Aviation Speak

An air traffic clearance is an authorization from air traffic control for an aircraft to operate in a specific way within controlled airspace. According to the Pilot/Controller Glossary in the Aeronautical Information Manual, air traffic clearances are issued specifically to prevent collisions between aircraft that controllers know about. But that doesn't mean you have to be in danger of colliding with another airplane before you get a clearance. In Class B airspace, for example, all aircraft must talk to air traffic control and follow the controllers' instructions. By arranging aircraft in an orderly fashion from the moment the aircraft enter the airspace, controllers can ensure that aircraft never get too close to each other.

Once you have accepted an air traffic clearance, you cannot deviate from it except in an emergency or if you have received an amended clearance. That's true whether you're flying IFR or VFR, but it doesn't mean that you, as pilot in command (PIC), have to give unquestioning obedience.

As PIC, you can request a different clearance if you have information that makes another course of action more practical or if aircraft equipment limitations make it impossible for you to comply. In the case of commercial operators, pilots may request a different clearance if company procedures forbid them to comply with the clearance they've been given. For example, if you operate out of busy airports with multiple runways and long taxi routes, you may have heard controllers issue a clearance to land on one runway followed by the pilot requesting to land on a different runway. This typically happens because the requested runway is closer to the aircraft's destination, such as an FBO or gate, and can save time and fuel spent taxiing.

Any time you don't understand a clearance or consider it unacceptable for safety reasons, you can request an amended clearance or request clarification. For example, if you are cleared to land and hold short but you have not participated in those operations in the past or are at all uncertain about your ability to comply, you can, and should, refuse the clearance.

To the extent that it is both practical and safe to do so, controllers are supposed to honor the requests of pilots. That doesn't mean that you'll get everything you ask for, but it does mean that it can't hurt to ask. In fact, it's your responsibility to ask for an amended clearance if a controller's instruction would cause you to break a rule or regulation or place the flight in jeopardy. As pilot in command, you have ultimate responsibility for the operation of the aircraft even when you've received instructions from controllers. If a controller instructs you to fly at 3,500 feet but you see that that altitude will put your VFR flight into clouds, you must inform the controller and request a different altitude.