airliner actual weight vs GTOW?

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alanbreck

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AlanB
Airline captains please explain this to me ....

I'm not a platinum member with any airline or anything, but I've flown commercial countless times.
And on just 2 occasions, something strange (to my mind) has taken place...

After boarding everyone, the cabin crew has said something like "sorry we are overweight, and need one person (or two people) to de-plane. We will offer you an alternative itinerary and a voucher for X dollars to be used in the future".

So how does an airline crew know they are overweight? Surely there is not a "weight on wheels" sensor? No doubt that would be technologically feasible, but I expect there are numerous reasons why that wouldn't make sense. I always assumed they factored the weight of warm bodies on board by using "FAA average human weight" times the number on the manifest.

But we've all been on plenty of airline flights where there wasn't a single empty seat. So what changes to make that not an option? Is there a scenario where "extra weight in baggage area" or "higher fuel level in tanks" means they have to depart with X number of empty seats in the cabin to offset that configuration?

I supposed they probably know the weight of each item of checked luggage, and any other freight, but does that explain it? "We have 500 lbs more than usual down below, so have to lighten the cabin by X amount"?

Obviously in a 4 seat GA plane we often know we have to leave a seat (or 2 or 3) empty due to fuel load or baggage load. But we come up with that by knowing pretty precise weight of warm bodies.

---------------------------------

To make things stranger, on one of the two occasions where I've experienced this, they didn't get any volunteers. So they chose a name from the passenger list and forced that person off the plane.

But get this, a few minutes pass, and than another person comes on the plane, in some sort of airline attire (I forget if it was apparent what sort of crew person they were), and takes the seat which had just been vacated. And then we depart ... full plane. (but not overweight somehow???)

Sounds sketchy.
 
Transport category airplanes have many factors affecting TO weight. Temperature, density altitude, runway length, slope, contamination, departure procedure, etc. Just because the airplane has a MGTOW of xxxxx doesn't necesarily mean it can be loaded to that each and every takeoff.

Also destination weather mins come into play with required alternates. This may mean taking more fuel than originally planned. Or even an MEL item on the plane could require additional fuel to be carried.

Lots of variables.
 
For the second part i think the most logical explanation would be that they needed that pilot to fly to that destination.
 
Andy Appeltans said:
For the second part i think the most logical explanation would be that they needed that pilot to fly to that destination.
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If that were the case, he would already have a positive space seat in the cabin.

a lot of things vhappen that don’t make a lot of sense.
 
I have only seen this on some smaller jets or turboprops. Only once in a larger plane, and that was due to a malfunction that did not allow the desired flap setting for takeoff. So they had to offload pax and baggage to take off from a high (Co Springs), hot (summer) day with less flaps.

A weight on wheels is possible
 
Weight and Balance falls under an OpSpec in Part 121/135 (A096, A097,A098 or A099). Whichever program (Actual Weight or Average Weight) is authorized the carrier must use that system, there is no allowance to intermix the two.

So the carrier performs a W&B and finds it's outside the operating envelope, a change must be made before the PIC can accept it.
 
Most big aircraft have the capability to self weigh via gear strut pneumatic pressures or strain gauges within the gear assemblies. I know the combat aircraft I work on has a reading that I can access as a maintenance guy, that shows current weight of the aircraft, no matter the load on it. If we are doing a defuel or certain refuel operations, I can watch the a/c weight change as the fuel load changes.
 
The weight you can take off with on a given runway changes day by day with weather. On both ends. We might have plenty of weight allowance on takeoff, but are near the max we can land with on the destination end. Further, carryons are included in passenger average weights but the quantity of checked bags and weight of cargo is not. That changes each flight, as does the amount of fuel we need.

As to why they took a passenger off and appeared to put a crew member on, it can be a couple of things. There’s a lot going on behind the scenes right up until push. It could be that they recalculated the numbers after the volunteer passenger accepted the voucher, and were able to squeeze one more on. It could also be that the crew member was going to go on the plane either way, but was going to sit in a jumpseat. Removing one passenger did in fact reduce the take off weight, and the crewmember decided to sit in the cabin instead of the jumpseat.
 
