B744 Weight and Balance

[skol]

I recently flew a sector where we ran short of fuel, our contingency was gone by 2/3 of the sector. No adverse winds, offtrack excursions or lower altitudes and the only possible explanation was that the aircraft was overloaded.
I'm curious if other 744 drivers have a weight and balance system installed that can check the weight on departure.
Apparently a system is built into the aircraft, it's a matter of paying for software and certification.
Thanks.

[Pitch Up Authority]

Yes we do have such a system. Some airlines actually have two.

I am not familiar with your flight plan. How long was your flight and at what time did you burn all that much?

[skol]

It was about a 10 hour sector, we burnt all the contingency around 2200 kgs after 6 hours.
I know some aircraft had them installed years ago but would certainly be handy to check load sheet against actual weight.

[tartare]

Question from ignorant small plane driver.
How is weight and balance calculated in a 744?
Assume it has a datum, just like small a/c's plus max/minimum weights of fuel allowed in each tank (I understand there are tailwing tanks).
How do you factor in pax weight/luggage/freight weight?
Assume there is a preformulated loadsheet which ground crew fill in for you and you sign off?
Is this process digitised in any way?

[skol]

tartare,
Loadsheet is done in the office and sent to the aircraft via ACARS either VHF or SATCOM just before engine start.
It does have a stabiliser tank, fuel is loaded in accordance with the fuelling manual, 9 tanks total.
%MAC is entered into the Flight Management Computer to calculate trim setting.
Cheers

[By George]

I often wonder about the use of standard weights, ie the aircraft is heavier than you think, thus the loss of burn.

[skol]

George,
Me too, average weight has to have increased over the years and when I watch the punters get off they seem to have heaps of carry-on bags to avoid paying for excess baggage.

[spannersatcx]

Apparently a system is built into the aircraft, it's a matter of paying for software and certification.
Thanks.

Not quite that simple, there is a lot of hardware and wiring and a computer to install.

[Feather #3]

If it was ex-LAX to Oz, that's not unusual!

G'day ;)

[skol]

This was across the Atlantic, not usually a problem, a/c normally lands with amost of the contingency.
Interesting though, you guys have the same problem on the Pacific eating up the contingency.
Cheers

[decurion]

In the past accident investigation boards have often recommended the development of onboard weight & balance systems. Attempts to develop an onboard weight and balance system go back to the 1940’s. Unfortunately, many of those attempts failed to deliver a system that was accurate and reliable enough. Therefore implementation of onboard weight and balance systems has been limited. In 1998, an evaluation of the reliability of onboard weight and balance systems conducted by the FAA showed that (cargo) operators had concerns with onboard weight and balance systems. Operators noted reliability problems resulting in unnecessary delays and maintenance burden. However, their biggest concern was with the accuracy of the system. Large differences were noted between the centre of gravity position determined by the onboard systems and the centre of gravity position determined by the operator’s primary weight and balance method. These large differences and the reliability problems resulted in a lack of confidence in the system by the flight crew. These issues generally result from the wide range of the operating environment the onboard systems have to deal with. In many cases the system was deactivated by the operators due to these reliability and accuracy problems. Therefore the FAA stated (in 1998) that the results of its evaluation did not support imposing a requirement to install a system that displays airplane weight and balance and gross weight in the cockpit of transport-category cargo airplanes.

Specifications drafted for onboard weight and balance systems state that the system shall be capable of measuring the gross weight within an accuracy of 1% and the aircraft centre of gravity within 1% of the mean aerodynamic chord (see FAA AC-120-27E). For a large jumbo jet with a mean aerodynamic chord of 8.3 meter this would mean that the system shall be accurate within 83 mm. The system must also self-detect any fault which would significantly degrade the accuracy, so that the probability of an undetected catastrophic fault is less than 1 in a billion flight hours. It is clear that these requirements are very though demands for a primary onboard weight and balance system. Also maintainability of the system is often a problem. This is something that cannot be ignored if an onboard weight and balance system is to become successful.

The most typical onboard weighing system consist of a set of strain sensing transducers in each main wheel and nose wheel axle, a weight and balance computer, and an indicator of the ground attitude of the aircraft. The strain sensors measure the amount each axle deflects and send these data into the computer, where signals from all of the transducers and the ground attitude sensor are integrated. The technical aspects of onboard aircraft weighing systems are too complex to discuss here in great detail. However, it can be easily understood that designing a certifiable primary onboard weight and balance system that works with high a reliability and accuracy under various harsh conditions (high and low temperatures for instance) is very difficult. Despite these difficulties, systems (not primary) for onboard aircraft weighing are available for a number of (mainly) large transport aircraft such as the Boeing B747-400, the MD-11 and the Airbus A300, A320, A330/340. There are a number of examples of incidents in which onboard weight and balance systems saved the day. Indeed an accurate onboard weight & balance system can help in mitigating most weight and balance related occurrences. However, some example incidents also showed their weakness if they are not properly used. New patents are filled regularly for onboard weight and balance assessment systems showing that the ideal system has not been developed yet. Still these systems are often too expensive to be introduced on all aircraft types and for now they are mostly used on large aircraft. However a secondary weight and balance systems could still be of some value in preventing weight and balance related accidents. On some civil transport aircraft such secondary systems are standard.

The advances of an onboard weighing system go further than safety only. In fact the operator can gain more operational flexibility and reduce cost. In theory an onboard weight and balance system should measure the actual weight and centre of gravity location of an aircraft. As a result an operator may not need to include certain curtailments to the loading envelope to account for variables such as passenger seating variation or variation in passenger weight giving more flexibility. However, an operator still needs to curtail the loading envelope for any system tolerances that may result in centre of gravity or weight errors.

As an alternative to onboard systems there are efforts to develop systems to rapidly weigh and automatically track passenger and baggage weight and location data as passengers board aircraft. The rapid development in different technological advances such as hand-held devices and wireless bar code scanners indicate that it may be feasible to compile actual weight data and account for the weight location, which can result in a reliable calculation of actual aircraft weight and balance.