Wind Gusts?
I am trying to understand wind gusts, in particular those lower-level ones we encounter when we are landing or taking off. What is the actual cause of these? Are they sudden changes in the velocity of the wind (e.g. going rapidly from a 20-knot headwind to a 10-knot headwind and then back)? Or are they rapid changes in wind direction (e.g. going from a 20-knot headwind to 20-knot crosswind and then back)? Plus, do they always lead to lower-level wind shear problems?
—Elliot Perlman, Providence, Rhode Island
We referred your note to Tim Vasquez, our meteorologist. He observed that this is likely a part of a broader article on wind in general, which we have done, but it might be time to revisit the topic. If so, he’ll be sure to include a discussion of gusts. Meanwhile, he offered this:
In the U.S. gusts are defined strictly according to variations in the wind velocity. In my early days as a weather observer in the 1990s we looked at the RO-362 wind recorder and examined only the speed chart (not the direction chart) to decide whether to encode a gust. If the difference between lulls and peaks is 10 knots or greater, then reporting a gust is warranted. The direction chart was only reviewed to get information on the prevailing wind direction and any evidence of wind shifts.
It’s still done much the same way by ASOS. Large variations due to gusts are coincidental but are definitely common. Gusts are a manifestation of turbulent flow in three-dimensional space, as whirls and eddies develop and propagate downstream, so variations in both speed and direction are always expected.
The root cause of gusts comes from the wind departing from laminar flow. This is normally due to thermal instability (typically when a cold air mass rolls out across warm or sun-heated terrain) and can also occur due to friction, caused by things like trees, buildings, and hills. Strong vertical wind shear is one of the things we look for that amplifies this kind of turbulent flow.
While it is true that gusts can cause aircraft to experience episodes of wind shear, when evaluating the potential for gusts and turbulence, meteorologists always start with the prevailing environmental winds (or the synoptic scale wind field) and measure the vertical shear within a deep layer, using radiosonde or model data. Knots of shear per kilometer of vertical depth would be an example of a useful measurement, though in practice we use either bulk shear or a more scientific s-1.
Whichever unit we pick, it gives us some idea of the potential for the atmosphere to generate these eddies. We then combine this with instability data and our experience with the local terrain, and also consult model-derived turbulence guidance. All of this tells us if you will encounter gusty conditions or turbulence.
If the background shear and instability parameters are concentrated close to the ground (or, of course, in situations where we were dealing with thunderstorm outflow), this is generally when we would advise pilots of the potential for significant airspeed losses or gains on the approach. —Tim Vasquez
New Pop-Up Procedure?
Has something changed in the way Center handles pop-up clearances?
The other day I took off from a small airport near El Paso, Texas, and got flight following to Odessa Schlemeyer (KODO). On V16 just south of the Guadalupe Mountains it became clear that I would need an IFR clearance to descend through a layer of clouds. I’ve done this in the past and have received an immediate clearance with only a question about souls on board and color of the aircraft.
This time, however, the controller asked if I was capable of IFR and if I wanted to declare an emergency. I assured him I was qualified and, no, I did not want to declare an emergency. Satisfied, he gave me the clearance and the remainder of the flight proceeded normally.
So, was this simply one controller being extra careful, or has there been a policy change at the FAA?
—Bill Kirkwood, El Paso, Texas
Mac Lawler, one of our resident controllers, tackled this question:
The controller was technically correct to ask if the pilot was capable and qualified for IFR flight and was right out of “the book.” The controller’s question about declaring an emergency was unnecessary though, given the circumstance.
The only reason an emergency would need to be declared, either by the pilot or on the pilot’s behalf by the controller, would be if the pilot chose not to fly VFR to a nearby airport with VFR conditions or if the pilot couldn’t do so. If an emergency is declared, the controller is then technically allowed to provide radar services to a VFR aircraft in IFR conditions.
I’d speculate that this controller either just got ahead of himself or simply conflated the appropriate guidance in the controllers’ guidebook, 7110.65 (now in version AA). Controllers, just like pilots, are human and don’t perfectly remember every aspect of their training, particularly in situations they don’t encounter often. I’ve included the appropriate section from the “Point 65.” (below)
GPS Jamming Practices
In his article on GPS jamming, (June 2024), Bob Teter mentions the “famous” case (better word choice would have been “well known” case) about the truck at Newark Airport with an illegal jammer being left on. He explained that the jammer was to prevent the truck’s logging and mapping system from reporting its location.
In the next paragraph he states, “There are also brief outages due to long-haul truckers inadvertently jamming the GPS signals at the airport.” My question: Are we supposed to connect the intents of the “famous trucker” and the larger group of long-haul truckers traveling near EWR? Is this a common practice among long-haul truckers? It is not clear (to me).
Second question and totally unrelated is about airport hot spots. The subject comes up frequently on check rides. I was discussing this with a student yesterday and I asked, “Where would you find explanations for the hot spots?” the obvious answer is in the Chart Supplement. Unfortunately that would just be too logical and is wrong. Can you or anyone else at IFR explain why this information is not included in each airport’s page in the Chart Supplement? You can always point out that this is the same FAA that gives us “the last day of the month after the month” deadlines but in practice uses the last day of the month.
—Richard Cassel, Los Angeles, California
Bob Teter replies:
First, thanks for taking the time to read and comment on the article. I agree that “famous” could have been described as “well known” (as a plus, it would have increased my word count). Seriously, the comment about “long haul truckers” was not meant to imply that they all, or even a large percentage, use illegal GPS jammers. I knew the manager at Honeywell, who handled the GPS local area augmentation system (LAAS) project. He explained that sporadic LAAC problems at EWR were due to GPS jammers used by “long haul truckers.” However, to my knowledge, the drivers with illegal jammers were never caught, except for the truck driver who parked at EWR and inadvertently left the jammer on.
To that, we would personally add that all this happened early in the age of equipping trucks with GPS for the purpose of monitoring them. While the drivers using jammers wasn’t “common practice” it certainly wasn’t unusual. I’d imagine that by now the practice has diminished as the GPS monitoring has gained acceptance.
To your second question, Lee Smith, our resident TERPSter, replied:
I could easily come up with an long list of ways FAA publications could be improved. However, the hot spot descriptions are in the Chart Supplement in the section just before the airport diagrams and in each Terminal Procedure Publication. It would be nice if it was included on the airport page of the Chart Supplement, but the info is there.
I’m sure there’s no specific reason that information isn’t more sensibly located, but between the need to change charting standards and changing automation systems that produce the charts, there probably hasn’t been much motivation to change it. One could raise an issue at the Aeronautical Charting Meeting—a government-industry-public forum to make suggestions like that.
We read ’em all and try to answer most e-mail, but it can take a month or more. Please be sure to include your full name and location. Contact us at Frank@IFR-Magazine.com.
