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## Jeeves

302 posts

Ultimate Geek

#1547013 5-May-2016 09:26

frankv:

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.

Some do. But there's too many variables for them to be taken over and above estimated weights following standard guidelines. Gust of wind comes along while weighing? Oh look the leeward side just gained 500kg. Plane gets iced up before taxiing to the de-ice station - thats 200kg of ice. etc.

This is all a storm in tea cup over an article about a self-entitled whiner who doesn't appreciate the complexities around operating a safe airline.

## MikeB4

15555 posts

Uber Geek
Inactive user

#1547018 5-May-2016 09:36

AirNZ has been successfully running their business for more than five minutes, I am sure they know the capabilities of their fleet and also know the various regulations the must comply with. This story is just someone after their 15 minutes and a free gift.

## Batman

22933 posts

Uber Geek

Trusted

#1547035 5-May-2016 10:38

MikeB4:

AirNZ has been successfully running their business for more than five minutes, I am sure they know the capabilities of their fleet and also know the various regulations the must comply with. This story is just someone after their 15 minutes and a free gift.

Agreed. But still interests me that I am lucky to be only a few kgs heavier than what air NZ thinks I am. I have to say they are very PC in their weight estimation :)

Involuntary autocorrect in operation on mobile device. Apologies in advance.

## Dingbatt

4574 posts

Uber Geek

#1547069 5-May-2016 11:01

MikeB4:

AirNZ has been successfully running their business for more than five minutes, I am sure they know the capabilities of their fleet and also know the various regulations the must comply with. This story is just someone after their 15 minutes and a free gift.

Goodness me, imagine Stuff or the NZ Herald (shabby little tabloid) creating a storm in a tea cup over someone's complaint on social media!

Yes I feel sorry for people who's holiday plans are ruined by circumstance. But that is what travel insurance is for, so at least there isn't financial loss added to the hurt. In the past I have experienced fellow passengers being asked to volunteer to be offloaded if there is a weight problem. The reports don't say if that was the case this time.

A little bit of research shows;

Aircraft (generally) don't have load cells in their undercarriage,
as such there is no starting interlock,
Bags are weighed after they disappear off the conveyer and are put in containers.

Airlines are periodically required to carry out statistical analysis to validate the average weights they use. I imagine this is difficult to do and would end up with social media posts (that are picked up by the media) when people are offended at being asked to step on some scales before boarding.

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## CitizenErased

184 posts

Master Geek

#1547090 5-May-2016 11:53

No one's mentioned the alternative option is to offload some freight, but that's never going to happen. Freight is what keeps the airline in business; passengers are just the cherry on the top.

## hairy1

2974 posts

Uber Geek

Trusted

#1547181 5-May-2016 13:13

Hello everyone.

I had a long post written about how load control works and flight plan fuel is determined this but decided against posting it.

If I can just say that if you don't know how something works then it may not be the best idea to post how you think it works.

Cheers, Matt.

My views (except when I am looking out their windows) are not those of my employer.

## hairy1

2974 posts

Uber Geek

Trusted

#1547191 5-May-2016 13:36

CitizenErased:

No one's mentioned the alternative option is to offload some freight, but that's never going to happen. Freight is what keeps the airline in business; passengers are just the cherry on the top.

Nope. Not on a A320 to the islands.

My views (except when I am looking out their windows) are not those of my employer.

## elpenguino

1460 posts

Uber Geek

Subscriber

#1547341 5-May-2016 20:39

hairy1:

If I can just say that if you don't know how something works then it may not be the best idea to post how you think it works.

Cheers, Matt.

That would make the internet a much quieter place. Can i still talk about politics?

## Scott3

1134 posts

Uber Geek

#1547352 5-May-2016 21:31

Weights for passengers and hand baggage are estimated only, however the plane can't fly with even 1kg more estimated weight than the limit.

Air NZ oversold the plane. It probably had some last minute urgent freight on it.

Overselling is standard airline practice, If everybody shows to a full flight, and brings their full check in allocation, people or freight will have to be bumped. In this case Air NZ chose to bump one passenger at their home airport, rather than bumping fright, or leaving 5 passengers bags behind.

