Air NZ - how they get plane weights?

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Batman

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#195799 4-May-2016 20:29

So apparently if a plane that has a max take off weight of 77 tonne reaches 77 tonne [who knows how they know it's reached 77 tonnes), you kick off the 70kg person (+30kg luggage) who booked their ticket last. That 100kg will convert a suddenly unsafe plane to a suddenly safe plane?

I'm sure there is more to this so please enlighten me :)

http://www.stuff.co.nz/travel/travel-troubles/79604676/air-new-zealand-denies-woman-from-boarding-heavy-tongan-flight

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#1546839 4-May-2016 20:33

it no different to a crane that has a safe working load of 2.5 tonne or what ever and you put 2499kg vs 2501kg on it.

the extra weight may put undue stress on components and increase the chance of failure to an unacceptable level.

safe working loads and weights are there to prevent premature failures due to loading they were not designed for

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#1546841 4-May-2016 20:37

It's illegal to fly an overloaded aircraft.

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#1546855 4-May-2016 21:09

I don't suppose they weigh the plane? Just adding up some random (and some measured) numbers?

And they must have gotten 77,050 kg?

firefuze

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#1546859 4-May-2016 21:17

The plane doesn't sit out on a weigh station if that is what you are asking?

The only weights where averages are used are for passengers, everything else is measured.

That aside, I do strongly feel this could have been dealt with a lot better, such as remove luggage etc. But once the aircraft reaches its max take off weight its black and white - its illegal as mentioned above (and unsafe).

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#1546862 4-May-2016 21:19

This is part of the weight and balance section of CAR 121 (121 relates to large aircraft transport operations)

121.303 Goods, passenger, and baggage weights

(a) Subject to paragraphs (b), (c), and (d), a holder of an air operator certificate must ensure that for every air operation conducted under the authority of the certificate the weights of the following items that are carried on the aeroplane are established:

(1) the total actual weight of goods, excluding baggage:

(2) the total weight of passengers and their carry-on baggage:

(3) the total weight of crew members and their carry-on baggage:

(4) the total weight of checked baggage.

(b) The total weight of passengers and their carry-on baggage must be established by using only 1 of the following:

(1) the actual weight of every passenger and their carry-on baggage:

(2) a standard weight for every passenger and their carry-on baggage that is established by the certificate holder and detailed in the certificate holder’s exposition:

(3) the following applicable standard weight for every passenger and their carry-on baggage:

(i) 15 kg for a child under 2 years of age:

(ii) 46 kg for a child of the age of 2 years and under the age of 13 years:

(iii) 86 kg for a person of or over the age of 13 years.

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#1546868 4-May-2016 21:37

It's unlikely she would have been the only passenger removed.

It's not uncommon to find people bumped off planes due to weight restrictions when freight takes priority. I'm aware of a few cases last year where Qantas were bumping people ex WLG because perishable freight took priority.

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djtOtago

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#1546869 4-May-2016 21:38

Or they could have removed 100kg of fuel.
For an Airbus A320 thats only about 2 minutes flight time lost.

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#1546872 4-May-2016 21:47

firefuze:

The only weights where averages are used are for passengers, everything else is measured.

Volt:

This is part of the weight and balance section of CAR 121 (121 relates to large aircraft transport operations)

121.303 Goods, passenger, and baggage weights...

(3) the following applicable standard weight for every passenger and their carry-on baggage:

(i) 15 kg for a child under 2 years of age:

(ii) 46 kg for a child of the age of 2 years and under the age of 13 years:

(iii) 86 kg for a person of or over the age of 13 years.

Ah, so it is very prescriptive, for a 200 pax plane the max error one could get is ... 17000 kg +/- ... ? 3 tonnes if every person happens to be 15kg heavier.

Ok that makes sense now if everything is measured other than the passengers.

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#1546873 4-May-2016 21:48

djtOtago: Or they could have removed 100kg of fuel.
For an Airbus A320 thats only about 2 minutes flight time lost.

I was thinking the same, but didn't know what 100kg of fuel meant for the flight. Presumably they have cut the fuel to the lowest they could, but if it's only 2 minutes [at max capacity, - as I presume fuel consumption would depend on a lot of things, including weight of plane] ... I guess safety is safety, maybe that 2 minutes were policy.

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#1546929 5-May-2016 00:51

Since the flight was going to Tonga. My guess is that they completely fill the planes fuel tanks. So the plane can fly NZ - Tonga, and Back to NZ without needing to refuel. If that was the case they could easily have removed quite alot of fuel to save weight. And then refuel in Tonga, but I bet the reason is jet fuel is probably expensive there.

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#1546932 5-May-2016 01:28

I'm still struggling with the title of this thread. How is Air NZ's requirement to offload a passenger so they can carry more (required) fuel a maths fail? If anything it shows a failure by people to understand how an aeroplane works. As for 2 minutes of fuel, that equates to about 5 nautical miles at typical approach speeds, so I guess you would be happy to swim the last bit to the airport? Yes, an exaggeration, because of reserves are carried, but just as an airliner isn't allowed to takeoff overweight, it's not allowed to depart with insufficient fuel to get to its destination or a suitable alternate airport if it's destination closes unexpectedly. The article even references the fact that a more distant alternate was required than would normally have been used.
So there's some maths for you,

more distant alternate = more fuel required

So

Total Weight(W) = Empty Weight(E) + fuel(F) + payload(P)

Where the first two (W and E) are fixed, then maths says if F increases then P must decrease.

