Gemuser From Australia, joined Nov 2003, 6285 posts, RR: 6
Reply 3, posted (11 years 7 months 2 weeks 1 day 4 hours ago) and read 6196 times:
All aircraft have a number of maximum, and sometimes minium weights specified as part of their certification. One of these weights is Maximum Gross Takeoff Weight (MGTOW), which is just what it says it is. Another is the Maximum Landing Weght (MLW).
Sometimes these are the same or very similar. Other times, eg B744 & I presume, A340-500, the differance is very large. In these cases a fuel dumping system is necessary if , for some reason (ie an ermengecy) it is decided not to complete the flight.
There are other weights eg Empty Weight, Maximum Zero Fuel Weight and Maximum Taxi Weight. The latter are pretty self explanarty, the first is actually quite complex with many variables depending on aircraft type.
OPNLguy From , joined Dec 1969, posts, RR:
Reply 4, posted (11 years 7 months 2 weeks 1 day 4 hours ago) and read 6129 times:
>>>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-IAH, 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-IAH 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.
Vortex From United States of America, joined Jul 2004, 99 posts, RR: 0
Reply 5, posted (11 years 7 months 2 weeks 1 day 3 hours ago) and read 6042 times:
One thing not mentioned, but that is important concerning max landing weight is that if the plane lands with a weight that is above the max landing weight, the Plane must undergo a very stringent structural inspection before it will be allowed to fly again. So if there is an emergency, such as engine shut down shortly after take-off, the plane can dump fuel and land at or below the max landing weight.
Warren747sp From United States of America, joined Feb 2004, 1210 posts, RR: 0
Reply 7, posted (11 years 7 months 2 weeks 16 hours ago) and read 5589 times:
Unless you are a super strong Airbus say A332 like NW NRT dep flight which was unable to dump fuel and landed with a full load of fuel. only fried the brakes!
I personally would prefer if they can dump fuel like the others.
Long and the short of it, airplanes can and do land "heavy" at times, but the risk of landing gear failure or damage, brake fires and/or blown tires is greater. There's no "law" against it - it's just procedure to dump fuel if you can in an emergency in order to prevent these sorts of problems on landing. But sometimes the pilots don't have much of a choice.
One example of a high-profile overweight landing I can recall is United Flight 811, which had its main cargo door blow off and a big hole ripped in the side of the plane. They turned the plane around and landed as quickly as they could get on the ground. They dumped what fuel they could but could not hold altitude so they landed above maximum landing weight. The pilots were concerned about structural failure (especially with a big, gaping hole in the side of their airplane) and they also had to land at a reduced flap setting due to damage, but they had no choice but to come in pretty hot. They managed to land and then stop the plane safely, though, despite coming in extremely fast and not having full reverse thrust available. So it is definitely possible, even in a pretty dire situation like that.
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