paco1980
Posts: 13
Joined: Mon Jan 12, 2009 10:28 am

Increase Of MTOW And MZFW

Wed Feb 04, 2009 3:06 pm

Hi All,

(MTOW: Max Take Off Weight)
(MZFW: Max Zero Fuel Weight)

I am wondering how could having different MTOW or MZFW for the same Aircraft type happen...

As MTOW and MZFW are structural limitations, this means that the Aircraft would break if taking off heavier than MTOW and the bending moments in the wings will damage the Aircraft if the Aircraft is loaded (excluding fuel) heavier than MZFW....

I can understand that some airlines want to operate with a MTOW lower than the manufacturer MTOW for landing fees purposes, this is as long as it is lower than the manufacturer's MTOW...

Plus, for a specific Aircraft, how can the manufacturer pratically increase the MTOW and MZFW? Only by running the certification tests with the new values? There should be a maximum (MTOW or MZFW) where a complete redesign of the Aircraft is unavoidable, isn't it?

If not, what kind of quick redesign could the manufacturer do to increase these values?

Any idea ?
Thanks in advance,

Paco
 
474218
Posts: 4510
Joined: Mon Oct 10, 2005 12:27 pm

RE: Increase Of Mtow And Mzfw

Wed Feb 04, 2009 6:21 pm



Quoting Paco1980 (Thread starter):
I am wondering how could having different MTOW or MZFW for the same Aircraft type happen...

By beefing up the structure, landing gear, brakes, wheels and tires, additional fuel capacity, up graded engines, etc.

The original L-1011-1 had a MTOW of 430,000 lbs and a MZFW of 325,000 lbs.

Over the years Lockheed developed a service kits that would allow increasing the MTOW to 440, 450, 466, 470, 474 or 510,000 lbs. With a increase of the MZFW of up to 338.000 lbs.

Each increase in MTOW required additional modifications.

Having said that there were other ways of increasing the MTOW.

The L-1011-500 has a MTOW of 496,000 lbs. But if you paid an addition fee to Lockheed they would provide an analysis and revised manuals that would allow operation at 504 or 510,000 lbs. There was no actual modification to the airframe.
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Increase Of Mtow And Mzfw

Wed Feb 04, 2009 9:13 pm



Quoting Paco1980 (Thread starter):
As MTOW and MZFW are structural limitations, this means that the Aircraft would break if taking off heavier than MTOW and the bending moments in the wings will damage the Aircraft if the Aircraft is loaded (excluding fuel) heavier than MZFW....

That's not what MTOW or MZFW mean. You could safely load an aircraft considerably above MTOW/MZFW and take off, for a couple of reasons. The major one is that the strength requirements on the wings are driven by 50% above the highest load the wings will ever seen in service, which would be hitting some high-g condition while loaded to a particular weight (probably MZFW, but it might be MTOW). Since you don't actually see those kinds of loads during a normal takeoff, you could get off the ground at probably a much higher weight without snapping anything.

MTOW/MZFW is primarily a certification limit, not a true structural limit (although it's obviously bounded by structural limits).

There's also the roll of fatigue...many aircraft components are sized on fatigue, not strength. As a result, they may be massively overstrength for one-time loads, but going above the certified limits will significantly reduce your cycle life and throw your damage tolerance assessments out the window.

Quoting Paco1980 (Thread starter):
Plus, for a specific Aircraft, how can the manufacturer pratically increase the MTOW and MZFW? Only by running the certification tests with the new values?

Analysis is the most common way. If you get better materials data, or your load cases change, you can redo all the calculations that got you to MTOW/MZFW in the first place and show that you can increase it while remaining within the requirements.

You can keep doing that until you run out of margin on something...then you reenforce the something, as 474218 describes, then keep going up until you hit the next limit.

Tom.
 
paco1980
Posts: 13
Joined: Mon Jan 12, 2009 10:28 am

RE: Increase Of Mtow And Mzfw

Thu Feb 05, 2009 10:19 am

OK, thanks all.

