User avatar
kitplane01
Topic Author
Posts: 1336
Joined: Thu Jun 16, 2016 5:58 am

What sets MTOW of an airliner

Sun Feb 17, 2019 6:46 am

What sets the MTOW of an airliner?

Suppose I want to add additional fuel in the wings (and there is space). I want to increase MTOW, but not landing weight or zero fuel weight. What must I change?

Yes, it's set by what one can get certified, but for modern airliners what is the governing limit in actual cases?

I assume it's either strength of the wing, or climb gradient with one engine out. So, for one engine out at gross weight for reasonable hot/high conditions, are modern airliners operating at the required climb gradient, or do they noticeably exceed it?
 
WIederling
Posts: 8472
Joined: Sun Sep 13, 2015 2:15 pm

Re: What sets MTOW of an airliner

Sun Feb 17, 2019 10:02 am

Gears, load carrying capability, force entry into structure, runway loading
engine thrust take off run 1 engine out, initial climb,
brake limits for take off abort "Max energy",
( this probably can turn into a major whammy: more mass PLUS more speed E=M/2 * V² !! )
wheel speed limit,
wing structure limits, center wingbox should show reduced loads.
Murphy is an optimist
 
747Whale
Posts: 725
Joined: Fri Dec 07, 2018 7:41 pm

Re: What sets MTOW of an airliner

Sun Feb 17, 2019 6:33 pm

The MGTOW, or the maximum gross takeoff weight, is limited by the lesser of the aircraft limitations (maximum certification), runway length (given temperature and pressure altitude), and after takeoff performance. There are several "segments" to takeoff performance, and each holds a minimum climb gradient for certification, but also for the specific departure from the specific runway. The aircraft must be able to meet the climb gradients prescribed for that runway, to ensure obstacle separation. Additionally, the takeoff must be planned such that an engine can be lost on takeoff and the takeoff continued with all the climb criteria met, but also that an engine can be lost on the runway and the aircraft stopped at its takeoff weight. Thus either accelerate-go, or accelerate-stop distances can become limiting, or both combined as a balanced runway length.

You said that you want to increase takeoff weight by adding fuel, but don't want to increase landing weight. If you increase your weight at takeoff, you increase your weight at landing. Thus the arrival weight must be such that given the forecast conditions (wind, temperature, runway surface condition), one can plan to land and stop in 60% of the available landing distance (which also has multiple definitions). If the runway is wet or contaminated, 115% of the planned landing distance must fit within 60% of the destination runway. This means that the destination runway is also limiting for takeoff weight. Also factoring into it are alternate airport requirements, and additional fuel needed, which may ultimately mean reducing less payload, if weight restricted, but in need of additional reserve fuel.

Factoring into all of that is the trip planning, which will look at winds aloft, temperatures aloft, etc, and determine what kind of fuel burn is anticipated; it maybe that with headwinds and above-standard temperatures, a higher fuel burn is required, and if already weight-limited, then to get the necessary fuel, the only other option is to reduce payload.

Typically the weight range for the takeoff can also be influenced by aircraft configuration; greater flap settings may mean becoming airborne sooner, but with reduced second-or third segment climb gradient, which also must be balanced with the needs after departure.

Additional factors will come into play, such as inoperative antiskid or brake units, which will make a longer accelerate-stop distance, and may further limit weight. Likewise a departure with a tailwind will have a similar effect. Elevated temperatures or higher pressure altitudes (and consequent density altitude when combined with temperature) also mean reduced climb gradients, longer acceleration distances, longer stopping distances, and may require reduced weight, or a lower takeoff weight.
 
User avatar
kitplane01
Topic Author
Posts: 1336
Joined: Thu Jun 16, 2016 5:58 am

Re: What sets MTOW of an airliner

Sun Feb 17, 2019 7:53 pm

WIederling wrote:
Gears, load carrying capability, force entry into structure, runway loading
engine thrust take off run 1 engine out, initial climb,
brake limits for take off abort "Max energy",
( this probably can turn into a major whammy: more mass PLUS more speed E=M/2 * V² !! )
wheel speed limit,
wing structure limits, center wingbox should show reduced loads.


I don't think much of that is true.

If I'm not increasing maximum landing weight, then I probably don't need to increase gear strength.

I don't think maximum takeoff weight is a a function of runway loading. I agree that certain weights for certain airplanes at certain airports are pavement limited. That's an operating limitation, not a maximum gross weight limitation.

Wheel speed limits are real. I wonder if that's the actual problem for most aircraft for small gross weight increases.

The brake energy limit is also real. One aborts a takeoff at MTOW, so the brakes need to handle.

If you're adding weight out along the wing, I wonder if that really requires a strengthen wing center box.
 
User avatar
kitplane01
Topic Author
Posts: 1336
Joined: Thu Jun 16, 2016 5:58 am

Re: What sets MTOW of an airliner

Sun Feb 17, 2019 7:58 pm

747Whale wrote:
The MGTOW, or the maximum gross takeoff weight, is limited by the lesser of the aircraft limitations (maximum certification), runway length (given temperature and pressure altitude), and after takeoff performance. There are several "segments" to takeoff performance, and each holds a minimum climb gradient for certification, but also for the specific departure from the specific runway. The aircraft must be able to meet the climb gradients prescribed for that runway, to ensure obstacle separation. Additionally, the takeoff must be planned such that an engine can be lost on takeoff and the takeoff continued with all the climb criteria met, but also that an engine can be lost on the runway and the aircraft stopped at its takeoff weight. Thus either accelerate-go, or accelerate-stop distances can become limiting, or both combined as a balanced runway length.


Sure. But lets assume a really long runway.

Now what's the actual limitation that governs? Is it first segment climb gradient with one engine out? Is it wing structure? For most airplanes, what's the actual limitation that governs, the first limitation that is reached?

747Whale wrote:
You said that you want to increase takeoff weight by adding fuel, but don't want to increase landing weight. If you increase your weight at takeoff, you increase your weight at landing.


I don't think that's true. Instead, I think one might fly further with these larger fuel tanks, and arrive at the same landing weight.
 
mmo
Posts: 1763
Joined: Thu Apr 18, 2013 3:04 pm

Re: What sets MTOW of an airliner

Sun Feb 17, 2019 9:13 pm

kitplane01 wrote:
[
I don't think much of that is true.

If I'm not increasing maximum landing weight, then I probably don't need to increase gear strength.

I don't think maximum takeoff weight is a a function of runway loading. I agree that certain weights for certain airplanes at certain airports are pavement limited. That's an operating limitation, not a maximum gross weight limitation.

Wheel speed limits are real. I wonder if that's the actual problem for most aircraft for small gross weight increases.

The brake energy limit is also real. One aborts a takeoff at MTOW, so the brakes need to handle.

If you're adding weight out along the wing, I wonder if that really requires a strengthen wing center box.


I think you are a little confused about a couple of things. First of all just assume the MTOW is 200 tonnes and you want to add 25 tonnes of fuel. The certification requirements specify you have to able to do an immediate return and landing so, you need to certify the landing gear, tires and other components for the additional 25 tonnes.

