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PHXWRLD
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Aviation Terms

Fri Apr 03, 2020 10:42 pm

I asked a question about the A220 range, and the responses were great, but I found myself not understanding a lot of the terms. Can somebody explain what MTOW, OEW, etc. mean? I know what they stand for (Max Take Off Weight), but I don't understand how they can be increased/decreased and how they impact and aircraft's range. Thanks in advance for the responses!
 
Wingtips56
Posts: 1252
Joined: Thu Dec 16, 2010 1:26 am

Re: Aviation Terms

Fri Apr 03, 2020 11:39 pm

Those involved in flight operations will trump whatever I say, but from my background as an airport ops agent, these terms came into play at my level.
MTOW/MGTOW (maximum gross takeoff weight) has a fixed limit according to the aircraft certification. That's the allowed grand total of the aircraft, crew and load in perfect conditions. However this can be reduced on a flight by flight basis by several factors; temperature on the ground, for instance. On a hot day in PHX, the MTOW is greatly reduced because the air is thinner. So can be departures from high elevation airports, where the air is thinner and lift reduced, or winds affecting take off or landing direction when staring into a mountain range. Fuel weight is also affected by temperature; which I remember from high school physics ages ago but don't remember the details.
OEW is Operating Empty Weight; it can vary by what each airline calls it, but it's usually means the measured weight of the aircraft and equipment plus minimum operating crew weight (zero fuel but essential oil and other fluids). The load limit is the difference between these weights, and that is fuel, passengers, bags/cargo and additional crew combined. Pax/bag/cargo/mail load can take a hit when more fuel is required for headwinds, routing (around weather) and additional hold/diversion fuel based on conditions both at the destination airport and occasionally the departure airport (e.g. ability to take off in fog with less visibility than is required to land), should the flight need to return/engine out, etc., as well as the Maximum Landing Weight, which is generally less than the take off weight.

Weight Restrictions affecting the load are employed when necessary, and that means having to leave empty seats, bumping bags and cargo. Non-revs are the first bumped.

The weight affects the range as do headwinds and routing factors. Long taxi times can burn fuel, and of course, the heavier the flight, the more it burns on take off, cutting into the range. Occasionally that requires a fuel stop.
Worked for WestAir, Apollo Airways, Desert Pacific, Western, AirCal and American Airlines (Retired). Flight Memory: 181 airports, 92 airlines, 78 a/c types, 403 routes, 58 countries (by air), 6 continents. 1,119,414 passenger miles.

Home airport : CEC
 
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VirginFlyer
Posts: 5545
Joined: Sun Sep 10, 2000 12:27 pm

Re: Aviation Terms

Fri Apr 03, 2020 11:45 pm

The maximum take-off weight of an aircraft is the maximum weight at which it is permitted to take-off. This may be a certification limitation (generally a structural limitation to avoid overstressing components, but other things like maximum revolution rate of the undercarriage) or a performance limitation (for instance due to runway length or obstacle clearance). In terms of increasing MTOW, you would be referring to a certification limit. You might do this by making the limiting structure stronger to allow a higher maximum take off weight before reaching another structural limitation.

OEW is the operating empty weight of an aircraft. Basically it is the weight of the airframe, fluids (oil, hydraulics, etc), undrainable fuel, and any other equipment required for flight. So if you take your MTOW and subtract the OEW, the difference you are left with is the total fuel and payload you can carry.

In terms of how MTOW affects range, basically if you increase the MTOW for the same OEW, you can carry more fuel for the same passenger/freight load (or more passengers and freight for the same fuel load). Now, there may be some prices to pay for this - you may make the basic structure of the aircraft heavier (so the OEW increases) which means if you don't need the extra range or payload you are carrying dead weight around. Many jurisdictions also charge airway fees by registered MTOW of the aircraft, so you may have to pay greater charges. Something some operators do to get around this last one is what is known as a "paper derate" - basically limit the aircraft to a lower MTOW in its documentation and systems, so that its capability is reduced and you don't have to pay such high charges.

