klemmi85
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Actual T/O Weight A Mystery?

Tue Jun 23, 2009 5:43 pm

Hi,

something just came to my mind when I was reading the Thomas Cook balancing thread in Civil Aviation forum.

We all know, there is a MTOW which may not be exceeded. So let's pretend you know the empty weight of the aircraft (we'll use fictional figures)

Empty weight is: 100.000 lbs

Then you begin to add fuel...

Empty weight + fuel weight is: 150.000 lbs

No you add the cargo which weighs 25.000 lbs,
making a total of: 175.000 lbs.

Okay, that's perfectly understandable.


Now the part I'm interested in.....

The MTOW of our fictional aircraft is 200.000 lbs.

One thing you still have to do is get your pax onboard. Okay... How do they know about the actual T/O weight in the end? I mean you are not weight before you enter the aircraft, neither asked. How can they be sure about the actual T/O weight / about not exceeding MTOW / about the precise fuel burn determined also bei weight and about not exceeding the maximum landing weight when an emergency landing has to be carried out?

I guess they estimate on basis of some research which came up with an average value but you never now for sure, is this right?

regards,
Dennis

[Edited 2009-06-23 10:43:38]
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wilco737
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RE: Actual T/O Weight A Mystery?

Tue Jun 23, 2009 5:49 pm



Quoting Klemmi85 (Thread starter):
How do they know about the actual T/O weight in the end? I mean you are not weight before you enter the aircraft, neither asked. How can they be sure about the actual T/O weight / about not exceeding MTOW / about the precise fuel burn determined also bei weight and about not exceeding the maximum landing weight when an emergency landing has to be carried out?

For passenger + handluggage there is a standard weight. Varies for each airline. I cannot remember what we used on the 737 as I am flying cargo now, but I think it is 100kgs. But you are right, you don't know the actual TOW 100% correctly.
For the fuel burn, it will be higher, but not significantly and you have a so called "contingency fuel" on board which gives you a little extra fuel for unforeseen circumstances. So if you are a little heavier than expected, you burn a little bit mor fuel, but this is covered in the cont fuel.

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Goldenshield
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RE: Actual T/O Weight A Mystery?

Wed Jun 24, 2009 1:16 am



Quoting Klemmi85 (Thread starter):
How can they be sure about the actual T/O weight / about not exceeding MTOW / about the precise fuel burn determined also bei weight and about not exceeding the maximum landing weight when an emergency landing has to be carried out?

To expand on Wilco's answer, everything is done in on paper using standard weights. if the computed values do not exceed MTOW, MLDW, or enroute drift-down MTOW limitations, then you are good to go.

As far as exceeding MLDW in an emergency, aircraft are tested up to their MTOW. In actual service, if a flight lands above MLDW, then inspections are carried out to ensure that no damage has occured (unlikely, but still required.)
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zappbrannigan
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RE: Actual T/O Weight A Mystery?

Wed Jun 24, 2009 2:40 am

You'll never know exactly what it is, and in many cases it's probably technically incorrect. You may have a load of pax which are, on average, above the standard pax weight used - and just as significantly, they're carrying a lot more weight in carry-on baggage than the airline specifies as a maximum - many pax simply ignore this weight requirement, and while I can't speak for overseas, here you're unlikely to get "caught" unless the bag's dimensions are too large, or a crew member helps you with the bag and notices how heavy it is. I frequently see people carrying a good 30kg on board, when the maximum allowed is 2 x 7kg bags.

Take a standard B737 load of, say, 160 pax - if on average they're 4kg over the standard weight, and each passenger is carrying an average of 4kg more carry-on baggage than allowed (probably realistic figures on some flights) - that's an extra 1.3 tonne which would not automatically be accounted for.

I'm no engineer, but I'm sure performance-wise this is acknowledged somehow. If taking off at 500kg above MTOW (note well: not deliberately doing this, but this happening outside anybody's control or knowledge) was extremely dangerous (not just bad practice, but very likely to result in an incident), then everybody would be weighed on arrival, and every aircraft would sit on a massive scale at the ramp so as to determine precise TOW. Deliberately taking off above MTOW is completely unacceptable, but technically, it must happen a fair bit.