The wing structure would most likely need beefing up to handle the 225 tonne landing weight but it could be over engineered but it would have to be load tested.

It is not quite as siple as you think.
If we weren't all crazy we'd all go insane!
 
747Whale
Posts: 725
Joined: Fri Dec 07, 2018 7:41 pm

Re: What sets MTOW of an airliner

Sun Feb 17, 2019 9:36 pm

kitplane01 wrote:
Sure. But lets assume a really long runway.


Then why did you bother to ask the question?

There are numerous factors that govern takeoff weight. I gave them to you. Answered your question.

Ok, now you want to eliminate the runway. In other words, runway length is no longer the limiting factor. Everything else I told you becomes the limiting factor. Again, the most limiting of all the factors I mentioned is what dictates the takeoff weight. If it's not the runway, then it's one of the others. The landing weight maybe one of them, and the landing weight, in turn, is dictated by the factors I provided. Read the reply.

kitplane01 wrote:
Now what's the actual limitation that governs? Is it first segment climb gradient with one engine out? Is it wing structure? For most airplanes, what's the actual limitation that governs, the first limitation that is reached?


The question has been answered for you.

The most limiting factor is what determines takeoff weight. If none of those factors is limiting, then the maximum weight for which the aircraft is certified will be limiting.

First segment climb is not the most restrictive; it lasts until 35' above the runway. Second segment climb is typically the most critical, and begins at 35' through 400'. It may be limiting.

Prior to to departure, a runway analysis must be performed using actual conditions; actual length, surface condition, winds, temperature, gradient, departure obstacles, departure procedure, etc. That is part of the picture, but it continues beyond, all the way to the destination.

kitplane01 wrote:
I don't think that's true. Instead, I think one might fly further with these larger fuel tanks, and arrive at the same landing weight.


No, it's true, else I wouldn't have said it.

Your response, however, makes no sense, whatsoever.

If one is scheduled to depart airport A, and fly to airport B, why would one "fly further and arrive at the same landing weight?"

One isn't flying further. One is flying from A to B. You asked about airline operations. That's what we do. Takeoff at A and go to B. We don't carry excess fuel as a rule, unless tankering fuel is necessary. There's no need for additional fuel beyond the trip fuel, and any additional imposed by regulation, company policy or procedure, and/or captain's discretionary.

Again, the limiting factor for any given flight is exactly what you were told.
 
GalaxyFlyer
Posts: 3352
Joined: Fri Jan 01, 2016 4:44 am

Re: What sets MTOW of an airliner

Mon Feb 18, 2019 4:11 am

The actual limitation is any or all of these, depending on the plane’s design, certification authority and conditions. From a design perspective, landing gear and attachment structure are often limiting. Gear are expensive and heavy, usually not massively over-built, so gross weight increases often require upgrade with testing. Yes, it has to meet criteria at certified MTOW for emergency return.

Then, if new engines, weight and structure considerations. The C-5 re-engining required beefing up the crown and torque deck to accept increased asymmetric loads during OEI operations, in addition to weight of heavier engines and pylons—about 20,000 pounds in the end. At one point, Vmcg couldn’t be held to the TF-39 numbers without one of several solutions-reduced outboard thrust, sympathetic thrust reduction when loss of opposite engine was detected or automatic rudder control to contain the asymmetric thrust. I’m not sure which they choose, but bet they lived with higher Vmcg as it was pretty low anyway.

Vmcg can be a limiting case. The Lear 75 has more thrust which resulted in higher Vmcg. The Vmcg increase made wet runway performance notably worse than original power, it also resulted in no better dry performance unless at near gross weight. Let’s solve the Vmcg problem with more authority out of the rudder, oops, the structure won’t support more stress from rudder loads.

The increase in gross weight for the Challenger 300 to 350 required upgraded gear and testing to prove it met standards.
 
User avatar
kitplane01
Topic Author
Posts: 1336
Joined: Thu Jun 16, 2016 5:58 am

Re: What sets MTOW of an airliner

Mon Feb 18, 2019 5:24 am

mmo wrote:
kitplane01 wrote:
[
I don't think much of that is true.

If I'm not increasing maximum landing weight, then I probably don't need to increase gear strength.

I don't think maximum takeoff weight is a a function of runway loading. I agree that certain weights for certain airplanes at certain airports are pavement limited. That's an operating limitation, not a maximum gross weight limitation.

Wheel speed limits are real. I wonder if that's the actual problem for most aircraft for small gross weight increases.

The brake energy limit is also real. One aborts a takeoff at MTOW, so the brakes need to handle.

If you're adding weight out along the wing, I wonder if that really requires a strengthen wing center box.


I think you are a little confused about a couple of things. First of all just assume the MTOW is 200 tonnes and you want to add 25 tonnes of fuel. The certification requirements specify you have to able to do an immediate return and landing so, you need to certify the landing gear, tires and other components for the additional 25 tonnes.

The wing structure would most likely need beefing up to handle the 225 tonne landing weight but it could be over engineered but it would have to be load tested.

It is not quite as siple as you think.


I am often confused, and things are complex.

But I note that many airplanes have a max take off weight MUCH higher than their max landing weight. For example, the 787 can take off at 560,000 lbs and has a max landing weight of only 445,000. I'm not asking how to raise the max landing weight, just the max takeoff weight.

I do know that there are procedures for landing above the max landing weight. Where does the FAA make requirements for immediate return? I've not seen that rule. Can you point it out to me?
 
User avatar
kitplane01
Topic Author
Posts: 1336
Joined: Thu Jun 16, 2016 5:58 am

Re: What sets MTOW of an airliner

Mon Feb 18, 2019 5:42 am

I think you put thought into your answer. I appreciate that.

747Whale wrote:
kitplane01 wrote:
Sure. But lets assume a really long runway.


Then why did you bother to ask the question?


I think we're miscommunication. I'm not asking "what governs the maximum takeoff weight for a particular runway at a particular temperature and pressure". I'm asking "what governs the maximum certified takeoff weight".

747Whale wrote:

You asked about airline operations. That's what we do.


Actually, I'm trying NOT to ask about airline operations. I'm trying to ask about aircraft certifications.


747Whale wrote:
Again, the limiting factor for any given flight is exactly what you were told.


I believe you. (And I already knew all that.) But that's not what I'm trying to ask.

I'll offer a specific example.

Suppose I wanted to certify the 787-10 to a higher maximum takeoff weight. I have found a place to put more fuel in the wings. I don't want to add cabin load, and I don't want to increase the maximum landing weight. I just want to be able to add more fuel without reducing cabin payload, so I can fly further. I'm OK with increasing runway requirements.

Which, of them many rules, will cause me difficulties? Is it the planes ability to do a second segment climb on one engine? Is it the brakes for an rejected takeoff? Is it wing structure (and why, since the weight is in the wings and not in the fuselage)?

