Once everyone have boarded an airplane the stewardesses usually go through the cabin and make a headcount to confirm that the right number of people are on board. But what would happen if the count fails and either there are too many or too few people? Would the airplane be de-boarded and forced to go through the boarding gate again?
For clarity, there isn't a "wrong number of people," there's a mismatch between the manifest (number of people that are supposed to be on the airplane) and number of bodies aboard.
It must be reconciled. How depends on company procedure and practicality. Some things I have seen:
- The first thing done is a recount, preferably by two people.
- If the onboard count is higher than the manifest and passengers do not show boarded, the Agent will call those passengers by name.
- If the onboard count is lower than the manifest and there are obvious empty seats, the Agent will call those passengers by name on board and presumably in the lounge.
- Some airlines use handhelds that show the boarded seat map so they can easily compare boarded seats to actual butts in seats. They can do the same with a printout.
- If there are more people onboard than seats, they'll first try to verify seat assignments of those affected, meaning those with duplicate seats.
- If things are really wonky, they'll ask everyone to sit in their assigned seat, then check boarding passes.
I've seen it a couple of times where children boarded as lap children are seated on a less than full flight. I figure it's because they're counting empty seats to accommodate stand-bys, so they end up with a seat count mismatch.
The most important regulatory issue is the count matches in case recovery is required. It's important to know who is onboard but this is less of a "security" issue since presumably any person onboard was screened at the terminal entrance.