October 18, 2012
When you're asking ATC for deviations around weather, it's useful to know where they can easily bend and where it'll take some negotiation and planning.
Saying pilots will deviate around thunderstorms is like stating a heavy metal concert is going to get loud. It's inevitable. As a radar controller working aircraft during thunderstorm season, it can feel like being in that concert's mosh pit when the lights go down and the guitars crank up. There's slightly less noise and grunge, but it still takes awareness and flexibility to work through the chaos.
ATC's first move is providing information. We verify each pilot has the pertinent ATIS, describe the size, intensity, and movement of precipitation along the route, and issue recent PIREPs concerning turbulence and icing. Essentially, ATC paints a picture of the weather conditions ahead.
Once we put the brush down, it's up to the pilot to make requests―such as deviations or reroutes―based on the view out the window, onboard weather displays and the data provided by ATC.
For both aviator and ATC, the desired result is the same: a safe arrival at the flight's intended destination. Of course, just like every day can't be clear and a million, the means of achieving that goal may not be ideal. Participants on either end of the radio need to be flexible and work with the options available.
Merely getting the aircraft off the ground can demand a creative, coordinated effort between a Tower and its overlying radar facility. Standard departure headings and procedures often get thrown to the wind in order to provide the aircraft with the safest possible avenue for its climb.
If you're taxiing out and notice that the cell off the active runway's end appears particularly nasty, don't hesitate to request a specific heading, or even a different runway. Do you have onboard radar? Traffic permitting, Tower may allow you to taxi onto the runway and get a look with your own gear. The controller may also offer recommendations based upon reports from previous traffic or news from the radar room.
Whatever you decide, Tower needs to coordinate it with us radar folks. I've never denied a custom heading request without a good reason. I figure the pilot wouldn't ask for it without cause to do so in the first place.
Just recognize that it often comes down to choosing the lesser of two evils: You may be forced to decide between, say, departing one runway with a powerful quartering tailwind and departing another runway into the wind but directly into heavy precipitation. Choose wisely.
Encountering areas of weather once airborne leaves you with a few primary options, all of which are specified in AIM 7-1-14 (b)(1).
If cloud buildups are spaced far enough apart where you could maneuver around them, request ATC approval prior to deviating off course: "Center, Baron Niner Niner Eight, request 10 degrees right to get around some buildups." I'd suggest even asking for a little extra. If you only need 15 degrees off course to get around a big CB, ask for 20. Wiggle room doesn't hurt and may save another transmission during a time when the frequency may be bursting with deviation requests.
Don't hesitate to ask for a deviation if you need one. Our radar doesn't paint clouds, only precipitation. Your request to deviate may be our first warning that a storm's forming out there. Also be aware that we may restrict how far you can deviate to prevent airspace or traffic conflicts. The format may be something like, "No further right than a 300 heading," or, "Remain within five miles of the airway." Once you're done deviating, be sure to tell ATC: "Approach, Baron Niner Niner Eight, we're turning back on course."
Sometimes hunting and pecking your way through weather may not be enough. What if, instead of a few widely scattered cells, you're staring down a huge front sweeping across the state like a soggy floor mop? Request a new routing around it. We can amend your flight plan and re-clear you on a new route that will take you around the nastiness. If you're unfamiliar with the area, we can give you vectors to point you in the right direction.
Aircraft weaving unpredictably through buildups still need to be separated from one another. The solution is simple: altitude. Their tracks on our scope may appear as if someone chucked a plate of spaghetti at the glass, but as long as each IFR aircraft is vertically separated by at least 1000 feet, we don't have to worry about them conflicting.
The need for altitude separation is what precludes controllers from just saying, "Deviations approved." Saying the latter by itself gives the pilot permission to deviate both laterally and vertically―not good if I have other aircraft above or below. In practice, I restrict the type of deviations and reiterate the assigned altitude just so there's no confusion: "Baron Niner Niner Eight, deviations left and right of course approved, maintain 7000." If you do need a different altitude, request it as soon as possible so it can be sorted out, and don't expect it to come as readily as lateral deviations.
