Tuesday, November 22, 2016
The 'plane' truth: Why wind, snow, or lightning can delay that trip to Maui
You've probably been there at some point: You're all set to hit that tropical beach, but instead you're stuck on the tarmac as the winds blow, or rain patters, or thunder crackles, or worse yet, snow falls. Or your antsy toddler is getting restless after being in the air for four hours, yet your expected arrival in Seattle in 25 minutes is stretching much longer as the pilot hits the brakes and you slowly spin around the region.
Seattle is lucky that we live in a temperate climate where severe weather and powerful blizzards are unheard of, but that doesn't mean Mother Nature can't toss our airport operations a few curve balls now and then, turning that 3 hour flight into something much more.
With the busy Thanksgiving weekend travel time upon us, I asked David Bieger, who is the Meteorologist in Charge at Seattle's Air Route Traffic Control Center what kind of weather conditions does it take to start causing delays or -- perish the though -- cancellations at our nation's airports, with special emphasis on Seattle.
First a quick background on how pilots and airlines get much of their weather information: There are 21 regional Air Traffic Control centers spread across the nation, which are responsible for providing weather information to pilots en route from one airport to another.
The Seattle center, which is based in Auburn, is responsible for about 300,000 square miles of air space across six Northwestern states. Each office has a Center Weather Service Unit (CWSU) that consists of a meteorologist in charge and three other aviation forecasters which provide forecasts 16 hours a day, 7 days a week. (The overnight hours are mostly just cargo flights so the staffing is during peak air traffic demand periods.) In addition, the center also provides weather support for take-off and landings at Sea-Tac Airport and airports within a 35 mile radius. (Other regional airports get their weather info from the aviation forecasts put out by the local office of the National Weather Service).
Bieger says they work in close collaboration with the local National Weather Service forecast office, the national Aviation Weather Center in Kansas City and the FAA's main air traffic command center just outside Washington, D.C. to get the best and most consistent weather information to those in need.
So, what kind of weather will slow airport operations?
Bieger says moderate-to-strong winds, generally greater than 40 knots (45 mph), and especially in the lowest 5,000 feet of the atmosphere, can lead to significant delays for traffic arriving at Sea-Tac.
"To avoid congestion due to these strong winds, traffic management coordinators build in extra space between aircraft (typically [as planes get set to arrive] below 15,000 feet) in order to maintain safe separation as they line up on approach to the airport," Bieger said. "This increased spacing reduces the number of aircraft that can land at the airport, and increases the amount of time between one airplane landing and the next."
And that means delays for incoming flights, which in turn could delay that plane's next flight if it's on a tight turnaround.
Bieger says some planes coming into Seattle begin to get put in their arrival line around Southern Oregon and northern California. As the planes all get lined up, they have to maintain specific speeds at certain altitudes to keep their safe distances. And this is where even changes in the winds at higher altitudes can affect travel times.
"When these airplanes encounter a sudden change in wind speed and/or direction it can lead to something air traffic managers call compression," he said. That's when the wind causes planes to speed up or slow down relative to others in line and air traffic controllers will have to slow other planes to adjust.
Bieger says if you live in the city, you likely deal with similar impacts on your morning drive into work:
"You’re cruising along at 65 mph on I-5 keeping five or six car lengths between you and vehicle ahead of you. Suddenly the speed limit drops to 45 mph as you begin to enter heavy traffic going into downtown. You slam your brakes to keep from running into the cars in front of you which, in turn, causes the cars behind you to slam their brakes to keep from hitting you. The spacing between you and the other cars is very quickly reduced, as is your speed. This is what compression is like for aircraft on approach. Now, if you were following at eight or nine car lengths you would have more time to react to the traffic jam, and could slow down sooner, but you wouldn’t get there as fast…your arrival at the traffic jam would be delayed."
He says it's worst when the winds are right down the runway -- even as little as 25 mph.
"The relationship between speed and direction is somewhat inverse; as the wind direction becomes more of a crosswind it takes stronger winds to cause impacts from compression, and vice versa. So a 20 knot wind from the southeast may have the same level of impact as a 40 knot wind from the southwest, and it may be more impactful at Boeing Field than at Sea-Tac (Airport) because their runways are oriented a little differently and the types of aircraft operating out of each airport are quite different in size and weight."
