Crosswind Limits

[HiFlyChick]

As for landing in tailwinds exceeding 10 knots in a slower GA airplane, I think attempting that, in anything other than a full emergency, is much more foolish than attempting a stronger crosswind. Most winds exceeding 10 knots are gusty, so a 10G15 wind on the tail is going to make a neat touchdown and rollout a misery in a slower GA plane.

Not sure what you fly, so it's hard to get a handle on what you define as "slower" GA aircraft

Certainly not in a light single, and I'm not saying take 20 kts of tailwind, but in light twin with gobs of runway, I don't see any great harm in going a little over 10 kts if there is a compelling reason. I agree that it can turn into a bad habit to get in to just take the closest runway regardless of the tailwind, but to add to the list that Redneck_pilot86 gave, taking a runway with an ILS over one with a localizer or LNAV (no V) when the weather is crap is another reasonable choice. Or taking a landing off the approach instead of circling in low weather to come around for the into wind runway.

I had this argument with a guy one time who said that I was acting as a"test pilot" because my performance graphs only went to 10 kts of tailwind (or did they include tailwind at all - I forget). He did not think that any form of extrapolation was valid. I tried to make the point that if my calculated landing distance is something like 3000 ft with 10 kts of tailwind and I have 8000 ft of runway, I should be able to make a generalized extrapolation and say that with 12 kts of tailwind, I can certainly land in something significantly less than 8000 ft. Heck, even if you doubled the calculated distance (which obviously 2 extra kts is not going to do), you would still have lots of room. He was adamant that it was test pilot territory and a total no-no.

By that flawed reasoning, the charts also say that they apply to a dry paved level runway. I've seen some that mention corrections to apply for turf or upslope/downslope, but I've never seen any that give any magic sort of number for calculations for a wet runway. Nor would I expect to, since the amount of standing water on a runway will change the distance - was it a little shower that went through just before landing or a deluge? So am I acting like a hot shot test pilot because I land on a wet runway? It seems that there is a certain amount of common sense that can be applied here.

I guess I was just concerned that it was such an absolute in that guy's mind without thought for making a decision based on safety of the situation (i.e. my circling example from above). And saying I was acting like a test pilot seemed rather overly-dramatic to me.....

[Rookie50]

Quartering Tailwinds certainly make for squirrely conditions in a light single on the runway.

[C-GKNT]

FAA certification only required (?requires? sorry, I don't have a reference handy) demonstrating landing in a crosswind that is 20% of Vso (Stall speed in landing configuration).

For example, on my Aerostar Vso is 77Knots and the demonstrated crosswind limit is 15 Knots (20% of Vso). It can certainly handle more than that. This tells me that during certification, either the test pilots could not find a runway with greater than a 15 Knot crosswind...or couldn't be bothered to try harder.

In my Mooney (a M20E), the demonstrated crosswind is only 11Knots. I have landed it in 25G45 at a 70 deg crosswind. Not that I would want to do this again but just to say that the "demonstrated crosswind" in many of these aircraft was the least that needed to be done for certification.

I think that you will find that for many other aircraft the demonstrated crosswind is also exactly 20% of Vso.

Glenn

[PilotDAR]

You mean friction then.

Well, no, I meant drag. Friction can be a source of drag - like brakes, or friction drag of the aircraft skin. Drag is the opposing force to thrust, and hampers acceleration.

Any force which opposes thrust, will oppose it more if you're going faster. Surface drag, in some cases, can make or brake a takeoff. The more slowly you're safely airborne, the better, if any form of drag associated with the surface is a concern.

If you're plane is old enough that the limitations are not explicitly presented as limitations - you're still bound by them. They're expressed in the flight manual or POH as a convenience to the pilot, so the pilot does not have to refer back to the Type Certificate for the aircraft. If you would know not to exceed a weight or C of G limitation, then you would also know not to exceed a speed, or any other operating limitation. For the times I am required to test to 110% of Vne, or overweight, I request a flight permit to explicitly permit that exceedence. That flight permit will come with some rather onerous conditions, appropriate in the interests of safety.

C-GKNT has it right. The manufacturer will demonstrate what is required for certification of their design - unlikely any more. There's no extra points for them demonstrating more when it comes to crosswind capability. If they were to publish a number greater than the requirement, it's not going to help them sell more planes, but it will create more liability if a pilot wrecks a plane at that higher cross wind speed, and then sues the airframe manufacturer.....

As for the regulator's perspective, why make a wind speed a limit, when other than for weather observation airports, any other location would be a "they said, we said" situation? If a pilot goes off the side of the runway, and the regulator really feels the need to take enforcement for some reason, I'm sure they'll find a way. Otherwise what's the advantage of making an issue? It just makes pilot second guess their decision to land. This is a pilot training and competence issue, regulation would not really help....

[photofly]

Drag is the opposing force to thrust, and hampers acceleration.