For Part 91 aircraft (which is the aircraft I really know, like Pipers, etc) the problem is probably climb rate with one engine. Pure lack of horsepower with one engine out is what limits maximum gross weight for most Part 91 twins.

But what about transport aircraft? I know all of these things are limitations, but for real airplanes, which of these problems is the limitation I'll likely reach first?
Last edited by kitplane01 on Mon Feb 18, 2019 5:51 am, edited 1 time in total.
 
User avatar
kitplane01
Topic Author
Posts: 1336
Joined: Thu Jun 16, 2016 5:58 am

Re: What sets MTOW of an airliner

Mon Feb 18, 2019 5:49 am

GalaxyFlyer wrote:
The actual limitation is any or all of these, depending on the plane’s design, certification authority and conditions. From a design perspective, landing gear and attachment structure are often limiting. Gear are expensive and heavy, usually not massively over-built, so gross weight increases often require upgrade with testing. Yes, it has to meet criteria at certified MTOW for emergency return.


Can you tell me more about this? For Part 91 aircraft, I know about the drop test, but I thought that was done a maximum landing weight, and not maximum take off weight (though for almost all Part 91 aircraft, they're the same).

What are the requirements for landing at above certified maximum landing weight? And do they typically require more strength than the requirements for maximum landing weight tests?

GalaxyFlyer wrote:
The increase in gross weight for the Challenger 300 to 350 required upgraded gear and testing to prove it met standards.


It's interesting data, but not the situation I'm asking about. It's not just more fuel in the wings. (i.e. keep the maximum zero fuel weight constant, but increase the maximum takeoff weight).

Challenger 300
Maximum takeoff weight: 38,850 lb (17,622 kg)
Maximum landing weight: 33,750 lb (15,309 kg)
Maximum zero fuel weight: 27,000 lb (12,247 kg)

Challenger 350
Maximum Take-Off (MTOW) 40’600 18'416
Maximum Landing (MLW) 34'150 15'490
Maximum Zero Fuel (MZFW) 28'200
 
747Whale
Posts: 725
Joined: Fri Dec 07, 2018 7:41 pm

Re: What sets MTOW of an airliner

Mon Feb 18, 2019 5:56 am

We routinely takeoff at weights considerably above our maximum landing weight. It's for that reason that part of our departure briefing covers the fuel dump time necessary to reach max landing weight, and also the possibility of returning to land sooner than that.

I am familiar with a crew that recently had to make a return to land at a weight and speed higher than anticipated, leading to the failure of all tires. Under the circumstance, it was necessary.

Maximum takeoff weight is usually limited by the flight, operationally.

So far as the actual maximum limitation for the aircraft, without regard to any other factors, the design of the aircraft comes into play, and yes, it is complex. The 747's max ramp weight wasn't far over the max takeoff weight; the difference is accounted in fuel burned in taxi. When fueling and loading, however, the entire aircraft can flex, and if done out of order or improperly, the doors won't close because the aircraft has flexed too much. The limitations on what is loaded and when and where are deep in the design of the specific aircraft, whether keel beams and trunions or supporting structure, wing bending moment, tire limitations and gear limitations (the 747 uses 18 wheels for a reason) or many other factors, one doesn't simply up the fuel and fly further.

In the 747, for example (Classic), we typically filled wing tanks only; the center fuel didn't get filled that often ,and just as well. It's a big, big tank. It holds more weight in fuel than a DC9 fully loaded. the wing tanks take 230,000 lbs of fuel. Most airplanes have fuel balance and imbalance limitations, in part due to control and trim, and in part due to wing bending moment; many aircraft need enough fuel out there to keep the wings from flexing up that it limits what can or can't be put on board, and the weight. Add weight to the fuselage, you're increasing that bending moment again; the weight is really needed out in the wings. When the wings are full, there aren't a lot of other places to put fuel; consequently many airliners use center fuel tanks of varying configurations. These have a similar effect on the wings, however, as loading the fuselage with people and/or cargo.

If the object is to simply load the airplane up with fuel and go, then some can go a long, long time. Most aircraft, however, need to earn enough to pay their keep, which means paying passengers and cargo. This means more revenue payload, and less fuel, or carrying just enough fuel. For every extra 1000 lbs of fuel, about 4% of that fuel will be burned just to carry the fuel. There's a penalty attached to tankering or carrying extra fuel, to say nothing of the fact that heavy airliners can't climb that high, which means that fuel burn will be higher during the intial part of a flight, until the aircraft can burn off enough fuel to step-climb higher to a more efficient altitude. This can also limit the flight and weight.

Pavement bearing capacity is certainly an issue, and that has todo with more than just weight. The number of wheels and their configuration, whether single or in tandem, etc, accounts for how the weight is distributed, and if a tight turn is done on a runway, some aircraft can tear up the runway. When the aircraft is designed with a particular gear configuration, it may be fine up to the existing gross weight, but might require a different landing gear configuration or type of gear to distribute the extra weight.

When the much-maligned MD-11 was developed, it weighed nearly 50% more than the original DC-10. It added a fourth landing gear below the fuselage, between the mains. That gear added a lot more than just weight, but considerable extra complexity in operation, locaiton, structural change, and an armor plate to account for a tire explosion, which could have a direct impact on the center accessory section where all hydraulics and bleeds come together, as well as batteries, etc. Also added was a tail fuel tank, and some had additional fuselage auxiliary tanks, to add to the existing 2. Putting cargo or fuel in the wrong place during loading could cause the airplane to fall on its tail. It also became the airliner that had the fasted approach speeds of any airliner in production, or otherwise, thanks in large part to the weight increase. It wasn't a simple matter of just adding weight, though.

Weight is limited by the gust loads imposed as part of the certification criteria, which comes back to wing flex, bending moment, and other factors. If the wings already have the fuel space taken up with tans, fuselage tanks are the only space remaining, and that affects the strength and load on the wings, especially in the event of vertical gusts, which are a critical part of design criteria.

In actual practice, however, it's nearly always operational factors that limit the takeoff weight.

kitplane01 wrote:
Actually, I'm trying NOT to ask about airline operations. I'm trying to ask about aircraft certifications.


That's exactly what you asked about to start the thread when you asked what sets the maximum takeoff weight of an airliner. Remember?

kitplane01 wrote:
What sets the MTOW of an airliner?


kitplane01 wrote:
Which, of them many rules, will cause me difficulties? Is it the planes ability to do a second segment climb on one engine? Is it the brakes for an rejected takeoff? Is it wing structure (and why, since the weight is in the wings and not in the fuselage)?

For Part 91 aircraft (which is the aircraft I really know, like Pipers, etc) the problem is probably climb rate with one engine. Pure lack of horsepower with one engine out is what limits maximum gross weight for most Part 91 twins.

But what about transport aircraft? I know all of these things are limitations, but for real airplanes, which of these problems is the limitation I'll likely reach first?


That really depends on the aircraft.

