Weight Distribution

[Martu]

Question for the pilots / flight literate on the forum:
I've often wondered how important the distribution of passengers on an aircraft is, in terms of how it affect the weight distribution and balance.

For example, when I've been on particularly empty flights, the Cabin Crew have announced that it's fine to move to a different seat once we're up, but that during take off and landing we need to be in our assigned seats.

Is this due to weight distibution issues, in which case wouldn't this still be the case during cruising? Or is it so that people can be identified should the worst happen during take-off? :sad:

[AtoBsafely]

Before every flight we have to confirm that the planned load is within the allowable centre of gravity range for takeoff and landing (as well as weight limits).

Balance is most critical during takeoff and landing because the speed is low (less control authority) and also that is when the most critical pitching manoeuvres occur. If the CG was too far aft it is very easy to overrotate and have a tail strike on takeoff, or over-flare in the landing. If the CG is too far forward the aircraft doesn't want to respond at all, which can be even worse.

[Martu]

Interesting, thanks for the answer. Is it mainly a fore / aft CG issue, or is left / right balance important as well (I'm sure there is a more technically correct way to describe this - lateral balance?, port / starboard maybe...) - or is that something that can be corrected easily by the pilot?

[AircraftOperations]

More fore and aft I believe, as unless you have a large amount of weight (large number of pax or pallets), you are only going to be a few feet from the centre of the aircraft from nose to tail.

I have been on smaller regional aircraft before where crew have tried to distribute pax across both sides of the aisle when the cabin was 1/2 full.
Whether there was any science in it, I doubt it.

[AtoBsafely]

Lateral balance is not calculated as everything in the fuselage is close to the centre of gravity and any imbalance there can be trimmed out easily. There are limits for fuel imbalances in the wings though.

Most airlines use an averaging system approved by their regulatory authority, which will stipulate an average per person, checked bag and carry on. The weight per person may be refined on the basis of season (winter/summer), and male/female. If the average weights are obviously not going to work - say a charter for a football team, then individual weights should be used. Average weights may not work very well on smaller aircraft because of the small sample.

Freight is always weighed.

The weight of an airliner is calculated for takeoff and landing - to the lb/kg. After we start engines we work to the the smallest unit on the fuel gauge - usually in hundreds of lb/kg.

[42psi]

Lateral balance is most certainly relevent on some a/c .. such as B74F...