iPad Air charger gets hot while charging

It is not unusual for an iPad or its Power Adapter to become warm when being used - more so when using some types of iPad case. High screen brightness and/or CPU intensive Apps will cause the iPad to create more heat. While charging the iPad, a Power Adapter can be expected to become warm - more so when the iPad is also being used during a charging cycle.

The key point is that whilst your iPad may become very warm in normal use, the iPad (or Power Adapter) should not become uncomfortably hot to handle. If either are becoming too hot to hold, you should take your iPad and Power Adapter along to the Genius Bar at your local Apple Store - where the technicians can examine and test your iPad for faults.

Genius Bar Reservation and Apple Support Options - Apple

Yes. Energy transfer causes heat. Chargers will get hot when charging a device. It should still be within normal temps though. If it gets so hot as to not be able to be touched, then there may be an issue.

Otherwise, it's normal for it to be warm or even a bit hot, while still being able to be touched.