Bruce Landsberg, executive director of the AOPA Air Safety Foundation, has told controllers, "unable."
A sage bit of advice has been passed along through generations of pilots: Don't bust your posterior, and don't let anyone else bust your posterior for you. The relationship between pilots and air traffic controllers is excellent. We work on the same team to achieve the same objective—to get to the destination safely. However, there are times when the pilot, as the final authority for the safe operation of the aircraft (FAR 91.3), must advise ATC that a particular course of action just isn't going to work and will put a flight in peril. Controllers have the option to refuse a pilot's initial request, but the regulations give the pilot the last word.
The Aeronautical Information Manual (AIM) offers some additional clarification: "If ATC issues a clearance that would cause a pilot to deviate from a rule or regulation, or in the pilot's opinion, would place the aircraft in jeopardy, it is the pilot's responsibility to request an amended clearance" (AIM emphasis). That seems reasonably clear, but there is a common-sense addition to keep anarchy to a minimum. The paragraph goes on to say, "When a pilot requests a different course of action, however, the pilot is expected to cooperate so as to preclude the disruption of traffic flow or creation of conflicting patterns." So the trick is knowing how and when to exercise your options.
Depending on where one learns to fly, most new pilots are introduced to the miracle of ATC when arriving at or departing from a towered airport. The instructions are crisp, the phraseology professional, and the controllers are masters of all they survey. It takes some seasoning to learn that the experts on the other side of the radio and radar scope nearly always put their pants on one leg at a time, just like us. We should always question each other when something seems amiss. It's not a sign of disrespect but of caution and verification.
In many cases, particularly at radar facilities, the people on the ground have little idea of the flight conditions beyond what has been relayed by pilots. They can't see the boiling clouds, can't feel the turbulence, or observe the ice building on the wings. Without that information, a controller can easily issue a clearance that will ruin your day—or decline to issue a clearance that will save it. It's nothing personal. They are just reacting to a traffic situation, which is everything to a controller. As pilots, however, everything that affects our aircraft should be of interest.
Thunderstorms are a constant source of discussion between pilots and ATC. ATC radar's primary purpose is to show aircraft, and the controller's primary function is to separate traffic. There is certainty to what happens if two aircraft collide, while tangling with a boomer may not always be disastrous. This is not to say that ATC will deliberately put you into weather if they know it's there. Usually they don't, but they always react to traffic conflicts.
The Northeast Corridor is part of what is sometimes referred to as the golden triangle. In thunderstorm season other descriptions come more readily to mind. On a trip from Boston down to the Washington, D.C., area a few summers ago, we had been given the preferred IFR route, which takes one well to the west to avoid the big three New York airports of Kennedy, La Guardia, and Newark. There were storms to the east over the city and we were enjoying the ride to the west. But ATC was having a tough time keeping the flow going as the afternoon rush wore on. Too many airplanes were going to the same place at the same time.
Farther west, a second line of storms developed rapidly and we began to query the controller about a turn to the south. The airspace was saturated and south was not currently an option. Flight watch confirmed that the line was indeed building, with tops to 45,000 feet, and no breaks were visible for about 150 miles. On our westerly course, the weather was now just 50 miles away and was clearly visible, both out the window and on airborne radar. There was no way anyone would fly through the line deliberately, and a second request to turn south was made. The controller was still unable because of heavy traffic. I advised that in another 20 miles we would have to turn and he should plan on that. Shortly before reaching the must-turn point our clearance to the south was approved.
In a situation where there is time to work things out, give ATC a warning that a new path will be needed. The controller may or may not be able to see the weather on the scope—but if the clouds look bad don't go there, and don't let anyone put you there. Timing is everything, and in a dynamic situation the more advance coordination there is, the less disruptive a change will be. It's much better to say, "Piper Two-One-Kilo is going to need a 20-degree turn to the left in a few miles" than "Piper Two-One-Kilo is getting pounded and needs to divert to the nearest airport, if the aircraft holds together that long."