What if you've flown into some especially dangerous conditions? ATC authority ends when you're faced with a weather emergency. FAR 91.123 grants you full authority to deviate from ATC clearances, headings and altitudes in the interest of self-preservation. But do try and keep us in the loop as to what you're forced to do so we can adjust as needed.
Flexibility vs. Limitations
While ATC can be flexible, airspace and separation requirements may force a controller to deny a pilot's request. Conflicting traffic may be a factor. Say you're getting knocked about inside a turbulent layer at 7000 feet and want to climb up out of it. The controller has another aircraft at 8000, opposite direction. Approving your climb would endanger you, violate separation requirements and put the controller in hot water.
There are also physical airspace considerations. If you're hoping to deviate into another controller's airspace, your controller must call up the neighboring controller and ask for permission. Normally, it's not an issue. However, a conflict due to traffic or active MOA means your controller has no choice but to turn you away from that airspace. If he lets you violate it, that's an operational deviation that can severely impact his career. As in war, one needs to know when to attack and when to retreat. You try something. It doesn't work out. Move on to the next option as quickly as you can.
It sucks, it's inconvenient ... and I had it happen the other day. A departing regional jet I was working was deviating to the northeast around a massive thunderstorm. My airspace sits against an active restricted area owned by another facility. I asked the other guys for permission to let the jet run through their space. "Unable," they said, "there's an active military mission." I had to hook the RJ back to the south―away from perfectly clear sky― and take him 20 miles west before he could head north again. The pilot was annoyed. I was annoyed. It's just the way it goes sometimes.
The rules are the rules. You wouldn't want ATC forcing you to shoot an approach below minimums just because it's convenient for their sequence. We don't want to get busted for letting you violate some else's airspace. So, while covering our respective butts, we try to find the most optimal, legal solution. So here's a tip for the wise: Look ahead as far as you might think your deviation is going to take you. If you see an MOA or restricted area, ask as early as you can if it's active. You can even ask if a deviation 50 miles off course is going to be a problem with other airspace. Better to know in advance and factor that into your overall plan.
Getting Down In It
Working arrivals on a nasty day can be especially entertaining. Favored runways change multiple times an hour as fast-moving storms kick the winds and visibility around. Controllers just have to roll with it, keeping the aircraft on frequency informed of the conditions. There's also the wildcard aspect of each pilot or aircraft's legal instrument capabilities.
When I get an inbound, I just ensure he's got the weather and then specifically ask for his approach request. I've learned to expect the unexpected.
During a recent weather day, I had three airliners request three different approaches to three different runways, none of which were the advertised RNAV Runway 36. The first wasn't RNAV-authorized, so he wanted a localizer to Runway 9. The second was RNAV-authorized, but the touchdown Runway Visual Range on Runway 36 was below his company RNAV minimums. A VOR to Runway 27 for him. The last couldn't shoot a non-precision approach with the weather we had in the area, so he needed an ILS to Runway 18.
I could hear the "WTF?" in the Tower controller's voice when I called for her approval. After a quick chuckle, I got the go ahead. From there it was just a matter of timing each arrival so that the preceding aircraft was arriving over the threshold to their requested runway while the next one was at least three miles out.
The ATC Plan B
The approach itself is only part of the equation. Concern for its outcome doesn't end with the pilot. The controller may be considering headings and altitudes for a potential missed approach, possible holding fixes if the pilot wants to wait before trying again, and dropping in a new proposed flight plan to the aircraft's alternate if the weather doesn't appear to be letting up.
Just like your efforts working the weather from the cockpit, we're working the situation from the scope. It's all about staying ahead of the game.
Tarrance Kramer sequences aircraft amid the daily thunderstorms of the southeastern U.S.