I hope this is a helpful explanation for you - feel free to ask for any further clarification and I'll do my best to help, as I am sure will other users (who have probably already answered this before I have hit send on this!)

V/F
It is not for him to pride himself who loveth his own country, but rather for him who loveth the whole world. The earth is but one country, and mankind its citizens. —Bahá'u'lláh
 
PSAatSAN4Ever
Posts: 1092
Joined: Thu Sep 13, 2018 5:38 pm

Re: Aviation Terms

Fri Apr 03, 2020 11:55 pm

Wingtips56 wrote:
Those involved in flight operations will trump whatever I say, but from my background as an airport ops agent, these terms came into play at my level.
MTOW/MGTOW (maximum gross takeoff weight) has a fixed limit according to the aircraft certification. That's the allowed grand total of the aircraft, crew and load in perfect conditions. However this can be reduced on a flight by flight basis by several factors; temperature on the ground, for instance. On a hot day in PHX, the MTOW is greatly reduced because the air is thinner. So can be departures from high elevation airports, where the air is thinner and lift reduced, or winds affecting take off or landing direction when staring into a mountain range. Fuel weight is also affected by temperature; which I remember from high school physics ages ago but don't remember the details.
OEW is Operating Empty Weight; it can vary by what each airline calls it, but it's usually means the measured weight of the aircraft and equipment plus minimum operating crew weight (zero fuel but essential oil and other fluids). The load limit is the difference between these weights, and that is fuel, passengers, bags/cargo and additional crew combined. Pax/bag/cargo/mail load can take a hit when more fuel is required for headwinds, routing (around weather) and additional hold/diversion fuel based on conditions both at the destination airport and occasionally the departure airport (e.g. ability to take off in fog with less visibility than is required to land), should the flight need to return/engine out, etc., as well as the Maximum Landing Weight, which is generally less than the take off weight.

Weight Restrictions affecting the load are employed when necessary, and that means having to leave empty seats, bumping bags and cargo. Non-revs are the first bumped.

The weight affects the range as do headwinds and routing factors. Long taxi times can burn fuel, and of course, the heavier the flight, the more it burns on take off, cutting into the range. Occasionally that requires a fuel stop.


Outstanding explanation! If I can add to this, here's a couple of things I know as well:

1) Another reason for an accurate weight check is distribution. Weight needs to be loaded both fore and aft.

2) At SAN, a plane taking off is required to be able to carry out the worst-case scenario: the loss of an engine after commitment to taking off. Is it V2 that is the point at which the plane cannot be stopped safely on the remaining runway? Anyway, the plane must be able to continue the take-off, clear Point Loma, and circle for an emergency landing. All on one engine.

This used to be a deal breaker for SAN, as the transition to long-distance flying is becoming very two-engined. The 787 was designed to fulfill this requirement, fully loaded, entirely on one engine. And BA has sent 777's galore here, both -200 and -300, confident that their planes could easily handle the task. LH, on the other hand, being a smaller-and-still-developing market from Germany, finds an A340 a better choice, as an engine-out situation at SAN would lose only 25% of the thrust instead of 50%, and that adds to the capacity of what can go inside the airplane. The A330 could do it, but not necessarily as fully loaded, which equals a loss in revenue.

By having this "security blanket", LH can accurately predict weight, and knowing this on a regular basis maximizes yields. Kind of a silly equivalency, I agree, but still something that has to be known by everyone at the airline.
 
PHXWRLD
Topic Author
Posts: 70
Joined: Mon Dec 30, 2019 1:49 pm

Re: Aviation Terms

Sat Apr 04, 2020 12:16 am

VirginFlyer wrote:
The maximum take-off weight of an aircraft is the maximum weight at which it is permitted to take-off. This may be a certification limitation (generally a structural limitation to avoid overstressing components, but other things like maximum revolution rate of the undercarriage) or a performance limitation (for instance due to runway length or obstacle clearance). In terms of increasing MTOW, you would be referring to a certification limit. You might do this by making the limiting structure stronger to allow a higher maximum take off weight before reaching another structural limitation.