A similar situation involving benign clouds can develop with a VFR pilot operating in Class B or C airspace. Pilots not on an IFR flight plan are expected to maintain VFR—period. If an assigned heading or altitude is going to put the airplane too close to a cloud, then advise the controller that you are "unable to maintain VFR" and suggest an alternative heading or altitude.
There are many exceptionally sharp nonpilot controllers, but they likely won't have the depth of understanding that an aviator will. Asking for the impossible isn't done with malice but with ignorance. Some years ago a Cessna 310 experienced an engine failure near Denver's Stapleton Airport. The pilot promptly advised ATC and was assigned straight-in approach to the nearest runway. The controller, rethinking the plan and to accommodate some airline departures, then cleared the twin to a different runway that would require some maneuvering. The pilot acknowledged and set about getting lined up on the new runway. The traffic situation changed again and the controller reassigned the original runway. The pilot attempted to comply and crashed on short final. The controller did not understand the gravity of the situation, and the pilot did not understand that it was his prerogative to refuse a clearance by advising "unable" when the first change was requested. He may also have been ignorant of how much danger he was in.
Some pilots try to accommodate a controller's request to hold short of a crossing runway (LAHSO—land-and-hold-short operations). Slide through an intersection that you've been told to hold short of, and—well, life could get interesting. If the airplane is heavy, you're new to the aircraft, experiencing a gusty arrival, or anything creates an uncertainty, just advise "unable." ATC will be happier with that answer than having to roll the crash trucks. You may be asked to circle or hold until traffic thins out, but that is a fair trade.
Similarly, there may be traffic on short final when the tower clears you for immediate takeoff. If you're ready, fine. If not, advise "unable" and don't hurry the checklist for the sake of expediency.
Wake turbulence is a well-defined hazard. Big airplanes churn and curl the air, making it impassable for small aircraft behind and below them. When flying from predominantly air carrier airports, light-aircraft pilots must be especially vigilant. The tower typically is used to heavy iron. Big airplanes don't usually pose a grave threat to other big airplanes, although there are a few exceptions. On several occasions, I've been cleared for takeoff immediately after a large aircraft and have requested an immediate turn to avoid the wake. In most cases it was approved. At one really busy airport, though, following a stated departure procedure was questionable. Twice, the controller repeated that I was to maintain runway heading. "Unable" was the key word to help the tower understand that neither of us would be happy with the outcome if I departed straight out. The turn was granted.
Another weather scenario involves ice. Pilots should inform ATC before ice degrades the airplane's performance. In ice-protected aircraft there is a bit more time, but in unprotected aircraft during an inadvertent encounter, request an altitude change immediately. ATC is usually quite cooperative when you explain why you need the adjustment. However, there will be times when a controller doesn't understand or has a traffic conflict that precludes giving you what you want. This calls for patience, persistence, and negotiation. Sometimes suggesting an off-course heading allows an immediate altitude change but sometimes it doesn't. If things aren't going your way, and the aircraft is becoming badly contaminated, the willingness to declare an emergency may be the only solution. This is an admission that the game has been played too long, but there have been many accidents because the pilot was unwilling to say "unable."
A good example of a proactive strategy is when the flight is on top of an icing cloud layer and ATC procedures call for a descent perhaps 40 miles out. You may have determined that 15 miles and a direct turn onto the localizer are all that's needed, with most of the maneuvering done above the cloud deck. Advise ATC in advance that you'd like to stay high and are willing to hold if necessary to solve any traffic conflict issue. Do not accept a clearance that compromises safety. You may have to do some explaining if it's a significant disruption, but at least you'll be around to do the explaining. Let's be clear, though, that this is not to encourage flight into icing conditions in unapproved aircraft. If you're down to one safe course of action, then insist on everything that ATC can provide and deal with the lawyers later.
ATC and pilots function as a team and need to cover for each other. Controllers seldom make mistakes—generally less often than pilots, in my observation—but no one is infallible. A declined clearance may be just what's needed.
See also the index of "Safety Pilot" articles, organized by subject.
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