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wave heights and ground speed for downwind take offs

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wave heights and ground speed for downwind take offs

Unread postby CFII » Fri Nov 16, 2012 4:11 pm

Where are seasoned pilots' wind and wave height limits and reasons why, for any given float displacement for downwind or crosswind takeoffs? Such as acceptable amounts of water pounding on floats generating higher maintenance costs and of course, acceptable safety ranges.
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Re: wave heights and ground speed for downwind take offs

Unread postby RKittine » Fri Nov 16, 2012 4:22 pm

Not being as "Seasoned" as a lot of the guys on here, I not the best on this. I have heard that 1/2 the overall height of the floats is a good Max Wave height. Especially with students, I am very conservative on wave heights and after seeing what a friends J-4 looked like after it had caught a wake wave and went over, I got more cautious. With wind, it depends where I entend to take off from and when I expect to land. At Island Bob's, the channel where we take off and land (one way only for each) has protective tress on either side that block the wind. I am not worried about down wind take offs, cross winds or tailwinds. At other spots, if I have 360 degrees of operating area, I am less concerned then say a river with nothing on the banks to block a cross wind, so I guess with all of that said, for me at least, it varies with conditions and mission, but when hearing what the floats sound like when landing or taking off on rough water, it is hard to imagine that rough water does not cuase more maintenance problems and damage.
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Re: wave heights and ground speed for downwind take offs

Unread postby de Haviman » Sun Nov 18, 2012 9:15 am

10 knots downwind max, but could be as low as zero depending on a multitude of variables including aircraft type, take off weight, available length, obstacles, terrain, OAT, external load, etc.
If the conditions are such that you have to ask if it's a good idea, it probably isn't. Spend ten minutes taxiing.
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Re: wave heights and ground speed for downwind take offs

Unread postby skimmerone » Sun Nov 18, 2012 11:01 am

During training in the Lake amphibian we generally get to do a downwind take off and landing...very carefully. The wind is probably not more than 5 knots. It is done to demonstrate the extra distance that is needed and the extra water speed that is needed to lift off, plus the lowered climb angle needed to clear an obstacle. We discuss it at length beforehand. We like a steady breeze without gusts if possible and the instructor (me) stays right on top of the situation. The proper attitude is critical to a safe lift off and climb out and also on the landing. Not recommended unless the wind is very light. Under upwind conditions, the Lake is capable of 1.5 foot waves, but it is governed by the proficiency of the pilot. The Lake Renegade is more comfortable in even bigger waves due to its stretched hull, a slightly deeper V, and its heavier weight.
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Re: wave heights and ground speed for downwind take offs

Unread postby CFII » Sun Nov 18, 2012 2:07 pm

That's more the info I'm looking for, skimmerone, thanks.

So it's a combination of wave height and tailwind, as would be expected, for a given aircraft with all other factors equal and a proficient pilot. If one was on sheltered water with smaller waves and no obstacles, tailwind tolerated for takeoff could be greater than normal.

skimmerone wrote:During training in the Lake amphibian we generally get to do a downwind take off and landing...very carefully. The wind is probably not more than 5 knots. It is done to demonstrate the extra distance that is needed and the extra water speed that is needed to lift off, plus the lowered climb angle needed to clear an obstacle. We discuss it at length beforehand. We like a steady breeze without gusts if possible and the instructor (me) stays right on top of the situation. The proper attitude is critical to a safe lift off and climb out and also on the landing. Not recommended unless the wind is very light. Under upwind conditions, the Lake is capable of 1.5 foot waves, but it is governed by the proficiency of the pilot. The Lake Renegade is more comfortable in even bigger waves due to its stretched hull, a slightly deeper V, and its heavier weight.
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Re: wave heights and ground speed for downwind take offs

Unread postby de Haviman » Tue Nov 20, 2012 3:25 pm

For most bodies of water, the question of wave height for downwind takeoff is a moot point. Generally speaking, the amount of wind required to generate a wave of any consequence is strong enough to negate the notion of taking off downwind.
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Re: wave heights and ground speed for downwind take offs

Unread postby CFII » Tue Nov 20, 2012 9:16 pm

de Haviman wrote:For most bodies of water, the question of wave height for downwind takeoff is a moot point. Generally speaking, the amount of wind required to generate a wave of any consequence is strong enough to negate the notion of taking off downwind.


It is those bodies of water and situations where it is not a moot point, that information on experience is desired.
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Re: wave heights and ground speed for downwind take offs

Unread postby de Haviman » Wed Nov 21, 2012 12:20 am

Well I must admit I am stumped. I can't think of a situation that would require a downwind take off where the wind is light enough to allow for one, but is generating large enough waves to make it impossible. Have you encountered such a situation? You have piqued my curiosity.
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Re: wave heights and ground speed for downwind take offs

Unread postby CFII » Wed Nov 21, 2012 2:15 am

There are situations where waves will be following through on a main body of water from a predominating wind but, a localized wind blowing from the opposite direction from land with obstacle clearance and/or noise abatement concerns into a harbor, calls for consideration of a possible tailwind takeoff.
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Re: wave heights and ground speed for downwind take offs

Unread postby kevinsky18 » Wed Nov 21, 2012 3:51 am

I can think of lots of situations where there are big waves and small winds.

First not all waves are created by winds. Tides, boat wake, etc. In fact some of the worst wave action I've landed in was during wake fest in Kelowna. The wind was calm but the hundreds of boats produced some serious chop.

Second waves don't stop exactly when the wind stops. I've seen plenty of occasions where waves persist for a long time after a wind stops or as mentioned before two opposing winds collide.

As a general rule if the waves are big enough for me to worry about I don’t like to take a cross wind. I worry about the combined banging force of the waves and side load of correcting for the cross wind folding a float underneath me.

Of course landing parallel to large swells is a different ball game as well but I don’t think that’s what is being discussed here.
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Re: wave heights and ground speed for downwind take offs

Unread postby de Haviman » Wed Nov 21, 2012 10:37 am

Interesting responses.
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Re: wave heights and ground speed for downwind take offs

Unread postby RKittine » Wed Nov 21, 2012 6:23 pm

And watch out for accidental landings in the water park Wave Pools.
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Re: wave heights and ground speed for downwind take offs

Unread postby Float Pilot » Thu Nov 07, 2013 11:47 pm

There is a formula that says:
Take 20% of your floats overall length.
That is the danger point for waves if that height is measured from the bottom of the trough to the peak wave height.

So my PK-2300s are 16.8 feet long. So in theory a 3.4 foot wave is the dangerous. ( remember it is from the trough to the crest, so 1.7 feet above the regular level and 1.7 feet below )

I was able to get a Super Cub off of Tustumena Lake with swells and waves that were pretty high. I only did so because I had totally goofed up and had the plane in an unsheltered beach when the waves came up. I almost did not make it out of there. Every time I would think I was doing OK, the next wave would toss me into the air and I would stall because I was not going fast enough. Then I would slam into the next wave and take water into the air intake and into the prop. After a mile of smashing the plane over the swells I was able to catch a good wave and get the heck out of there.

As for a tail wind, in some underpowered light planes it is darn near impossible to get up on the step with anything over a 3-4 knot tail wind.
Even in a plane like a Beaver, a 5-6 knot-tail wind can double your water run distance.

There are more than a few lakes along the coastal mountains which are surrounded by mountains on three sides. Depending on the time of day, there is a good chance that either your take-off run or landing will be with a tail-wind.

In a plane like a PA-11, J3 or Cessna 172, I try to get up on the step into the wind and then step-turn towards down-wind for my escape take-off.
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Re: wave heights and ground speed for downwind take offs

Unread postby CFII » Fri Nov 08, 2013 2:56 am

Float Pilot wrote:There is a formula that says:
Take 20% of your floats overall length.
That is the danger point for waves if that height is measured from the bottom of the trough to the peak wave height.

So my PK-2300s are 16.8 feet long. So in theory a 3.4 foot wave is the dangerous. ( remember it is from the trough to the crest, so 1.7 feet above the regular level and 1.7 feet below )

I was able to get a Super Cub off of Tustumena Lake with swells and waves that were pretty high. I only did so because I had totally goofed up and had the plane in an unsheltered beach when the waves came up. I almost did not make it out of there. Every time I would think I was doing OK, the next wave would toss me into the air and I would stall because I was not going fast enough. Then I would slam into the next wave and take water into the air intake and into the prop. After a mile of smashing the plane over the swells I was able to catch a good wave and get the heck out of there.

As for a tail wind, in some underpowered light planes it is darn near impossible to get up on the step with anything over a 3-4 knot tail wind.
Even in a plane like a Beaver, a 5-6 knot-tail wind can double your water run distance.

There are more than a few lakes along the coastal mountains which are surrounded by mountains on three sides. Depending on the time of day, there is a good chance that either your take-off run or landing will be with a tail-wind.

In a plane like a PA-11, J3 or Cessna 172, I try to get up on the step into the wind and then step-turn towards down-wind for my escape take-off.


Good reply.
I originally asked because I had a CFI preaching to me vehemently that the tailwind didn't make any difference in any airplane and thought it was BS.

I've done the wave pounding the aircraft into the air takeoff technique quite a few times and your 20% float formula seems about right at the extreme high wave end, except maybe with an unusually good power to weight ratio.

I've also heard formula variations based on vertical float height at the step.
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Re: wave heights and ground speed for downwind take offs

Unread postby Tim McCormack » Fri Nov 08, 2013 10:28 am

Float Pilot wrote:In a plane like a PA-11, J3 or Cessna 172, I try to get up on the step into the wind and then step-turn towards down-wind for my escape take-off.


That's better than going the other way where you'd have centrifugal force AND the wind trying to turn you over.
It's amazing how much longer the take-off run is in even a slight downwind. I can remember reading a slight wind wrong and trying to get on the step with a good load in the plane and not being able to. At first I thought there was something wrong with the plane or I was overloaded. As soon as I turned into the wind it climbed onto the step as I expected.
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Re: wave heights and ground speed for downwind take offs

Unread postby Float Pilot » Fri Nov 08, 2013 5:30 pm

That's better than going the other way where you'd have centrifugal force AND the wind trying to turn you over.


That is for sure. The wind in that situation helps keep the plane from banking ( tipping) outward from the centrifugal force.
Even then I have been on-the-step during a turn to downwind and then been slammed back into a mushy plow by a tail-wind gust.
Even Beavers do not like much of a tail wind. They fly like a giant Cub anyway...


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I've also heard formula variations based on vertical float height at the step.



I went out in the cold and measured my PK 2300s from the keel to the deck in the step area. It is between 24 to 28 inches.
I is hard to tell without somebody else to help measure.

I wonder what formula you should follow for that...

Actually a two foot wave is a lot of wave for a lake. So I would be more than happy to call it quits when the waves are the same height as the height of my floats.
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Re: wave heights and ground speed for downwind take offs

Unread postby snoboy58 » Mon Sep 12, 2016 5:07 pm

My general rule of thumb is a max of 4 knot of tail wind for take off and landing. I've done 5-6 knots but start getting uncomfortable landing with that much in our 172XP, as it creates too much ground speed and float friction increases rapidly. Takeoff runs can take almost twice as long wit more than 4 kts of tail wind. A wave height of half a float is our clubs limit and it seems to be quite accurate. We have a 12 knt operating limit for pilots with under 30 PIC hrs and a 15 knt operating limit for the airplane. At 55 hrs while I'm certainly not a master I have enough SES time that I find those rules very acceptable. Heavy boat traffic on a large lake even with no wind can be just as treacherous bringing large swells traveling in different directions. JMHO
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