Why are airline seatbelts lap-belt only?

I’ve wondered for a while now. What is the reasoning behind this decision? Wouldn’t a shoulder belt and lap belt be safer? Or do planes fly so fast the shoulder belt would end up cutting through one’s torso in the event of a crash, or something equally nasty? Is the suddenly deceleration brought on by smacking into your seatbelt worse than the sudden deceleration from smacking into the seat in front of you?

#1 - If they were shoulder-belts, no one would wear them, which would defeat their purpose because…

#2 - Airplane seatbelts aren’t really intended to do much in the event of a crash. They’re mainly to keep your butt planted in the chair in the case of rough turbulence (which is much more unpredictable and likely to occur than a crash.) If the plane goes down, what good is being strapped to it going to do you, and how would a shoulder-strap change any of that?

You’re thinking in car terms. Are you expecting to rear-end the plane in front of you or something?

Well, it’s true that airplane crashes most often involve the plane travelling forward, and therefore one of the largest magnitude vectors of [negative] acceleration is forward for the passenger. Of course, “forward” might also be straight down…

Airline crashes normally come with some warning to give the occupant time to assume the bent over forward position which minimizes the kinetic energy release of the vulnerable head wrt to the immediate surroundings . A shoulder harness would prevent someone from assuming this position.

I would also venture to say that a downward impact on a vertical spinal column would do more damage than if the spinal column was horizontal.

Perhaps I shouldn’t tell you this but those guys up in the flight deck have the full monty- lap and shoulder straps. So obviously it would be safer, but lap belts are a compromise between safety and worrying the passenger to death about a possible crash. Probably a better idea would be to have the seats facing rearwards, then in the case of a rapid deceleration you would be pushed into the seat back.
All this supposes that the seats stay attached to the seat rails of course which is quite important too.

The “brace” position is much more efficient at keeping you alive than a shoulder belt. Also, it’s much easier to escape from wreckage with only a lap belt since you can’t get it tangled around your arms.

military airliners here - VC10s, don’t know what they use now - did have the seats facing rearwards, but their passengers are under orders, and have to take what they are given. Commercial passengers don’t like it, and would desert the airline for someone else.
I once travelled in a cab from Edinburgh to Connell Ferry sitting in the rear-facing seats, and got sick as a dog. Once I was in the front-facing seats I was alright.

Like Shalmanese says, the brace position is safer, plus the added safety of being able to easily get out of your belt if you somehow survive the crash. (In most situations, if you die in the crash, it isn’t because the seatbelt didn’t do it’s job). The pilots have shoulder harnesses because they have to fly the plane when coming in hard, and can’t assume the ‘crash’ position, and if they are trying to land in a field, clipping trees might cause them to fly forward, but they still need to be able to do last second ditch efforts, and not be flung head first into the instrument panel.

The “brace position” works only in first class and business. For people seated in economy–where sardine compression is the norm–the brace position is impossible to achieve, unless you’re Tom Thumb’s little brother.

In economy, you’re basically pressing your forehead against a flat-panel television monitor, which isn’t the best surface.

And what do you base this on?

The brace position, among other things, allows your limbs to flail around, which frequenlty results in broken bones. About the only thing it really does is keep your torso from snapping forwards and causing internal injuries by pre-folding your body.

I fly regularly with a four-point harness. I can’t recall ever “getting tangled” in it. They are designed so that as soon as you pull the latch the shoulder straps automatically detach from the lap belt and fall free. No fumbling required.

(Which is a good thing, since the arrangement on the airplane I’ve been flying this summer is supposed to allow you to unbelt and squeeze out the door while burdened with a parachute, all before the airplane wreckage hits the ground. Me, I’m trying hard to not require actual testing of this feature)

Safest seating/belt arrangement would be facing towards the rear with a full four or even five point harness. The general public, however, will not stand for that. My friends among cabin attendants tell me it’s hard enough just to get the average tourist to sit down and use a lap belt. A safety ssytem is useless if you can’t get people to use it, so the lap-belt-only is a compromise.

And emergency exit rows. Now I’m curious – whenever I ask for the emergency exit row, do the airline employees understand that at 6’2" those seats are just more comfortable, or are they thinking that I’m a paranoid dork? Or probably not thinking about it at all…

They do like to have someone fairly able bodied in those seats for obvious reasons.