Beg to differ... drag is the aerodynamic force that acts parallel to (and opposite to) the relative wind. Nothing to do with the direction of the thrust

PS looked in various Cessna 1xx POH's - there's nothing expressed in section 2 "Limitations" about not landing with a tailwind, or any maximum crosswind. Maximum demonstrated crosswind is first stated in Section 4, "Normal Procedures"

[PilotDAR]

PS looked in various Cessna 1xx POH's - there's nothing expressed in section 2 "Limitations" about not landing with a tailwind, or any maximum crosswind. Maximum demonstrated crosswind is first stated in Section 4, "Normal Procedures"

I entirely agree. I have never seen a reference to wind associated with a limitation in any flight manual I have ever read. However, any limitations expressed in a flight manual must be followed, as previously described. As I review the Cessna owners manuals/POH/flight manuals I have, dating back to 1957, they all explicitly express limitations, and mostly in a common format. It's just the location in the document moves around a little, until established by convention in the 1970's.

Beg to differ... drag is the aerodynamic force that acts parallel to (and opposite to) the relative wind. Nothing to do with the direction of the thrust

I beg to differ back. "relative wind" acts only upon a stationary aircraft, as "wind" is a horizontal motion of the air relative to the earth, caused by atmospheric pressure differences. I have successfully flown aircraft in zero wind conditions, so there was no relative wind whatever.

"Relative airflow" acts on an airfoil, and otherwise to a moving aircraft, (as described on the second page of the TC Flight Training Manual).

Relative airflow is always parallel with, and opposite to the flight path of the aircraft... The flight angle is not necessarily the same as the angle of attack....

The flight path of an aircraft will closely oppose the thrust, though not always exactly so. Thus the relative airflow will closely oppose thrust.

"Drag" (quote from Wikipedia, 'cause I'm too lazy to retype it):

In fluid dynamics, drag (sometimes called air resistance, a type of friction, or fluid resistance, another type of friction or fluid friction) refers to forces acting opposite to the relative motion of any object moving with respect to a surrounding fluid.

Relative airflow in powered level flight is opposed to thrust "approximately". Relative airflow is the motion of the aircraft through the fluid (air). Drag opposes motion through the fluid, and thus relative airflow, so drag opposes thrust in the case of a powered aircraft in level flight. Drag increases with relative airflow, which will be very close to speed.

Drag is not limited to a reaction with the air, it is a reaction with a "fluid". From my experience, water, acting upon hulls is drag, "fluid" tires acting against a hard surface has the effect of drag, as do somewhat firm tires moving through very long grass, or pools of standing water.

At a more practical level, if you takeoff without making the most advantage of any wind which might prevail, the aircraft will be moving across the surface faster than it might otherwise, and that difference might be enough to prevent a takeoff at all. I have very certainly seen both floatplanes on the water, and wheel planes in mud, which could not attain flying speed. More relative airflow might have helped, less surface drag might have enabled that, suggesting the benefits of takeoff into any prevailing wind might be better.

[photofly]

tl;dr

But refer to the Flight Training Guide for the definition of drag

[PilotDAR]

...drag by the resistance of the air to the passage through it of the aircraft, and all it's component parts

Same page.

Any pilot who limits themselves to this definition will be correct, in the context of flight, but may be unpleasantly surprised when the affects of drag relative to the surface are not considered at the beginning of a takeoff. And it's takeoffs we're talking about.....

Does not thrust overcome drag (as well as inertia) at the very beginning of the takeoff? In still air, would there be any aerodynamic drag yet? There's drag from somewhere, and a water pilot, or mud strip pilot can certainly tell you where!

[AuxBatOn]

PilotDAR,

Since water is a fluid, the force opposite to the relative airflow created by the movement of a seaplane in the water is considered Drag, as well as the force relative to the airflow created by the movement of the same seaplane in the air.

The force parallel to the takeoff surface created by (initially) the contact between the aircraft and the surface is called Static Friction. Once overcome and the wheels start turning, it becomes Dynamic Friction. It is a force, just not drag. As the aircraft generates lift and the apparent weight of the aircraft over the takeoff surface becomes less and less, the Dynamic Friction reduces.

I know what you are talking about, but you are not using proper terminology which can lead to confusion.

[photofly]

Drag is parallel to and opposite the motion; no need to bring thrust into it. Deffo an error to say drag is opposite to thrust; that really will mislead people!

Here's some flight where thrust and drag are in very different directions:
https://youtube.com/watch?v=1gvUYLA0ucQ

[PilotDAR]

So honestly, does the pilot dragging his aircraft along on little more than pure thrust, support examples of fixed wing flight, which promote understanding for new pilots? I'm also a helicopter pilot, and I'm not bringing those dynamics into it.... I certainly expect that flying on the side of the fuselage pilot was not doing it downwind! Why would that be?

Is the continuous deflection of a tire while it rolls along the ground more a "friction" event? Or a "fluid" event? Honest question.... Which model promotes better understanding? I can grease wheel bearings to overcome friction. Can I increase tire pressure to overcome friction? If aircraft tires were rigid solid, I'd be on board with the analogy, but as long as they deflect when they roll, or they roll through "fluid" long grass or mud, I still gravitate toward the fluid "drag" vision of the retarding force much more than friction. Put the brakes on, and I'm with the friction model.....

Pilots need to consider factors which will affect their safe flight, and place them in appropriate context. Visualizing retarding forces (including drag) is a part of making good decisions when it comes to performance expectations. Sometimes performance expectations are really important, so the basic understanding should be too....