A few weeks ago we needed to boot a passenger due to weight. By the time the agents pulled the jet bridge back and got a volunteer, we had burned enough fuel with the APU to keep them on.
The Airbus does not “self weigh”. We send the number of passengers and bags to a big computer in the sky. In about a minute it tells us our weight & CG.
I don’t believe I’ve ever touched max weight from an airplane limitation standpoint. It’s always been limited by performance, or landing weight.
 
Kritchlow said:
A few weeks ago we needed to boot a passenger due to weight. By the time the agents pulled the jet bridge back and got a volunteer, we had burned enough fuel with the APU to keep them on.
The Airbus does not “self weigh”. We send the number of passengers and bags to a big computer in the sky. In about a minute it tells us our weight & CG.
I don’t believe I’ve ever touched max weight from an airplane limitation standpoint. It’s always been limited by performance, or landing weight.
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In other words. You do an estimate. I’m betting a lot of airliners are flying overweight. And the FAA looks the other way
 
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Kinder said:
In other words. You do an estimate. I’m betting a lot of airliners are flying overweight. And the FAA looks the other way
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Yea, you really don’t understand the system. It’s all on the up & up. No looking the other way required.

ETA: now they break the peeps into zones, but several years back we just gave the number of peeps, and the computer assumed even distribution for balance purposes. As far as I know this is still approved, but now we break it into 2-3 sections to improve efficiency.
 
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alanbreck said:
To make things stranger, on one of the two occasions where I've experienced this, they didn't get any volunteers. So they chose a name from the passenger list and forced that person off the plane.

But get this, a few minutes pass, and than another person comes on the plane, in some sort of airline attire (I forget if it was apparent what sort of crew person they were), and takes the seat which had just been vacated. And then we depart ... full plane. (but not overweight somehow???)

Sounds sketchy.
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It's not sketchy. There is a lot of different permutations of passenger weights, cargo weights, last-minute bags showing up, crewmembers that have to be repositioned, and fuel loads that could change last minute as well.


Kinder said:
In other words. You do an estimate. I’m betting a lot of airliners are flying overweight. And the FAA looks the other way
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The FAA makes us revalidate our weight and balance process every year. We have to kick off people a lot more frequently now than years past due to the higher average weights we are required to use. Which is the topic of this very thread. So the FAA is doing the exact opposite of "looking the other way".
 
A number of years (decades!) ago I was flying on TWA from my home in Colorado Springs to Boston via STL on a very hot summer day (before the long runway was built at COS). The 727 could not take off from COS with the passenger load and enough fuel to get to STL so we had to stop in Kansas City for more fuel.
 
Arrow76R said:
A number of years (decades!) ago I was flying on TWA from my home in Colorado Springs to Boston via STL on a very hot summer day (before the long runway was built at COS). The 727 could not take off from COS with the passenger load and enough fuel to get to STL so we had to stop in Kansas City for more fuel.
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Just a swag…. They used much different procedures back then than they do now.
 
Doc Holliday said:
LOL.

Do you understand how average weights work? Or how the cargo is weighed? Or how carry on baggage is calculated?

https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_120-27F.pdf
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Doc Holliday said:
LOL.

Do you understand how average weights work? Or how the cargo is weighed? Or how carry on baggage is calculated?

https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_120-27F.pdf
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average weight for a male with his carry on is now 200 lb. HA! I’ll bet it’s a lot more than that. I would like to see every one weighed at the gate and your fare adjusted accordingly!
 
