Personal Minimums / Wind and Gusts

[Matthew]

Not a typo.

Fly an approach at 1.3 Vs0 like you're supposed to, with a 0.3 Vs0 gust headwind. The gust quits. What's your airspeed now?

0.3 Vs0 is less than 20 knots in a Cessna or Piper trainer. That's not all that an extreme gust factor.

I remember going through all that math with my CFI.

Vs0 in the Archer I flew yesterday is 49kts. 1.3Vs0 = 64. 0.3Vs0 = 15kts

My understanding is the gust factor is the difference between the gust and winds. Yesterday I was flying in 19G25, so a 6kt gust factor, it would have needed to be 19G34 to reach the 0.3Vs0 number.

My CFI beat it into my head to always add half the gust factor to that 1.3Vs0 value.

So, if you add half the gust factor to 1.3Vs0, you'll theoretically stay >=15% above Vs0 if the gust quits.

But that's theoretically. Once you do get gust factors that approach 0.3Vs0, there's probably a pretty significant measurement error in the wind numbers, so that 15% margin might not really be 15%. One of my "Flying for Dummies" (or whatever) books suggests adding a little extra for the "wife and kids factor".

Real world vs theoretical don't always match.

 

[Henning]

Huh? I've had several occasions where I've Maintained at or near 123mph Va and have the stall horn sound as I get hammered by gusts or shear or whatever your favorite descriptor is. Since airplanes fly in relative wind I'd say my speed must be changing. The airspeed indicator sure indicates such.

Watch your ASI, forget the stall horn in turbulence and gusts, it's not accurate due to the vertical components of the airflow. Just watch the ASI and tell me what it does. Unless I am changing pitch, my ASI stays steady to within a knot or two. The plane accelerates pretty quickly to match the wind conditions, far faster than you will drop.

 

[Stewartb]

I did tell you. It was all over the place. The only value I cared about was that the maximum reading was at or below my Va. Other than that I was busy keeping the airplane in control.

 

[Henning]

I did tell you. It was all over the place. The only value I cared about was that the maximum reading was at or below my Va. Other than that I was busy keeping the airplane in control.

Oh, because you're bouncing, yeah, hard to read a round dial in that. That's why I find it easiest to just trim for my bottom final speed and just correct attitude with the yoke and control vertcal path with the throttle. If I'm trimmed for 1.3Vso, I can do whatever I have to do to maintain attitude and glide path, and when I get to the fence, as long as I was not fighting the elevator trim, I will be on my airspeed no matter what.

 

[winrose46]

I encourage my students to test their limits with me in the plane. I think most low time PP still have crosswind issues and they need to work on it until they are comfortable. I know of an individual who has flown multiple times over the north pole and 3 times in the past two years he has blown a tire upon landing. He still does not know how to land in a crosswind and has blamed his incidents upon gusts. His last one was on runway 30 which closed that runway. I landed 20 minutes later on 23 without any issues (25 knot gusts at 320). I teach my students to attempt to land and if they can not track down the runway centerline, go around and reattempt with less flaps and a higher landing speed to increase rudder effectiveness.

 

[Henning]

I really think a bit of aerobatics should be involved in PP. The problems most people have with crosswinds comes from being timid with the controls and being afraid to run them to the stops. A bit of aerobatics gets you over that real quick as you experience what the plane does through the whole range of control deflections.