So suddenly changing winds as planes line up their approach can create delays, as can certain stronger winds near the runway. But even if the winds are relatively light at the airport itself, there's still a scenario that can cause some delays: A simple wind shift.
Pilots like to take off and land into a headwind because that provides extra lift if needed. Thus, Sea-Tac will align the traffic so that planes take off and land into the prevailing headwind -- usually a south wind on cloudy/rainy days and a north wind on sunny days. But a frontal passage, or a drift in the Puget Sound Convergence Zone can flip the winds around quite quickly.
"Typically, for Sea-Tac, winds of 8 knots or greater (~9 mph) will create enough of a tailwind that airplanes will reject landing, and the airport will have to change the direction of traffic flow," Bieger said. This means shifting the traffic from, say, landing from the north to landing from the south. "This takes between 20 and 30 minutes to accomplish as it requires controllers to not only re-sequence airborne aircraft but also change the way ground traffic moves around the airport," Bieger said.
And what about crosswinds? Sea-Tac's runways are aligned with the dominant southerly/northerly winds, but what about days of a rare east or west wind?
"Different aircraft types have different thresholds at which a crosswind will impact them, but in general a crosswind will cause aircraft to drift away from the runway laterally," Bieger said. "Crosswinds make it a challenge to stay lined up on the centerline of the runway while landing (we call this crabbing into the wind), but for some aircraft it may prevent a safe landing altogether resulting in a missed approach or a diversion. This is generally a bigger problem for lighter aircraft, as your typical 737 can handle a maximum crosswind of 35 knots (~40 mph) in ideal conditions."
To Recap: Windy days can cause delays at the airport, but rarely any closures.
Bieger says snow and ice can present a threat to both planes in the sky, and on the ground, but in different ways.
"For airborne aircraft, snow reduces visibility below five miles at a lower intensity than rain, so even a light snowfall can lead to delays," Bieger said. "This effect is much worse during the day than at night and in colder temperatures. Visual approaches aren't possible at that point, and air traffic must be slowed down with increased separation between aircraft. As with heavy and extreme liquid precipitation, expect to have low ceilings as well and possibly strong winds, which in concert can lead to major delays."
As for ice (especially clear icing, which Bieger says is the most dangerous kind), airplanes may experience changes in performance due to the increase in weight as well as changes to the aerodynamics of the wing.
"The good news is that commercial aircraft are equipped with de-icing equipment, but during severe icing even that equipment may not be able to keep up with accumulation," Bieger said. He added his office works to identify where these weather conditions might be present and work to relay that to pilots to avoid to those areas.
On the ground, runways and taxiways must be kept cleared. "For wet snow (or slush), it takes less than a half inch to contaminate a runway, reduce braking action, and increase the landing distance for aircraft," he said. "For freezing rain, a light glaze will do the same."
That means airports have to be diligent in plowing runways.
"If nothing is done to mitigate this threat, or pilots are unaware of the hazard, they may not be able to come to a full stop soon enough or may lose traction entirely," he said. "And, of course, if snow accumulates faster than it can be cleared aircraft will be completely unable to take-off or land."
Freezing rain is even worse because it's harder to get rid of, creates severe traction problems, and aerodynamic issues on the planes themselves. And as Seattle saw during its major freezing rain event in January, 2012, it can cover the entire airport and its outside aircraft with thick coats of ice that is a losing battle to de-ice. Portland's Airport has had to close a number of times over the years due to ice storms as they get a cold east surface wind from the Columbia Gorge that interacts with mild, heavy rains off the Pacific Ocean, creating an intense, and prolonged freezing rain mess.
"(An airport) closure is far more likely in a freezing rain event," Bieger said.
And even if the airport is open, getting the planes de-iced is no easy task as it's a time-consuming process that must be done at just the right time before the plane leaves. "Too early, and the efficacy of the fluid will wane and it will have to be reapplied. In that case, or in waiting too long to apply, the flight can be delayed," Bieger said. And then there's the question of if they even have enough de-icer on hand!
"The biggest problem with the fluid is that it's often purchased during the summer when prices are reasonable, and climatology (historical and outlooks) weight heavily into how much fluid is purchased. Run short and airfield managers may find themselves scrambling to find more to purchase, which can be very difficult, especially in an active winter season."