Kinder said:
Really, I worked for NWA for 20 years as a mech, mostly on 747s I’ve seen what goes on
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iu
 
Kinder said:
Really, I worked for NWA for 20 years as a mech, mostly on 747s I’ve seen what goes on
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Hay, have you ever even touched an airliner other when riding in one. You are clueless
 
Kinder said:
I replied to Doc Holliday
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You replied to yourself. :rolleyes:

That's OK, I once knew a guy that drove the lav truck, he claimed he knew everything about the airline and the CEO even would call him for advise. ;)
 
Doc Holliday said:
You replied to yourself. :rolleyes:

That's OK, I once knew a guy that drove the lav truck, he claimed he knew everything about the airline and the CEO even would call him for advise. ;)
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I did heavy structural work in bay 5 and 6 at MSP. We did C and D checks on the passenger and cargo 747s
 
Often the load is limited before boarding & with cargo. The could be a handful of seats ‘blocked’ with more wanting to get on, or ‘standbys’. It comes down to takeoff data, which includes many details, enroute, and arrival fuel.

I do Central America a fair amount, you may have one runway, winds favor the direction towards terrain, though can easily be better off the reciprocal with 8 kts(or more) of tailwind. They don’t call this airplane the ‘runway inspector’ for nothing. Then after the 4:45 hour flight you need fuel for an alternate. People wonder why are there 15 open seats when more are trying to get on? Yes, I’ve heard flights loaded up & ready, then waiting for better winds. I’ve seen dispatch plan the takeoff weight with 12 knots of tailwind, but now we have a steady 18+. One may say, ‘why not take off into that headwind’? Well at 80 degrees, with the rising terrain, even ‘bleeds off’, not an option.

Then more so with a light passenger load, off the gate you get a message from load planning you have to move some passengers. This is to do with C.G. issues, measuring with the micrometer & cutting with the ax.

A lot of details go into the calculated takeoff date. There are a few ‘tricks’ to bend it in your favor. I don’t really care for a ‘wallowing’ takeoff. I prefer a crisper takeoff. BWTHDIK
 
I once talked to a mechanic who said he could fly, ‘because he knew hydraulics’. He said you ‘pull back on the stick to go up’ (military). I didn’t ask him about coming back to land.
 
sourdough44 said:
I once talked to a mechanic who said he could fly, ‘because he knew hydraulics’. He said you ‘pull back on the stick to go up’ (military). I didn’t ask him about coming back to land.
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I also have my commercial pilot license and have been flying since 1968 and have my own aircraft since 1981
 
Kinder said:
I did heavy structural work in bay 5 and 6 at MSP. We did C and D checks on the passenger and cargo 747s
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I fly airliners and even have an A&P.

I don’t know squat about mx at the airline because it’s not my job. I have a general idea about them because I’m in the business but I can’t speak with authority on the subject.

Are you really sure you actually know what goes on today in regards to operational control and dispatch?
 
Tarheelpilot said:
I fly airliners and even have an A&P.

I don’t know squat about mx at the airline because it’s not my job. I have a general idea about them because I’m in the business but I can’t speak with authority on the subject.

Are you really sure you actually know what goes on today in regards to operational control and dispatch?
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All I was saying was that the FAA average male weight with carry on baggage at 200 lbs is unrealistic
 
Happened a decent amount on times on the CRJ-700/900 and more prevalent on the 200. Most of the time we were able to move passengers around or shift cargo from one bin to the other and we made it work. 757 can basically do anything and is the best engineered plane IMO. Talking off hot and heavy is no problem. 767 has some issues on our longer flights from Africa and some deep Europe stuff, especially when it starts getting warmer. We are typically at max gross on those. There’s also some stations that are notorious for having lots of cargo and that will also affect how many passengers we can get on.
 
@alanbreck , I'm somewhat wary that this response will get lost in the downward spiraling toilet bowl that this thread has become, but your question is a good one.

I think what it's important to realize is that large, transport-category aircraft are operating far closer to their limits/margins than, say, a 172 generally is. What I mean is this - when you takeoff in a 172 from a 5000 ft runway, the exact amount of weight you have on board, or the density altitude, or the humidity, or the flap setting, or the engine power, etc., just doesn't really matter too much. You have almost 10 times the runway needed in many scenarios. But if you instead operated regularly off of a 1000 ft runway with trees at the end, all of a sudden all those factors become much more critical. Instead of just briefly looking at the performance tables for max gross, you'd instead be calculating what your takeoff distance, climb speed, climb rate, all of that is at your actual weight. And then maybe you'd decide to take 10 fewer gallons or something like that.

