c5load
Topic Author
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How Is Mtow Determined?

Sun Jun 13, 2010 2:04 am

Will an airplane not takeoff if it is beyond MTOW at all? How is it determined what the MTOW is on an airplane? I'm not even going to guess on a formula for it, but is there one?
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ajd1992
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RE: How Is Mtow Determined?

Sun Jun 13, 2010 11:01 am

It will take off, but you risk exceeding max tyre speed (more lift needed down to higher weight, and you need to go faster on the ground to get the needed lift), you'll need a lot more runway, and even if it did take off, it'd fly like a pig with lead weights inside it.
 
FredT
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RE: How Is Mtow Determined?

Mon Jun 14, 2010 3:54 pm

Maximum design take off weight is typically a structural limitation. In other words, above (design) MTOW you'll overstress components. Theoretically it could be a performance limitation, i e the maximum weight at which the aircraft can satisfy the performance requirements, but then the airframe would be overdesigned.

MTOW can also be a strictly legal limit, i e the aircraft itself is capable of being certified to a higher MTOW (the design MTOW is higher) but you have to change the paperwork to legally take off at the higher weight. This can be desirable when fees are based on MTOW. The paper raising your MTOW can have a pretty hefty price per kg... and may also come with changed maintenance requirements.

In yet other cases, you can exchange crucial components to raise the MTOW.
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N353SK
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RE: How Is Mtow Determined?

Mon Jun 14, 2010 4:09 pm

Quoting FredT (Reply 2):
MTOW can also be a strictly legal limit

For example, the first few DC-9s originally had an MTOW of 80,000 lbs. because at the time a higher MTOW would have required a third crewmember.

Also, the Beechcraft King Air 200 has an MTOW of 12,500 because that is the maximum MTOW allowable without requiring a type rating. The military version of the same plane has an MTOW aroud 14,000.
 
reins485
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RE: How Is Mtow Determined?

Tue Jun 15, 2010 3:56 am

Quoting FredT (Reply 2):
MTOW can also be a strictly legal limit

I have heard a story that AA had a lower MTOW on the 767, when they first got them. They later determined that they could make more money by raising the MTOW and carrying more cargo with the revenue from the extra cargo being higher than the cost of the additional maintenance checks.
 
thegeek
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RE: How Is Mtow Determined?

Tue Jun 15, 2010 4:36 am

Putting it another way, they think of a number and then check a list of parameters to see if that number is acceptable. Or maybe I'm just cynical. I would have thought it would be the other way round: they go through the list of critical parameters to find which is the most limiting, and then make the smallest MTOW based on those parameters the actual MTOW.

A few things which can limit MTOW:
Engine out climb versus drag
Tyre speed versus lift at sea level
Elevator authority
Vertical stabiliser effectiveness
Braking capability

And that's about all I can think of off the top of my head. Probably I have missed some. All other things such as runway length and obstacles are only limitations which apply at given fields.
 
vikkyvik
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RE: How Is Mtow Determined?

Tue Jun 15, 2010 10:53 pm

Quoting thegeek (Reply 5):
Putting it another way, they think of a number and then check a list of parameters to see if that number is acceptable. Or maybe I'm just cynical

Nah, not cynical - it's an iterative process in airplane design.

One of my professors probably put it best:

"You don't know what the airplane weighs until you know what it weighs."

Simply put, you have a given design mission that an airplane should fulfill (say, to carry a certain amount of payload a certain range). So that's your starting point for design. From there, you have to define everything else about the aircraft in order to maximize efficiency on your design mission (and perhaps take into account other important factors, like performance from high elevation airfields or airline requests or whatever). Eventually (hopefully) you'll arrive at a refined, finessed design that accomplishes your design mission at maximum efficiency (and, of course, that someone wants to buy!)

At every stage of the iteration, you'll have an MTOW. TOW in general is OEW + Payload + Fuel. Now obviously, fuel depends on TOW. So if you change your TOW, that changes your fuel, which again changes your TOW, which again changes your fuel, etc., etc. until the numbers converge.

If you have a hard and fast design mission, then your MTOW will likely be the minimum possible weight to achieve that mission.
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474218
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RE: How Is Mtow Determined?

Tue Jun 15, 2010 11:37 pm

When designing a new airliner the first thing you do is determine the MTOW. You then design the wing to provide the lift, the engines to provide the thrust, the landing gear to support the weight, etc.
 
tdscanuck
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RE: How Is Mtow Determined?

Wed Jun 16, 2010 5:19 am

Quoting c5load (Thread starter):
Will an airplane not takeoff if it is beyond MTOW at all?

It absolutely will. Testing beyond MTOW is part of the certification process.

Quoting c5load (Thread starter):
How is it determined what the MTOW is on an airplane?

Unless you've screwed up your design, it's whatever weight will take some piece of primary structure past failure load at the worst-case load case (likely maximum positive g), divided by 1.5.

Quoting c5load (Thread starter):
I'm not even going to guess on a formula for it, but is there one?

There is (see above), but the trick is in figuring out where the critical primary structure is. The formulas for that are relatively simple, actually applying them to something as complex as an airplane is really hard.

Quoting thegeek (Reply 5):
A few things which can limit MTOW:
Engine out climb versus drag
Tyre speed versus lift at sea level
Elevator authority
Vertical stabiliser effectiveness
Braking capability

Those should only be TOW limits. If any of those are limiting MTOW, then you've screwed up your structural design and your aircraft is too heavy.

Tom.
 
thegeek
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RE: How Is Mtow Determined?

Wed Jun 16, 2010 5:54 am

Quoting tdscanuck (Reply 8):
Those should only be TOW limits. If any of those are limiting MTOW, then you've screwed up your structural design and your aircraft is too heavy.

Ok, I think I see what you are saying here.

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