MySQL query does not use index

The following query:

EXPLAIN 
SELECT 
    fdb . * , 
    TIME_FORMAT( fdb.scheme,  '%H:%i' ) AS scheme, 
    TIME_FORMAT( fdb.actual,  '%H:%i' ) AS actual, 
    TIME_FORMAT( fdb.baggage, '%H:%i' ) AS baggage, 
    TIME_FORMAT( fdb.baggage_handled,  '%H:%i' ) AS baggage_handled, 
    ff . * , TIME_FORMAT( ff.actual_saved,  '%H:%i' ) AS actual_saved, 
    TIME_FORMAT( ff.baggage_saved,  '%H:%i' ) AS baggage_saved, 
    TIME_FORMAT( ff.baggage_handled_saved,  '%H:%i' ) AS baggage_handled_saved, 
    ap.device_id, 
    ap.device_token, 
    ap.device_language, 
    ap.app_edition, 
    ap.receive_status_notifications, 
    ap.receive_time_notifications, 
    ap.receive_luggage_notifications, 
    ap.receive_gate_notifications, 
    ap.receive_runway_notifications, 
    ap.receive_plane_notifications
FROM flights_database fdb
    JOIN flights_followed ff ON fdb.flight_id = ff.flight_id
    JOIN apns_users ap ON ff.device_id = ap.device_id
                        AND ap.app_edition =  '1'

After using Explain, it is clear the query uses table scans:

There are various keys and pusher_idx contains both device_id and flight_id. Why is this index not used?

In this case mysql optimizer changed the order of your tables (it can do it - they are all INNER JOINS):

SELECT 
    ...
FROM flights_followed ff
    JOIN flights_database fdb ON fdb.flight_id = ff.flight_id
    JOIN apns_users ap ON ff.device_id = ap.device_id
                        AND ap.app_edition =  '1'

So as soon as flights_followed is not filtered by any own predicate (with new join order it's other tables joined to it, so it's they that are filtered) - it decides to perform a full scan.