Aviationstack at https://api.aviationstack.com/v1 tracks flights in real time and serves airport, airline, and route reference data.
Sign up at aviationstack.com for an access key, passed as access_key in the query string. In the examples it's written {{AVIATIONSTACK_ACCESS_KEY}}. The free plan has a monthly call limit.
Full reference: https://aviationstack.com/documentation
By IATA flight code.
curl "https://api.aviationstack.com/v1/flights?access_key={{AVIATIONSTACK_ACCESS_KEY}}&flight_iata=AA100&limit=5"
curl "https://api.aviationstack.com/v1/airports?access_key={{AVIATIONSTACK_ACCESS_KEY}}&search=denver"