly) accurate indication of the lift of the wing under all conditions. But what about when the Pitot tube is 24" off the runway? I just don't know how ground effect affects IAS, if at all. I have read that the induced drag goes down by almost 50% within 1/10 of wing span over the ground. That would be the same as adding power. So maybe the float I experienced was due to that, not some characteristic of the cold air. I'm still learning...

Anyway, here's a short video clip showing the difference in ground speeds during a landing at BJC (8,000' density altitude) and LQK (800' DA). Indicated airspeed was 75 Kts during the flare on both landings. At Denver, the centerline stripes were just a blur.

https://www.dropbox.com/s/2um5zel7g3kitmx/Density Altitude Landing.mp4?dl=0
 
Scott Black gets an A+. That is one of the most beneficial aspects of our airspeed indicators showing indicated speed. If we always use the same speed on the airspeed indicator no matter the environment, our approach will be equivalently safe. Granted our ground speed will vary. This can be a real gotcha when a pilot used to flying at sea level flies into a high-elevation airport. The pilot's instincts will tell him he is coming in too fast to land even though the airspeed indicator is at the same value. There have been many stall spin accidents in the patterns at mountain airports when a flatlander pilot tries to slow to a speed he is used to sensing rather than nailing the same indicated approach speed used back home. Trust the airspeed indicator despite the trees passing by much faster (or slower) than you are used to. Same exact thing applies to hotter or colder ambient temps. Trust the instrument.

Jim

This makes more sense to me. I was always under the impression that cold, dense air produced more lift. You're saying the ASI compensates and provides a constant speed for all conditions. It is my understanding that the colder air is providing more lift and accounts for slower landing GS when cold, low, etc. and higher GS when hot, high, etc., as explained by Jim above. It's just that we don't have to compensate for it when using the ASI to set landing speeds.

Larry
 
Do airplanes tend to float when landing in cold weather?

Depends on how cold, and for how long it has been cold. If it's been cold for quite a while the water will be frozen and you'll find your airplane will float quite nicely atop a layer of that stuff called "ice"! :)

(Sorry, I just couldn't resist!)
 
couldn't resist too

Do airplanes tend to float when landing in cold weather?

Depends on how cold, and for how long it has been cold. If it's been cold for quite a while the water will be frozen and you'll find your airplane will float quite nicely atop a layer of that stuff called "ice"! :)

(Sorry, I just couldn't resist!)

Nothing wrong with floating when cold. Go Float! No wake! :D

Raquette%20Lake%20-%201%201.jpg


5%20mph%20no%20wake%20-%201.jpg
 
braking action?

Go Float!
So Vlad, how was your braking action on that lake? LOL! ...and what about directional control? Did you ever have to step out of the aircraft and push the tail around to aim the plane where you wanted to go?
 
If your AOA system is reliable, in a position where you can see it, and you've become comfortable flying AOA, then that is your most reliable reference on any approach. The wing stalls at the same AOA in a particular configuration (like full flaps) regardless of air temperature.
 
AoA video

If your AOA system is reliable, in a position where you can see it, and you've become comfortable flying AOA, then that is your most reliable reference on any approach.
Thanks, Scott. I have an AFS Sport system and, so far, it's never let me down. On my plane, three amber LEDs seems to work well on final approach. On my bouncy cold weather landing, I was only seeing 1 amber LED. This landing wasn't so bad - I only floated a little bit. It's the same landing I posted earlier, but from the cockpit cam:

https://www.dropbox.com/s/c8dhrcv61gldync/Cold Wx Cockpit.mp4?dl=0
 
So Vlad, how was your braking action on that lake? LOL! ...and what about directional control? Did you ever have to step out of the aircraft and push the tail around to aim the plane where you wanted to go?