I just think of the number of westbound LAX-SYD/MEL sectors I've been a pax on where the captain has announced "we're right on maximum weight tonight".
 
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Starlionblue
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RE: Actual T/O Weight A Mystery?

Wed Jun 24, 2009 2:43 am

As Zappbranigan says, you're probably never EXACTLY sure. But it's close enough.

If weight and balance are especially sensitive, like on small aircraft, calculations tend to be more precise. If you think about it, an extra 500 kg on a 747 is a rather small proportion of aircraft weight. An extra 50 kg behind the CG on a Cessna 172 probably has a noticeable impact on flying characteristics.
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Goldenshield
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RE: Actual T/O Weight A Mystery?

Wed Jun 24, 2009 2:45 am



Quoting Zappbrannigan (Reply 3):
If taking off at 500kg above MTOW (note well: not deliberately doing this, but this happening outside anybody's control or knowledge) was extremely dangerous (not just bad practice, but very likely to result in an incident), then everybody would be weighed on arrival,

That would be the day when they start installing scales on the taxiways, making the airport like a trucker weigh station on the highway/freeways.
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klemmi85
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RE: Actual T/O Weight A Mystery?

Wed Jun 24, 2009 7:47 am

Hm I see, so there is some headroom to play with and it won't be hurting much to be a little bit unprecise.

Just being curious now and a little of topic, but

Quoting Starlionblue (Reply 4):

mentioned the CG.

If in a 744 everyone would storm to the front / or aft / of the aircraft. What would happen? Would there be some significant impact to the flight or would the A/P just re-trim the aircraft accordingly?

Having 300pax, each at 100kg totals to 30 tonnes  Smile
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zappbrannigan
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RE: Actual T/O Weight A Mystery?

Wed Jun 24, 2009 9:38 am



Quoting Klemmi85 (Reply 6):
Hm I see, so there is some headroom to play with and it won't be hurting much to be a little bit unprecise.

You're correct, although this is probably not the best way of putting it - it's as precise as it can realistically be, and pilots and dispatchers would never deliberately be unprecise. Stating it like that implies that pilots are quite happy to depart if "we're only a few kilos over" - which isn't the case. We don't allow for any headroom - but realistically, there has to be a tiny bit of headroom there.

If I'm flying an aircraft and departing at its MTOW of 4,000kg (properly calculated), and I discovered after the flight that someone had secretly weighed it on the ramp and the TOW was actually 4,010kg - I wouldn't be surprised, nor would I think that I'd put anyone in danger. Doesn't mean that on my next flight, if I calculate TOW at 4,010kg that I'd agree to depart.
 
klemmi85
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RE: Actual T/O Weight A Mystery?

Wed Jun 24, 2009 12:05 pm



Quoting Zappbrannigan (Reply 7):

See your point. Didn't want to say that pilots don't care in the end cause I'm sure they do and they have to, no doubt. Of course, they calculate as good as possible so that the difference from calculated T/O weight to actual T/O weight is as small as possible.

But thanks for pointing that out, nobody should think these limits are just made up because they look good.

regards,
Dennis
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dispatchguy
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RE: Actual T/O Weight A Mystery?

Wed Jun 24, 2009 3:38 pm

Just remember in this biz, measure with a micrometer and smash the hell out of it with a sledgehammer...

As long as the paperwork is legal....
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FredT
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RE: Actual T/O Weight A Mystery?

Wed Jun 24, 2009 8:56 pm



Quoting Starlionblue (Reply 4):
If weight and balance are especially sensitive, like on small aircraft, calculations tend to be more precise. If you think about it, an extra 500 kg on a 747 is a rather small proportion of aircraft weight. An extra 50 kg behind the CG on a Cessna 172 probably has a noticeable impact on flying characteristics.

Which is why the stipulated standard weights are higher for smaller aircraft.
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Tugger
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RE: Actual T/O Weight A Mystery?