I'll give you an example in a small aircraft that I fly. The Air Tractor 802 is a single engine turbine airplane with a gross weight of 12,500 lbs. It's used for hauling fuel, spraying crops, fertilizing and seeding, and fighting fire. I use it on fires. When used on fires, the gross weight is 16,000 lbs. It can't be landed that way, as it's well over its landing weight, but fortunately up to half of that gross weight is rapidly disposable. Consequently, if we launch on a fire but get cancelled, we have to jettison the load before returning to land. In that airplane, the landing gear is replaced every few years, as are wing attach bolts and other components, regardless of whether it's been operated off rough fields, or if its done much flying.

The gross weight increase isn't available to anyone choosing to operate the airplane to those weights; it's applicable only for certain operations. It doesn't mean that someone can go load the airplane like that and spray a field: it's not an increase to the certification of the aircraft, but only for specific operations.

The nature of those operations, however, is that fires occur in the mountain west and it's often hot and high, so carrying the full load out, especially in an area surrounded by rising terrain, isn't always possible. With a full 302 gallon fuel load, the airplane won't get its full 800 gallons of retardant before reaching the 16,000 lb gross weight, and when it gets there, it's definitely performance limited. Go fly one under those circumstances, and you may find yourself going grey a bit prematurely. You'll see what I mean.

Increase the weight on any given aircraft without increasing the available thrust, wing area, and other factors, and you're going to run into performance limitations and many other factors. In the case of the 802, depending on the way it's loaded, there are also speed limitations that apply, and other large aircraft have speed limits for certain fuel loadings.

Again, it's aircraft-specific and there's no one correct answer to your question, which is best addressed to those who designed the particular airplane in question, whatever it may be. They're the ones who were part of a team of a thousand who spent years doing it...for that one airplane.
 
WIederling
Posts: 8472
Joined: Sun Sep 13, 2015 2:15 pm

Re: What sets MTOW of an airliner

Mon Feb 18, 2019 11:32 am

kitplane01 wrote:
I don't think much of that is true.

then checking all but one item.
Thank you.

if you increase MTOW for a plane that already maxes out regularly available pavement loading limits ( 77W, 789/10 ) ...
Murphy is an optimist
 
GalaxyFlyer
Posts: 3352
Joined: Fri Jan 01, 2016 4:44 am

Re: What sets MTOW of an airliner

Mon Feb 18, 2019 3:49 pm

Here’s one of several FAA documents on certification. Engineers spend a lifetime in the field, hard to put that in a forum post. For MTOW, landing velocity is 6fps, at MLW, its 9 FPS, plus a proof margin of, I believe, 150 percent.

https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_25-7D.pdf#page31

For any given day, any of the performance standards can be limiting—second segment climb or obstacle often being the limit, but runway will be if it’s short enough. Second segment is 2.5% for twins, 3.0% for quads. The limit applies even if taking off over water, it’s not obstacle clearance.
 
User avatar
kitplane01
Topic Author
Posts: 1336
Joined: Thu Jun 16, 2016 5:58 am

Re: What sets MTOW of an airliner

Mon Feb 18, 2019 6:03 pm

GalaxyFlyer wrote:
Here’s one of several FAA documents on certification. Engineers spend a lifetime in the field, hard to put that in a forum post. For MTOW, landing velocity is 6fps, at MLW, its 9 FPS, plus a proof margin of, I believe, 150 percent.

https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_25-7D.pdf#page31

For any given day, any of the performance standards can be limiting—second segment climb or obstacle often being the limit, but runway will be if it’s short enough. Second segment is 2.5% for twins, 3.0% for quads. The limit applies even if taking off over water, it’s not obstacle clearance.


Awesome much thanks.

If 2.5% is safe for twins, why is it not also safe for quads? That makes no sense.
 
trijetsonly
Posts: 671
Joined: Tue Jul 21, 2009 10:38 pm

Re: What sets MTOW of an airliner

Mon Feb 18, 2019 6:28 pm

The Maximum Design Takeoff Weight depends on the center of gravity.
It is a function where several forward CG and aft CG limitations meet at the top the the operational envelope.

Forward CG limitations include nose gear strength and tail empennage shear and bending limitations (remember: a forward cg means that a high moment of inertia from the elevator and horizontal stabilizer is necessary).
The function is quite linear: Increasing the weight with the same center of gravity increases the nose gear load and tail shear/bending limitation. That means, that the aircraft center of gravity needs to move aftwards to sustain the maximum loads.

Aft CG limitations include maximum main gear load, maneuverability, tail tipping considerations, stabilizer airflow and several other.
For simplicity let's say that the main gear load is the most limiting.
With increasing weight the aircraft center of gravity needs to move forward to sustain the maximum allowable load.

That is how you get the maximum design takeoff weight, where the forward CG limits and the aft CG limits meet.

As others have noted, that is usually not what you get as an operator as other limits are more restrictive (airport surface loads, tire limits, performance...)

The closest image I could find on google is the following one:
Image
you can nicely see how the forward and aft CG limits get closer together torwards the maximum takeoff weight. There are weight variants of several types where the MRW is basically just a point, not a line.

In case you have access to myBoeingFleet and their training material: They have nice presentations about this and how the MTOW is determined.
Happy Landings
 
747Whale
Posts: 725
Joined: Fri Dec 07, 2018 7:41 pm

Re: What sets MTOW of an airliner

Mon Feb 18, 2019 6:39 pm

kitplane01 wrote:

If 2.5% is safe for twins, why is it not also safe for quads? That makes no sense.


2.4% for two-engine aircraft, 2.7 for 3-engine aircraft, and 3.0 for four-engine aircraft.

With a two-engine aircraft, when one engine is lost, only one remains.

With a three-engine aircraft, when one engine is lost, two remain.

With a four-engine aircraft, when one is lost, three remain.

Think about it.
 
GalaxyFlyer
Posts: 3352
Joined: Fri Jan 01, 2016 4:44 am

Re: What sets MTOW of an airliner

Mon Feb 18, 2019 7:33 pm

Yes, 2.4%, stand corrected. IRS, I can’t remember s**t. It’s not “safety” so much as setting a minimum gradient. As I said, if departing over the 600’ cliff with 3,000 miles of ocean in front of you, you still have to have OEI performance available.
 
GalaxyFlyer
Posts: 3352
Joined: Fri Jan 01, 2016 4:44 am

Re: What sets MTOW of an airliner

Mon Feb 18, 2019 7:43 pm

You might find this interesting.

http://code7700.com/takeoff_climb_gradient.htm

Your question is way too open-ended involving various structures, certification standards (variable depending on authority and amendments the cert was done under); performance, also variable on certification amendment and OpsSpecs, and finally the specific operating conditions. I always had a hard time explaining why their 6000 mile jet wasn’t when leaving Aspen or Sion or Selatar, pick your region

GF
 
User avatar
kitplane01
Topic Author
Posts: 1336
Joined: Thu Jun 16, 2016 5:58 am

Re: What sets MTOW of an airliner

Mon Feb 18, 2019 11:33 pm

747Whale wrote:
kitplane01 wrote:

If 2.5% is safe for twins, why is it not also safe for quads? That makes no sense.


