How is Maximum Landing Weight calculated?

[Airmann]

So how is it calculated?

I read somewhere that go around climb requirements must be considered. As well as approach speeds. But what about structural considerations, gloadings? I mean most aircraft are capable of landing above MLW. Must be done carefully of course with an inspection afterward.

If anyone has a little more insight?

[Wizofoz]

Like MTOW, MLW has both performance and structural considerations.

However, you are going to be limited be structural MLW than performance, unless you are landing in unusual conditions, such as a short and/or contaminated runway, very hot/high or with significant obstacles effecting the missed approach path.

Landing over structural MLW is an option in emergencies.

[giggitygiggity]

Does anyone know why easyjet MLW for a A319 is 61 tonnes whereas the official Airbus figures suggest the MLW for an A319 is 62.5 tonnes? Is that just easyjet being overprotective or is it some mod/design difference, or perhaps my Airbus source is wrong...?

[Johnny F@rt Pants]

Can't help you with the EZY Airbus, but on our 737-800's we have an artificially lowered MLM which reduces the final approach speed sufficiently to be able to classify the aeroplane as Cat C rather than Cat D. Perhaps that has something to do with it.

[Airmann]

Originally Posted by Wizofoz

Like MTOW, MLW has both performance and structural considerations.

However, you are going to be limited be structural MLW than performance, unless you are landing in unusual conditions, such as a short and/or contaminated runway, very hot/high or with significant obstacles effecting the missed approach path.

Landing over structural MLW is an option in emergencies.

Thanks for the response.

Any idea of specifically what the manufacturers are looking at with regards to structural considerations. Is it load on the landing gear? Airframe vibrations? I know that Airbus has landing limits of 600fpm below MLW and 360fpm above (are least the Airbus I have flown.

[Owain Glyndwr]

It is not calculated so much as chosen. MLW is defined in the initial specification and the aircraft has to be designed to meet all applicable requirements at that weight. Landing gear loads will figure highly in that assessment - airframe vibrations are usually high airspeed problems.
The choice of MLW will depend on how one expects the machine to be used.

On a dedicated long range usage for example MLW mignt be something like MZFW plus an arbitrary but sensible fuel allowance - say total fuel reserves uploaded for a flight starting at MTOW but none of them consumed.
If the aircraft is expected to fly a series of short sectors through the day without refuelling then MLW could be close to MTOW to cover the first segment.
On something like a Cessna where the total fuel is a relatively small percentage of the total weight then MLW and MTOW may have the same value.

These are just examples to illustrate the thinking.

[MarkerInbound]

The 600 fpm and 360 fpm impact are found in Part 25, all transport category jets have to meet those standards. Boeing used to let the airline pick the MLW (within limits.) Some airlines picked an artificially low MLW as some airports set their landing fees based on the aircraft's MLW. In a prior job we had several aircraft where this had been done. We needed a higher MLW. Sent a checked to Boeing and the MLW jumped up 4500 lbs with no physical change to the plane except a new data plate.

[pattern_is_full]

Airmann is no doubt primarily interested in the technical calculations.

But as GG, JFP, and OG allude to, there are operational reasons for picking a MLW.

It's been mentioned here before that some operators "derate" their own weight limits so that they fall into a lower weight category for weight-based landing fees. As well as landing category, etc.

Airbus says the plane can take the landing forces at 62.5 tonnes. EZ decides they'll never need to land above 61 tonnes - so why not list that as their own limit, and pocket the savings?

[KenV]

Max Landing Wt is driven by many considerations. The two biggest drivers are (generally) fuselage bending moment and wing downward bending moment. The former is related to the max zero fuel weight of the aircraft and driven by payload.. Wing downward bending moment is driven by the amount of fuel in the wings at landing, particularly the outer wing tanks. The details of these factors depend on configuration, for example low wing vs high wing aircraft. Landing gear are attached to the wing spars on low wing aircraft and attached to the fuselage on high wing aircraft and the loads and load paths are fundamentally different, especially the instantaneous moments when the aircraft shifts from wing borne flight to wheel born taxi.

