Wait a minute: Demonstrated X-wind Velocity

[Goose Egg]

Hey all,

Today I had a little time and I thought I'd sit down and figure out just how much crosswind I had landed in a few weeks ago. To my nearest estimation, it was a crosswind component of 22 knots (good story). The maximum demonstrated value for the Cessna 172R is 15 knots, and so here's the question.

If I, your average 1000 hour CFI, can land a C-172 in a 22 knot crosswind, then why can't those super-test-pilot types that help determine the value in the book? It's not like it was a daring feat of piloting skill. It was just a landing. So what's the deal with that demonstrated value? A test pilot could have certainly duplicated what I had done.

-Goose

 

[Jedi_Cheese]

Hey all,

Today I had a little time and I thought I'd sit down and figure out just how much crosswind I had landed in a few weeks ago. To my nearest estimation, it was a crosswind component of 22 knots (good story). The maximum demonstrated value for the Cessna 172R is 15 knots, and so here's the question.

If I, your average 1000 hour CFI, can land a C-172 in a 22 knot crosswind, then why can't those super-test-pilot types that help determine the value in the book? It's not like it was a daring feat of piloting skill. It was just a landing. So what's the deal with that demonstrated value? A test pilot could have certainly duplicated what I had done.

-Goose

They get to the test flight where they need to demonstrate x-winds, and run with it. If the winds they can find are 15kts when the testing schedule calls for x-wind component demo, they go with 15kts. If they can get 17kts, then 17kts it is. They do find something reasonable and don't just demonstrate 5kts of x-wind.

 

[nosehair]

If I, your average 1000 hour CFI, can land a C-172 in a 22 knot crosswind, then why can't those super-test-pilot types that help determine the value in the book?

The "maximum demonstrated cross-wind" is not the highest wind that the super-duper-test-pilot-type can manage. I don't know what it is.

Some say it is just whatever wind was blowing that day and the test pilot could handle it, others say different things and I don't recall any official publication that defines it. It is another source of OWT. Old Wives' Tales.

But it isn't the strongest crosswind that the airplane is capable of.

I also think the aircraft company has 'liability' in mind when establishing this number.

 

[flyf15]

I also think the aircraft company has 'liability' in mind when establishing this number.

I'd lean much more towards this. Its a fine line between accurately displaying what the aircraft is capable of and telling Joe "I fly one weekend a month" Pilot he should go try to land in some strong crosswinds. For a 172, 15kts is something that is more than you'll see on 98% of your landings yet it is something managable with moderate pilot skill. You could probably do a 30kt crosswind in a 172 if you really really were on top of things, but then if you publish that number in a book and Joe Pilot does it and kills himself in the process, you might have some legal trouble to work out. Going above the 15kt demonstrated number, I personally feel, is a wakeup call that says "Hey, you're starting to get in some pretty strong wind for this aircraft type, make sure you're up to the challenge before you do it."

 

[pgcfii2002]

I've always been told its the maximum crosswind that was demonstrated by the engineers during certification.

It's not the maximum the aircraft can tolerate though....

 

[AC560]

§ 25.233 Directional stability and control.

(a) There may be no uncontrollable ground-looping tendency in 90° cross winds, up to a wind velocity of 20 knots or 0.2 V SR0, whichever is greater, except that the wind velocity need not exceed 25 knots at any speed at which the airplane may be expected to be operated on the ground. This may be shown while establishing the 90° cross component of wind velocity required by §25.237.
(b) Landplanes must be satisfactorily controllable, without exceptional piloting skill or alertness, in power-off landings at normal landing speed, without using brakes or engine power to maintain a straight path. This may be shown during power-off landings made in conjunction with other tests.
(c) The airplane must have adequate directional control during taxiing. This may be shown during taxiing prior to takeoffs made in conjunction with other tests.

§ 25.237 Wind velocities.

(a) For landplanes and amphibians, a 90-degree cross component of wind velocity, demonstrated to be safe for takeoff and landing, must be established for dry runways and must be at least 20 knots or 0.2 VSR0, whichever is greater, except that it need not exceed 25 knots.
(b) For seaplanes and amphibians, the following applies:
(1) A 90-degree cross component of wind velocity, up to which takeoff and landing is safe under all water conditions that may reasonably be expected in normal operation, must be established and must be at least 20 knots or 0.2 VSR0, whichever is greater, except that it need not exceed 25 knots.
(2) A wind velocity, for which taxiing is safe in any direction under all water conditions that may reasonably be expected in normal operation, must be established and must be at least 20 knots or 0.2 VSR0, whichever is greater, except that it need not exceed 25 knots.

 

[Magic1872]

its the strongest X-wind the test pilot could find on Demo X-wind cert

 

[regionalcap]

§ 25.233 Directional stability and control.