2.4% for two-engine aircraft, 2.7 for 3-engine aircraft, and 3.0 for four-engine aircraft.

With a two-engine aircraft, when one engine is lost, only one remains.

With a three-engine aircraft, when one engine is lost, two remain.

With a four-engine aircraft, when one is lost, three remain.

Think about it.


I understand why a quad has more power after one engine fails. But that's not my question. My question was clearly stated: If 2.4% is safe for twins, why is it not also safe for quads? Think about it.
 
747Whale
Posts: 725
Joined: Fri Dec 07, 2018 7:41 pm

Re: What sets MTOW of an airliner

Mon Feb 18, 2019 11:55 pm

kitplane01 wrote:

I understand why a quad has more power after one engine fails. But that's not my question. My question was clearly stated: If 2.4% is safe for twins, why is it not also safe for quads? Think about it.


Your attitude is wearing a bit thin.

Why don't you read about it for yourself and then you can figure it out.

https://www.ecfr.gov/cgi-bin/text-idx?S ... 1&rgn=div8

Because aircraft with additional engines and performance are capable, higher standards are set. It's regulatory.

Read the regulation. I don't really care whether you choose to think about it or not.

Second segment climb:

(b) Takeoff; landing gear retracted. In the takeoff configuration existing at the point of the flight path at which the landing gear is fully retracted, and in the configuration used in §25.111 but without ground effect:

(1) The steady gradient of climb may not be less than 2.4 percent for two-engine airplanes, 2.7 percent for three-engine airplanes, and 3.0 percent for four-engine airplanes, at V2 with:

(i) The critical engine inoperative, the remaining engines at the takeoff power or thrust available at the time the landing gear is fully retracted, determined under §25.111, unless there is a more critical power operating condition existing later along the flight path but before the point where the airplane reaches a height of 400 feet above the takeoff surface; and

(ii) The weight equal to the weight existing when the airplane's landing gear is fully retracted, determined under §25.111.
 
GalaxyFlyer
Posts: 3352
Joined: Fri Jan 01, 2016 4:44 am

Re: What sets MTOW of an airliner

Tue Feb 19, 2019 12:58 am

It’s NOT a safety issue, it’s purely a certification standard. As tris and quads have more remaining power in the OEI case, the regulators writing FAR 25 raised the minimum performance standard.

GF
 
User avatar
kitplane01
Topic Author
Posts: 1336
Joined: Thu Jun 16, 2016 5:58 am

Re: What sets MTOW of an airliner

Tue Feb 19, 2019 3:17 am

747Whale wrote:
kitplane01 wrote:

I understand why a quad has more power after one engine fails. But that's not my question. My question was clearly stated: If 2.4% is safe for twins, why is it not also safe for quads? Think about it.


Your attitude is wearing a bit thin.

747Whale wrote:
Why don't you read about it for yourself and then you can figure it out.

https://www.ecfr.gov/cgi-bin/text-idx?S ... 1&rgn=div8


Because aircraft with additional engines and performance are capable, higher standards are set. It's regulatory.

Read the regulation. I don't really care whether you choose to think about it or not.



Because the thing you posted doesn't answer the question. You keep answering a different question than the one I'm clearly asking. That's wearing a bit thin.

Your link says *that* a quad is expected to have higher performance. It doesn't say *why*. If 2.4% climb gradient is safe for a twin, why is the same climb gradient deemed unsafe for a quad?
 
User avatar
kitplane01
Topic Author
Posts: 1336
Joined: Thu Jun 16, 2016 5:58 am

Re: What sets MTOW of an airliner

Tue Feb 19, 2019 3:20 am

GalaxyFlyer wrote:
It’s NOT a safety issue, it’s purely a certification standard. As tris and quads have more remaining power in the OEI case, the regulators writing FAR 25 raised the minimum performance standard.

GF


I honestly don't understand that. I thought the point of the one-engine-out climb gradient *was* safety. If not, what is the point? And if so, why is a 2.4% climb gradient for twin safe, and for a quad unsafe?

Of course I understand a quad can reach the requirement more easily, but I thought the point was to discover the minimum safe standard, and enforce that.
 
747Whale
Posts: 725
Joined: Fri Dec 07, 2018 7:41 pm

Re: What sets MTOW of an airliner

Tue Feb 19, 2019 3:59 am

Think of the concept that where much is given, much is required.

Those are minimum gradients to meet for certification purposes. Nothing prevents a design from doing better.

Consider the required climb gradient for a single engine airplane, following an engine failure.
 
GalaxyFlyer
Posts: 3352
Joined: Fri Jan 01, 2016 4:44 am

Re: What sets MTOW of an airliner

Tue Feb 19, 2019 4:10 am

Remember, the point to provide an acceptable performance margin in the event of engine failure. If you have three operating engines, you should expect more than if only one is left operating.

GF
 
User avatar
kitplane01
Topic Author
Posts: 1336
Joined: Thu Jun 16, 2016 5:58 am

Re: What sets MTOW of an airliner

Tue Feb 19, 2019 5:15 am

GalaxyFlyer wrote:
Remember, the point to provide an acceptable performance margin in the event of engine failure. If you have three operating engines, you should expect more than if only one is left operating.

GF


I might *expect* it, but I would not *require* it.

If "Remember, the point to provide an acceptable performance margin in the event of engine failure" I would decide what an acceptable performance margin is, and require all transport planes to meet that margin. I would not impose a different limit on different planes.

I would figure out what the minimum safe climb gradient for a transport aircraft is, and require all such planes to meet that limit. I would not impose some other limit on a subset of planes. All transport planes would have the same requirement, regardless of engine number.

If 2.4% is good enough for a 777, it should be good enough for an A340. I would understand if an A340 greatly exceeded the limit, but would require both the 777 and the A340 to meet the same standard.

(I'm assuming we're talking large airliners here. Part 91 Piper Cubs have different rules, including a maximum stall speed, to handle engine out.)
 
Woodreau
Posts: 1776
Joined: Mon Sep 24, 2001 6:44 am

Re: What sets MTOW of an airliner

Tue Feb 19, 2019 5:44 am

kitplane01 wrote:

I might *expect* it, but I would not *require* it.


The FAA requires a 4-engine aircraft to maintain a minimum of 3.0% gradient for OEI at the most unfavorable condition for certification under part 25.

If a manufacturer building a new 4-engine aircraft can only demonstrate a 2.4% gradient with OEI, then the aircraft won't get certified under Part 25 or it can choose to certify the aircraft under Part 23, where there is no required climb gradient, only that a number be published (it can be a negative climb gradient, like your Part 91 Piper piston twins.)

When you become the FAA administrator, you can change the Part 25 certification rules to make it 2.4% for all aircraft, 2-engine, 3-engine, 4-engine.
Last edited by Woodreau on Tue Feb 19, 2019 6:05 am, edited 1 time in total.
Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.
 
