1. In an airplane, the primary motion in the event of a sudden stop (crash) is vertical, not horizontal. The forward motion of an airplane is less critical than the vertical motion (landing impact) because the cabin of the airplane is essentially a "closed box" that is pressurized. In a car crash, the force is primarily horizontal and can cause severe injuries or death if the occupants are not properly restrained.
2. The height of an airplane cabin and the location of the seats make shoulder seat belts impractical. In a car, the seats are close to the floor and the roof is relatively low, making it easy to install and use shoulder seat belts. In an airplane, the seats are often higher off the floor and the ceiling is much higher, making it difficult to reach and use shoulder seat belts.
3. The cost of installing and maintaining shoulder seat belts in an airplane would be significant. Airlines would have to retrofit their entire fleet of aircraft, and the ongoing costs of maintenance and inspection would be substantial.
4. Shoulder belts can be uncomfortable and restrictive for passengers. In a car, passengers can adjust their seat belts to be comfortable, but in an airplane, the seat belts are often fixed in place. This can be uncomfortable for passengers, especially on long flights.
For these reasons, airplanes do not have shoulder seat belts like cars. However, airplanes are still equipped with a variety of other safety features to protect passengers in the event of an accident, such as lap belts, airbags, and emergency exits.
