Why do car seatbelts have one strap whereas airplane pilot seatbelts have two straps?
In: Engineering
Comes down to how you are most likely to die.
You are very likely to survive the plane crash. You are less likely to survive the ensuing fire, and will probably die of smoke inhalation.
A complicated seatbelt is the last thing you need as your skin melts and your lungs fill with carbon monoxide.
With car seatbelts, they are designed to keep your head from smashing through the steering wheel and into the dashboard. That kind of thing is pretty fatal most of the time. Cars usually don’t catch fire and explode after a crash. Which is why seatbelts in cars are designed to save you from the collision most likely to kill you (the one with the dashboard).
Cars travel on one planar field of motion. One strap.
Planes travel on two. Two straps.
Think about it
Car seat belts are optimized to reduce injuries in case of a crash.
Pilot seats are made to keep the pilot in his position at any cost, even in a falling and wildly spinning plane.
Car self belts are meant to protect you in a crash.
Airplane seat belts is just there so when there’s a crash, the police can identify the bodies if they’re seated at the right place
Pilot harnesses are actually a 5-point harness; there’s a strap that comes up between the legs, with a 4-part buckle assembly, and then a lap belt (L and R straps) and two shoulder straps (L and R) go into that.
You find this same sort of harness in the seats of high-speed race cars.
The use-case is the same: keep the person/people in control of the vehicle firmly in place despite the extreme forces exerted on them trying to pull them out of their seats in some extreme conditions that either may (pilot) or will (racecar driver) occur in normal operation.
It’s a cost/function formula.
Let’s start with planes: There are three types (oversimplifying) of seat belts. Planes don’t crash very often, so the seat belts aren’t about saving your life in the event of a crash. They do, however, experience turbulence, shaking the plane pretty badly. If you look at a passenger seat, it’s just a single strap to keep you from flying out of your seat when the plane drops. It’s the cheapest option, but as effective as it needs to be. The stewardesses get two straps to hold the upright while rear facing. This lets them have a good idea of what is going on in the plane, even in an emergency. Then, move to the pilots, and they have a 5 point harness. As the plane drops or shakes, they need to be upright, in place and able to control the plane.
Now, as we move to a car, you’ll notice a two strap system in most passenger vehicles. Cars are more likely to crash than a plane, and passengers are more likely to survive. These three point systems hold you in place to prevent whiplash and other spine injuries. However, since most of the time you’re not in an accident, they are flexible enough to let you comfortably move around. In the case of a crash, the driver mostly needs to be able to hit the breaks and remain in their seat, so a 5 point harness is overkill. A simpler belt will be more likely to be used, and doesn’t loose much over a 5 point, plus is cheaper and fits more people.
We’ve actually seen a move from a single strap seatbelt for cars, to the two belt system in the front seat, to a two belt system in 2 of the three rear seats as well, to now almost all cars have a two belt system in all three seats. This is because cars are generally traveling faster, and are safer and are more expensive. You’re more likely to survive the impact, and it will have more force, so there is an added benefit to a more secure strap – with cars costing more, you’re less likely to notice the extra cost.
Further proof: race car drivers use the 5 point harness. They are going faster, subjected to stronger forces, and need to have control of their car in much more extreme scenarios. Plus, the cars are built for them, so the straps don’t need to be as easily adjustable. They aren’t reaching in the back for a soda, or adjusting the radio – so comfort isn’t as important.
Cars are more likely to take on abrupt but survivable impacts. The seatbelts on a plane aren’t for a crash from the air so much as to keep you in your seat during heavy turbulence.
Racecars and pilots use the same type of seat belt because they need to keep control of their vehicle/plane in cases of extreme maneuvers. It’s called a 5-point harness. Your average passenger car could benefit from this, but it would be highly inconvenient for the driver and passengers. This would increase the cases of people not using seatbelts at all. Imagine a woman wearing a dress; She wouldn’t be able to use a 5-point harness as one strap needs to be between the legs.
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