SAAFNAV
Posts: 567
Joined: Wed Mar 17, 2010 5:41 pm

Re: What sets MTOW of an airliner

Tue Feb 19, 2019 6:01 am

kitplane01 wrote:
(I'm assuming we're talking large airliners here. Part 91 Piper Cubs have different rules, including a maximum stall speed, to handle engine out.)


You keep on confusing Part 91/121/135 (Operating Rules) with Part 23 (Normal Category Airplanes) and Part 25 (Transport Category Airplanes) certification aspects.

You could fly a B777 as a private business jet under Part 91, and still have to comply to the 2.5% climb gradient. Similarly, if you fly sight-seeing charters in your Cub in a commercial operation, you would do it under Part 135. The plane itself will still be under Part 23 regulations.
L-382 Loadmaster; ex C-130B Navigator
 
747Whale
Posts: 725
Joined: Fri Dec 07, 2018 7:41 pm

Re: What sets MTOW of an airliner

Tue Feb 19, 2019 6:48 am

A multi engine airplane under Part 23 doesn't have to have a positive climb gradient with an engine-out; as a result there are light twins that drift down following an engine failure, but that have dismal single engine performance; it's also the reason why we usually teach light twin pilots not to attempt a go around on one engine.

Transport category airplanes are another matter. They are required to meet minimum standards. It's not just about what's safe. One could argue that anything that meets diverse climb criteria (200'/nm) qualifies. Part 25 doesn't set that as the standard. Single engine aircraft aren't certificated under Part 25, but even if they were, a single engine airplane could only have the glide as the "climb gradient" after an engine failure. Clearly it's a non-starter. Nobody could ask it to do more, because it's not capable.

With 2.4% set as the minimum climb gradient for a two-engine airplane, it's a minimum standard that the airplane must meet, as previously given, to get Part 25 certification (or in other words, to earn a type certificate under the regulation). If you want to build it and sell it under the regulation, that's the minimum second segment climb standard.

If you want to build a three engine transport, then there's a different standard. Three engine aircraft still have two engines after engine failure. A two engine airplane is down to one, and doesn't lose half it's performance, but 70-80%, as much of the performance of the second engine is lost addressing sideslip and associated drag, the drag of the windmilling engine, rudder displacement and spoiler and aileron displacement, etc. Lose one engine, you don't lose half the climb performance. You lose a lot more...depending on the airplane. Lose the first engine on a three engine airplane, you have less of a performance loss.

A three engine aircraft isn't purchased for looks: it's purchased for performance. Additional engines don't really add much in terms of speed, but they do increase performance and capability. In fact, the formula for climb performance is excess power above that requried to maintain level flight for a given speed and configuration and trim. Excess power comes with additional engines; it's why they're present on many designs, and certainly as engines are lost on a 3 engine airplane, and then on a 4 engine airplane, a lesser and lesser percentage of total thrust is lost; the aircraft can be expected to do more, and for certification, it is required to do more.

In the theoretical single engine airplane, no climb performance may be expected. In the twin engine airplane, a minimal 2.4% gradient is required to gain certification. In the three engine airplane, which loses a lower percentage of total thrust and which offers greater capability, performance, redundancy, and remaining thrust after an engine failure, a higher value than the theoretical single engine airplane, and a higher value than the twin engine airplane, is required. In the case of the four engine airplane, which loses a smaller percentage of it's total thrust than the three, two, or one engine airplane, meets a slightly higher second segment target requirement for Part 25 certification, which is 3%.

Not really a hard concept to grasp.

Again it's the standard required of Part 25. It's a higher standard with considerably more stringent requirements than Part 23.

By comparison, one could say that a private pilot has minimum performance standards set forth, and that because this minimum standard exists, then there's no need for any different standard for a commercial pilot certificate holder, or an ATP. This is not true, however. The commercial pilot is held to a higher standard than the private pilot, and the ATP is held to a higher standard than the commercial pilot. In fact, at the ATP level, there's even a moral requirement codified in the regulation that's not found attached to any lesser level of certification. Clearly, more is expected. The greater the qualification, the more that is expected.

A Part 25 airplane has higher standards of performance and certification than a Part 23 airplane. Within Part 25, with regard to second segment climb performance, Four engine airplane is held to a higher standard than a three engine airplane, which is held to a higher standard than a two engine airplane. This really need not be confusing or surprising. The original poster continues to say it should not be this way, that he wouldn't assign such a requirement, but then the original poster doesn't have that option, didn't write the regulation, doesn't administer the regulation, doesn't operate Part 25 aircraft, or participate in their certification, design, creation, construction, or development. Otherwise he'd know all of this and wouldn't be asking. What the original poster thinks doesn't really matter because the simple answer as to the performance standard is that it's federal regulation and the existing standard minimum.

There is nothing that prevents an aircraft from operating with greater performance than the minimum.

When I did a checkout in the Sabreliner NA-265-60 (Sabre 60), I was required to fly a single engine ILS and then fly a missed approach. As I pushed the power up and began climbing out on the missed approach, my rate of climb was in excess of 3,000 fpm. I retarded the power as I didn't have very far to climb to reach the missed approach altitude, which required a level off, flight to a VOR, and a hold entry. The examiner yelped and asked why I was pulling the power back on the good engine. I felt had far more performance than I needed, and I didn't want to overspeed or bust an altitude. No need at the time for all that extra thrust. The airplane could clearly do far more than the minimum.

Conversely, if you ever get the chance to depart in a transport category airplane at gross weight, it can be a dog. The heaviest I've flown is the 747 at 830,000 lbs MGTOW. I've flown that airplane out of some fairly mountainous areas, including places like Afghanistan where very big barriers of dirt and rock are just about everywhere. I'll take all the performance I can get, thanks and I will take every bit of climb gradient that's available. The airplane was certified to meet the minimum standard with smaller engines, lower thrust ratings, and later versions of the Classic, having graduated from the -7A engines to the -7R and Q motors, had more capability, which was a very good thing.

If you want things to get interesting, wait until the second engine fails. In that event, the twin is in the same boat as a single engine airpalne with an engine failure, except that it's a whole lot bigger and heavier and faster and won't fare nearly as well when it goes down. The three engine airplane becomes a single engine airplane, and the four engine airplane becomes a twin, and it's in that scenario, seen in training, where things start getting "real."

Bear in mind that as engines start dropping offline in large airplanes, various systems are also lost; electrical sources, hydraulic pumps, bleed air. This also further complicates operation, as some flight controls or control capability is lost, fuel may end up stuck somewhere or a fuel imbalance may occur, and crossfeeding becomes necessary. Anti-ice may be unavailable for one wing or some surfaces, and alternate procedures may be needed for landing gear, flaps, brakes, steering, etc. There's a great deal more happening after an engine failure in a transport category aircraft, and many more implications and complexities, than that super cub or 172.

I've had nearly two dozen single engine failures over the years, which fortunately, have been largely non-events, from returns to runways to landing on a mountainside (in a forest fire). I've had more failures in multi engine airplanes, and while those did have their interesting moments, most were also non-events and none resulted in off field or forced landings. Each did involve more procedure, more checklist, and more consideration and each takeoff had been preceded with a briefing and a plan on what we would do if an engine failed. The plan for singles was a lot more simple and straightforward, and every student pilot practices it and plans for it, too.

