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FriscoHeavy
Topic Author
Posts: 1763
Joined: Tue May 27, 2014 4:31 pm

### How Often Are Planes Weighed?

How often do they get weighed?

I'm wondering due to possible performance implications.

The newly announced flight from FLL-DXB got me wondering. If the plane (in this case a 777) is a few thousand pounds heavier than believed, could this have any consequences on takeoff?

Let's say the performance numbers on this flight calculate that the takeoff roll is 8,500 ft (on a 9,000 ft runway).

However, let's say that in reality, the plane's weight (minus passengers and cargo) is 3,000 lbs heavier than thought and the cargo/passengers/bags is 4,000 lbs heavier than planned since average weights are used).

This totals 7,000 lbs over the numbers used for takeoff calculations. Is it possible this difference could have a material impact on the takeoff roll?

On a longer runway, this wouldn't be a problem....but when you are lifting off near the end of the runway like at FLL, could this be problematic?
Whatever

Natflyer
Posts: 642
Joined: Fri Oct 11, 2013 9:29 pm

### Re: How Often Are Planes Weighed?

In EASA land at least every 4 years.
https://www.easa.europa.eu/system/files ... pdf#page11

Aircraft, like people tend to put on weight with age.

rmilstre
Posts: 25
Joined: Mon Sep 12, 2016 11:42 pm

### Re: How Often Are Planes Weighed?

I'll avoid answering your question, but focus instead on the risks involved. In your example, you're (potentially) concerned about being 7000 pounds overweight. The lightest listed MTOW for a B777 on wikipedia (YMMV) is 545,000 pounds. In this example, then, you're talking about 1.3% overweight. From an engineering perspective, if you're cutting things so fine that being wrong by 1% results in a crash, you're cutting things far too fine. And the safety record of the global aviation industry has proven that, in general, the margins are much greater than 1%.

This is not to say that the MTOW should be knowingly exceeded. There may be margin there, but that margin is not a fixed or a priori known number. The catch is, you also don't know a priori how much of it you've used. You've already listed some "margin consumer" (fat pax w/ overly heavy bags, planes that are overweight, error in the current temperature measurement, temperature gradient along the runway, contaminants in the fuel that prevent engines from generating predicted thrust, etc.). They're all small items in the grand scale of things, but they can add up. (Some can cancel out, too, if you have light bags and the far end of the runway is colder than the near end, for example).

As an aside, even if you weigh the aircraft on a regular basis, I would not be surprised to learn that the scales themselves are only certified to be 1% accurate. So, when the scales say it weighs 296,826 lbs, does that mean 296,826 or does it actually weigh 299, 105 lbs?

The detailed design process accounts for that delta.

-R

Okie
Posts: 4146
Joined: Wed Jul 16, 2003 11:30 am

### Re: How Often Are Planes Weighed?

MTOW of a Boeing 777-300ER is 775,000lbs. Your 7,000lbs would be less than 1% of the MTOW.
Lets hope they have more than 1% fudge factor in their performance calculations.

Okie

FriscoHeavy
Topic Author
Posts: 1763
Joined: Tue May 27, 2014 4:31 pm

### Re: How Often Are Planes Weighed?

Thanks for the replies so far.

I wouldn't be concerned from a technical standpoint per say as I know 1% wouldn't normally be a big deal.

I just didn't know if being 7,000 lbs off in this case could cause the needed runway to exceed 9,000 ft (again, referring to my instance above) or unable to lift off the runway in time if an engine was lost right before rotation.

On a longer runway, it wouldn't be problematic to have a few more miles per hour to the speed to get airborne.
Whatever

rmilstre
Posts: 25
Joined: Mon Sep 12, 2016 11:42 pm

### Re: How Often Are Planes Weighed?

First order, assuming a linear system (which isn't really true, but is probably close enough in the first order), your 7K overweight flight that should have taken of in 8500 feet (per your number) is gonna take of in ~8600 feet. No worries, mate!

