I was travelling by air today, and I realized that there's an important difference between safety belts in cars and planes. A safety belt in a car moves from the chest on one side to the hip on the other side, while safety belts in a plane move from one hip to the other. Safety belts on the plane also seem to be quite flimsy compared to those in cars.
So what is the real purpose of safety belts in a plane? Is it just a design feature they forgot to change?
The purpose of seat belts on aircraft are to keep you secure in case of turbulence. You become an airborne projectile once this happens so not only can you hurt yourself but you could hurt others.
The National Transportation Safety Board has opened an investigation into what caused a United Airlines jetliner that departed from Dulles International Airport to experience severe turbulence.
A United Airlines spokesman said 25 people were injured, including four crew members. The crew members were flight attendants.
A spokeswoman for one Denver hospital said it treated seven people Tuesday from the United flight. Dee Martinez, a spokeswoman for Denver Health Medical Center, said all of the patients were treated and released. They suffered from "moderate head, neck and back injuries," Martinez said.
This article mentions that according to the FAA on average 60 people a year are injured a year because of turbulence.
According to Capt. Lim, each year about 58 U.S. passengers and flight attendants are injured by not wearing their seat belt.
It's somewhat faulty to compare "commercial airliner" seatbelts with "car" seatbelts. A more accurate comparison would be to compare commercial airliner safety equipment with bus safety equipment as a baseline, then add the air specific items. Both count on the seatback (or empty space in front of the first row) as a part of the safety mechanism, hence no shoulder belts needed. Air travel adds the issue of turbulence strong enough to toss one into the ceiling, hence the lap belts (although, arguably, they should be required in buses too in the event of a rollover).
Now, if you compare car seatbelts with private single-engine airplanes (which are car sized and have similar cabin arrangement as a car), you'll find that they have the same lap and shoulder harness configuration as a road-going car. Newer private airplane seatbelts even have integrated airbags in the lap portion (mostly because it's easier to put it there than to try to find a space on an already completely full instrument panel/yoke).
The "seeming flimsy" part of your question strikes me as odd and overly subjective. I've never seen an airplane seatbelt of any kind that's flimsy compared to a car. They use the same materials, in the same widths. The buckles may be different, but that's mostly and age of design issue, and has nothing to do with strength (they all have to meet similar requirements).
I remember watching a documentary about Aloha Airlines Flight 243 where a large part of forward portion of the airplane was ripped from the plane, and the cockpit lost communication with the rest of the plane (but was still able to fly and land the airplane). The only casualty could probably have survived if she was buckled in at the time. From Wikipedia:
The only fatality was flight attendant C.B. Lansing who was blown out of the airplane. Another 65 passengers and crew were injured.
Not exactly something to worry about considering how safe it is to fly, and I don't know if it's considered as real purpose of the belt. However if something like that were to happen, the belt should save you.