It should come as no surprise, then, that as complexity and performance becomes greater, especially as the number of engines increase and performance potential alongside (and redundancy and percentage of retained thrust or power, the minimum standards also increase.
 
WIederling
Posts: 8472
Joined: Sun Sep 13, 2015 2:15 pm

Re: What sets MTOW of an airliner

Tue Feb 19, 2019 9:34 am

kitplane01 wrote:
If 2.5% is safe for twins, why is it not also safe for quads? That makes no sense.


IMU that was a bit of tilting the table to advantage TWINs when it was opportune ?
Murphy is an optimist
 
WIederling
Posts: 8472
Joined: Sun Sep 13, 2015 2:15 pm

Re: What sets MTOW of an airliner

Tue Feb 19, 2019 9:55 am

747Whale wrote:
When I did a checkout in the Sabreliner NA-265-60 (Sabre 60), I was required to fly a single engine ILS and then fly a missed approach. As I pushed the power up and began climbing out on the missed approach, my rate of climb was in excess of 3,000 fpm.


Remember the weight? Cert requirements are for MTOW ( Here 8t vs empty: 4t )
Murphy is an optimist
 
747Whale
Posts: 725
Joined: Fri Dec 07, 2018 7:41 pm

Re: What sets MTOW of an airliner

Tue Feb 19, 2019 10:45 am

True, but in that airplane even at MGTOW, there's an abundance of thrust.
 
User avatar
kitplane01
Topic Author
Posts: 1336
Joined: Thu Jun 16, 2016 5:58 am

Re: What sets MTOW of an airliner

Tue Feb 19, 2019 11:04 pm

My question is “if a 2.4% climb gradient is safe for a single engine out twin, why is that same climb gradient unsafe for a quad with one engine out?”

Could you please finish this sentence: A 2.4% client gradient is safe for a single engine out twin, but unsafe for a single engine out quad because _______

Actually, let’s just move on. I am frustrated at our mutual inability to communicate, and my guess is you are too.

Peace
 
User avatar
AirKevin
Posts: 483
Joined: Wed Apr 26, 2017 2:18 am

Re: What sets MTOW of an airliner

Wed Feb 20, 2019 12:15 am

kitplane01 wrote:
My question is “if a 2.4% climb gradient is safe for a single engine out twin, why is that same climb gradient unsafe for a quad with one engine out?”

Could you please finish this sentence: A 2.4% client gradient is safe for a single engine out twin, but unsafe for a single engine out quad because _______

Actually, let’s just move on. I am frustrated at our mutual inability to communicate, and my guess is you are too.

Maybe I missed it, but I don't think anybody said it was unsafe as much as it is it's a certification requirement for the aircraft. I guess that's a question you'd have to ask the FAA as to why they made that a rule for aircraft certification in the first place.
Captain Kevin
 
GalaxyFlyer
Posts: 3352
Joined: Fri Jan 01, 2016 4:44 am

Re: What sets MTOW of an airliner

Wed Feb 20, 2019 12:52 am

It’s NOT a safety issue, it’s a CERTIFICATION requirement, PERIOD.

GF
 
GalaxyFlyer
Posts: 3352
Joined: Fri Jan 01, 2016 4:44 am

Re: What sets MTOW of an airliner

Wed Feb 20, 2019 1:47 am

I suppose you could at it this way, if a twin loses a second engine, it crashes. If the quad loses a second engine that extra climb performance translates into some time to land. By way of saying a twin loses an engine, it has 50% loss of thrust and probably about only 20% excess thrust over that required to maintain flight at V2. A quad loses only 25% of its total thrust but retains likely 33% of its excess thrust over that required for level flight at V2. It should have a higher gradient.

GF
 
RJMAZ
Posts: 1520
Joined: Sat Jul 09, 2016 2:54 am

Re: What sets MTOW of an airliner

Wed Feb 20, 2019 11:37 am

kitplane01 wrote:
I'll offer a specific example.

Suppose I wanted to certify the 787-10 to a higher maximum takeoff weight. I have found a place to put more fuel in the wings. I don't want to add cabin load, and I don't want to increase the maximum landing weight. I just want to be able to add more fuel without reducing cabin payload, so I can fly further.

In this case most of the limitation will be the initial climb rate. This can be solved simply by a thrust bump.

Another problem would be that the takeoff speed would be higher and this requires tyres that are rated for a higher speed. Again this only a small issue.

Of course runway length would increase significantly. The pavement loading would also increase but all this will do is limit the aircraft to fewer airports.

The wing would see higher loads so it will probably need a bit of strengthening. The engines at a higher thrust rating will be putting more loads on the mount. Not a game changet. Plenty of little things.

The 787-10 would never take off with a full tank of fuel as it can only carry 20T of payload with a full tank. So Boeing would not have to find extra space in the wings they can simply put more fuel in the tanks.
 
B777LRF
Posts: 2423
Joined: Sun Nov 02, 2008 4:23 am

Re: What sets MTOW of an airliner

Wed Feb 20, 2019 12:12 pm

In extremely simplistic terms the limits are governed as follows:

MTOW: Engine power
MLW: Landing gear
MZFW: Wing bending
Signature. You just read one.
 
User avatar
kitplane01
Topic Author
Posts: 1336
Joined: Thu Jun 16, 2016 5:58 am

Re: What sets MTOW of an airliner

Wed Feb 20, 2019 9:50 pm

trijetsonly wrote:
The closest image I could find on google is the following one:
Image


Well that’s the most interesting picture I’ve seen today.

I noticed there are two limits for the aft CG situations. One for takeoff and the other four flights/landing. How does that work for a go around, especially a go around that occurs after you touch the actual runway?

It seems you could have a situation where you were within limits for a go around, and out of limits for national take off
 
User avatar
kitplane01
Topic Author
Posts: 1336
Joined: Thu Jun 16, 2016 5:58 am

Re: What sets MTOW of an airliner

Wed Feb 20, 2019 9:52 pm

AirKevin wrote:
kitplane01 wrote:
My question is “if a 2.4% climb gradient is safe for a single engine out twin, why is that same climb gradient unsafe for a quad with one engine out?”

Could you please finish this sentence: A 2.4% client gradient is safe for a single engine out twin, but unsafe for a single engine out quad because _______

Actually, let’s just move on. I am frustrated at our mutual inability to communicate, and my guess is you are too.

Maybe I missed it, but I don't think anybody said it was unsafe as much as it is it's a certification requirement for the aircraft. I guess that's a question you'd have to ask the FAA as to why they made that a rule for aircraft certification in the first place.


