flyrocoak
Topic Author
Posts: 46
Joined: Wed Jun 15, 2016 3:14 pm

Gain of 15 knots at 300'

Sat Feb 09, 2019 1:42 am

Apologies in advance if I have the wrong forum on this.

Our region is under a high wind warning today, with guests to 60, which is not unusual for ROC this time of year. After hearing/watching several go-arounds at ROC today due to pilot reported wind shear, I listed to ATC and heard approach informing pilots of the wind speed, '28 knots with 39 knot gusts', and then informed them that there was a "gain of 15 knots at 300 feet". I'm assuming a 'gain of 15 knots at 300 feet' means the windspeed (headwind in this case) is faster at the lower elevation.

Assuming the pilots are flying into the wind, I was wondering how a 15 knot change in speed, close to landing, affects the piloting of the aircraft. If I understand this correctly, If it were a sudden loss in wind speed, it seems to me that the pilots would potentially increase airspeed to account for the upcoming drop, with lift being the concern. But, for a sudden 15 knot increase in headwind, how does that affect the operation of the flight? Why the cautionary announcement by ATC? What is the concern by having head winds suddenly increase as you are approaching the runway? Is it the opposite of losing lift, you are gaining lift and you have to suddenly account for a potential increased rate of descent?
 
N353SK
Posts: 1016
Joined: Thu Jun 08, 2006 5:08 am

Re: Gain of 15 knots at 300'

Sat Feb 09, 2019 2:48 am

The concern is that if there's a 15 knot gain there's also a 15 knot loss nearby!
 
747Whale
Posts: 725
Joined: Fri Dec 07, 2018 7:41 pm

Re: Gain of 15 knots at 300'

Sat Feb 09, 2019 3:10 am

flyrocoak wrote:
After hearing/watching several go-arounds at ROC today due to pilot reported wind shear, I listed to ATC and heard approach informing pilots of the wind speed, '28 knots with 39 knot gusts', and then informed them that there was a "gain of 15 knots at 300 feet". I'm assuming a 'gain of 15 knots at 300 feet' means the windspeed (headwind in this case) is faster at the lower elevation.


This is a windshear advisory, and the controller is alerting traffic that previous aircraft have reported an airspeed increase of 15 knots at 300'. This alerts crews to be aware of the speed increase, but as noted by a poster above, wind gusts mean that increases can occur, but wind can also decrease. It's the nature of gusting conditions. A speed increase is an increasing-performance windshear. Windshear simply means a change in velocity or direction. An increase in indicated airspeed may occur when descending into a layer of air which has a headwind. Groundspeed hasn't changed and won't immediately, but an increasing headwind will cause a bump in airspeed, and a tendency to climb above the glideslope. Crews will plan on adjusting their approach speed to compensate for windy or gusting conditions, but will also be apprised of the gusting conditions, rather than being surprised by them.

It's reasonable that aircraft receiving an increase of 15 knots wind in descend through 300' would experience a loss of 15 knots while climbing through 300' going in the same direction, which is something departing crews will want to know.

flyrocoak wrote:
Assuming the pilots are flying into the wind, I was wondering how a 15 knot change in speed, close to landing, affects the piloting of the aircraft. If I understand this correctly, If it were a sudden loss in wind speed, it seems to me that the pilots would potentially increase airspeed to account for the upcoming drop, with lift being the concern. But, for a sudden 15 knot increase in headwind, how does that affect the operation of the flight? Why the cautionary announcement by ATC? What is the concern by having head winds suddenly increase as you are approaching the runway? Is it the opposite of losing lift, you are gaining lift and you have to suddenly account for a potential increased rate of descent?


Normally an approach is conducted using a Vref speed, or reference speed. Vref is typically 1.3 Vso, or 1.3 times the stall speed at that weight and configuration (gear, flaps, slats, etc). A typical approach is conducted at Vref+5 knots. Our aircraft manual directs doing wind additives for approach speeds based on adding half of the reported steady wind greater than 20 knots, or all of the gust value above the wind steady state, up to 20 knots additive speed. In the case you've described, a steady state windspeed would be 28 knots, which is 8 knots above 20, meaning half of that gets added, or 4 knots. However, looking at the gust above steady state, (28 gusting 39), the gust is 11 knots. In this case, the additive would be 11 knots above Vref+5, so a total of Vref+16 knots. There are other variations; that's the company policy for my aircraft. Some say all the steady state plus half the gust. The point is that a cushion is added due to varying airspeed.

