If you're an instrument pilot, you've no doubt called ATC for an IFR clearance before taking off. But if you're an airline pilot, chances are, you no longer call ATC for your clearance. At least not at most major airports in the US.

That's right, you're not scribbling your clearance on a piece of paper as fast as possible (remember the CRAFT acronym?). Instead, you're probably using something called a Pre-Departure Clearance (PDC).

What Is A PDC?

A pre-departure clearance is basically an electronic version of the the IFR clearance you would normally from get from ATC by talking to them over your radio.

How A PDC Works

So how does a PDC make it from ATC to the aircraft? There are a few steps:

The pre-departure clearance starts in the tower cab. For example, at Atlanta Hartsfield airport, the tower cab has a computer that handles the clearances for the airlines that are part of the PDC program (not all airlines use PDC, but many do).

The tower cab computer has something called a "passive tap" on the flight data input/output line used by airlines. In short, it's able to pick up the flight plans that airline dispatchers are putting through their systems for upcoming flights.

Issuing The Clearance

When a controller is ready to issue a clearance for an aircraft that is getting close to pushback from the gate, they send an IFR clearance to the airline's dispatcher through the computer. And, the clearance looks something like this:

The clearance is basically the same as what you would get if you called ATC, it's just in text format.

When ATC transmits the clearance, it's sent to the airline's dispatch center. And once it gets to dispatch, the clearance is automatically relayed to the airplane as well. That way, the pilots and the dispatcher have a copy of the clearance.

So how do the pilots get the clearance? There are a few different ways. In most cases, pilots get the PDC through their ACARS (Aircraft Communications Addressing and Reporting System) system. It might be displayed digitally on the ACARS screen, or printed out if the aircraft has an ACARS printer. If the pilots don't have ACARS, or it isn't working, they can get a printout from the gate agent's printer.

Why Digital?

So why are airports going through the process of issuing PDCs instead of the usual method of issuing clearances over the radio?

First off, ATC and the pilots have an exact copy of the flight plan, which means there's less possibility of confusion or mis-communication of the clearance. If you've ever flown into a large airport, or monitored clearance delivery at a major airport, you know things move pretty quickly over the radio.

Second, it cuts down on radio delays. Clearances can be long, especially if they're amended and the controller needs to read the entire clearance to the pilots.

What If It Doesn't Work?

Computers aren't perfect, and sometimes a PDC doesn't work. If that's the case, there's always the old-school method of calling clearance delivery over the radio:

Putting It All Together

Pre-Departure Clearances are a way for ATC to communicate IFR clearances to airline pilots and dispatchers without time-consuming voice communication. They make life a little easier, and less error prone too. And at the end of the day, who doesn't like that?