How is gross weight determined?

[Malybenek]

How is the gross weight measured in a modern passenger aircraft before it is dialled into the FMC?

Also, how do you determine the exact location of the centre of gravity which depends on the actual passengers and cargo distribution?

Thanks for any inputs.

Malybenek

[Earthmover]

It is quite straightforward actually.
The aircraft's basic weight is known from the time it rolls out of the factory, and its weight is regularly re-calculated at major engineering checks. So then various bits are added, such as the engine oil, library of manuals, all safety equipment etc, and whatever the crew composition is on that flight, and we get the APS weight (aircraft prepared for service) We start the loadsheet with that figure and add the catering and bars, all the passengers' weight, their bags and any cargo and this becomes the ZFW (Zero Fuel Weight - a nice term, because it actually describes itself perfectly, unlike some other aviation terms!) To this we add the fuel and we get the TOW or the Take off weight (plus an allowance for taxying)

As for the Cof G - well it is usually described as a percentage of MAC (Mean Aerodynamic Chord) which is a figure we can use easily to derive the setting for the stabilizer on Take - off. As you check-in, the computer integrates your weight (a standard value) your seating position, and the weight of your bags into the loadsheet which is presented to us with the correct TOW, MAC and Stabilizer setting when everyone and everything is on board - and off we go! If we have to do a manual (hand-written) loadsheet - we, the crew, decide how the aircraft is to be loaded - and locate the C of G on a graph, not difficult on my type (737).

Hope this is of some help. :)

[Malybenek]

Earthmover,

Many thanks for such a concise and clear explanation.
Appreciate it. :p :p :p

Malybenek

[Pilot Pete]

Earthmover

presented to us with the correct TOW, MAC and Stabilizer setting when everyone and everything is on board - and off we go!

not the last half a dozen I've been presented with....every single one of them had an error in them. Crap in, crap out!!!!!!

Be warned Malybenek!!!!!

PP

[Malybenek]

Pilot Pete,

Let's imagine a situation as follows:

You are in charge of a charter flight from somewhere (say in South America) and about to take off from a short runway whose elevation is 12000' and it's extremaly hot outside (50 Celsius).
You suspect that the guys who provide you with the loadsheet data are among the 'half a dozen' careless blokes and you can't rely on their information.
You sort of judge that the available runway lenght will do yet you are not quite sure if it will not require firewalling the throttles, etc.

How do you handle such a situation???

Do I exaggerate?
Please write something reassuring. ;)

Malybenek

[Earthmover]

Pilot Pete

Yeah, of course I've had a few of those - including one memorable occasion when not only were the weights wrong - but so was the aircraft type!

However, I sort of assumed malybenek was an enthusiast, rather than a student pilot and described the ideal. Perhaps he could let us know?

[Malybenek]

However, I sort of assumed malybenek was an enthusiast, rather than a student pilot and described the ideal. Perhaps he could let us know?

Correct, I am an enthusiast (a keen 737 fan) and aviation photographer. If I were a pilot I wouldn't have asked about issues that are so obvious to you PROs. ;)

Anyway, engineering is my background and that's why I'll always have thousands of questions about those flying miracles. :)

[FlyingForFun]

Hope you don't mind some input from someone who's not actually involved in commercial aviation... at least not yet. I think it might be important to distinguish between two types of error. There's the "this number is close enough" type of error, and then there's the "I screwed up" type of error.

"This number is close enough" refers to the fact that you can't know the exact details of everyone and everything on board. Standard weights are used for passengers, you can't account for passengers wandering around during the flight, and so on. In the 2-seater aircraft I fly, this type of error is not acceptable - passengers don't move around in flight, and if there's any doubt about whether I'm overweight or not I'll weight myself, my pax and my baggage. On the other hand, this type of error is always present in transport aircraft, and the weight+balance limits are designed with enough tolerance to deal with the errors safely.