-R

Florianopolis
Posts: 333
Joined: Sun Mar 29, 2015 2:54 pm

### Re: How Often Are Planes Weighed?

Random fact: Did you know the Concorde crash was overloaded and out of CG? Probably didn't make a difference, though.

Anyway, here's the FAA circular on determining weight and balance for you to peruse at your leisure. It may answer questions you hadn't even considered - like what about assigned passenger seating vs free seating?
http://www.faa.gov/documentLibrary/medi ... 20-27E.pdf

I'll note a few things:
1: The frequency of weighing FAA-regulated commercial aircraft is generally at least every 36 months. Operators may establish fleet weights with a sample set of identical airplanes, as long as they're all within 0.5% of each other.
2: The statistical likelihood of capturing the "true" weight with standard average passenger weights increases with more samples. This means weight and balance derived from standard average weights is less likely to be correct as the airplane shrinks, to the point that commercial service on airplanes with fewer than 5 passengers must actually weigh everything.
3: The minimum confidence interval required for average passenger and bag weight surveys is 95%. The error goal is 1% or 2%, and surveys must use valid sample populations. For example, not being biased towards connecting passengers, or direction of travel (passengers returning from vacation destinations presumably having heavier suitcases because of souvenirs).
4: The FAA does validity surveys to ensure their standard averages are accurate, and they'll change their standard average when it deviates from "true" by 2%.
5: Where your passenger load is obviously non-standard, such as chartering a sports team, an airline may establish standard average weights for the non-standard passenger profile, or may use actual weights (the FAA even recommends using roster weights!)
6: The standard average female flight attendant weighs 160 pounds.
7: Where using actual weights, the FAA has this gem (the ten pounds is for clothing, btw):

Let's not forget that aircraft performance variation comes from all over the place. A wing leading edge covered in smashed bugs will have different stall characteristics vs a clean and polished airplane. Fuel density and energy content varies. Fresh and aged engines have large differences in performance. Catering weight tends to start concentrated in galleys and distribute throughout the cabin over the course of a flight. Tire treads and brakes wear, sink water drains out masts, cabin air weighs less at higher altitude, passenger weight goes out the outlet valve in exhaled water vapor.

http://en.wikipedia.org/wiki/Air_Midwest_Flight_5481
http://www.faa.gov/regulations_policies ... 0_ch04.pdf

trijetsonly
Posts: 735
Joined: Tue Jul 21, 2009 10:38 pm

### Re: How Often Are Planes Weighed?

Usually, weighing airplanes depends on the size of the fleet, the kind of operation and the responsible authority.
In EASA world it has to be done between every 4 and 9 years. Large airlines usually weigh during heavy checks every 5 to 6 years as lots of modification are done and you don't want do get an additional downtime when the 9 years are full. For weighing the plane has to be defuled, drained, cleaned. Especially defueling Boeing planes for weighing is usually one day or more what gets you an expensive downtime.

The second important thing is, that an operator always has to ensure that he is aware of the current weight and CG. Most changes are covered by recalculating the current weight and CG, based on what has been removed or installed. But at one point you can't be it sufficiently exact anymore (for example after structural repairs) and weighing is necessary. Thats why the weighing during heavy checks is useful.
Happy Landings

Woodreau
Posts: 1924
Joined: Mon Sep 24, 2001 6:44 am

### Re: How Often Are Planes Weighed?

The performance numbers for takeoff allow for the aircraft to take off with one engine inoperative. So if the numbers are run for FLL's shorter runway allow for a two engine takeoff, it also accounts for takeoff performance with an engine failure right exactly at V1.
Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.

