>>>Why is landing with full fuel tanks outlawed?
It's not, nor is it even possible. Even if an aircraft took off with full tanks (say, 35,000 lbs.), and had to come back around and land, it'd be landing with less than 35,000 lbs. and thus not "full".
The reason for fuel dumping capability (which is actually used pretty infrequently) is to allow an aircraft to get itself down to max landing weight in the event of a premature landing is needed.
There are various aircraft weight limits out there, but two major ones are max takeoff weight (what you can actually get off the ground) and max landing weight (what's the most the aircraft can weigh upon landing). One could theoretically load an aircraft right to it's max takeoff weight (full payload, pax, cargo, fuel, etc.) and then launch for a flight to a destination only :30 away, but the aircraft would be overweight for landing when it got there.
The way to deal with this is to take what the fuel burn getting from A-B is, and then add that to your max landing weight is, and use the resultant number is as your max takeoff weight (that is limited by landing). As an example, using a 737-200. You're going DFW
, and can get 115,000 lbs. off the ground at DFW
. The max landing weight at IAH
is 103,000, and the fuel burn DFW
is 5,000. If you load the aircraft up all the way to 115,000 and takeoff, you're going to arrive at IAH
weight 110,000, or about 7,000 lbs overweight. So, the way to prevent this is to take the 103,000 max landing weight at IAH
, and add the 5,000 burn to get there, and come up with 108,000, and limit the aircraft to that 108,000 leaving DFW
. Thus, you takeoff only at 108,000, burn 5,000, and arrive at 103,000. That 108,000 is the max takeoff weight "limited by landing".
As you can see, trip distance is a factor here. The longer the flight gets, the more fuel it burns. If a flight DFW
-XYZ burned 12,000 lbs, the 115,000 max takeoff weight, and the takeoff weight limited by landing (103,000 plus 12,000) would be the same. On a 737-200, that 12,000 burn will make that DFW
-XYZ about 2:15 in duration.
Aircraft capable of long-haul flying have an even greater "spread" between max takeoff weight and max landing weight. If one of these is on a long flight and then unexpectedly has to cut that flight short (like to a medical emergency mentioned in the photo's text above) it needs a way to get the aircraft's weight down to max landing weight, hence the fuel dump system.
Most smaller airline aircraft do not have fuel dump systems even installed, and can land (in an emergency) at weights above the normal maximum, as long as maintenance inspections are conducted afterwards.
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.