B737-800
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 149
Situational Awareness
No Specific Anomaly Occurred All Types
Aircraft; Human Factors; Procedure
Upon arriving at the gate for this flight; I asked the Agent how many Passengers we were taking out. He said; '25.' I then asked him if he was planning on using the three zone loading schedule instead of the single zone for the -800. He said he was; and we entered into a discussion concerning weight and balance on the -800. I told him I had heard of -800 being loaded out of trim when the Passengers load was light and the load sheet calculated as a single zone instead of the three-zone method. He said he had heard of the issue too which was why he was using the three-zone method; but that he knew a lot of Agents weren't familiar with the problem. He then proceeded to show me on the gate computer that if he entered my flight's weight and balance using the single-zone method; the computer would show the plane in trim. He then switched to the three-zone method and put most of the Passengers in zone one; where they would actually be sitting on the plane instead of evenly dispersing themselves throughout the plane. The load sheet wouldn't calculate and instead showed the error message of the aircraft being out of trim.  The problem is this: if a -800 load sheet is calculated using the single-zone method with a light Passenger load then the plane shows in trim because the load sheet program assumes the Passenger load to be evenly distributed throughout the aircraft. But with a light Passenger load; most Passengers sit up front. Furthermore; the -800 forward cargo bin is loaded by the Ramp before the aft cargo bin. This will create an out-of-trim aircraft; which would have been my situation had the Agent decided to use the single-zone method and had I not bothered to inquire about it. When it is used properly and a warning appears saying the aircraft is out of trim; then either Passengers or cargo must be moved; or perhaps fuel added. This of course will lead in some cases to a late push which there exists operational pressure; especially for the Agents; to guard against. So the quick and easy thing to do is simply use a single zone at all times and the plane shows in trim. Of course an out-of-trim aircraft is not only illegal; but dangerous. I do not believe the Agents are adequately trained to understand the severity of this problem; and the Pilots likewise have not been made aware this issue might exist. Today I had an Agent who was already aware of the issue and ready to perform the correct procedure even if I had not asked. However; there doesn't seem to be a consensus among Agents of when the three-zone rule must be applied and when a single-zone rule can be applied. I believe this is a serious safety issue which needs to be immediately addressed to ensure no -800 flies while potentially being out of trim.Both Ground Ops and Flight Ops need to work together to ensure this issue is clear for all operators and that the necessary procedure is in place so that an -800 doesn't fly out of trim. And perhaps it's as simple as determining at which Passenger loading this needs to occur and have that be made obvious for everyone. For example; 'If flying an -800 with less than 100 Passengers; ensure three-zone loading is accomplished.' The [manual] as it's currently written provides no guidance for the Pilots in this area. It simply states that the Company Form is used for flights that require the three-zone loading schedule and that it's not required for single-zone or specialty flights. This is simply stating the obvious and provides no guidance for the requirements to use one method over the other.
B737-800 Captain described a situation he encountered under which a light passenger load can cause a nose heavy; out of CG situation unless the gate agent takes special precautions to disperse the passengers throughout the cabin.