There are regulations that discuss the use of safety belts, shoulder harnesses, and child restraint systems. The requirements distinguish between crewmember use and passenger use. For purposes of the J-3, there will only be one required crewmember, and since the J-3 is a two-seat aircraft, only one passenger, right? Maybe not. Let's review the regulation.
Federal Aviation Regulation 91.105(a) says that each required flight crewmember must keep his or her seat belt fastened during takeoff and landing, and while en route. FAR 91.105(b) says that during takeoff and landing, the required flight crewmember must keep his or her shoulder harness fastened while at the crewmember stations, but need not keep it fastened while en route. The harness requirement only applies if the crewmember's seat is, in fact, equipped with a shoulder harness. There is an exception that might apply to our single-pilot, small-aircraft operations. Because a shoulder harness may interfere with piloting duties, the regulations do not mandate that a required flight crewmember must fasten the shoulder harness if the pilot would be unable to perform required duties with the shoulder harness fastened.
For passengers, FAR 91.107(a) imposes similar use requirements but with one additional condition and one exception. In addition to takeoffs and landings, passengers must also use their seat belts and shoulder harnesses during taxi. But, passengers are not required to use seat belts or shoulder harnesses while en route (although it is probably a good idea for them to do so). Note that these are requirements for passengers, not the pilot in command. As the pilot, it is your responsibility to ensure that each person on board is briefed about the use of belts and harnesses and that each person is notified to fasten his or her belt and harness. And, it is not required in the regulation that the pilot be the one to give the briefing or the notification, just that the pilot is to ensure that it is done. Practically speaking, though, in our small-aircraft operations, it usually is the pilot who gives the instructions on how to use the safety belt and the announcement to fasten them.
One of the exceptions to seating and belt requirements is that a child under the age of 2 may be held by an adult who is occupying an approved seat or berth. And, this is the exception that occurred to me in contemplating whether my son was mature enough to sit by himself in a seat or could I have someone hold him while I'm flying the Cub. According to a reasonable reading of the regulation, it would appear that the only choice I have is to have him sit by himself in the only other seat in the aircraft, with the belt securely fastened about him.
But, the FAA has explained in a Letter of Interpretation that the rule may allow me another option. The FAA says that compliance may not only depend on there being one person per seat, but it could depend on the combined weight of the persons in the seat and the ability for the seat and the seat belt and harness to accommodate the occupants. According to the FAA, "as long as approved safety belts are carried aboard the aircraft for all occupants, and the structural strength requirements for the seats are not exceeded, the seating of two persons whose combined weights does not exceed 170 pounds under one safety belt where the belt can be properly secured around both persons would not be a violation of the regulations for an operation under Part 91."
So that explains why we sometimes see two children strapped into one airplane seat.
Kathy Yodice is an attorney with Yodice Associates in Washington, D.C., which provides legal counsel to AOPA and administers AOPA's legal services plan. She is an instrument-rated private pilot.