OEW is the operating empty weight of an aircraft. Basically it is the weight of the airframe, fluids (oil, hydraulics, etc), undrainable fuel, and any other equipment required for flight. So if you take your MTOW and subtract the OEW, the difference you are left with is the total fuel and payload you can carry.

In terms of how MTOW affects range, basically if you increase the MTOW for the same OEW, you can carry more fuel for the same passenger/freight load (or more passengers and freight for the same fuel load). Now, there may be some prices to pay for this - you may make the basic structure of the aircraft heavier (so the OEW increases) which means if you don't need the extra range or payload you are carrying dead weight around. Many jurisdictions also charge airway fees by registered MTOW of the aircraft, so you may have to pay greater charges. Something some operators do to get around this last one is what is known as a "paper derate" - basically limit the aircraft to a lower MTOW in its documentation and systems, so that its capability is reduced and you don't have to pay such high charges.

I hope this is a helpful explanation for you - feel free to ask for any further clarification and I'll do my best to help, as I am sure will other users (who have probably already answered this before I have hit send on this!)

V/F


What I still don’t understand is how you increase the MTOW both between variants (e.g. 788 vs 789) and between aircraft of the same variant (pmUA and pmCO 772s). Also, why wouldn’t you make all aircraft have the highest possible MTOW for maximum range?
 
Wingtips56
Posts: 1252
Joined: Thu Dec 16, 2010 1:26 am

Re: Aviation Terms

Sat Apr 04, 2020 12:24 am

Oh yeah, length of the runway is a big factor too. A short runway can add a significant MTOW reduction and weight restriction, which means reduced capacity and blocking seats. That affects the profitability of a route.
And the choice of an aircraft for the route....a 739 runway hog might have the desired number of seats for the bean counters but a 73G or 738 can take off full without dragging it's behind. SAN sees that, as does SNA. More arriving passengers than departing, when you look at the stats.
In my experience, our departures from SMF rarely ever had a weight restriction on a 110 F degree day on an 8800 ft runway because we were at 20 feet above sea level and relatively "thick"air. However the day that the Air France Concorde charter (US promotional tour) took off, the departure time was moved up 90 minutes because the forecast was going to be too warm at original departure time. It used every inch of concrete.
But then we had a 737-200 on a hot day at LAS at Gross with 37 pax going to RNO, because of the higher elevation and dry air. And there was the day when all HP 737-100/200 were grounded at PHX because it was 122F, and those little bitty wings were just not enough.
Worked for WestAir, Apollo Airways, Desert Pacific, Western, AirCal and American Airlines (Retired). Flight Memory: 181 airports, 92 airlines, 78 a/c types, 403 routes, 58 countries (by air), 6 continents. 1,119,414 passenger miles.

Home airport : CEC
 
Wingtips56
Posts: 1252
Joined: Thu Dec 16, 2010 1:26 am

Re: Aviation Terms

Sat Apr 04, 2020 12:41 am

Sometimes a certified MGTOW is an artificial limit, controlled not by the actual capability of the aircraft but by paperwork to fit into a certain classification. For example, back in my day, there was a cut-off point with commuter aircraft classification at 12,500 lbs gross. One of the airlines I was working for was flying HPJ-137 Jetstreams with 18 seats. Low and behold years later, the planes all turned out to have been physically underweighed, so the OEW was understated by a bunch. They had to take the seats down to 15, removed the internal (decorator) bulkheads, window drapes, beverages and anything else (occasional, optional flight attendant) that could be eliminated to stay under 12,500. While clearly the plane had been flying much heavier just fine over the years, the paper-certification had been exceeded. Not sure if that's a difference such as we now see with Part 121 vs Part 135 and different regulation or if different.
The Jetstream 32, essentially the HPJ-Neo was certified at 16,204 lbs, and 19 seats, with bag pod. Probably not much more physically capable, the the paperwork again rules.
Someone else can chime in here.
On 787s and 772 of apparently similar models, engines and equipment (heavy seats, etc.) are different and affect operations. The airlines have to determine which options were better for them most of the time, which is why you see differences between operators.
Worked for WestAir, Apollo Airways, Desert Pacific, Western, AirCal and American Airlines (Retired). Flight Memory: 181 airports, 92 airlines, 78 a/c types, 403 routes, 58 countries (by air), 6 continents. 1,119,414 passenger miles.