Because the aircraft has mass and inertia, it can't physically increase velocity or decrease velocity over the ground in a gusty condition; changes in wind speed, or gusts, will produce airspeed variations and changes in performance. Flying a higher airspeed means more energy to press into the wind, which offers something in the loss of airspeed in a gust, and if one has 20 knots steady headwind, for example, one's groundspeed is 20 knots slower; increasing speed for a safety margin really only restores the actually speed of the aircraft on touchdown, which doesn't increase landing distance. It's padding the airspeed margin for added safety in flight and protection against performance loss without an appreciable change in landing distance or braking requirements. If the shear value is 15 knots in this case, an additive of 15 knots to the Vref+5 value means that airspeed may bump up to Vref+35...but it also accounts for a potential wind loss

If at some point the approach does become unstable (large airspeed variations can be instability, depending on the nature of the gusts, performance, and the loss or gain), a go-around or missed approach may also be required, and giving crews an alert of what other aircraft have experienced allows them to brief it, discuss it, and prepare for it in advance.
 
flyrocoak
Topic Author
Posts: 46
Joined: Wed Jun 15, 2016 3:14 pm

Re: Gain of 15 knots at 300'

Sat Feb 09, 2019 2:41 pm

Thanks for your detailed response. As I have no piloting experience, I appreciate your explaining it in a way that I understand. I would imagine crosswind landings in gusty conditions brings about other factors and conditions that must be accounted for that are not as straight forward.

ROC runway #28 is only 6400’ and usually only used heavily during strong wind events. My follow-up question would have been the impact to runway length, so thank’s for touching on that too. I live just under the 28 flightpath about a mile from the runway, so I get to see all this out my window. And, fortunately, only when they are landing, so the noise is minimal unless there is a go-around. Seeing the FedEx 763 last night just over my house, that low, was impressive. Especially that it can handle the short runway for landing. That one is distinct and louder, when it occurs; I hear it coming ;-)

As I am approaching about 500k lifetime miles flying, I started as a nervous flyer. Understanding the operations of the aircraft is what has made me feel comfortable and safe. Appreciate it.
 
GalaxyFlyer
Posts: 3718
Joined: Fri Jan 01, 2016 4:44 am

Re: Gain of 15 knots at 300'

Sat Feb 09, 2019 3:26 pm

Also, winter winds, while strong and potentially destabilizing to airspeed, do not have microbursts or derechos associated with thunderstorms. There’s wind shear, but not the potential disasters thunderstorms possess.

The string headwinds on final shorten the landing distances because the plane is landing at a lower ground speed.

GF
 
747Whale
Posts: 725
Joined: Fri Dec 07, 2018 7:41 pm

Re: Gain of 15 knots at 300'

Sat Feb 09, 2019 3:57 pm

True. Windshear can happen anywhere; it's just a change in direction or velocity, and windshear can be vertical or horizontal, close to the ground, or at altitude in cruise flight. Vertical shear is what's often felt as turbulence in various forms, horizontal shears are more of a cockpit indication, roughly speaking.

The microburst that GalaxyFlyer mentioned is something that comes from thunderstorms, or "convective activity." That's the lifting and falling of air based on heating from the earth's surface, sometimes thanks to terrain or tall buildings, sometimes due to surface heating like factories or forest fires and occurs in an unstable airmass. Air is heated, becomes less dense, rises, cools, and falls. When it does, the changing air properties change the ability to hold moisture and energy; hot air can hold more moisture, cool air not so much. When moisture is released, latent heat energy is released. When moisture evaporates, it causes cooling, and descending air falls faster the more it cools. Rain falling out of descending air causes the column of air to fall faster, and it can descend at very high rates of 6,000 feet per minute or more. When that column is encountered by an aircraft, it's a downdraft. When the column reaches the ground, it spreads out, just like pouring water on the ground, and as it spreads out, it forms a gust front. These are referred to as microbursts; downdrafts that reach the ground and can rob so much performance from an aircraft that it may not be able to overcome the loss.

When you hear about windshear warnings and predictive windshear and things like that, mostly we're taking about thunderstorms and big hazards like microbursts. If a airspeed increase occurs flying into a microburst, then it's what we call an increasing-performance windshear encounter. The airspeed jumps up, the aircraft has more lift, and it will try to climb above the glideslope. The nose must be lowered, power reduced, to stay on the glidepath to the runway. The aircraft will slow down with a headwind; it's physical energy decreases as it slows; it it has less inertia. Passing under the microburst, the wind is from above; a strong downdraft. At this point any extra benefit of the wind increase at the gust front is gone. The aircraft has less kinetic energy; it's slower, now has less airspeed, less power, and it's being driven downhill; this is all happening in a very short period of time. We're now into the downdraft phase of the microburst, and full power may not be enough to overcome.

