IFR Fix: What is a clearance?

April 14, 2014

A low sits squarely over Michigan, with snow falling on the Upper Peninsula and a tentacle-like cold front trailing southwest through central Texas. A warm front extends eastward, becoming stationary over New England. It’s 68 degrees in New York City. In Waterville, Maine, it’s 40 degrees.

At the Texas end of the frontal boundary, a similar pattern is seen. It’s 73 degrees in Houston, wind calm. To the west, in San Marcos, it’s 52 degrees with gusty northwest winds as the cold front pushes through. Nearby, in New Braunfels, the current observation contains these items: 36019G30KT 10SM -RA OVC016 16/13 A2980 RMK AO2 PK WND 36031/1238 LTG DSNT NW RAB14 PRESRR.

With your instrument proficiency check scheduled, and the first AOPA Regional Fly-In set for April 26 at San Marcos Municipal Airport, take this good opportunity to do some route planning, look up and study instrument approach procedures, and monitor area weather trends.

Discussing the pros and cons of possible routes during your review, your CFII poses an unexpected question: "What is an air traffic clearance?"

Most pilots can paraphrase the first 27 words of the 218-word definition of an air traffic clearance in the Pilot/Controller Glossary: "An authorization by air traffic control for the purpose of preventing collision between known aircraft, for an aircraft to proceed under specified traffic conditions within controlled airspace."

The rest is more opaque—especially as applicable to weather scenarios or other unforeseen circumstances.

For example, the April 8 IFR Fix poll question asked whether a flight that has been cleared to "deviate as necessary" by departure, then cleared  "direct GSP" by center, may still deviate. About 62 percent of 845 respondents said no, 38 percent said yes.

The scenario, reported to the Aviation Safety Reporting System, arose when a jet crew cleared direct GSP (Greenville-Spartanburg) by the center deviated five degrees to avoid weather ahead.

"We had in our minds the clearance from Departure Control that we could deviate as necessary," a pilot reported.

That wasn’t how ATC saw it.

Center called, and "reminded us that our last clearance from him was direct GSP which we had accepted."

The glossary’s definition of a clearance covers a lot of contingencies; the basic idea is avoiding uncertainty by clearly accepting ATC’s instructions, or requesting an alternative if safety necessitates.

If you haven’t read those 218 words lately, click here before flying.

Dan Namowitz

Dan Namowitz | Aviation Writer

Dan Namowitz is an aviation writer and flight instructor. He has been a pilot since 1985 and an instructor since 1990.