Home airport : CEC
 
Wingtips56
Posts: 1252
Joined: Thu Dec 16, 2010 1:26 am

Re: Aviation Terms

Sat Apr 04, 2020 12:41 am

Wingtips56 wrote:
Sometimes a certified MGTOW is an artificial limit, controlled not by the actual capability of the aircraft but by paperwork to fit into a certain classification. For example, back in my day, there was a cut-off point with commuter aircraft classification at 12,500 lbs gross. One of the airlines I was working for was flying HPJ-137 Jetstreams with 18 seats. Low and behold years later, the planes all turned out to have been physically underweighed, so the OEW was understated by a bunch. They had to take the seats down to 15, removed the internal (decorator) bulkheads, window drapes, beverages and anything else (occasional, optional flight attendant) that could be eliminated to stay under 12,500. While clearly the plane had been flying much heavier just fine over the years, the paper-certification had been exceeded. Not sure if that's a difference such as we now see with Part 121 vs Part 135 and different regulation or if different.
The Jetstream 32, essentially the HPJ-Neo was certified at 16,204 lbs, and 19 seats, with bag pod. Probably not much more physically capable, the the paperwork again rules.
Someone else can chime in here.
On 787s and 772 of apparently similar models, engines and equipment (heavy seats, etc.) are different and affect operations. The airlines have to determine which options were better for them most of the time, which is why you see differences between operators.
One may want more weight capability at a lesser distance.
Worked for WestAir, Apollo Airways, Desert Pacific, Western, AirCal and American Airlines (Retired). Flight Memory: 181 airports, 92 airlines, 78 a/c types, 403 routes, 58 countries (by air), 6 continents. 1,119,414 passenger miles.

Home airport : CEC
 
GalaxyFlyer
Posts: 5331
Joined: Fri Jan 01, 2016 4:44 am

Re: Aviation Terms

Sat Apr 04, 2020 2:40 am

Sometimes the MTOW or MLW is artificially reduced to reduce landing and handling fees or allow access to airports. The Global 7500 has several max weights to allow operations at airports that put restrictions on aircraft weight. The Global 6000 has two weights, 99,500 and 74,999. The lower weight allows ops at Naples,
Fl, Scottsdale, AZ and Sydney Kingsford Smith during noise curfew hours.
 
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airportugal310
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Re: Aviation Terms

Sat Apr 04, 2020 3:57 am

Just another reporter looking to take everything you say and spin it somehow against the industry...some of these questions are so obvious
“They bought their tickets, they knew what they were getting into. I say, let 'em crash.”
 
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VirginFlyer
Posts: 5545
Joined: Sun Sep 10, 2000 12:27 pm

Re: Aviation Terms

Sat Apr 04, 2020 4:09 am

airportugal310 wrote:
Just another reporter looking to take everything you say and spin it somehow against the industry...some of these questions are so obvious

Sorry could you explain a bit more?

V/F
It is not for him to pride himself who loveth his own country, but rather for him who loveth the whole world. The earth is but one country, and mankind its citizens. —Bahá'u'lláh
 
wetpantsmcgee
Posts: 80
Joined: Fri May 20, 2011 1:23 am

Re: Aviation Terms

Sat Apr 04, 2020 4:21 am

Would be nice to have a sticky thread filled with decoded acronyms. I've been in the industry 20 years and I still see some that leave me stumped.
 
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VirginFlyer
Posts: 5545
Joined: Sun Sep 10, 2000 12:27 pm

Re: Aviation Terms

Sat Apr 04, 2020 4:54 am

PHXWRLD wrote:
VirginFlyer wrote:
The maximum take-off weight of an aircraft is the maximum weight at which it is permitted to take-off. This may be a certification limitation (generally a structural limitation to avoid overstressing components, but other things like maximum revolution rate of the undercarriage) or a performance limitation (for instance due to runway length or obstacle clearance). In terms of increasing MTOW, you would be referring to a certification limit. You might do this by making the limiting structure stronger to allow a higher maximum take off weight before reaching another structural limitation.