Wed Jun 24, 2009 9:56 pm

So why don't they just put load sensors in the landing gear and get the exact weight? Everything here seems to be based on calculation when its seems relatively easy to just weigh the aircraft.

Thanks,
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Viscount724
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RE: Actual T/O Weight A Mystery?

Wed Jun 24, 2009 10:04 pm

After the following US Airways Express Beech 1900D crash in 2003, where there was an indication the aircraft may have been overweight (in addition to other maintenance-related issues), airlines operating 19-seat and smaller aircraft were required to sample actual passenger and baggage weights for some time to see how accurate the average weights were. As a result of the sampling data, the FAA increased the average weights, presumably only for small commuter types. See the "Follow-up/safety actions" sections in the following link.
http://aviation-safety.net/database/record.php?id=20030108-0
 
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Starlionblue
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RE: Actual T/O Weight A Mystery?

Wed Jun 24, 2009 11:55 pm



Quoting Klemmi85 (Reply 6):
If in a 744 everyone would storm to the front / or aft / of the aircraft. What would happen? Would there be some significant impact to the flight or would the A/P just re-trim the aircraft accordingly?

There was a whole thread on this. It was a while ago though and I can't remember the conclusions.

Quoting Tugger (Reply 11):
So why don't they just put load sensors in the landing gear and get the exact weight? Everything here seems to be based on calculation when its seems relatively easy to just weigh the aircraft.

I think I recall a poster saying this is not as easy as it would seem.
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zappbrannigan
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RE: Actual T/O Weight A Mystery?

Thu Jun 25, 2009 7:16 am



Quoting Tugger (Reply 11):
So why don't they just put load sensors in the landing gear and get the exact weight? Everything here seems to be based on calculation when its seems relatively easy to just weigh the aircraft.

Many, many problems with this. Have a search of some other threads - it wouldn't work with any realistic, cost-effective system.
 
borism
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RE: Actual T/O Weight A Mystery?

Thu Jun 25, 2009 9:02 pm



Quoting Tugger (Reply 11):
So why don't they just put load sensors in the landing gear and get the exact weight? Everything here seems to be based on calculation when its seems relatively easy to just weigh the aircraft.

I wondered about this quite often too. But if you think about it:

1. Precision of such system, as there is very little room to place accurate scale mechanism anywhere near the MLG, will be worse than manual calculations done now.
2. If you will want more precise system you'll have to add weight.
3. Finally it all adds up as cost. That's no good for commercial enterprise. I guess it's not as expensive to have loadmasters and dispatchers as it was to have flight mechanics?

But I recall hearing that modern airliners actually do estimate weight on MLG? But I guess that was not the case with EK A340-500 that overrun at MEL due to erroneous TOW input?
 
airbuske
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RE: Actual T/O Weight A Mystery?

Thu Jun 25, 2009 9:15 pm

Some airplanes have strain gauge transducers on the landing gear struts to weigh it, but I don't know how accurate they are.
 
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longhauler
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RE: Actual T/O Weight A Mystery?

Fri Jun 26, 2009 1:56 am

Standard weights are used as suggested above, but most airlines take it a step further. It also depends on the time of year, the time of day, and the actual city pair flown. A Monday morning YYZ-YUL passenger weighs less than a Saturday YYZ-MBJ passenger!

Then ... audits are performed from time to time to confirm, correct and adjust the standard weights. Twice in the last year, on flights I was flying, a weigh scale attached to a computer was in the bridge, and every passenger, crew-member and carry on bag was weighed.

As you state above, the only variable is passengers/crew, everything else, including the aircraft itself is weighed.
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zappbrannigan
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RE: Actual T/O Weight A Mystery?

Fri Jun 26, 2009 2:18 am



Quoting Borism (Reply 15):
But I recall hearing that modern airliners actually do estimate weight on MLG? But I guess that was not the case with EK A340-500 that overrun at MEL due to erroneous TOW input?

Don't know about that - but it would work only as an "idiot proofing" system - when, like the EK flight, entered ZFW is off by something in the order of 100 tonne. A system like this wouldn't require massive levels of accuracy and calibration - it'd only be there to detect huge anomalies between entered and sensed data. It'd still have to be calibrated frequently though - not sure it'd be worth it.

