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I find myself thinking often, if the seatbelts in airlines are designed just to keep the passengers in seat or to really act as a safety belt like in cars, because in cars which go at 300kmph speed, you have the 5 point seat belt. Why isn't similar approach used for airlines, where the risk is too high.

Plus articles like these, suggest heavier / obese passengers are at more risk.

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    $\begingroup$ This is not a duplicate of the existing seatbelt question: That questionis about the locking mechanism while this one is about the choice between 2/3/4/5-point harness. $\endgroup$
    – Jan Hudec
    Oct 7, 2015 at 13:11
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    $\begingroup$ As a side note, heaver/obese people are more at risk for everything. (Discussion for a different SE, however.) $\endgroup$
    – FreeMan
    Oct 8, 2015 at 18:45
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    $\begingroup$ @FreeMan: Except for starving after a successful emergency landing in a remote area … $\endgroup$
    – chirlu
    Sep 6, 2016 at 14:18

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For a proper answer we first need to define what "safe enough" is. If you want a significant reduction in risk over current standards, improving the restraint will not be enough.

Your observation is absolutely correct: The lap belt is the minimum the airlines get away with, and you would be much better protected in some kinds of accident by a proper 5-point harness. But fitting the harness alone will not achieve much: To stay in place in a crash, all seats need to be reinforced, so they can tolerate the increased loads at the shoulder belt attachment points. This will add weight and cut into the profits of airlines, so it will only happen when it is mandated.

Please read this answer for more details. It concerns GA aircraft, but much applies to airliner seats as well. The better the restraints are, the more deceleration load can be survived. Note that the points in the Eiband diagrams shown in this answer are from tests with additional leg, chest and arm restraints. If all you have is a lap belt, you might probably be able to tolerate only half as many gs.

The better alternative to a 5-point harness would be to turn around all seats. This would help most to reduce injuries and improve chances of survival in a crash landing.

Now I have to contradict @FreeMan: While the brace position will put all the load on a small strip around a person's hips and lets the arms flail forward, a properly secured torso can take 10g for a duration of more than a second and up to 40g momentarily without ripping the head off. See below the famous sequence of Dr. John Stapp in a ride on his rocket sled "Sonic wind":

Dr. John Stapp in a deceleration test

Dr. John Stapp in a deceleration test (picture source)

He clearly had his head bobbing about, but even though he lost dental fillings and suffered hemorrhage in his eyes, the head stayed intact.

In the end, all what the lap belt is really good for is to keep you in your seat with negative acceleration due to gusts. This is by far the most common situation when a restraint is needed. The classical brace position (torso over legs, head down) is impossible anyway in economy class due to the ever reducing seat pitch.

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    $\begingroup$ Long ago, I was told by a Boeing engineer that the weak point is not the seatbelt - it's the seat. In high-energy crashes the seats often break off the rails and slide around; if the seats were more tightly secured, then people would get wounded by the lap belt. $\endgroup$
    – egid
    Oct 7, 2015 at 20:31
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    $\begingroup$ @egid: Sounds credible. Today's seats should match the strength of the belt, so either of them could fail first. $\endgroup$
    – Peter Kämpf
    Oct 7, 2015 at 22:08
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    $\begingroup$ @Firee: Yes, this is very true. Being fixed to a bulkhead increases safety of those seats even more. $\endgroup$
    – Peter Kämpf
    Oct 8, 2015 at 12:59
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    $\begingroup$ @egid - I've seen similar; the theory is that the most survivable crash is a "crash landing", either with gear up or at too great a rate of descent. In such an event, the gear are designed to snap off (otherwise they'd cause damage to fuel lines risking a fire), and the cargo hold is usually full, so the fuselage doesn't provide much absorption of the impact forces. Really the only thing keeping the passengers from a pelvic or spinal fracture is the spindly legs of the seat crumpling. $\endgroup$
    – KeithS
    Oct 8, 2015 at 15:32
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    $\begingroup$ @Firee Everything is a trade-off between safety and cost, or safety and convenience. To a first-order approximation, the number of fatalities in commercial aviation per year is zero. It just isn't worth spending money to make that number even closer to zero. $\endgroup$
    – David Richerby
    Sep 4, 2016 at 10:36
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Are the current seat belts used in commercial airlines safe enough?

Not really, the current seat belt standard is aimed mostly to protect against aircraft movements encountered in flight, not during impact which are very uncommon, contrary to automotive industry.

Current FAA regulations impose the seat and seat restraints to protect from 16g deceleration using a complex test scenario and simpler 9g test (only the latter before 2009). Some seat belts are fitted with an airbag that expands forwards.

Why?

With a passenger not keen on taking time to understand and adjust a harness, and not willing to remain restrained by a harness for a long time, the compromise is to have a simple belt for flight turbulence, and use the brace position against impact, albeit:

  • This combination has been proven effective mostly in frontal collision (e.g. in the Hudson ditching).
  • Lap belt have shown vertebrae problems in car accidents due to the rapid deceleration of the lumbar region:
    enter image description here
    (Source).
    Wikipedia article on seat belt:

    Evidence of the potential of lap belts to cause separation of the lumbar vertebrae and the sometimes associated paralysis, or "seat belt syndrome", led to progressive revision of passenger safety regulations in nearly all developed countries to require 3-point belts first in all outboard seating positions and eventually in all seating positions in passenger vehicles. [...] Besides regulatory changes, "seat belt syndrome" has led to tremendous liability for vehicle manufacturers.

