By now we've got a pretty good handle on how a modern jetliner is designed to fly. We've got a beautiful swept wing, nicely optimized for flying at .80 mach at 35,000 feet.
And there, lies our problem. A wing that's designed to fly fast isn't very good at flying slow. It won't even fly until somewhere around 200 knots. Unless we're using the dry lake bed at Edwards AFB, we just don't have enough runway at most airports to reach 200 knots.
So, we need a way to reconfigure our wing for low speed flight to let us takeoff and land in a reasonable distance. That's what the flaps do for us.
The primary purpose of wing flaps is to increase lift. They change the shape of the wing, turning its cross section from something knife-like to something more curved. If you're into trivia, the aerodynamic term for this is "camber". A fatter, curved wing produces more lift at lower speeds (but more drag at higher speeds) so we can get the same weight of airplane to fly at a slower speed.
They have another trick as well. Most airliners used a specific type of flap called a Fowler Flap. These don't just change the shape of the wing, they actually make the wing bigger. It's like putting a couple extra pieces of wing out the back. This can be pretty significant. The flaps on my B-52 were actually equal in size to a DC-9 wing. More wing means we have more lift which once again means we can fly the same weight of airplane at a slower speed. Simple enough.
This gives us the best of both worlds. We can have our efficient high speed wing but change its shape to a wing that lets us take off and land at much slower speeds.
The flaps on the 757 are so effective that lightly loaded we can fly final approach at 115 knots. With a decent headwind our ground-speed on final approach might be under 100 knots. I've landed at Munich while being passed by cars on the Autobahn.
The first few degrees of flap extension give you mostly extra lift, plus a little bit of extra drag. As the flaps extend further and further you end up getting a lot of extra drag and just a little bit of extra lift. That's why the last couple flap positions are normally only used on final approach when we want the extra drag.
The other cost of having flaps is added weight and complexity. We spend most of the flight carrying around something we only use for takeoffs and landings. That unfortunately is just the cost of doing business.
On most airliners the flaps are driven by hydraulics. One or more hydraulic motors turns a series of shafts and gearboxes that move the flaps up and down. The trailing edge flaps also move in and out along their tracks. Those canoe shaped fairings on the bottom of the Airbus wing cover the flap tracks.
From the cockpit it's all pretty simple. We have a handle to move the flaps and a gauge that shows us the flap position. There are also some bells and whistles that are supposed to warn us if we try to do something stupid like takeoff or land without the flaps in the correct position.
If you're sitting near the wing you have a pretty good view of the flaps. Before takeoff you should see the leading edge devices extend and just a little bit of the flaps. On a 757 that's flaps 5. This gives us good takeoff and climb performance. On a short runway we may need more flaps and we can go all the way to 20 degrees. This will get us off the ground sooner but hurt our climb performance later because of the extra drag.
After takeoff we normally level off 1000 feet above the ground and retract the flaps so we can accelerate to climb speed.
On landing you'll see the flaps extended incrementally as the plane slows. You'll initially see the leading edge come down and the flaps extend partially. Prior to starting down the glide slope you'll see the trailing edge flaps come down half way or so. Each airline has different procedures, but sometime shortly before or shortly after starting down the glide slope you should see the flaps extend fully. You'll probably notice some extra noise from the air going over the flaps.
The flaps are normally pretty reliable. Because we like redundancy the flaps may run off multiple hydraulic systems (Airbus) or have electric motors for backup (Boeing). Unless the flap mechanism itself breaks, we will usually have some way to lower them, at least partially.
The KC-135 was so old fashioned that the flaps could be cranked by hand. It took a while but you'd get them down eventually.
If we do have a problem with them it's probably one of two things:
1. We move the handle and something doesn't move.
We call this condition a "disagreement". The flaps don't go to where the handle is positioned. Could be the leading edge (front) or trailing edge (back) or even both.
We may be able to use an alternate means to work the flaps. On the 757 we have electric backups which will slooooowly move them. If they're stuck, we land with whatever we've got. We'll probably burn gas until we get down to a lighter weight and try to find a nice long runway somewhere.
This happened to me on an A310 once, but it was a temporary glitch and we got them moving again.
2. We move the handle and only one side moves.
This is the bad one. We call this an "asymmetry". This would make the plane want to roll, because one wing is producing more lift than the other.
Note that the flap indicator has a left and a right indication. Normally they would move together but in this picture they're not.
We handle this problem the same way as the other, except in this case we're probably not going to get the stuck portion of the flaps to move. Something is probably physically jammed or broken to make the flaps move asymmetrically. We'll end up with just the leading edge or just the trailing edge and land accordingly.
Same drill as before. Burn fuel and find a long runway because landing speed will be quite a bit higher than normal.
That's about it. Now you can amaze your seatmates by looking out at the wing and saying "I bet you didn't know that's a double-slotted Fowler Flap out there."