And I thought I was done with this :-)

OK, if it’s not a certification requirement related to safety, why is it a certification requirement? I will agree that I have the assumption that certification requirements are enforcing safety rules. If this is not about safety, what’s it about?
 
trijetsonly
Posts: 671
Joined: Tue Jul 21, 2009 10:38 pm

Re: What sets MTOW of an airliner

Wed Feb 20, 2019 10:19 pm

kitplane01 wrote:
Well that’s the most interesting picture I’ve seen today.

I noticed there are two limits for the aft CG situations. One for takeoff and the other four flights/landing. How does that work for a go around, especially a go around that occurs after you touch the actual runway?

It seems you could have a situation where you were within limits for a go around, and out of limits for national take off


The background is the tip up moment when setting takeoff thrust and a reduced steerabilty.
With the thrust vector causing a tip up moment and the acceleration causing a tip up moment it is possible that the remaining load on the nose gear is not high enough to keep the track or steer when the speed is still to low for aerodynamic steering.
During go arounds that should be no problem, as there is aerodynamic steerability.
The only problem might occur when the go around is initiated after so much deceleration that the rudder has no effect. But I can't think of that situation in commercial aviation.

On Boeing widebodys (747, 777 and 787 at least) there is an additional Aft limit for takeoff thrust setting while beeing static on the runway. Aft of that limit only rolling takeoffs are allowed with gradual power setting.
Happy Landings
 
flipdewaf
Posts: 2784
Joined: Thu Jul 20, 2006 6:28 am

Re: What sets MTOW of an airliner

Wed Feb 20, 2019 10:26 pm

There is lots of detail going on in this thread and its probably worth noting that in an ideal world there are lots of things limiting the MTOW but really you want them all to happen at the same time to mean you haven't over engineered it too greatly, if there was one thing that limited it then you'd better believe that someone is going to be after you to remove that bottleneck or that the other things should be "toned down" so as to be a better match.

Fred
Image
 
trijetsonly
Posts: 671
Joined: Tue Jul 21, 2009 10:38 pm

Re: What sets MTOW of an airliner

Wed Feb 20, 2019 10:54 pm

flipdewaf wrote:
There is lots of detail going on in this thread and its probably worth noting that in an ideal world there are lots of things limiting the MTOW but really you want them all to happen at the same time to mean you haven't over engineered it too greatly, if there was one thing that limited it then you'd better believe that someone is going to be after you to remove that bottleneck or that the other things should be "toned down" so as to be a better match.

Fred


That must be one of the longest single sentences that i've ever read :o
Happy Landings
 
flipdewaf
Posts: 2784
Joined: Thu Jul 20, 2006 6:28 am

Re: What sets MTOW of an airliner

Wed Feb 20, 2019 10:58 pm

trijetsonly wrote:
flipdewaf wrote:
There is lots of detail going on in this thread and its probably worth noting that in an ideal world there are lots of things limiting the MTOW but really you want them all to happen at the same time to mean you haven't over engineered it too greatly, if there was one thing that limited it then you'd better believe that someone is going to be after you to remove that bottleneck or that the other things should be "toned down" so as to be a better match.

Fred


That must be one of the longest single sentences that i've ever read :o
I have big lungs and I'm a nightmare on a conference call!
Image
 
stratclub
Posts: 1286
Joined: Fri Jan 05, 2018 10:38 pm

Re: What sets MTOW of an airliner

Thu Feb 21, 2019 6:33 am

trijetsonly wrote:
flipdewaf wrote:
There is lots of detail going on in this thread and its probably worth noting that in an ideal world there are lots of things limiting the MTOW but really you want them all to happen at the same time to mean you haven't over engineered it too greatly, if there was one thing that limited it then you'd better believe that someone is going to be after you to remove that bottleneck or that the other things should be "toned down" so as to be a better match.

Fred


That must be one of the longest single sentences that i've ever read :o

I have seen car forum single sentence posts that were 4 or more times as long of rambling with a single period for punctuation. At least Flip's post was somewhat coherent and I thought not excessively long for the thought I think he was trying to make. :o :o
 
User avatar
AirKevin
Posts: 483
Joined: Wed Apr 26, 2017 2:18 am

Re: What sets MTOW of an airliner

Thu Feb 21, 2019 6:48 am

kitplane01 wrote:
OK, if it’s not a certification requirement related to safety, why is it a certification requirement? I will agree that I have the assumption that certification requirements are enforcing safety rules. If this is not about safety, what’s it about?

That's probably something only the FAA would know. Think most of us wouldn't know why it's a requirement, only that it is one.
Captain Kevin
 
stratclub
Posts: 1286
Joined: Fri Jan 05, 2018 10:38 pm

Re: What sets MTOW of an airliner

Thu Feb 21, 2019 7:21 am

kitplane01 wrote:
AirKevin wrote:
kitplane01 wrote:
My question is “if a 2.4% climb gradient is safe for a single engine out twin, why is that same climb gradient unsafe for a quad with one engine out?”

Could you please finish this sentence: A 2.4% client gradient is safe for a single engine out twin, but unsafe for a single engine out quad because _______

Actually, let’s just move on. I am frustrated at our mutual inability to communicate, and my guess is you are too.

Maybe I missed it, but I don't think anybody said it was unsafe as much as it is it's a certification requirement for the aircraft. I guess that's a question you'd have to ask the FAA as to why they made that a rule for aircraft certification in the first place.


And I thought I was done with this :-)

OK, if it’s not a certification requirement related to safety, why is it a certification requirement? I will agree that I have the assumption that certification requirements are enforcing safety rules. If this is not about safety, what’s it about?

You could try reading the appropriate CFAR's. The answer you seek is there if you look hard enough. The Net result of certification requirements is safety. That doesn't mean that everything ever written by the FAA has to be directly proven to be linked to safety. Some requirements might even be written in a way to enhance enforceability and clarity of the requirement.

Report back after you do some FAR reading and can answer your question.......................
 
Harry765
Posts: 18
Joined: Mon Jan 28, 2019 7:11 am

Re: What sets MTOW of an airliner

Thu Feb 21, 2019 9:29 am

The maximum takeoff weight (MTOW) or maximum gross takeoff weight (MGTOW) or maximum takeoff mass (MTOM) of an aircraft is the maximum weight at which the pilot is allowed to attempt to take off, due to structural or other limits. The analogous term for rockets is gross lift-off mass, or GLOW.
 
WIederling
Posts: 8472
Joined: Sun Sep 13, 2015 2:15 pm

Re: What sets MTOW of an airliner

Thu Feb 21, 2019 10:13 am

Harry765 wrote:
The maximum takeoff weight (MTOW) or maximum gross takeoff weight (MGTOW) or maximum takeoff mass (MTOM) of an aircraft is the maximum weight at which the pilot is allowed to attempt to take off, due to structural or other limits. The analogous term for rockets is gross lift-off mass, or GLOW.


Which as a fun detraction need not be the maximum weight for flying.
( cue military planes being tankered after takeoff beyond MTOW.
or in the civil domain the piggy back Short Mayo Composite arrangement. )
Murphy is an optimist

Who is online

Users browsing this forum: agentskelly, GalaxyFlyer and 17 guests

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos