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Ultralight/LSA/Microlight Take-off Headwind Limitations

In the POH for our TL Ultralight Sting TL20 there is the normal section:

Demonstrated crosswind performance Section 5.3.6
# Demonstrated XW speed at 90 degrees is 15 knots
# Maximum headwind at take-off is 12 knots
# Maximum tailwind at take-off is 5 knots.

Pilots are reminded that the above values should be considered in the light of pilot
experience and currency.

The interesting/unusual bit here is the headwind at take-off ‘limitation’. I have never come across this before and until today had kept it in mind but not really given it a second thought, especially given that the aircraft is very easy to land well in excess of the demonstrated cross wind vector. The wind at EGHH this evening was directly down the runway at 14kts (from the ATIS). With two pilots (current on the aircraft) onboard we decided to see whether the headwind was really a consideration. I should probably add that not all versions of the POH have this content either, before appearing in someway reckless.

Normal rotation speed for the aircraft is 45kts and once the nose wheel is in the air it accelerates very quickly. The normal technique to is to climb away at 65kts to a safe height before retracting flaps and accelerating to 75kts. The pitch angle in the initial climb is always quite steep and the stall speed in this configuration is 41kts.

Predictably the take-off was ‘brisk’, normally it takes a few seconds into the take-off roll for the airspeed indications on the EFIS to come alive, today we hit 45kts almost as soon as full power was applied! We had decided given the normal climb out is quite steep, that I would simply avoid exceeding the flap limiting speed (75kts with take-off flap) and see how it went… Well: Maintaining even that created quite an uncomfortable deck angle, albeit with a particularly rapid climb. With flaps retracted the pitch attitude could be made more comfortable whilst remaining a rocket-ship rate of climb.

Todays wind was fairly constant, but I am not sure I would want to be in those attitudes in an aircraft with low inertia in gusty conditions. It seems that the 12kt headwind ‘limitation’ is a good one for comfort and I would now be more cautious in conditions where it might be exceeded. Landing on the other hand, when the wind had increased to 16kts down the runway was a much simpler affair – helicopters have landed with more ground speed I imagine

Does anyone else have similar experiences, similar limitations or thoughts?

Last Edited by Whiskey_Bravo at 22 Apr 21:51

Certainly climb angle would be affected by a strong headwind but it’s not clear to me how aircraft pitch attitude could be affected. Assuming it’s a steady wind, all airspeeds used during climb should also be unaffected as should the rate of climb. The aircraft is unaware there’s a steady headwind once airborne. In gusty winds, as you say, a light wind loading aircraft can become a handful.

Demonstrated capability is not the same thing as a certification ‘limitation’ (which has a specific regulatory meaning) although there may be very good reason to consider it. As the POH says, the values are to be considered in light of pilot experience and currency.

I agree, established in the climb this was true but in the very early stages a greater pitch angle was required to control the airspeed – maybe because the wind speed 50-200ft up was also increasing. Still trying to get my brain around this one as what you say is clearly correct.

I think the guy writing POH was not aware of fact that only cross wind component is required nowdays. In the past POH were including total wind limitation but in the newer POHs approved by CAAs/EASA you find only crosswind and it make sense. Some people are simply not aware of this. But this is not answering the question and the “stupidity” (if you pardon stronger term) will not help you from insurance problem when you were trying to depart in 15 kts wind alligned with runway.

LKKU, LKTB

In the context of the OP the only motivation I can think of for the max demo headwind figure is not the headwind itself, but the wind gradient (i.e. wind shear).

Every time there is wind, there will be wind shear.

The wind shear on departure works in your favour i.e. it does not cause a sinkage (which is so dangerous to jets and other planes whose engines take a while to wind up) but it causes a change in pitch (a pitch-up) which if the pilot is not expecting it could cause a stall. Obviously almost every departure is done into a headwind so it is normal to see this pitch-up.

But that’s a normal part of flying.

I recall that Concorde had a 10kt tailwind landing limit but I don’t understand why because a landing into a tailwind produces a wind shear which works in your favour. Maybe it was limited by the tyres because a tailwind means you are landing at a higher GS.

Administrator
Shoreham EGKA, United Kingdom

Wind shear was exactly my thought when reading it initially and is the reason I would now work on the basis that 15kts from any direction is a safe planning limit (obviously not as a tail component!) as I think a departure above this would be mighty uncomfortable in this particular aircraft. Horses for courses

Low inertia and high angle of attack do not make reassuring bed fellows.

In this instance (slightly experimental) we did comprehensively brief the departure and by using the flap limit speed were easily 30kts beyond the stalling speed for the configuration throughout everything but the very initial climb.

Having had the experience, evidently a strong headwind on departure in this type is much more of a consideration than cross wind as it is very easy to land in a stronger cross component than the demonstrated. The effect of head wind on landing was almost comical with the GPS stopping flight logging 2 miles from the threshold, presumably because the ground speed was so low and the resulting landing predictably used up very little runway but very easily managed.

Other circuit traffic is also a consideration at smaller airfields. Due to the short take off roll and massive rates (by SEP standards) achieved it would be very easy to come into conflict with another aircraft making a crosswind join, similar to what occurred at Peter’s home base a few years ago.

Hopefully this shared experience will be useful for anyone else flying ‘very light’ types.

I think it has to be wind shear related because apart from the interaction between (a) the inertia of the aircraft’s mass and (b) a vertical wind gradient an aircraft is totally unaware of the wind, once the wheels are off the ground and it has reached a steady state in the air.

Administrator
Shoreham EGKA, United Kingdom

There is a gradient in wind speed rising from the surface so the higher pitch angle would seem to be reasonable for a short climb after takeoff, with the effect also I agree proportional to the inertia of the plane.

In my experience with lightly loaded aircraft the biggest issue with strong winds aligned with the runway is not take-off or landing, but taxiing. For instance after you land your Cub etc into a 25 kt wind, its useful to have somebody on each wing as the plane is moved to its tie down! Less of an issue with a nose wheel, but still a problem.

Last Edited by Silvaire at 23 Apr 23:56
8 Posts
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