When you add into that the additional safety factors that part 121 operators must comply with, like being able to stop on the runway during an aborted takeoff, or climb rate with an engine failure, then you see that lots of these airliners are often operating fairly close to the "limit". Granted, this is a "safety factored" limit, but a limit nonetheless. So a little difference in weight or temperature or whatever can cross that line, and that becomes a no-go until something is changed (like removing one passenger).
 
Kinder said:
All I was saying was that the FAA average male weight with carry on baggage at 200 lbs is unrealistic
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OK. So tell us about how you assisted with the sampling data that's required for the average weight OpSpecs and what was the conclusion of that run to determine the 200 lb average male was unrealistic?
 
RussR said:
@alanbreck , I'm somewhat wary that this response will get lost in the downward spiraling toilet bowl that this thread has become, but your question is a good one.

I think what it's important to realize is that large, transport-category aircraft are operating far closer to their limits/margins than, say, a 172 generally is. What I mean is this - when you takeoff in a 172 from a 5000 ft runway, the exact amount of weight you have on board, or the density altitude, or the humidity, or the flap setting, or the engine power, etc., just doesn't really matter too much. You have almost 10 times the runway needed in many scenarios. But if you instead operated regularly off of a 1000 ft runway with trees at the end, all of a sudden all those factors become much more critical. Instead of just briefly looking at the performance tables for max gross, you'd instead be calculating what your takeoff distance, climb speed, climb rate, all of that is at your actual weight. And then maybe you'd decide to take 10 fewer gallons or something like that.

When you add into that the additional safety factors that part 121 operators must comply with, like being able to stop on the runway during an aborted takeoff, or climb rate with an engine failure, then you see that lots of these airliners are often operating fairly close to the "limit". Granted, this is a "safety factored" limit, but a limit nonetheless. So a little difference in weight or temperature or whatever can cross that line, and that becomes a no-go until something is changed (like removing one passenger).
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Yes but this is all on paper, garbage in garbage out.
 
Doc Holliday said:
OK. So tell us about how you assisted with the sampling data that's required for the average weight OpSpecs and what was the conclusion of that run to determine the 200 lb average male was unrealistic?
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Have you looked around you lately? People on this forum are constantly complaining that they don’t fit in a number of airplanes.
 
Doc Holliday said:
OK. So tell us about how you assisted with the sampling data that's required for the average weight OpSpecs and what was the conclusion of that run to determine the 200 lb average male was unrealistic?
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And that they can’t take more than 2 people in a 4 place plane without being over gross
 
Kinder said:
Have you looked around you lately? People on this forum are constantly complaining that they don’t fit in a number of airplanes.
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Ah, OK. I wasn't aware AC 120-27F used the "search PoA forum" as a means of determining average passenger weight.

Thanks. o_O
 
Craig said:
Most big aircraft have the capability to self weigh via gear strut pneumatic pressures or strain gauges within the gear assemblies. I know the combat aircraft I work on has a reading that I can access as a maintenance guy, that shows current weight of the aircraft, no matter the load on it. If we are doing a defuel or certain refuel operations, I can watch the a/c weight change as the fuel load changes.
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But how sensitive is it? I have never seen any aircraft oleo that didn't have some stickiness to it, and the large airplanes I worked on sure wouldn't respond to the weight change of one or two people. Maybe not even ten people.
 
Dan Thomas said:
But how sensitive is it? I have never seen any aircraft oleo that didn't have some stickiness to it, and the large airplanes I worked on sure wouldn't respond to the weight change of one or two people. Maybe not even ten people.
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Correct, but the airlines have go by something to do W&B. I suspect that they fly overweight a lot but there is enough safety factor built in that they get away with it
 
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