Does it matter?

[th3k0p]

A few hours ago I was on a flight back from my hometown to where I was studying. One thing that caught my attention during that flight was when the plane was taxiing to the runway, the stewardess said something I have never heard before (or maybe I did but I wasn't paying too much attention).

"Changing seats without permission is not allowed due to the weight distribution and the balance of the plane"

Something like that. I can't seem to remember in detail.

Does passenger's weight and where they are seating affects the plane's weight distribution and balance? For cargo maybe this can make sense (to me) but for passenger's I'm quite unclear. The reason why I'm having a hard time coming to a conclusion is because lets say for example the plane (in my case A320) was:

- All the weight minus the passenger's (payload,fuel etc..) 60,000 Kg (132277 lbs for all you "imperials" out there)
- Assume there are 80 passenger's and the average passenger weight was 60 kg, A320 has 150 seats = 60 x 80 = 4800 kg (10582 lbs)
- Assume that all 80 passenger's is not seated in an organized manner in the plane (more up front - less at the back, more at the back - less up front)

So with only 8% contributed to the total weight of the plane, does seating arrangement really matter? Will it affect the plane's stability? Am I over thinking? hahaha :P

What do you think? Doesn't have to be correct, I just wanna know your opinion.. Maybe someone can shed some light to this.

-k0p

[Shadeborne]

I wouldn't expect weight distribution to matter much on an aircraft as large as an A320, but on small aircraft (such as the CRJ-200 I just flew on) it supposedly does matter, at least on takeoff and landing.

[Wingman]

Good topic...

What I'd be curious to know -- and it would affect the answer to your question -- is the center of gravity (CG) envelope of the A320 (or whatever aircraft is in question).

Every pound (kilogram) moved forward or aft of the datum point (some imaginary point along the longitudinal axis of the aircraft) moves the calculated center of gravity (by multiplying the weight by the difference in distance to the datum point). The aircraft can only fly safely with a CG that remains within an identified limit in the Pilots Operating Handbook of the aircraft (POH).

Different CG configurations of an aircraft are managed by (among other things) elevator trim. Trim is set at some point prior to takeoff (see the POH and checklists) based on the calculated CG and weight of the aircraft.

Sudden changes of CG can and have resulted in aircraft accidents and incidents. One such event that comes to mind is National Airlines flight 102, a 747 taking off in Afghanistan. Some of the 747's cargo (vehicles) came lose and rolled aft while the aircraft began its initial climb from the runway. The resulting movement of the CG too far aft prevented any corrective control action (primarily including elevator and elevator trim) that the pilots could have taken to avoid a stall and subsequent crash. [Wiki] [YouTube]

To your original point, though, it does appear very unlikely that an average person (or even an above-average person) could move far enough fore or aft of the CG to result in an out-of-flight-envelope condition, hence my question about the CG envelope. How much forward/aft of the CG would 8% of the mass have to move to result in such a condition, and would it be within reasonable limits (inside the cabin) or highly unlikely (passengers hanging on to the tail, outside the cabin)?

Great, all that monologue and all I've done is repeated your question. It's food for thought, at least.

[th3k0p]

Interesting point there Wingman.. You mentioned about the NA flight 102 in Afghanistan, that could be more easily concluded because cargo has a large weight contribution to the overall aircraft's weight. From a passenger's perspective.. I find it quite difficult for 8% of contribution to the total aircraft's weight to have any significant impact on the CG regardless of the position they are seated in (not taking into account the "passengers hanging on to the tail, outside the cabin") hahah!

Even if the 8% had any sort of effect to the aircraft's CG the pilots would probably issue the use of ballast weights to balance it again (very uncommon/ If there are few bags etc.. on the cargo hold)

It becomes a food for thought however if it came to this situation; The reason passenger's are told to sit and buckle up besides the obvious safety reasons are (in my opinion) so that whatever the CG calculations were at the beginning (before takeoff) remains the same and we are then allowed to move freely after a climb of 10,000 feet-ish because the movement of passengers is of lesser concern balance wise and the elevator is fully active and the autopilot will keep the plane in trim/balance. What if.. (for whatever reason) all the passengers in the plane did not buckle up and moved around (like lose cargo's)? Will the 8% be more of an impact then? Will the CG become imbalanced? Very unlikely that this situation will ever happen hahaha but just for thought.

Thanks for sharing Shadeborne. I believe you've got a point there too. Larger aircraft needs much more than 8% of imbalanced weight distribution to have any affect on the CG imo.

-k0p

[Sound]

Seems to me that there will be more than 80 passengers on any A380 aircraft. More like 300?

*edit. I see now you mentioned A320. I'm not an airbus guy, so i don;t know how many passengers they hold.