(a) There may be no uncontrollable ground-looping tendency in 90° cross winds, up to a wind velocity of 20 knots or 0.2 V SR0, whichever is greater, except that the wind velocity need not exceed 25 knots at any speed at which the airplane may be expected to be operated on the ground. This may be shown while establishing the 90° cross component of wind velocity required by §25.237.
(b) Landplanes must be satisfactorily controllable, without exceptional piloting skill or alertness, in power-off landings at normal landing speed, without using brakes or engine power to maintain a straight path. This may be shown during power-off landings made in conjunction with other tests.
(c) The airplane must have adequate directional control during taxiing. This may be shown during taxiing prior to takeoffs made in conjunction with other tests.

§ 25.237 Wind velocities.

(a) For landplanes and amphibians, a 90-degree cross component of wind velocity, demonstrated to be safe for takeoff and landing, must be established for dry runways and must be at least 20 knots or 0.2 VSR0, whichever is greater, except that it need not exceed 25 knots.
(b) For seaplanes and amphibians, the following applies:
(1) A 90-degree cross component of wind velocity, up to which takeoff and landing is safe under all water conditions that may reasonably be expected in normal operation, must be established and must be at least 20 knots or 0.2 VSR0, whichever is greater, except that it need not exceed 25 knots.
(2) A wind velocity, for which taxiing is safe in any direction under all water conditions that may reasonably be expected in normal operation, must be established and must be at least 20 knots or 0.2 VSR0, whichever is greater, except that it need not exceed 25 knots.

FAR 25 is for transport category aircraft.

 

[F9 Buff]

Have you guys noticed that when you land with a windspeed over the demonstrated value that you start running out of rudder?

I am thinking the manufacturers don't want to tempt people into sideloading their planes.

 

[AC560]

FAR 25 is for transport category aircraft.

Similar stuff under part 23 as well.

 

[Goose Egg]

Have you guys noticed that when you land with a windspeed over the demonstrated value that you start running out of rudder?

Not quite yet... That 22 knot crosswind, I had to use a lot of rudder, but I didn't have to use all of it. And that's the most I've ever been in.

But then again, I do fly also fly a Grob G109B motorglider, which is set up with a conventional gear configuration. With that long wingspan, you can't do a lot of banking close to the ground, and I can see how you could run out of rudder real quick. Crosswind technique for the Grob is touch down with the upwind wheel first, then use opposite aileron to allow the downwind wheel to touch, then use divebrakes to kill all the lift, then add aileron back into the wind. It can get a little crazy.

Oh, and I should probably mention that I like to land Cessnas with full flaps in a lot of wind. I've noticed that I like to have the extra drag--otherwise it just seems like it floats forever. This is contrary to what I've heard a lot of instructors say--that minimal or no flaps is better in a strong crosswind. I've tried it both ways; I like a lot of flaps better.

-Goose

 

[midlifeflyer]

The "maximum demonstrated cross-wind" is not the highest wind that the super-duper-test-pilot-type can manage. I don't know what it is.

Some say it is just whatever wind was blowing that day and the test pilot could handle it, others say different things and I don't recall any official publication that defines it. It is another source of OWT. Old Wives' Tales.

But it isn't the strongest crosswind that the airplane is capable of.

I also think the aircraft company has 'liability' in mind when establishing this number.

My guess is that it's a cross between liability and marketing. Between "look how good our airplane is in a crosswind" and "opps, better not make it sound =too= good." Although it probably is a factor, I think the liability reason tends to get overblown a bit - mostly because I think these numbers have historically been pretty consistent since before the "liability crisis." My 1958 Comanche shows 20 mpg (17 kts) and that was back in the days when "strict liability" for product defects was only a glimmer in a tort lawyers eye.

 

[mattpilot]

About a month ago i landed a 152 at a nice little airport that had winds blowing directly across it at 22 Gusting 29.

It was a very... uhh... challenging experience.

 

[PURPLEHAZE21]

It's a pre calculated number based on a formula. Essentially its a percentage of your stall speed in the landing configuration. Lets be real...do you think they paid a test pilot to sit in an airplane and wait for a Xwind day... The number is derived so you can theoretically land within "safe limits". If you exceed the crosswind and crash, the manufacturer says your an "idiot" and there not responsible. If you crash prior to the limit, it's pilot error.

 

[UndauntedFlyer]

Have you guys noticed that when you land with a windspeed over the demonstrated value that you start running out of rudder?

I am thinking the manufacturers don't want to tempt people into sideloading their planes.

I have never noticed this. There is usually a good bid of rudder left on most light aircraft at 17k of crosswind if the airplane is flown onto the runway. The max demonstrated crosswind is a really no where near the max possible crosswind if the airplane is flown onto the runway wing low, followed by flying the nose-wheel onto the runway to further prevent weather-cocking along with opposite runner. One thing to understand is that the max demonstrated crosswind is not a limit. It is only a figure that is usually the same for most light aircraft (around 17K) and it is a product of the manufacturers’ legal department. It says that any properly certified student pilot or higher should be able to handle crosswinds up to that figure.