Seattle is one of the rainiest spots in the nation by frequency of rain, but it takes quite a bit of rain before it causes issues.
Bieger says it's when rain rates hit about 1 inch-per-hour, or 50 decibels on the Doppler Radar (usually denoted by red colors on most radars) that things get dicey. "Heavy to extreme rainfall will cause aviation impacts due to reduced visibility and decreased braking action from ponding on the runway and taxiways," he said. Just ask vice-president elect Mike Pence.
But being in rain country, Sea-Tac is prepared as their runways are grooved and crowned. "This allows the runway to direct large amounts of water off the runway surface very quickly," Bieger said. "This keeps the rainfall from pooling on the runway which increases braking action and controlability."
Still, while the planes can land just fine, rain will still cause delays due to lower visibility.
"We’ll start getting significant delays when visibility drops below five miles, and (cloud) ceiling drops below 1,000 feet (though delays will occur anytime ceilings drop below 5,000 feet) because traffic managers need to add even more spacing between aircraft doing instrument approaches when visual separation is not possible," Bieger said. "In a situation with heavy to extreme precipitation, there are usually additional impacts from weather that work together to create delays. Things get worse when we introduce strong winds into the mix."
That would likely include our typical fall and winter storms around Seattle.
WHAT IF WE ADD IN SOME LIGHTNING?
Contrary to what you might think, lightning isn't much of a danger to planes as they're designed to take the impact and dissipate the strike. On average, each plane gets hit once a year and yes, it even happens around Seattle.
But flying through thunderstorms presents other problems in the air -- namely low visibility, turbulent winds, hail and if close to the ground, wind shear. "Hail is far more impactful (than lightning) and can do serious damage to aircraft," Bieger said. So those cells are generally not fun to fly through, and Bieger's team do their best to steer pilots clear of the storms while in transit. But if thunderstorms are near the airport, it can cause other issues.
Bieger says there are four main corridors where they line up planes for landing into Seattle (called "arrival gates"): Near Yakima, Olympia, Port Townsend, and Everett. If thunderstorms are present over one or more of those regions, they'll shuffle the air traffic to an alternate arrival slot to avoid the storms.
"This creates congestion as those aircraft must now be sequenced into the existing arrival stream which can lead to arrival delays," Bieger said. Vertical wind shear, where you can get an updraft right alongside a downdraft, is dangerous if present near the runways and can temporarily close an airport as well. "The risk of aircraft accidents is increased during this phenomenon; aircraft are often operating close to their stall speeds on final approach, and a sudden loss of lift near the surface can be unrecoverable," Bieger said.
ONE IMPACT THUNDERSTORMS HAVE AT AIRPORTS THAT YOU MIGHT NOT KNOW:
There is one issue thunderstorms bring up that I bet you would have never thought of as a reason for creating delays at airports: Fueling.
"Regulations prevent fueling aircraft with lightning within 10 miles of the airport," Bieger said. Makes sense. And if the storm is within 5 miles, it becomes unsafe for anyone on the ground to be outside.
"Passenger deplaning and baggage loading, among other ground activities, ceases, and aircraft are unable to take-off or land," Bieger said. "This may temporarily 'close' an airport to traffic which can result in arrival aircraft holding or diverting altogether if they have insufficient fuel to hold for long periods. The good news is that thunderstorms are transient, so very long delays aren’t usually expected."
Luckily for Seattle, this is a pretty rare occurrence. Most of our delays, Bieger says, are just from our gray skies.
"Sea-Tac is charged with more delays due to reduced ceilings and visibility annually than for winter precipitation, so for us that’s the far more impactful event in whole," he said.
SOMETIMES IT'S NOT SEATTLE'S FAULT:
Bieger says you can't always blame a weather delay on Seattle weather.
"Sometimes it’s not the weather at Seattle, but rather the weather at your destination, that delays flights," Bieger said. "Traffic managers with the FAA use several traffic management initiatives, such as ground delay programs, to safely and efficiently operate the system, and they work very closely with the CWSUs to ensure the least amount of negative impact to the industry as possible. We’re all on the side of the traveler and we all want to make sure you get where you’re going safely. "
Posted by Kathryn on 6:43:00 PM