So what I understand is:
- MTOW and MZFW are structural limits but are NOT the ones for with the Aircraft will collapse if being higher by one pound !  Wink
- This means that the manufacturer can still play with this margin to increase MTOW or MZFW if necessary by only playing with tests if not increased significantly
- If they want to increase significantly, the tests have to be done and the failing structure (brakes, landing gear,...) will have to be reinforced
- In all cases, the higher the MTOW the lower will be components life cycle

Cheers,

Paco
 
CosmicCruiser
Posts: 2049
Joined: Tue Feb 22, 2005 3:01 am

RE: Increase Of Mtow And Mzfw

Thu Feb 05, 2009 2:26 pm

The diff in MTGW for the MD-11 is nothing more than a cost option offered by Boeing for AFM data sets and FMS data. Nothing more.
 
n737aa
Posts: 226
Joined: Thu Aug 28, 2008 6:00 pm

RE: Increase Of Mtow And Mzfw

Thu Feb 05, 2009 7:08 pm

It can include many of the things mentioned here but all boils down to certification.
 
tbanger
Posts: 253
Joined: Tue Jul 27, 2004 6:11 pm

RE: Increase Of Mtow And Mzfw

Fri Feb 06, 2009 8:13 pm



Quoting CosmicCruiser (Reply 4):
The diff in MTGW for the MD-11 is nothing more than a cost option offered by Boeing for AFM data sets and FMS data. Nothing more.

Not always as simple as nothing more. The Saab 340 has a serviceable bulletin out that increases MTOW by 500lb/230kg. It is nothing more than an ammendment to the AMM, IPC, AFM, AOM, WBM, ACAP and ARM manuals.

However, there is a restriction or a requirement to have certain part number or certain status Main Wheels, Brakes and NLG components installed to comply with the SB.

I think different tyre pressures are also required.

At this stage we've only done the SB to our freighters.
 
wirelock
Posts: 98
Joined: Fri Sep 14, 2007 12:55 pm

RE: Increase Of Mtow And Mzfw

Fri Feb 06, 2009 11:28 pm

for the a320 the mtow is requested by the customer when the aircraft is delivered.
for example if you wanted the aircraft to fly BRU AMS all the time you would have a low MTOW so as to keep landing charges as low as possible.
if for instance the aircraft then changed operator and now the aircraft would be flying DUB ATH then the customer would contact airbus and ask them to increase the MTOW of the a/c.
depending on the requirement of the customer airbus would issue an SB to increase the MTOW of the aircraft. this SB once implemented would lead to a different Weight variant of the aircraft.
i think that the A320 has now 11 different weight variants. if the operator wanted a new weight variant then airbus would have to approve the SRM for this new WV(320 SRM is standardised for the fleet, you need to know the a/c WV to know if a particular repair, assesment is valid for your a/c). until then any structural damage, repairs or inspections would have to be directly approved by airbus engineering.
if another operator decides to reduce the MTOW again, then the highest WV that the aircraft has ever had must be used.
 
OPNLguy
Posts: 11191
Joined: Tue Jun 15, 1999 11:29 am

RE: Increase Of Mtow And Mzfw

Sat Feb 07, 2009 3:39 pm



Quoting Paco1980 (Thread starter):
I can understand that some airlines want to operate with a MTOW lower than the manufacturer MTOW for landing fees purposes,



Quoting Wirelock (Reply 7):
if you wanted the aircraft to fly BRU AMS all the time you would have a low MTOW so as to keep landing charges as low as possible.

I can't address how it's done everyplace on the planet, but here in the USA, landing fees don't have anything to do with takeoff weights or zero-fuel weights. Landing fees here are predicated on the max structural landing weight of the aircraft times a fee (usually expressed with respect to each 1,000 lbs).

For example, if the aircraft has a max structural landing weight of 150,000 lbs. and the landing fees at the airport are $2.00 per 1,000 lbs, the landing fee is $300. The fee will be the same $300 for every aircraft that has that same max structural landing weight, and it doesn't vary based on the actual load/weight on the aircraft. Whether the aircraft has a butt in every seat, or only has one pax, or even no paxs (like a ferry flight), the landing fee is the same $300 (in this example).
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Increase Of Mtow And Mzfw

Sat Feb 07, 2009 9:00 pm



Quoting OPNLguy (Reply 8):

I can't address how it's done everyplace on the planet, but here in the USA, landing fees don't have anything to do with takeoff weights or zero-fuel weights. Landing fees here are predicated on the max structural landing weight of the aircraft times a fee (usually expressed with respect to each 1,000 lbs).