"I screwed up" errors are the type Pilot Pete is talking about, where someone does something wrong, through incompetence, or a genuine human error. These are not acceptable, and airlines have procedures in place to ensure that, if they do happen, they are noticed well before take-off. I'm sure that the commercial pilots can describe the procedures in their particular airline. But I think it's fair to say that, in all cases, the final check is for the captain to look at the figures and make sure they are roughly what he's expecting.

FFF
--------------

[Malybenek]

But I think it's fair to say that, in all cases, the final check is for the captain to look at the figures and make sure they are roughly what he's expecting.

That explains why they are called 'CAPTAINS', doesn't it?

Thanks FFF for your post. :cool:

Malybenek

[Pilot Pete]

Flying for Fun

But I think it's fair to say that, in all cases, the final check is for the captain to look at the figures and make sure they are roughly what he's expecting.

You are correct in saying standard weights are used for pax and hand baggage (we had a European destination weight and a Non-EU weight). These have the tollerance that you mention. The rest of the weights that go to make up the load sheet are much more accurate, being actual weights, thus, if the crew know the number of passengers and the breakdown between males, females, kids and infants they can come up with their own ZFW and T/O weight which the load sheet should match exactly, not roughly. Hope that doesn't come across as being pedantic. Thus the Captain signs a correct loadsheet, not one presented with a figure that roughly matches what he expects. Usual errors are wrong amount of fuel (especially the amount allowed for taxy, incorrect APS weight (possibly out of date or for another a/c), incorrect representation of where the bags are actually stowed in the a/c holds, which leads to an incorrect trim position, etc etc.

Earthmover

yeah, it's the written word as opposed to speech - we once had a loadsheet for a Monarch flight from Palma to Luton for an Airbus.......we were a jmc 757 going to Manchester!!!!!!!

Malybenek

There is no guess work involved, we have a performance manual which contains all the required data for every runway we plan to depart from (if not we call Ops who can get the info for us). This covers all the conditions, dry, wet etc etc and all we do is enter the correct table with the outside air temperature and the column for the wind component (head or tail and speed from the ATIS). This result gives us a limiting weight to get safely airborne off that runway. We also check the climb limiting weight from this table (the weight you can actually carry to achieve the minimum climb profile) and also the Brake limiting weight which only usually comes into consideration on a shorter runway in conditions like you mention. This is for the rejected take off case and gives the maximum weight that we can have to stop from V1 with the available braking capability of the a/c. We then compare all these figures and take the most limiting weight as our 'maximum T/O weight'. If we've cocked up and we are above this weight then usually bags would be offloaded first to get us back below it. If we were a switched on crew ;) we would have thought about the conditions whilst sitting on the ramp during turnaround sipping tea and would see what we could do to ensure we didn't go over that weight; such as can we put on less fuel (obviously still at least plog fuel unless we plan to tech stop), use an en-route alternate which allows you to depart with less than 5% contingency fuel to replan at a pre-determined time in flight to see if you have made up fuel against the plog figure (which is often the case due shortcuts etc)so that you now have your 5% etc etc. Or plan to put less bags on in the first place and so on.

If there is 'excess' runway available i.e. using 'firewalled' levers would get you airborne halfway down the runway then we would look at the tables again and 'de-rate' our T/O thrust to a calculated figure to get us safely airborne using a bit more of that excess runway whilst saving wear on the engines, again the calculations are all based on deciding to stop at a particular speed that leaves us enough runway left to stop on or getting airborne at a speed that allows us to climb safely away at the minimum required climb profile in the worst case scenario; an engine failure just at your decision speed of 'V1'. These speeds are all taken from the same table as is the acceleration altitude (again, for the engine out case) and any emergency turn (if there is an obstacle that you cannot out-climb in the engine out case there will be a published turn away from it to a direction that will be safe for your single engined climb)

So there you have it, a bit drawn out, but there is no black art to it, just follow a set of calculation rules and comparisons to come up with a limiting weight for the runway and a set of associated speeds. All of the tables have an in built safety margin to allow for 'the real world'.

PP