The goal at this point is to prevent ground contact. The best known mishap regarding a microburst was Delta 191 at Dallas, Ft. Worth (one I remember very well). It was a victim of just such an event, and the reason that training systems all incorporate windshear training today. At t his point, the crew may have gone to maximum power to overcome the downdraft, the aircraft is slow, pitched up and slowing, and doing everything possible to prevent ground contact. The procedure is to fly the aircraft right to the stick shaker (stall warning) if needed; prevent the aircraft from touching the ground at all costs. There's been a bit of modification to the procedure over the last few years, but the main points remain: stay alive, stay airborne.

As the aircraft transitions to a tailwind, there's a further performance loss. Already slow, already low on energy, now the aircraft has a further airspeed loss, a tailwind, and the struggle gets worse. it will begin to move faster over the ground, but it's going to have a loss of airspeed at a time when it may already be on the edge of a stall, with the stall warning systems active. The aircraft will already be at maximum power; there's no more reserve to go get. As Scottie on Star trek would say, "I'm givin it all I can, captain; I can't give her no more."

If done correctly, and in time, the aircraft flies out of the microburst and gives adequate warning that others don't have to go through the same thing.

What we do is mostly prevent ever being there in the first place. We avoid landing when there are thunderstorms near the airport. We have predictive windshear capability on many transport category aircraft, that works as part of the radar to map airflow by noting the velocity and direction and change in both of moisture on radar returns, to predict where windshear might be. Many airports have LLWAS, or a low level windshear alerting system, which is a doppler radar system that works in a similar manner to the onboard predictive windshear system in the aircraft, but on a broader scale ,with more capability. Air Traffic Control broadcasts windshear warnings, etc.

What GalaxyFlyer noted was that the windshear encounter I've just described is typical of thunderstorms, which are seldom found in the wintertime (they can be). Gusty conditions are common in the winter, and don't have all the components of the microburst; typically they're simply wind changes that can produce airspeed loss or gain, and make the flying a bit more demanding. Increases in airspeed may delay touching down, or may result in a touchdown a bit faster than one planned (eg, airspeed loss), resulting in some of the entertaining videos one sees on youtube, in really gusty conditions. The approach and landing is kept as stable as possible, with small corrections for changing wind and conditions.

The 15 knot wind increase warning is just a heads up of what to expect, or at least, what others encountered.

I should add that conditions don't have to be turbulent and particularly gusty to have that speed change. In the wintertime, in cold air, a point where there's a marked temperature change will typically have a windspeed change, too. There are different wind properties in the two parcels of air. It's not uncommon to have an inversion, in which the air gets colder close to the ground. The cold air is trapped by warm air above. One might have a 15 knot tailwind on approach, but the air beneath the inversion is still, calm; when the airplane passes from the tailwind to calm air, it will be seen as a 15 knot speed increase. Why? The tailwind is pushing the airplane an extra 15 knots over the ground, and when it hits the still air, that speed will register as extra airspeed (think of it as punching a pillow, when your fist encounters the increasing resistance of the air or pillow, if that makes sense). The aircraft hasn't actually encountered a headwind, but the reaction will be the same. There are a lot of variations on a theme.

Typically there would be a turbulent layer where the tailwind meets the calm air; one air parcel moving over another causes friction or turbulence at the boundary, and once into the calm air, it maybe perfectly still.

When flying an approach, we have indications in the cockpit showing the wind vector. We can look at the wind vector of a tailwind, for example, and look at the reported surface wind at the runway, and determine what kind of change we might get. In most cases, the wind decreases as we get closer to the ground (friction from the earth's surface, and interference from obstacles, terrain, etc, slow the wind velocity close to the ground), so much of the time the wind actually gets to be less, though it often gets a bit more turbulent as the air is spilling around trees, buildings, hangars, etc, assuming there's any significant wind to begin with.

As you can guess, a headwind means the aircraft touches down with a slower groundspeed, consequently a shorter landing roll, and a tailwind has the opposite effect. A crosswind can go either way, depending on how much the crew is doing to counter the crosswind, and if it delays touchdown. In most cases, it's a simple transition from straightening the nose to point down the runway while lowering a wing into the wind, ideally happening at the same moment as touchdown. After touchdown, spoilers are deployed to kill lift on the wing and put weight on the wheels, to prevent gusts from lifting a wing, and to ensure that the brakes can be as effective as possible.

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