Split/Second

This secret achievement is:

Frequent Flyer
In an event, wreck 3 rivals by crash landing a plane.

Must be done on Airport terminal. It's a two parter. On the first or second lap, you must use the route change that takes down the airport tower, and near the same area on the second run, you need to use the level 2 power play that brings the plane down, and it needs to crush at least 3 people.

Race on Airport Terminal.

Drift and draft your way to the control tower and then use the route change. Now drop back to about sixth place and start drifting and drafting again. When you reach the tower for the second time (still in sixth place) use the level 2 power play.

Odds are good that you'll take out everyone in front of you, giving you the achievement and an easy win...

Start a race on the Airport terminal. Drafting steadily fills your power bar, so do it often. Drifting is another much faster way to fill the power bar. Use a level 2 power play, which requires a full power bar to bring the airport tower down, before you turn towards it, on the previous straight area. You'll see a hexagonal red icon next to the tower. Right now you need to press to trigger a route switch.

The next lap, you have to use another level 2 power play around the same place where you used the first one. On this lap, it may be difficult to tell whether or not you're approaching the airstrip, which you can now drive on, since there's no longer an upright tower marking the way. I'd recommend practicing a couple times to get used to the location, memorizing a few important objects that give you a sense that you're there. A level 2 power play prompt will appear early far from the turn towards the airstrip. You need to press right away once you see this.

Try to place your self in 6th place long before you get to this point, as this seems to work very well according to other guides. If a car ahead of you crashes, don't worry about letting it catch up, as this might create a large gap in front of you, preventing you from getting enough cars in the path of the trap. Just make sure that you have at least three cars reasonably close together in front of you, behind you or in a combination of both positions. If you get a double take down, you may still be able to get a triple since cars behind you may crash into the flight. Once you meet all of the right circumstances and trigger the trap, a flight will crash land on the airstrip and take out a majority or all of the cars ahead of you, and also possibly some behind you. A triple take down will unlock the achievement.