I used to fly on Twin Otters regularly. What happened was you walked out to the plane and handed your carry-on, such as a brief case, to the copilot. They put it in a compartment at the rear of the plane. Then you board via a few steps.

It was be possible to take something aboard, but there was no overhead compartment so this left only under the seat in front.

As the flights are short and scenic on these types of planes, it really didn't matter.

I used to fly on Twin Otters regularly. What happened was you walked out to the plane and handed your carry-on, such as a brief case, to the copilot. They put it in a compartment at the rear of the plane. Then you board via a few steps.

It was be possible to take something aboard, but there was no overhead compartment so this left only under the seat in front.

As the flights are short and scenic on these types of planes, it really didn't matter.