Braking action zero directional control minimum minimorum. I did push the aircraft to aim in right direction. :D

Raquette%20Lake%202017%20-%201%205.jpg
 
Once again I floated on a cold day

Today I landed at AVL in 0C weather and once again I floated several thousand feet before touching down. It was very frustrating but she just didn't want to land. Before taking off my idle speed was 830 RPM where on a warmer day it would have been around 700. So more power at idle must be part of the problem. My IAS was right on the normal 80 KIAS, but when I flared, she just kept flying....and flying...and flying. A gust ballooned me up off the runway, my AoA was going off, and I had to add power lest I stall and the runway rise up to smite me. I was all set to go around but, after easing the power back off, gave it one more try. Amazingly, the bi*ch finally touched down 4,000' down the runway. Sheesh... Two years ago I checked the calibration of my ASI, but I intend to do it again when it's cold and see if it's different. Here's the embarrassing video, along with my "good as it gets" landing at GMU 1 hour prior. The only difference that I could see was that the OAT was 40F at GMU vs 30F at AVL. Ground speeds were similar and headwinds were ~5 Kts. at both airports:

Floaty 30F landing at AVL

Normal 40F landing at GMU 1 hr earlier
 
Has nothing to do with engine performance

Airplanes float(fly) until stall speed is achieved. Ground effect has a bit to do with float, but if you slow down, it will land, hot or cold.
 
My IAS was right on the normal 80 KIAS, but when I flared, she just kept flying....and flying...and flying.

An 80kt Vref? You might explore a Vref based on 1.3 times your stall speed and see how that works. A typical Vref for my -6A is in the low 60s, sometimes upper 50s if I'm light (computed using a TOLD app I wrote for my phone/iPad).
 
high sink rate

An 80kt Vref? You might explore a Vref based on 1.3 times your stall speed and see how that works.
That would be 78 KIAS, which is just about what I was indicating. The Lancair is different than the RV in that it develops a high sink rate between Vref and Vso. I try to avoid that regime until the wheels are <6" above the runway. :eek: I guess you watched the "good" landing using the same airspeeds. I'm still searching for an explanation why that one worked and the colder one an hour later was ugly.
 
Some people might not be seeing the good landing because the URL is missing a ":" after the https.

https://www.dropbox.com/s/5kj6n1xzsvhvrcq/2017-2-4 GMU Landing.mp4?dl=0

I try to fly the same IAS on final during all temperature conditions I've flown in from ~15F to ~105F and I've never noticed any increased tendency to float when cold. Even if temperature had a significant aerodynamic effect on float, 10F is pretty insignificant.

My impression from the video (based on little knowledge of your airplane) is that you are flying the approach too fast even in the good video. You round out and fly down the runway at least 500 feet and still touch the nose wheel simultaneously with the mains. In planes I'm familiar with this is at least several degrees AoA below stall. It seems you are carrying more energy on final than you need. Forgive my ignorance if this is just how you are supposed to land a Lancair.

If it were my plane, I'd make sure I know exactly what the stall speed is at different flap settings and start flying my approach slower 5 knots at a time until I feel like I have just enough energy to flare and feel for the runway for a comfortable (not too long) amount of time. I'd probably try practice landings a few thousand feet AGL at these reduced speeds to build confidence.

I imagine this speed might end up being lower than you currently feel comfortable with if you start consistently touching down close to minimum speed on the mains with the nose wheel off the runway.

Also, I'd try to fly the approach aiming closer to the threshold so I'm not flaring ~1000 feet down the runway.
 
thanks

Chris-- Thanks for your constructive criticism and fixing the link. I'm not sure how the ":" went away on a cut-and-paste.
 
more armchair quarterbacking

well John, since you posted on a public forum, you get the whole enchilada!

as others have observed, we are mostly not Lancair drivers.

Is there another notch of flap to be had? that's usually the 'drag' that would reduce the float.
If you are carrying power to flatten the approach, as you say the sink rate is dramatic, you have to bleed off all that energy.....= float, or just plain flying level in ground effect.
If your idle is higher in cold temps, you have a rich mixture. Try leaning if you can ( FI not applicable).
I saw the nose pointed at the aim point on the runway....I never saw a 'flare', which dramatically increases drag, with minimal lift, at or near the magic number ( stall),

If you have an AoA, you can practice all this at altitude, then try above the runway, where you might have to add another 'dot' or two to your pitch.

