737-700 Captain. Weight disrubution

[kehenno]

Please could any Captain of a 737-700 explain the fact (if there is one) that if there were 118 passengers on a capacity plane of 148, (737-700), that on take off, any pax that had moved to a spare seat would have to return to their original seat, yet landing did not matter. Is this a regular occurance or just at a captains disgrestion. I flew as a pursar on a A320 and never had to move a pax ever.

Thanks

[flyboyike]

Consider: what changes between takeoff and landing?

[MrBernoulli]

"737-700 Captain. Weight distribution"

Mostly round his waist and chin!

[Drop The Dunlops]

I think what you're referring to here is the 'free seating' policy that most UK users of the 737-700 have in place.

If the load is less than full, we like to spread the load throughout the cabin evenly. This is because the trim setting on take-off is calculated based on an even spread of passengers.

If we do not do this, on take-off, the plane will be either nose heavy (too many pax at front) or tail heavy (too many pax at back). This means the plane will either be very difficult to rotate at the correct speed or will try to rotate too early, depending on the loading bias.

It's not so critical on landing as we trim as necessary to keep the airplane at the correct attitiude and speed, providing the load is not out of CofG limits of course.

All airlines that have allocated seating calculate the correct trim based on actual position of the passengers, therefore negating the need to reposition pax once on board.

The whole scenario is not really that crucial on a shorter airplane like the -700, but certainly is on the -800/-900. I would imagine any airlines operating the free seating policy on the 320/321 have the same concerns.


DTD.

[Mr Levitator]

Take Off MAC and subsequently Take Off Trim setting is calculated on the allocated seating configuration. Change this before getting airbourne and these setting just won't work. If pax move in flt, the autopilot will gradually trim with these small changes and keeps the aircraft in trim, starting landing run "in trim". Hence moving before Takeoff, there is no way to be "in trim" other than using the seating configuration allocated by the airkiosk and subsequently the mass and ballance computers.

Mr L.

[ft]

Another fact for the ongoing investigation:

I do not have W&B info on the -700 available, but I had a gander in the WBM for the -500. The aft limit for flight and landing in the lower half of the weight envelope is roughly 1%MAC aft of that for takeoff. Perhaps this carries through to the -700. Anyone with the facts on hand?

Rgds,
/Fred

[j_davey]

put simply landing is a transient maneuver and will have varying trim inputs, therefore any small variation in Cg will not be terribly important, correct me if i`m wrong.

[ft]

j_davey,
unless you are at either end of the envelope to begin with, you'll be fine. Too far aft and you could be looking at overrotating the flare and having a tail strike. Too far fwd, and you could end up running out of energy too soon in the flare and wheelbarrowing it in.

I have a feeling this is not news to you, but perhaps it will be to others reading this thread.

Rgds,
/Fred

[j_davey]

for sure!, and if you are close to w&b limits there would normally be a comment on the Loadsheet saying for example close to wb limits - observe at lmc or if a manual loadsheet, you`d have to be in the shaded area that says "check cabin" if so, then u`d want people in their assigned seats otherwise all is good i suspect!

take it easy.
jd.

[john_tullamarine]

From a practical point of view ..

(a) don't go outside the certification envelope limits. This puts a significant load on the system designer and the users to make sure that the system is both workable and valid.

(b) don't calculate too much of a misload so that the stab setting is too far wide of the mark. A small mis-trim is not going to be a problem .. but a major mis-trim might well be a show stopper ... try a max aft CG takeoff in the sim with the stab set for a max forward load .. guaranteed to get everyone's undivided attention if the pony rears its head at (VR - a lot of knots).

Main thing is for the pilot to use the loading system in the manner intended by the designer and (hoperfully) published in operational documents ..

[j_davey]

i`ve heard stories of one or two 737-800`s sitting on their tail when takeoff thrust was applied due to a very tail heavy cg, is this true? i can see it in my head due to the moment of the force from those low slung engines but find it hard to believe that anyone could grossly miss-load an aircraft this way....

[FlightDetent]

I have seen pictures scratched tail on T/O due to gross mis loading. It is possible.

[Mac the Knife]

Forgive me asking, but when individual passenger weights can vary from 45k to 145+kg, how do you work it if you just count warm bodies?

Mac

[john_tullamarine]

how do you work it if you just count warm bodies?


You don't.

The loading system should account for appropriate population variations in the error analysis and the basic construction of the design if necessary. No simple answer as to how this might be done as there is a range of techniques available to achieve the desired goal.