OEW is the operating empty weight of an aircraft. Basically it is the weight of the airframe, fluids (oil, hydraulics, etc), undrainable fuel, and any other equipment required for flight. So if you take your MTOW and subtract the OEW, the difference you are left with is the total fuel and payload you can carry.

In terms of how MTOW affects range, basically if you increase the MTOW for the same OEW, you can carry more fuel for the same passenger/freight load (or more passengers and freight for the same fuel load). Now, there may be some prices to pay for this - you may make the basic structure of the aircraft heavier (so the OEW increases) which means if you don't need the extra range or payload you are carrying dead weight around. Many jurisdictions also charge airway fees by registered MTOW of the aircraft, so you may have to pay greater charges. Something some operators do to get around this last one is what is known as a "paper derate" - basically limit the aircraft to a lower MTOW in its documentation and systems, so that its capability is reduced and you don't have to pay such high charges.

I hope this is a helpful explanation for you - feel free to ask for any further clarification and I'll do my best to help, as I am sure will other users (who have probably already answered this before I have hit send on this!)

V/F


What I still don’t understand is how you increase the MTOW both between variants (e.g. 788 vs 789) and between aircraft of the same variant (pmUA and pmCO 772s). Also, why wouldn’t you make all aircraft have the highest possible MTOW for maximum range?

Ok, let's address these one at a time:

  • How to increase MTOW between variants (eg 787-8 and 787-9)? The primary difference will be structural. Basically if a 787-9 has a higher MTOW than the -8, that means more load has to be transmitted through the structure, especially the wing root. So the wing root on the 787-9 will have been built with more material to enable it to take the higher load. Now on the -10, they have stretched the fuselage (which will raise the empty weight of the aircraft) without raising the MTOW, meaning the aircraft won't be able to carry as much fuel with a full payload. There may also be some aerodynamic refinements needed to allow an aircraft to achieve a higher MTOW, or system changes (again, one which I have heard come up as a limitation is the revolution rate of the landing gear - for an identical aircraft the heavier you are, the faster you will be at take-off and landing).
  • Why do aircraft of the same variant have different MTOWs? There are a few factors which may be in play here, and without knowing too much about Continental and United's 777-200s I wouldn't want to point to which one it is in that particular case. But in general, you might be looking at the paper de-rate described above. Where there are different engine options available (either different manufacturers, or the same manufacturer at different thrust ratings) you will end up with different MTOWs.
  • Why not maximise MTOW for maximum range? Certainly if the key requirement for your route is maximum range then you would want to do that. But let's say you don't need to fly the maximum theoretical range of a type to achieve your mission, then if your aircraft is set up for its highest possible MTOW you will either be carrying around unnecessary structural weight, excess engine thrust, or both. Excess engine thrust means increased maintenance cost. If you never need more than 90% of the engine's thrust for a particular mission, then you can derate the engine to that and save on expenses. Likewise, remember a lot of charges are based on MTOW. If you mission requires a regular take-off weight of 110 tonnes, there is no point flying an aircraft with an MTOW of 140 tonnes if you can paper de-rate it to, say, 120 tonnes. For some aircraft types there will be additional benefits to this paper derate due to increased maintenance intervals for certain parts. Basically the golden rule of aviation is you don't want to be carrying around any more weight than you really need to. An airline won't generally be able to optimise a type one specific route, but it will try to optimise it to a set of routes.

V/F
It is not for him to pride himself who loveth his own country, but rather for him who loveth the whole world. The earth is but one country, and mankind its citizens. —Bahá'u'lláh
 
Yeastbeast
Posts: 36
Joined: Mon Nov 05, 2018 9:03 pm

Re: Aviation Terms

Sat Apr 04, 2020 6:02 am

It would be nice if Anet had a glossary of terms, acronyms, and definitions we could use to look these things up.

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