I find speed, sink, and float a big issue with my RV-9a, all of which can be addressed...I too am learning on every landing, and if I fly the correct speeds and pitch on the approach, the landing is guaranteed to be a good one, on the mains, stick in the belly, minimal tire wear etc etc.....all good stuff.

is there a transition trainer on the Lancair? I'll bet one circuit & landing with them would answer a lot of questions.

enjoy the cold 'n clear while you can!!! :)
 
Good comments

Is there another notch of flap to be had?
Nope. What you saw was all I have: 20 degrees. Later model Lancairs have 30 degrees. Maybe it's not a coincidence that the L-320s do better at landing on the mains. As you saw, I always do "three point" landings. There's no "flare" like one would do in a Cessna. I land the Lancair just like I would a floatplane: Hold a level attitude just above the surface, ease off the power, and let it settle. Normally, that all happens immediately after round-out, but as you saw, it sure didn't this time.

If your idle is higher in cold temps, you have a rich mixture.
I've set both the idle speed and mixture, but that was done when the weather was temperate so maybe I need to re-do it when the weather gets cold?

If you have an AoA, you can practice all this at altitude, then try above the runway, where you might have to add another 'dot' or two to your pitch.
I have an AFS Sport system and it works really well. I just uploaded the cockpit video of the same landing so you can see/hear the AoA in action: Cold Weather Landing from Cockpit Cam

The landing gear in a Lancair isn't nearly as robust as in your RV-9. The wheels are tiny, there's only maybe 3" of travel in the stuts, and the last thing I want to see are the gear trunions breaking through the top of the wing skins if I drop it in from 24". :eek:

I fly the correct speeds and pitch on the approach, the landing is guaranteed to be a good one
That's my belief but I'm still missing something. In the above video I had a stabilized approach at 82 KTs, which was maybe 2 Kts too high. I held in the usual tiny amount of power, but I'm now thinking that was too much. Doing the math, it looks like on a zero-degree day my engine would put out roughly 5% more horsepower power than on a standard day. That's something I haven't been considering, but will in the future. I'm still learning, obviously.

enjoy the cold 'n clear while you can!!! :)
I DO really like climbing out at 2,000 fpm on 140 HP! :D
 
That would be 78 KIAS, which is just about what I was indicating. The Lancair is different than the RV in that it develops a high sink rate between Vref and Vso. I try to avoid that regime until the wheels are <6" above the runway. :eek: I guess you watched the "good" landing using the same airspeeds. I'm still searching for an explanation why that one worked and the colder one an hour later was ugly.

Sorry, video link didn't work on my phone and didn't realize you weren't in an RV, so i totally get the 80kt vref with a lancair.
 
As you saw, I always do "three point" landings. There's no "flare" like one would do in a Cessna. I land the Lancair just like I would a floatplane: Hold a level attitude just above the surface, ease off the power, and let it settle. Normally, that all happens immediately after round-out, but as you saw, it sure didn't this time.

I don't know Lancairs from my @ss, but I was taught never to land in a "three-point" attitude in a nosegear airplane. Mains only, hold the nosegear off, and then let it come gently down as elevator authority is lost.

Perhaps that (you're not reaching critical angle of attack, despite whatever your AOA is saying), combined with this:

I held in the usual tiny amount of power

is the root of the problem here? I believe I can *hear* the alternator whine decrease *after* you touch down, rather than before (in what should be the flare)? If you're trying to *land*, power above idle is working against you.

Basically, you're flying it onto the runway, rather than doing a proper flare and stall at 1-2" above the ground, under idle power, perhaps?

ETA: It's been ages since I got a seaplane rating, and never really used it, but I don't think they taught a *level* attitude during landing due to the possibility of the floats "digging in" and flipping the plane. It was always a nose-up attitude and then after landing, stick full aft.