I went and had a good look at the tarmac and stopway after that EK incident - very scary.
 
pygmalion
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RE: Actual T/O Weight A Mystery?

Sat Jun 27, 2009 6:10 pm

there are not insignificant margins built into the limits by the manufacturers.

Example: Sizing of the mainlnading gear. One key criteria is a maximum weight landing at or near late landing phase max descent rate. This calculation is a big factor in sizing the gear struts and the attachements of the gear to the wing and fueslage. Those attachements then also have a safety factor of at least 50% built in. Then there are also safety margins built into the weight and balance calculations so that at full CG fore and aft limits there is still control authority left over.

All in all, at the "flight manual" book limits, you are not at the ragged edge, you are at a safe, set distance back from the ragged edge.

All bets are off if you climb over the safety fence though.
 
VMCA
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RE: Actual T/O Weight A Mystery?

Sat Jun 27, 2009 6:48 pm



Quoting Borism (Reply 15):
But I recall hearing that modern airliners actually do estimate weight on MLG? But I guess that was not the case with EK A340-500 that overrun at MEL due to erroneous TOW input?

No they don't. There have been various systems tested but at the end of the day they are full of problems and provide inaccurate weights.

If you look at tail strikes incidents on takeoff, most are caused by incorrect performance being calculated. If you look further, you will find the error is generally a round number, for example, 50, 000 lbs or 100 tons. Right now, the passenger weight is estimated, on the generous side and all the containers are weighted. There are ways of minimizing the potential for errors. Some airlines have dispatch ACARS the performance data for all runways based on the weights calculated by load control. The performance data is also loaded via ACARS into the FMS.

The performance margins are more than ample for any type of "rounding" errors that would occur. So, it's not quite on the "ragged edge" as was described.
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zappbrannigan
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RE: Actual T/O Weight A Mystery?

Sun Jun 28, 2009 12:53 am



Quoting Pygmalion (Reply 19):
All in all, at the "flight manual" book limits, you are not at the ragged edge, you are at a safe, set distance back from the ragged edge.

All bets are off if you climb over the safety fence though.

Nicely summed up.
 
jackmidd
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RE: Actual T/O Weight A Mystery?

Fri May 13, 2011 10:29 am

I was about to ask this question about actual weight vs estimated weight but it appears to have been answered. I was also going to ask how a pilot can determine takeoff thrust without having an accurate weight, here in Aberdeen we have a very small runway yet i've been on what appeared (hand luggage) to be a very heavy A321, takeoff thrust must've been 100% needless to say it was reduced almost instantly after gear retraction. A 321 departing LHR feels to go along the runway for twice the distance on a much lower thrust setting, are these settings determined by an average too?
 
Northwest727
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RE: Actual T/O Weight A Mystery?

Fri May 13, 2011 2:37 pm

Quoting Viscount724 (Reply 12):

Standard pax weights were 170lbs summer time, and 175lbs winter. At a regional airline I interned at, the standard was 190lbs summer, and 195lbs winter time.

Further proof that Americans' waistlines are grossly expanding.
 
kingairta
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RE: Actual T/O Weight A Mystery?

Fri May 13, 2011 3:28 pm

Ahhhh weight and balance.

That's my forte.

If I don't have access to actuall passenger weights I estimate the weight to be 200 pounds. Why 200? Well this also includes any carry on bag they may have with them. You'd be surprised at often this number works out in reality. If I'm getting too close for comfort on my max weight (13,500 on our C-12B) then I'll get a more accurate weight. The only time it ever really comes into play as being an issue is when the plane is high, hot and heavy. Single engine performance is horrible in those conditions.

And in those cases when we are at or over we can't leave people behind so we lose some fuel and schedule a fuel stop if it's required.

In the 20 years I've been doing weight and balance I could probably count on one hand the number of times I had as accurate of weight as your gonna get.