The reason there are no shoulder harnesses is there is no place to attach them. The seat backs of all airplane seat are designed with what is call a breakover point. This is the amount of force required for the seat back to fold forward. During a crash if you are thrown forward, the back of the seat in front of you will give, hopefully cushioning the impact. To install shoulder belts, the seat back would have to be rigid or additional structure would have to be installed. Both would add weight to an airplane. That would mean fewer passengers and cargo. During the assembly process of the 737, we occasionally breakover a seat back, a quick call to the vendor and they come out and fix the seat for us.

Personally, I think that safety is not a very important consideration when it comes to laying out the interior of the cabin. Most of the safety features are geared around making sure the plane doesn’t crash, and that it stays together if it does.

The seatbelts are cosmetic at best, and the emergency exits are a joke. I’m skinny and would struggle to get through an over-wing exit in a hurry. A larger person would get stuck like a cork in a bottle and prevent anyone behind them getting out. 4 or 5 seat wide blocks in economy would slow you down hugely.
Current arrangements are all about providing whatever safety is available without compromising the economics of the cabin.

As per broomstick and Mk VII, a really safe cabin would have rearward-facing seats with full harnesses. It would probably also have fewer seats further apart, more emergency exits, and those exits would be much wider than they are currently (more like the doors which are actually designed for people to get out of). Since passengers would hate it and airlines couldn’t afford it, it’s unlikely ever to appear.

I’d assume they do realize there’s more leg room there. I know I’ve asked for bulkhead/exit row seats for my 6’4" husband (and I’m only 5’9" but I have the same inseam length that he does, so I appreciate them too), and have never gotten a quizzical look about why that would make a difference.

Does the shoulder harness help one avoid this problem, somehow?

Re: the rearward facing seat idea. Apparently, one of the major airlines or aircraft manufacturers (I think it was Boeing, but my memory is not clear here) did a study about improving safety with rear-facing seats, but it seems they found that such an orientation proved unsatisfactory. I don’t recall all the specifics, but the study demonstrated that passengers were disoriented by facing rearward and more easily confused in the event of an emergency; so, rearward-facing seats were abandoned.

Could find exactly what I was looking for (my recollection is from a Discovery Channel program about airline safety or somesuch), but I did find this excerpt from a UK Select Committee report concerning rearward-facing seats:

I think it might have actually come from an article in one of the inflight magazine, admittedly, not the best of sources :(.

Truthfully - no.

You could tuck your hands/arms into the shoulder straps, which could reduce the incidence of busted arms, but your legs will still have a tendency to swing forward and smash themselves on the seat in front of you, which is just one way you can get seriously injured in a crash.

Because the full harness spreads the deacceleration forces over more of your torso (because there is a greater area of strapping) you are less likely to be injured. Instead of the forces being concentrated on your pelvis or, worse yet, your soft underbelly if the belt rides up higher than it should (and that’s where the internal injuries really start to happen) your ribcage, shoulders, and so forth take some of the force.

The brace position can also result in your whole upper body flopping around, which can lead to a head injury in economy seating all too easily. The full harness, however, keeps you further away from what’s in front of you, which is usually a good thing. It may, however, also leave you more vulnerable to flying objects in the cabin.

Which brings up another point - allowing children under two to be held in a passenger’s lap. This is stupid. There is no way a parent could continue to hold onto a child in a crash. The child stands a good chance of either being crushed between the parent and the seat in front of the parent, or becoming one of the flying objects loose in the cabin. But, again, people forced to buy a ticket/seat for the infant and also use and infant seat (pretty much what you’d use in a car) might elect not to fly at all and take to the road instead. Bizarrely enough, the odds of a child dying even while properly restrained in a car are significantly higher than expiring while sitting completely unrestrained in an airplane. So, statiscally the latter is safer than the former, although if you have the misfortune to be on a crashing plane your infant stands a much higher risk of death (almost certain) than you do. Such children are also more likely to be injured or killed in severe turbulence.

As for “where to attach the shoulder straps” - how about the ceiling? Some sort of rail system like what the seats are attached to?

But the public would never go for it.

The public would never go for it, and nor would anyone else. Can you imagine the amount od re-certification that would be needed? Every single seat type/seating configuration/aircraft type would need to be checked. Overhead luggae racks would have to be either eliminated or dratically re-designed (and re-certified). This would cost millions, take years and not yield a great benefit.