The whole "islanders are heavy" angle really has nothing to do with it.

Given it's 2016 planes should really have load-cells in their landing gear, or scales at the gate or runway approach, so the pilot can check the actual weight and weight balance against the limits and estimates made on the load sheet. Current system means a full plane that is carrying say a professional wrestling tour onboard will be dangerously overweight.

Of course with more accurate data, safety margins can be reduced, improving productivity.

## Technofreak

3978 posts

Uber Geek

Trusted

#1547363 5-May-2016 22:10

Scott3:

Air NZ oversold the plane. It probably had some last minute urgent freight on it.

I don't think it was a case of over selling or the need to add urgent freight.

The explanation given was there had been a late change in the alternate airport that had to be provided for, in the event the aircraft isn't able to land at the intended destination. This change required a greater fuel load than normal, the only option was to offload passengers.

This sort of thing happens regularly. Every day on some routes an airline will not know know till the day exactly how many passengers they will be able to carry on a particular flight. The en-route winds will dictate how much fuel is needed and can restrict the number of passengers carried.

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## Technofreak

3978 posts

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#1547366 5-May-2016 22:19

Scott3:

Weights for passengers and hand baggage are estimated only, however the plane can't fly with even 1kg more estimated weight than the limit.

Current system means a full plane that is carrying say a professional wrestling tour onboard will be dangerously overweight.

Of course with more accurate data, safety margins can be reduced, improving productivity.

Standard weights are used for passengers and hand luggage. This weight is regularly checked to ensure it is representative of the passenger population. It is not really an estimate as you allude to.

When it comes to sports teams and the like, a higher standard weight is used.

The data used now is pretty accurate.

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## nickt

84 posts

Master Geek

#1547367 5-May-2016 22:25

Dingbatt: Airlines are periodically required to carry out statistical analysis to validate the average weights they use. I imagine this is difficult to do and would end up with social media posts (that are picked up by the media) when people are offended at being asked to step on some scales before boarding.
I've saw this in action prior to a long haul international flight at Auckland. At a convenient narrow point just prior to the gate lounge, they were asking passengers if they would mind being weighed; hand luggage too. It was optional and there was no pushiness, but the 'someone in a uniform asked me to do something' effect meant most complied. Presumably a statistician still has to figure out if there's any correlation between weight and refusing the request, and how to correct for that.

## Linuxluver

5615 posts

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Trusted
Subscriber

#1547368 5-May-2016 22:26

joker97:

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

It's probably illegal to fly overweight....and their insurance company would laugh itself silly if anything happened and a claim was made.

It's also safer not to fly overweight......the top allowance being the maximum allowable.

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## Wheelbarrow01

1019 posts

Uber Geek

Trusted
Chorus

#1547405 6-May-2016 00:50

Technofreak:

Standard weights are used for passengers and hand luggage. This weight is regularly checked to ensure it is representative of the passenger population. It is not really an estimate as you allude to.

When it comes to sports teams and the like, a higher standard weight is used.

The data used now is pretty accurate.

Are you sure? When US Air 5481 crashed in 2003 due (in part) to overloading, the NTSB discovered that the FAA's approved passenger weight estimate not being reviewed since 1936 - that's 67 years! People in general have become taller and heavier since the 30's.

I'm not fat by any stretch of the imagination, yet I'm 93kgs (something which came as a great surprise to me recently), so even I am 13kg over the 80kg estimate quoted in the news article. Imagine for a moment that 168 Simons board an A320 to Australia. That's 2.1 tonnes of extra weight not being accounted for, so you had better hope the maximum take off weight tolerances have enough of a buffer - especially if all those Simons have 9kgs of carry on luggage instead of the prescribed 7kgs, plus 3 bottles of duty free spirits each - that's another tonne..... of course I'm being silly, but you get the idea.

The views expressed by me are not necessarily those of my employer Chorus NZ Ltd

## Technofreak

3978 posts

Uber Geek

Trusted

#1547419 6-May-2016 07:36

I'm talking about what happens in New Zealand, I have no idea what the rest of the world does.

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