[Piltdown Man]

All of the above are totally correct. But if these considerations are not factors, your MLW can often be increased. How? An operator will generally find the manufacturer rather accomodating. But they will have to have large amounts of cash thrown at them. You will almost certainly end up with reduced life cycles on many components and shorter maintenance intervals. You will often pay more for every landing as well. For some this will be worth it but for many it will not not.

PM

[compressor stall]

Lower MLW

A319 already at Cat C so no benefit there, but I wonder if in addition to the aforementioned landing fees further lowering the MLW allows longer intervals between undercarriage overhauls? Thus more cost reductions long term.

[Airmann]

Thanks for the responses. The picture is becoming clearer. Seems to be a number of factors:
1. Aircraft structural considerations
2. Aircraft operational considerations
3. Aircraft performance considerations
4. Operator financial considerations

I guess the last one is really a bit of a trick. For those of you flying aircraft with 'artificially' lowered MLW: What would you do if you had to land above your stated MLW but below the aircraft's actual design MLW. Would you follow the overweight landing checklist? I'm sure you would log the event, just to satisfy formalities.

[KenV]

Thanks for the responses. The picture is becoming clearer. Seems to be a number of factors:
1. Aircraft structural considerations
2. Aircraft operational considerations
3. Aircraft performance considerations
4. Operator financial considerations

You can increase MLW (up to a point) if you're willing to accept lower service life and/or increased maintenance actions. Perhaps an example will help.

The C-17 is designed for a 30,000 hr service life. That service life assumes a "load spectrum" of loads imposed by:
flt hours at cruise altitude
Flt hours at high speed cruise at low altitude (below 5000 ft)
number of pressurization cycles
number of touch and go landings
number of full stop landings
number of assault landings
number of rough airfield landings
and many more.

The actual load spectrum the C-17 sees is different than that assumed during design, flying many more hours at low altitude and making many more assault landings and landings into rough unpaved fields. To achieve the design life, the maintenance program has been modified (for the entire fleet) to account for the harder than planned for use. This benefits the RAF C-17s which don't do many (any?) tactical operations into unpaved fields, But that "benefit" also translates into higher than necessary maintenance costs. Interestingly, the C-17 fleet is now being maintained with an eye to achieving at least 3 design lifetimes. To put that into perspective, at current flight hour/utilization rates, this means that just over half the fleet will still be flying in 2117, just over a century from now!

BTW, C-17 MLW is the same as its MTOGW. But this is on paved fields and "normal" landings (2.5 degree glideslope with a flare before touchdown) When doing an assault landing (5 degree glideslope and no flare at touchdown) the MLW is (predictably) reduced. This limit is (primarily) driven by fuselage bending moment. Besides the gross weight limit of the aircraft at touchdown during an assault landing, there are limits on the amount of fuel allowed in the outer wing tanks. This is mostly driven by wing down bending moment. Since normal landings are much much less stressful than assault landings and the structure is designed for the worst case assault landing (into a short, rough, unpaved field) the structure can easily handle the normal landing at MLW. (In other words, no need to dump fuel if returning to base right after a heavy takeoff.) It also means that the structure does not fatigue at its design rate if the airframe only experiences "normal" landings. Which MIGHT mean reduced maintenance actions. But it comes at the price of an "overbuilt" (and therefore heavy) structure and landing gear components.

An airplane MAY be designed for a specific service life assuming a certain number of MTOGW takeoffs followed by a certain number of flight hours and a certain number of pressurization cycles with no specific MLW. Once that structure is designed, the MLW for that structure can be back calculated and that becomes the certified and/or warranted MLW.

I hope this made sense and was useful.

[Piltdown Man]

I guess the last one is really a bit of a trick. For those of you flying aircraft with 'artificially' lowered MLW: What would you do if you had to land above your stated MLW but below the aircraft's actual design MLW. Would you follow the overweight landing checklist? I'm sure you would log the event, just to satisfy formalities.

Assuming "had to land" means an emergency or abnormal operations then exactly as you say. Assuming a normal landing, the only outcome will be paperwork.

PM