FriscoHeavy
Topic Author
Posts: 1763
Joined: Tue May 27, 2014 4:31 pm

### Re: How Often Are Planes Weighed?

Really appreciate the detailed reaponses.
Whatever

bhill
Posts: 1835
Joined: Thu Sep 13, 2001 8:28 am

### Re: How Often Are Planes Weighed?

Huh...I always thought airliners had weight sensors in the LG that told the flight computer the current weight of the plane. Am I to understand that the authorities require a different method to certify an airplane?
Carpe Pices

AirBoat
Posts: 61
Joined: Sun Jan 18, 2015 11:58 am

### Re: How Often Are Planes Weighed?

How is this for margin...
A c130 was loaded kg instead of pounds and it still flew.....(2.2 x max load) this is from incident reports.

Natflyer
Posts: 642
Joined: Fri Oct 11, 2013 9:29 pm

### Re: How Often Are Planes Weighed?

bhill wrote:
Huh...I always thought airliners had weight sensors in the LG that told the flight computer the current weight of the plane. Am I to understand that the authorities require a different method to certify an airplane?

Yes you are. We had weight computers (sensors) in the DC-8 freighters way back when. Weren´t really accurate. So it is weighing and calculations from then on.

trijetsonly
Posts: 735
Joined: Tue Jul 21, 2009 10:38 pm

### Re: How Often Are Planes Weighed?

bhill wrote:
Huh...I always thought airliners had weight sensors in the LG that told the flight computer the current weight of the plane. Am I to understand that the authorities require a different method to certify an airplane?

Most airlines that I know got rid of the sensors in the late 70s, as they are usually faulty and cause delays because of dispatch disagrees.
Today, I'm only aware of Virgin Atlantic and several cargo airlines having an onboard weight & balance system.
Happy Landings

rmilstre
Posts: 25
Joined: Mon Sep 12, 2016 11:42 pm

### Re: How Often Are Planes Weighed?

AirBoat wrote:
How is this for margin...
A c130 was loaded kg instead of pounds and it still flew.....(2.2 x max load) this is from incident reports.

Wow. That's more margin than generally planned for in engineering. I bet that plane had very odd flight characteristics, even given the OEW of the aircraft didn't change!
-R

john7g2
Posts: 4
Joined: Sun Apr 17, 2016 5:24 pm

### Re: How Often Are Planes Weighed?

I left 11 years ago but if I remember correctly...
W&B engineering makes calculations of the expected growth (dust etc). Every few years (think it was 2) we'd weigh 10% of the fleet. We'd get directions of how much ballast to add (usually fwd end of C1), prep the airplane (sump the tanks, drain the lav, remove specific cabin items.. a long list) and then weigh the a/c. We'd compare to the engineering forecast to our reality, record the data and send the results back to engineering. Not sure where it went after that.

Load cells were located at each axle so a 767 would take 5 load cells and a 737 would take 3.

trijetsonly
Posts: 735
Joined: Tue Jul 21, 2009 10:38 pm

### Re: How Often Are Planes Weighed?

john7g2 wrote:
Every few years (think it was 2) we'd weigh 10% of the fleet.

The correct formula for fleets with 10 airplanes or more is (n+51)/10, where n is the fleetsize.
That means, that the calculated amount of planes had to be weighed during the past four years.

Example: Fleet consisting of 50 B737-800:
n = 50
(n+51)/10
(50+51)/10
=10,1

10,1 (rounded up to 11) planes of the fleet must have been weighing during the past 4 years. The 90 other planes can have weighing results up to 9 years old.
Happy Landings

litz
Posts: 2367
Joined: Wed Dec 24, 2003 6:01 am

### Re: How Often Are Planes Weighed?

This is an interesting question ... barring aircraft repairs adding weight (skin work, doublers, etc), kinda makes you wonder when they weight a plane, and it's noticeably off from what it should, what do they find as causes?

john7g2
Posts: 4
Joined: Sun Apr 17, 2016 5:24 pm

### Re: How Often Are Planes Weighed?

trijetsonly wrote:
The correct formula for fleets with 10 airplanes or more is...

Thanks for the correction. I was the one in the hangar doing the weighs and not that involved with the engineering aspect of it.

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