What gets planes in trouble isn't so much as over weight but out of CG limits. A couple hundred pounds over on my C-12 isn't really that much of an issue but if it's too tail heavy or too nose heavy then things can be scary.
 
GST
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RE: Actual T/O Weight A Mystery?

Sun May 15, 2011 9:38 am

Quoting Tugger (Reply 11):
So why don't they just put load sensors in the landing gear and get the exact weight? Everything here seems to be based on calculation when its seems relatively easy to just weigh the aircraft.

This is slowly coming in on some new GA aircraft where the only suspension is the flexing of the main gear legs. With a strain gauge on all gear legs you can calculate an exact aircraft mass and accurate CG location at any time when you are stationary (i.e. full weight on the wheels and not bouncing along an uneven surface to give you an oscillating result.

This is much more tricky in an airliner or more complex gear arrangement as you have shock absorbers on the legs or around them, perhaps a fluid pressure sensor in the shock absorber would work but it would be a real pain to maintain and replace if broken.

I also seen a concept advocating putting strain gauges on seat legs, and having them feed into a pax load calculation algorithm, but this would mean having to wire seats into the aircraft further than already, not to mention that strain gauge installation is an extremely skilled job requiring precise alignment and good bonding. You could very well have an incorrectly placed strain gauge feeding erroneous results and not even suspect it.
 
access-air
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RE: Actual T/O Weight A Mystery?

Sat May 21, 2011 3:54 pm

At the small commuter I used to work for, Propheter Aviation, we flew Cessna 402s and later Chieftains into and out of OHare. We ALWAYS had to ask our passengers their actual weight. The planes were really too small to allow anything bigger than a purse to be carried onboard. Aircraft seating less than 10 passengers were required to have actual passenger weights.
Some people didnt mind giving you their weight. One lady almost slapped me when I asked her for her weight. I asked politely. She wasnt too happy but she told me anyhow.. And of course, you will get those that are vain lie....In this case, Vanity = Death.....If they refused then you had to guess. Then of course they wanted to know what you guessed...LOL...I simply told them I was fair in my guess...

But getting back to the point of the thread. It would not surprise me one bit if EVERY airliner flying today goes out at or over gross weight. In addition to passengers being heavier, that also carry more. With the advent of ovehead bins in aircraft versus the old hat racks, we all know that what people carry on is probably the weight of a small child.

Ive been on a fully loaded (with passengers) American 727-200 taking off from OHare...It was hot and muggy that morning....I will bet that we used nearly every bit of runway 27L. And our climb was anemic at most. Now just think how much lighter that plane might have been without all that heavy carry-on baggage.....Perhaps not much but you never know...
With the exceptions medical devices such as my BiPap machine that I must now carry with me, We really should check our baggage. That way, it is weighed and weights would be more accurate.. But I guess since we all operate in the realm of faith, we will continue in the practice of "law of averages" when it comes to loading up airliners...That is until an engine pukes on takeoff and the airplane cannot continue flying and it crashes....

Just my two cents..

Access-Air
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Fabo
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RE: Actual T/O Weight A Mystery?

Sun May 22, 2011 4:12 pm

Quoting Access-Air (Reply 26):
Now just think how much lighter that plane might have been without all that heavy carry-on baggage.....Perhaps not much but you never know...

It would all just go down to the belly. But it would be weighed precisely, as you point out a bit later.

Quoting jackmidd (Reply 22):
I was about to ask this question about actual weight vs estimated weight but it appears to have been answered. I was also going to ask how a pilot can determine takeoff thrust without having an accurate weight, here in Aberdeen we have a very small runway yet i've been on what appeared (hand luggage) to be a very heavy A321, takeoff thrust must've been 100% needless to say it was reduced almost instantly after gear retraction. A 321 departing LHR feels to go along the runway for twice the distance on a much lower thrust setting, are these settings determined by an average too?

There are programs, that compute required power, considering runway length (both stop distance and go distance for engine out at critical point), weather, obstacles in climb, etc. They do do this based on the guesstimate of weight calculated by the method above, though. Still if we allow for 10% mistake in pax weight, that would probably account to far less than 5% of TOW. More like 2%.
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