I'd worry much more about the nosegear hitting *first*, causing porpoising and PIO, than I would flaring too high, unless the airframe and gear are considerably weaker and more fragile than I would expect in a properly design light aircraft.
 
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Thanks to all...

Thanks to all for your comments and I've learned a lot about reduced drag/increased lift in ground effect and increased power in cold weather. But now I see this thread moving away from the original question into "Lancair Landing Techniques". I'd certainly be glad to discuss that, but an RV forum really isn't the place. The management has been gracious enough already. Thanks again.
 
Just a thought

Thanks to all for your comments and I've learned a lot about reduced drag/increased lift in ground effect and increased power in cold weather. But now I see this thread moving away from the original question into "Lancair Landing Techniques". I'd certainly be glad to discuss that, but an RV forum really isn't the place. The management has been gracious enough already. Thanks again.

In cold air , since the air has more O2 in it, you are effectively making the mixture leaner. I have not read to see if your ship is injected or has a carb.

If injected, your normal idle rpm will be faster as it is like the 50 RPM gain as you pull the mixture half way out , but from the cold air.

If carbed, try landing with full carb heat as that will richen the mix and drop your RPM just a bit.

If that does not do it, just pull the mixture all the way out and try holding the mains 2" off as long as you can. In order to do this you will be progressively increasing the AOA and raising the nose wheel ever so gradually while holding the mains off 2" until stall.
 
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MA-3SPA carb

If carbed, try landing with full carb heat as that will richen the mix and drop your RPM just a bit.
Yes, my O-290-D2 is carbureted. I'm becoming convinced that I float more during cold weather due to the excess power the engine is putting out. So pulling carb heat and leaning would help counteract that. When I pull carb heat during runup, I get roughly a 250 RPM drop so my setup works well. The only concern I have is that if I had to go around, I wouldn't want to forget to remove carb heat; I made that mistake once as a student pilot and almost had a close encounter with an apartment complex so it's burned into my brain. :eek:

I'll go up and practice that.
 
Just curious...taking the AOA indicator out of the equation here, just what IS Vs0 (demonstrated via actual full-flap, gear-down stall at altitude)?

Forget the 82 KIAS for the moment...are you *sure* Vs0 is as high as you think it is?
 
I'm just reading through earlier posts on this thread. I approach at 62 to 65 Kts in a heavy fat IFR F1 Rocket. Yes that makes for a steep approach but I have enough airspeed left to round out and flare. Your IAS of 78 Kts in a 7A would instantly explain why you float down the runway. I think you are carrying way to much air speed and energy. I think the simple solution is to ride with some other 7A guys that operate out of short strips and just watch their numbers and see how it looks. It will be steeper and much slower.
 
I'm just reading through earlier posts on this thread. I approach at 62 to 65 Kts in a heavy fat IFR F1 Rocket. Yes that makes for a steep approach but I have enough airspeed left to round out and flare. Your IAS of 78 Kts in a 7A would instantly explain why you float down the runway. I think you are carrying way to much air speed and energy. I think the simple solution is to ride with some other 7A guys that operate out of short strips and just watch their numbers and see how it looks. It will be steeper and much slower.

He's already said he's flying a Lancair, not an RV.
 
No change in engine RPM with carb heat on final

The weather has been crappy so my landing-speed-quest was interrupted for a couple weeks. I went up today (OAT=45F) and flew wind triangles at 80 KIAS to verify my pitot-static system at that speed. I averaged three runs and as near as I could tell, my ASI was right on. The formula I use is very sensitive to ground speed so it's possible that there was an error of +/- 1 Kt but that's plenty good enough for me. I tried to land at 78 KIAS today but a gust of wind on short final bumped my speed up.

The important thing I learned was that pulling carb heat on short final had absolutely no effect on my engine RPM. It read 1030 at idle with or without carb heat. I take that to mean that the prop is driving the engine at 85 KIAS. Now that the weather is better, I'll continue testing and report back.
 
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