Although it is not visible on the outside, the headjoint cork is certainly an important component of the flute.
Line indicates proper cork alignment
The cork serves to balance intonation between octaves on the flute and can cause problems if it is not fitting properly. Because cork is a natural material, it can change over time - shrinking and deteriorating because of time and conditions. If a flute is played frequently outdoors, the natural "wear" of the cork will be accelerated. When the cork becomes worn, it will loosen within the headjoint. The loose cork then prevents students from producing a full sound because air will begin to leak. Proper position of the headjoint cork can be determined by using the flute tuning rod/swabstick marker, which should be in the middle of the embouchure hole. Powell's repair technician, Rachel Baker, recommends having the headjoint cork replaced every other year - or more frequently for extensive playing outdoors.