Oh, and @aredwood, your guess ^ is wrong

“We’ve arranged a society based on science and technology, in which nobody understands anything about science technology. Carl Sagan 1996

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#1546936 5-May-2016 05:19

there has been at least 1 crash caused by the "average" weight being used to calculate passenger total weight.

From Wiki

Although the pilots had totaled up the take-off weight of the aircraft before the flight and determined it to be within limits, the plane was actually overloaded and out of balance due to the use of incorrect Federal Aviation Administration (FAA)-approved passenger weight estimates. When checked, the National Transportation Safety Board found that the actual weight of an average passenger was more than 20 pounds (9 kg) greater than estimated. After checking the actual weight of baggage retrieved from the crash site and passengers (based on information from next-of-kin and the medical examiner), it was found that the aircraft was actually 580 pounds (264 kg) above its maximum allowable take-off weight with its center of gravity 5% to the rear of the allowable limit.

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#1546963 5-May-2016 07:55

Dingbatt: I'm still struggling with the title of this thread. How is Air NZ's requirement to offload a passenger so they can carry more (required) fuel a maths fail? If anything it shows a failure by people to understand how an aeroplane works. As for 2 minutes of fuel, that equates to about 5 nautical miles at typical approach speeds, so I guess you would be happy to swim the last bit to the airport? Yes, an exaggeration, because of reserves are carried, but just as an airliner isn't allowed to takeoff overweight, it's not allowed to depart with insufficient fuel to get to its destination or a suitable alternate airport if it's destination closes unexpectedly. The article even references the fact that a more distant alternate was required than would normally have been used.
So there's some maths for you,

more distant alternate = more fuel required

So

Total Weight(W) = Empty Weight(E) + fuel(F) + payload(P)

Where the first two (W and E) are fixed, then maths says if F increases then P must decrease.

Oh, and @aredwood, your guess ^ is wrong

I didn't understand how they calculated 77 tonnes with such precision that a skinny lady had to be taken off. to meet the threshold.

[Thanks for the pointer I've amended the title]

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#1546964 5-May-2016 07:57

Ramjet007:

there has been at least 1 crash caused by the "average" weight being used to calculate passenger total weight.

From Wiki

Although the pilots had totaled up the take-off weight of the aircraft before the flight and determined it to be within limits, the plane was actually overloaded and out of balance due to the use of incorrect Federal Aviation Administration (FAA)-approved passenger weight estimates. When checked, the National Transportation Safety Board found that the actual weight of an average passenger was more than 20 pounds (9 kg) greater than estimated. After checking the actual weight of baggage retrieved from the crash site and passengers (based on information from next-of-kin and the medical examiner), it was found that the aircraft was actually 580 pounds (264 kg) above its maximum allowable take-off weight with its center of gravity 5% to the rear of the allowable limit.

I'm pretty sure half the time I fly my baggage weights are either rounded up/down, totalled up (with rounding), and sometimes never recorded.

Having said that, I'm not sure what happens at end of the cargo belt, whether they weigh before loading.

I presume this is slightly different in different areas of the world.

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#1546998 5-May-2016 08:58

joker97:

Dingbatt: I'm still struggling with the title of this thread. How is Air NZ's requirement to offload a passenger so they can carry more (required) fuel a maths fail? If anything it shows a failure by people to understand how an aeroplane works. As for 2 minutes of fuel, that equates to about 5 nautical miles at typical approach speeds, so I guess you would be happy to swim the last bit to the airport? Yes, an exaggeration, because of reserves are carried, but just as an airliner isn't allowed to takeoff overweight, it's not allowed to depart with insufficient fuel to get to its destination or a suitable alternate airport if it's destination closes unexpectedly. The article even references the fact that a more distant alternate was required than would normally have been used.
So there's some maths for you,

more distant alternate = more fuel required

So

Total Weight(W) = Empty Weight(E) + fuel(F) + payload(P)

Where the first two (W and E) are fixed, then maths says if F increases then P must decrease.

Oh, and @aredwood, your guess ^ is wrong

I didn't understand how they calculated 77 tonnes with such precision that a skinny lady had to be taken off. to meet the threshold.

[Thanks for the pointer I've amended the title]

I suspect that they have load cells in the landing gear to measure the actual weight of the aircraft, and calculate its centre of gravity. Since individual passengers aren't weighed, there's no way of knowing how much fuel can be loaded to keep the aircraft under maximum takeoff weight, and the fuel is loaded before the passengers. So the amount of fuel (& freight) to load has to be a guesstimate, based on the average weight of a passenger. The ICAO-mandated guesstimate is 70kg... if the actual average weight is only 1kg above that, you're 300+kg in error. Offloading 100kg of fuel isn't an option.

I suspect also that it may not have been the pilot's decision... the airplane computers themselves probably won't allow it to start if it's overweight. I don't think it's too much to expect that the sensors have the precision to distinguish between 77000kg and 77100kg... that's .12%, so you could do it with a 10-bit A/D. Whether those sensors are accurate and repeatable enough to measure that weight to that accuracy is moot. I suspect that if they had got all the passengers to jump up and down a few times, they could probably have made it answer less than 77000.

I guess they could have offloaded the meals and/or drinks instead. Or sent all the passengers to use the toilets in the terminal. Or got some skinnier hosties.

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