Yes, but max landing weight should scale with MTOW and MZFW, since MLW has to be larger than MZFW but smaller than or equal to MTOW.

This whole discussion is about the certified limits, not the actuals. You can get exactly the same aircraft with difference max certified weights (including landing).

Tom.
 
OPNLguy
Posts: 11191
Joined: Tue Jun 15, 1999 11:29 am

RE: Increase Of Mtow And Mzfw

Sat Feb 07, 2009 11:04 pm



Quoting Tdscanuck (Reply 9):
Yes, but max landing weight should scale with MTOW and MZFW, since MLW has to be larger than MZFW but smaller than or equal to MTOW.

This whole discussion is about the certified limits, not the actuals. You can get exactly the same aircraft with difference max certified weights (including landing).

Thanks, but the above is completely irrelevant and out of context to the point I was making to the other two folks (them, and only them), i.e. that landing fees have zippo to do with MTOW or MZFW (your scalability or not) and are based on max structural landing weight (per thousand) times whatever the particular airport's rate is.
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Increase Of Mtow And Mzfw

Sun Feb 08, 2009 2:51 am



Quoting OPNLguy (Reply 10):
that landing fees have zippo to do with MTOW or MZFW (your scalability or not) and are based on max structural landing weight (per thousand) times whatever the particular airport's rate is.

Right, but max structural landing weight can be different for different certifications of the same basic aircraft, so that's going to directly affect landing fees.

Tom.
 
OPNLguy
Posts: 11191
Joined: Tue Jun 15, 1999 11:29 am

RE: Increase Of Mtow And Mzfw

Sun Feb 08, 2009 6:24 pm



Quoting Tdscanuck (Reply 11):
Right, but max structural landing weight can be different for different certifications of the same basic aircraft, so that's going to directly affect landing fees.

Let me recap...

In a thread regarding MTOW and MZFW, two posters mention landing fees, as if either MTOW or MZFW had something to do with their determination.

They don't, and I point this out in a message, directed at strictly at the two posters, and not at anyone else, nor having anything to do with the central thread topic of MTOW and MZFW.

Now, irrespective of whether an aircraft's max structural landing weight is Nnn,nnn pounds or whether the structure can actually handle more weight than that, or whether some variants of the same aircraft type might have higher max structural landing weights than others, the point remains that the max landing weight that's in an aircraft's AFM and the one that the airline is obstensibly complying with on a daily operational basis is the figure that's being used for multiplying times the landing fee rate to come up with the total landing fee at an airport. Period.

I can appreciate the fact that (according to your profile) you're in SEA and are involved in flight testing, but you should be able to readily understand the context of my reply to the two other folks who were under the mistaken impression that MTOW or MZFW had anything to do with calculation of landing fees. With all due respect, rather than nitpick my contributions (that were only trying to correct misconceptions), perhaps you could read my posts a little more carefully, and otherwise keep them within the context (and spirit) that they're being offered to others?
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: Increase Of Mtow And Mzfw

Wed Feb 11, 2009 1:29 pm

In my experience, however limited, it's MTOW which generally counts when determining landing fees. There are, however, places where it is based on MLW. I have no idea how the distribution between these two are in various parts of the world.

How about a quick semi-quantitative study. Google on

"landing fee MTOW" (3820 hits at the time of posting)

and "landing fee MLW" (1290 hits at the time of posting)

You're both right.
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
abnormal
Posts: 73
Joined: Tue Aug 21, 2007 12:14 pm

RE: Increase Of Mtow And Mzfw

Thu Feb 12, 2009 5:50 am

I deal with this question very often when the finance guys call up to clarify the basis for what we should be paying at various airports around the world. Outside of the US, the vast majority are based upon what MTOW. MLW based fees are much less common and a few fields simply charge based upon seating configuration or even passenger count.

I'm only guessing here but in the US enough of the fields are not publicly owned and these can charge lower landing fees than the major fields which keeps the avg ldg fees overall at relativelly bargain prices.

Another point to mention is that manufacturers recognize the need for less than the design MTOW so they commonly will market the plane at a lower certified design MTOW. To get the full design MTOW you simply pay the fees to have the AFMs revised and a tire pressure placard installed

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