Power down on full power

The symptoms that you describe suggest a failing/failed battery. 

Are you able to use the iPad while remaining connected to your Power Adapter? If yes, while monitoring the indicated battery state, disconnect the Power Adapter; if the indicated battery state suddenly falls, or the iPad shuts-off, this test demonstrates the iPad battery having reached the end of its usable life.

As already commented by sberman, due to its age, the iPad Air is classified by Apple as a “vintage” device. Unlike obsolete products that are entirely out of support, vintage items remain supported while parts remain available:

Obtaining service for your Apple product after an expired warranty - Apple Support

However, you may wish to consider that the cost of a battery replacement may be uneconomic - given the limited utility of older devices.

Yours is an original iPad Air, released by Apple 9½ years ago. It’s considered a vintage product, which Apple no longer supports.

Obtaining service for your Apple product after an expired warranty - Apple Support

You are very welcome.

Before consigning your iPad for recycling, you might be well advised to seek assistance directly from from Apple Support - and request a remote diagnostic test. You can contact the Support Team using the Support link at top-right or bottom-left of this page. Alternatively and perhaps easier, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:

https://apps.apple.com/app/apple-support/id1130498044

You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad - and should you choose to do so, investigate availability of a replacement battery:

Genius Bar Reservation and Apple Support Options - Apple

My pleasure.