When ATC Says ‘Unable’

Most controllers want to grant pilot requests. When they decline, it's not just for amusement or out of lassitude. There can be a lot going on backstage that a pilot can't see.


Judgment is the cornerstone of the air traffic control profession. This basic trait, common among thousands of successful air traffic controllers worldwide, allows controllers to face a variety of situations-including unusual pilot requests-and act appropriately.

As with the folks in the cockpits, a controller relies on situational awareness. She takes a multitude of things into account before approving a pilots request, asking herself questions like, If I approve this, will it create a conflict with another airplane? Will I violate someone elses airspace? Do I need to coordinate with another controller? Is this request illegal according to my regulations?

If she doesnt like the answer to any of the questions above, she may have to disapprove the pilots request. The reasons fluctuate between scenarios, but here are some of the more common obstacles that can prompt an Unable.

How Much is Too Much?

Workload is a prime factor. When the sky is black with airplanes and a controller is too busy to fully process the ramifications of what the pilot is asking, then he shouldnt be approving it. Otherwise, he might sink himself deeper into the muck.

This concept is integral to ATC training. Controllers are evaluated not just by how much traffic they can handle, but also by how well they know their personal limits. One of the old salts that I worked with said hed never recommend an ATC trainee for certification until hed seen that trainee get so busy he hadto call for help. It showed the newbie had recognized he wasnt superhuman.

Part of that education is learning the time and place to say, Unable. Ive seenATC trainees bomb their already busy certification checkrides by approving some strange request that cut the thin thread by which they were hanging. When a trainees supervisor has to take over because aggressive pilots talked the newbie into a bind, the trainee can forget about getting his ticket punched that day.

Majority Rules

Every Star Trek fan knows the famous Spock quote: The needs of the many outweigh the needs of the few. While ATCs control towers and radar facilities tend to be more 20th-century government issue than 23rd-century science fiction, the meaning still applies: If a pilots request is approved, will anyone else be negatively affected?

If I have a line of air carriers ready to go and a Cessna asks if he can practice holding at a fix in the middle of our departure corridor, thatll be a resounding No way. Apologies to the Cessna pilot, but Im not going to delay hundreds of paying passengers just because he needs to check a box on his flight-training syllabus. The Skyhawk can bore donuts in the sky for a while until the jets depart. Then he can have his proper holding pattern.

Its not a GA versus big air carrier thing. Its a balancing act between safety, efficiency, and order. If Ive got two Skyhawks in our active runways touch-and-go pattern, and a Delta 757 requests to land on our crossing runway, thats elementary: By briefly extending a downwind or upwind for each Skyhawk, I can easily build a hole for the Boeing to land between them.

Now lets change the parameters. What if I have six or seven Skyhawks in the pattern? What if three of them are student solos? Sure, I can opt to scatter my organized pattern into a storm of 360s from inexperienced pilots. Ill need to clear the Skyhawks out of both the airliners arrival path and the path hell take if he goes around (always leave an out). Because my Ground controller uses that inactive runway as a taxiway, Ill also need to take it from him and force him to issue hold-short instructions to all aircraft hes already cleared across it.

Or, I can just say unable to the crosswind-runway request and force the 757 to the active. Were talking about only a few extra flying miles for the big guy, a lot less headache for myself, and far better service for the little guys.

Hold the Line

On occasion, pilots request things that are just downright illegal or unsafe. The controller needs to stand fast, adhere to the rules and protect the pilot from, well,himself.

I once gave an Unable to a Bonanzas request to pass through an active bombing range. He actually argued with me over it! Then there was the King Air on vectors for an ILS who caught a glimpse of his destination airport through a break in the overcast and requested a visual approach, even though the airport was legally IFR. He complained about my Unable, too. I had a VFR Citation bizjet ask me for a climb to FL230 to test some onboard systems. For a Citation pilot, he seemed blissfully unaware of Class A airspace requirements and refused an IFR clearance. Sorry dude: Unable.

The rules themselves are constantly changing. Not long ago, the FAA instituted a ban on all opposite direction operations. Up until then August, if our advertised runway was 33 and an aircraft wanted to depart Runway 15, no problem. At the time of writing this, I cant approve it even if hes the only airplane moving for a hundred miles.

However, just because I cant approve one thing doesnt mean I cant find a workaround if the pilot really needs it. Due to our shorter runways and higher summer temperatures, some air carriers will have to leave passengers behind due to weight if they cant get a specific runway. This means I must temporarily make 15 the active-including coordinating a runway change with the radar guys and cutting a new ATIS. Its only a couple minutes work (twice, as I have to change it back) but it keeps some passengers from having their trip ruined or delayed, so Im glad to do it. In fact, I just did that very thing this morning.

Dodge and Weave

Nothing generates more pilot requests than adverse weather. When the nastiness rolls into town, we expect the frequencies to go crazy with aircraft needing deviations in headings and altitudes to escape thunderstorms, icing and other phenomena. Controllers are encouraged to give the pilots as much leeway as possible. After all, were not the ones careening through the angry atmosphere in a cramped metal tube.

Where it gets tricky is when the pilots desired heading or altitude conflicts with other traffic or another controllers airspace. ATC can only approve things as long as they dont compound the problem.

Suppose I have a Cirrus and a Cherokee at 10,000 and 8000, respectively, and a Twin Cessna cutting between them at 9000. Each requested deviations around a storm cell. Ive OKd them by telling each aircraft, Maintain [aircrafts current altitude.] Deviations left or right of course approved. That allows each plane to swerve at the pilots discretion, while maintaining the 1000-foot vertical separation Im required to have between IFR aircraft.

Now the Cirrus calls, tension bleeding into his voice, demanding a descent to escape unexpected icing conditions. Well, that I cant approve, with both the Twin and the Cherokee in the way. Id lose vertical separation and endanger those pilots. As much as I want to say otherwise, I would have to tell the Cirrus, Unable. What if he wants a turn to the west, directly at another controllers airspace, and my coworker is too busy to answer my coordination call? Unable. If I approve any of those, I would get flagged with an operation error or deviation.

However, the Cirrus pilot has the right to declare an emergency and do whatever he deems fit to ensure his own safety. FAR 91.3 says, In an emergency requiring immediate action, the pilot in command may deviate from any rule-in this case, his clearance to maintain 10,000-to the extent required to meet the emergency. I fully understand that the pilot needs to look out for himself, and if I see him start descending, Ill be vectoring the other guys out of his way.

Standing Up

Just like pilots, controllers all have different experience levels and tolerances for complexity. Yes, some controllers are too quick on the unable trigger. Believe me, they frustrate their ATC brothers and sisters just as much as they annoy pilots. Recalcitrant controllers hinder smooth operation and reflect poorly on the rest of the great controllers making things work.

But understand that for the vast majority of controllers, a justified unable is a sign of professional confidence, not weakness. It demonstrates their knowledge of the rules, the best flow of traffic and their own limits given the current situation. Outside these bounds, there is no safety, and thats what ATC exists to provide.

Tarrance Kramer does his best to grant requests working Tower and Approach somewhere in the southeastern U.S.

This article originally appeared in the October 2012 issue of IFR magazine.