From AOPA:

When ground and sim training are complete, it’s finally time to fly the airplane! Back in the day, the first step was to get some landings in an actual airplane, usually conducted in the middle of the night at a small outstation under the guidance of a specially trained pilot. Those days are largely gone because of cost and safety concerns (mostly cost). Simulators are now so good that the airlines and the FAA agree that “familiarization flights” are no longer needed.

Initial operating experience (IOE) is the term used to describe your first trip of several in an airplane under the watchful eye of a check airman (sometimes called a line check airman, or LCA). IOE is an exciting yet nerve-wracking experience. You’ll go to the airport, find the crew room, and go through the entire preflight routine. It will feel like you have no time at all to get everything you need to do done, but in no time you’ll be able to do it all with time to spare.

The LCA will be talking a mile a minute, trying to teach you as much as possible in as short a time as possible. At the gate, you’ll do a supervised walk-around, and then get in the cockpit and do your routine as you’ve trained for it in the sim. However, now you’ll be bombarded by other distractions that you didn’t have before, such as flight attendants who want to say hello or need you to order something they’re missing in the cabin. Mechanics may be nosing around, and ticket agents usually come down to see if you’re ready. It doesn’t help that you still haven’t perfected the routine, and you feel as if you’re running in mud. Meanwhile, the LCA keeps talking, and he’ll take over a lot of the little stuff to try to achieve an on-time departure.

You’ll be thinking about the fact that you’ll be flying the airplane for the first time with a cabin full of passengers who have no idea that you’ve never actually flown this airplane, but you can’t dwell on it. Time will feel very compressed as you’re dealing with ATC, busy frequencies, and weather you don’t see in the sim (especially good weather). Your first night in the hotel will probably be one of the best nights of sleep you’ve ever had, thanks to the exhaustion.

IOE is a lot of fun in addition to being a steep learning curve. You’re putting all of the pieces together and realizing the culmination of your dreams. At times it’s frustrating because you don’t realize going into it how much you still have to learn, and landing the airplane is totally different than the sim. But over a few trips, with several LCAs, it starts to fall into place. And no matter how many times you go through IOE in the future, it will never be as overwhelming as the first time. Nor will it be as fun.