Checking your instrument.
Damage Prevention and Self-Repair
Chances are that within the first year of having or renting) your flute, you'll have to take it in to be repaired. This problem may be easily fixable on your own though, and it might not be worth the cost of taking your flute in to a professional. There are also some ways here to prevent these kind of problems in the first place.
Prevention
1. Regular Cleaning
Keep your flute clean! A dirty flute is not a happy flute. Moisture buildup on the inside of your flute can cause damage of pads which will have to be replaced, and corrosion of the inside, cause your sound not to be as full. After every playing session, be sure to clean the inside of your flute and wipe any fingerprints off of the outside. If it starts getting corroded (a black buildup on a silver flute), send it in to a professional cleaner right away.
2. Proper Treatment
Many a time, I've seen people leave their flutes sitting on a stand or a chair, just waiting to be knocked down and damaged. If you have to leave your place for any length of time, make sure your flute is in a safe place! Put it somewhere where it's not going to get knocked down, or better yet, put it away. Just memorize the position in which your headjoint was and then you'll be able to keep your tuning the same as it was before. Never keep your flute in extreme temperatures or conditions for any length of time, and if it's an expensive one, keep it supervised at all times!
While putting it together, be sure not to grasp it too tightly, and not to hold it by the keys and rods. A lot of people put their flutes together by clutching the key system; THIS DOES EXTREME DAMAGE TO YOUR FLUTE OVER TIME!!!! If you put your flute together this way, expect to be taking it into the repair shop at least every two months, costing needless inconvenience and cost.
3. Take it in for a Regular Check-up and Overhaul
It is a really good idea to take your flute in to the shop, even if there's nothing wrong with it, about two times every year (once a year is okay too but twice is better). This allows the repair person to check out your flute, fix any alignment problems, clean it, and make it nice and happy. A really good repair shop will completely take apart your flute and put it back together, making sure there are no internal problems and allowing them to clean the most remote pieces. Don't let your flute go without a check-up for more than a year, it is very important to keep it in working condition and it avoids having problems when you least need them.
Always remember, to prevent damage in the first place, keep your flute clean, safe, and in good health
Simple Self Repair.
1. The Screws
Depending on what kind of flute you have, it will have tiny flathead screws at various points. On simpler models, there will only be screws on the end of the rods. Some flutes have individual screws for every key though, which makes it much easier to fix. If you are planning on doing self-repair, go to your local music store and buy a tiny, flathead instrument screwdriver.
2. The Springs
As you notice, while you're playing, pressing down one key can cause others to go down as well. When you let go of that key, the others that were pressed down go up as well. This motion is all caused by springs. The problem will be quite obvious when a spring is not in its proper place, as the keys that are supposed to move up and down will not and many of your notes will not play. To fix a spring, take a pen, a small screw-driver, or something long and slender and push it back into place (you'll notice that it's out of place if it looks like it's bent because it's on the wrong side of the little catch that holds it). It is very tedious to fix this, but sometimes its not worth sending it into a repair shop if its just a little out of place.
3. Adjusting Alignment
To adjust alignment on your own, you need to use the screws that are mentioned in #1 above. You'll notice alignment problems if some of your notes don't sound as good as they usually do, or if you notice that one of your keys doesn't go all the way down. To fix this try checking if any of the rods are loose. If they are, tighten them. If it's not the rods, and the keys have individual adjustment screws try adjusting the screw that goes with the problem key (the key that makes the other one go down); tightening cause the key's position to be lowered while loosening causes it to be raised. This is also very tedious work so if it doesn't work out, keep trying for a little bit, but if nothing at all works, it's an internal problem and you should take it into the shop.
The flute is a very fiddly instrument, so it takes A LOT of patience to play around with it. Just remember only to use little tiny adjustments, never do anything too rash or too harsh. Only fix little problems that you feel confident enough to deal with yourself; if it's beyond your capabilities, don't even try to fix it, just send it in to a professional. Doing self repair well takes a lot of experience, so start out with smaller problems, and gradually work your way up to bigger ones.
Hope this has helped