Seat belts & brace position question

[eigerjoch]

I've been following the Asiana thread and someone mentioned that in some seat configurations (e.g. business) different seat belts are used because of the seat angle (inwards rather than forwards, say).

And I was wondering about the reasoning behind lap belts on aircraft. I believe in an auto it's accepted that a lap belt is never as safe as an over the shoulder belt.
so are lap belts used on aircraft because a lap belt + brace position is actually safer than a shoulder to waist belt would be? or because shoulder belts would not be feasible/practical and so a brace is the next best thing?

(apologies if this is a stupid question, I'm new and it looked like this was the right place to ask)

[SpringHeeledJack]

Probably because the public wouldn't accept a shoulder/across the shoulder belt due to discomfort/annoyance. Although you're talking about take-off/landing, the reality these days is that the seatbelt ON requirement is usually for the whole flight. There was much complaining when car seatbelts were introduced and yet today almost everyone wears them without a revolution starting.



SHJ

[llondel]

For much of the time, a lap belt is probably adequate - if you're at FL360 it'll keep you in your seat during turbulence and negative-g, and any force serious enough that a harness is better would probably mean that the seat belt is low on your list of problems (fortunately not many commercial aircraft fly inverted, and then not usually for long). When you come close enough to the ground then a full harness is much safer to keep you in your seat because the potential for a high-g impact is much greater. If you notice, the crew have full harness belts.

When you're sat in a hold for half an hour you don't really want to be restrained in your seat, it would get uncomfortable. Car seat belts, with the inertia reel, have improved a lot from the fixed ones we used to have, but the added weight of several hundred of those would be significant.

[eigerjoch]

Thanks - so sounds like it is a matter of what's practical, the choice of belt. So if everyone was wearing full harnesses there would be no 'brace' instruction in the event of an incident? (I assume it'd be very difficult to even lean far enough forward with a harness on)

[GoldwingSpain]

On a lot of budget airlines there is not enough room to get into a suitable brace position.

[SpringHeeledJack]

On a lot of budget airlines there is not enough room to get into a suitable brace position.

And there is the reality, also by 'normal' airlines if one is larger than average in height. Yes the seat in front will tip forward, but still the standard brace position is hard to achieve outside of first/business/exit row seats. Take into account the many rotund passengers, even if just in the belly area and it gets more problematic to brace.



SHJ

[John Farley]

Like most things in airliners the seat and passenger restraint systems have to meet certification requirements. If you added a car lap and diaginal you would have to strengthen the seat back and if you were prepared to pay the weight and cost penaly of that then you would also need to strengthen the floor where the seat attaches. In aircraft design just like sex one thing leads to another.