why aircraft weight has so big difference between landing and takeoff? 3

[lisamaverick]

Hello everyone:

During searching for some information, I found out that the maximum takeoff weight and maximum landing weight are so different for an aircraft?
take an example for A380:
Maximum takeoff weight 560 tonnes Maximum landing weight 386 tonnes
So if A380 takeoff with 560 tonnes, it can not land anymore??

Anybody knows why???

Thanks in advance

 

[swearingen]

The answer is fuel - and you are correct, it cannot land safely at full weight. What they will do in an emergency is dump the fuel and then land.



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[GregLocock]

As to 'why'?

Takeoff is well defined event. Essentially the wheel load gradually reduces as the wings take over and provide lift.

On the other hand a landing is more of a controlled crash - the aircraft desends at a certain velocity and the supsension, and the flare-out, between them, decelerate the downward velocity. Sometimes it is gentle. And sometimes it isn't.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[lisamaverick]

hi swearingen,

Thanks. Now I am clear about this.

 

[rb1957]

whilst the plane does burn fuel during the flight, the "real" design reason (some planes have a design landing weight very similar to their take-off weight)is that landing loads are much higher than take-off loads and so it's efficient to use a lower landing weight to reduce the landing loads so that the landing gear can be as light as possible.

Another requirement (whne the landing weight is much lower than the take-off weight) is for a fuel dump system, so that if there is a problem the plane doesn't have to cruise around for hours waiting to burn off the fuel untill the weight reduces enough for a safe landing.

 

[monkeydog]

Hmmm,
That might explain my lost luggage, cheaper to dump luggage then the fuel.

 

[KENAT]

As rb1957 says the main reason is mass/weight saving.

To be able to land at max TO weight requires a very robust structure to absorb the landing 'impact' (both the landing gear it'self and the structural members it and potentially things like fuel tanks are attatched/bearing on to). This structure has a mass penalty.

Given, especially for a long range commercial aircraft and the like, you'll usually be landing with a lot less fuel than you took off with then to save mass the landing gear & structure is designed around this factor. For safety you then have to be able to get rid of fuel/payload quickly in an emergency.

A lot of military A/C especially bombers etc used to do the same, they assumed they'd always drop or jettison their bombs and had max landing weights significantly lower than max take off. As the cost of guided weapons etc has increased they now like to be able to bring back weapons that don't get dropped, especially for operations like policing no fly zones. This is a factor in why some A/C have been retired apparantly early and is a factor in why military A/C rarely take off with a full load of guided weapons. This is a major factor for carrier A/C which have much higher landing loads.

Think about Breguet range equation if you want to put some math behind the explanations. Essentially (if I recall correctly) the greater % fuel is of your take off weight the more efficient you're flying. The use of lightweight structures amplifies the % difference.

KENAT, probably the least qualified checker you'll ever meet...

 

[davidjh]

To clarify a few things: the quoted maximum Takeoff Weight ("MTOW") and Maximum Landing Weight ("MLW") are structural design values - ie; the airplane meets all the type certification ("TC") (eg;FAR/JAR) structural design criteria/requirements. On any given takeoff or landing the MTOW and/or MLW may be lesser because of the TC performance criteria (typically when the airport is "high and "hot", and/or the runway is "short"). The longer ranged airplanes have the larger difference between the structural MTOW and MLW - you will find that airplanes such the B737, or Embraer 140/170 have a measurably smaller difference.

In fact all FAR 25 certified airplanes to the more recent regulation levels must demonstrate the ability to takeoff at the stuctural MTOW and return for an "immediate" "safe" landing. The longer ranged airplanes will have a fuel dump system. It is common for the short ranged airplanes to not have one. On the long ranged airplanes the approach/landing speeds up near the MTOW become very fast, and brake energy and tire speed limits can be exceeded. Often the tire speed rating is set by this requirement.

At the structural MLW the airplane must be designed for a 10 ft/sec touchdown, and even at the MTOW must be designed for a 6 ft/sec touchdown. Most "inservice" touchdowns are in the 2-3 ft/sec realm. A 6 ft/sec touchdown is "firm to hard". Very few people have experienced a 10 ft/sec landing.

The performance requirments dictate that the operator must plan each flight so as to arrive at the destination at a weight where the MLW requirements (structural and performance) are met. So should a flight arrive at its destination above that weight the operator must either burn the fuel or dump fuel if able, but there are now severe enviromental regulations against dumping fuel. One of the more likely scenarios where this might occur would be flights arriving at Denver in the summertime.

It is "common" in the manufacturers flight test program to need to conduct a series of takeoffs starting at or above the stuctural MTOW which obviously requires landings made at/near the structural MTOW. In the controlled FT environment there is assurance these can "readily" be made below the 6 ft/sec design limit.