Three-point diagonal seat belts, which don't have this problem, are used for helicopters and light aircraft, but are not seen in airliners because the upper anchoring point is not easily available.

Better protection used in commercial aviation

Torso restraints are mandatory for the attendants in the US, but attendants are trained to make them effective and are expected to take the time to adjust them correctly so that actual additional benefits over the lap belt can be obtained.

enter image description here

AC No. 25.785-1B:

In accordance with § 121.311(g), each seat occupied by a flight attendant required by § 121.391(a) must have torso restraint (a combined pelvic restraint and double strap upper torso restraint with a single point release) that meets the requirements of § 25.785, except that any combined safety belt and shoulder harness approved and installed before March 6, 1980, may continue to be used.

also:

Passenger seats occupied by required flight attendants must fully comply with § 25.785(h), including the combined pelvic restraint and upper torso restraint.

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    $\begingroup$ @mins : Even if we don't consider a five point harness, wouldn't a simple seat-belt like approach, as in cars, would provide atleast some support to the torso? $\endgroup$
    – Firee
    Oct 8, 2015 at 10:39
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    $\begingroup$ @Firee, yes, they would. The seats, however, would have to be redesigned to be strong enough for the upper mount point to be useful. This can be done (many car seats have the belt anchored to the top of the seat), but it adds weight. Fixed weight in the air frame reduces cargo carrying capacity and increases fuel costs. It then becomes a trade off to see if/how often the added protection of the shoulder harness would benefit a passenger in a "typical" crash scenario. For in-flight safety in turbulent conditions, the lap belt is sufficient. $\endgroup$
    – FreeMan
    Oct 8, 2015 at 12:41
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    $\begingroup$ @FreeMan: I was reading this about harness use: "Accident experience has provided substantial evidence that the use of a shoulder harness in conjunction with a safety belt can reduce serious injuries to the head, neck, and upper torso of aircraft occupants and has the potential to reduce fatalities of occupants involved in an otherwise survivable accident." Source: Transport Canada AC 605-004 $\endgroup$
    – mins
    Aug 17, 2016 at 18:30
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I would postulate (but have no firm evidence to prove the point) that the brace position is actually safer than a 5-point harness would be for passengers in an aircraft accident.

  • The 5-point harness would hold your body snugly against the seat, but leave your head bobbing around as a 5-pound ball on a weak tether (your neck). The same issue would happen to you in your family car with a 3-point harness, and that's one of the reasons it has airbags installed.
  • The brace position has your head fairly firmly planted in the furthest forward position already, so your head doesn't whip forward during that sudden stop.

Why do race drivers wear a 5- or 6-point harness then? They also wear a HANS device which effectively "seat belts" their head to the car, as well. The HANS device leaves their head with 1-2 inches of forward movement and absorbs the stress of impact* with tethers designed for it.

*One recent F1 accident saw impact forces upwards of 25G. The driver unbuckled himself and got out of the car while the track marshals were still on their way. I'm sure he was sore, but was really no worse for the wear.

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    $\begingroup$ I would add that pilots do have 5-point harness, but the primary reason is that they need it for support in turbulence. Without the harness even moderate accelerations could cause inadvertent control inputs. $\endgroup$
    – Jan Hudec
    Oct 7, 2015 at 18:58
  • $\begingroup$ Note, I did indicate passengers in my answer. Your point about the pilots is quite valid. Of course, they have the same bobble-head problem the passengers do, but if the stop at the end of the flight is sudden enough, the pilots usually don't make it anyway. $\endgroup$
    – FreeMan
    Oct 7, 2015 at 20:42
  • $\begingroup$ As a F1 fan myself, i agree with the Hans device implementation a few years ago, it helps drivers withstand upto 30-40G impacts. $\endgroup$
    – Firee
    Oct 8, 2015 at 10:36
  • $\begingroup$ Counter to the points about HANS - part of the need for it is the significant additional weight of the crash-helmet that drivers wear. $\endgroup$
    – MikeB
    Aug 29 at 9:26
  • $\begingroup$ Yes, the helmet does add additional weight, @MikeB, however, your head is heavy enough on its own that a sudden stop will most likely cause notable damage to your neck without a helmet and could, potentially, break your neck. $\endgroup$
    – FreeMan
    Aug 29 at 12:56
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The short answer is that current airline seats and seatbelt systems are pretty safe when used in accordance with their design criteria. They are not merely decorative - they are tested and proven safety devices.

The longer answer requires looking at the regulations and understanding what the seat/seatbelt/aircraft interior design as an overall system is intended to do for passengers in the event of an incident.

The current seat belts on FAA-certificated transport-category (airliner) aircraft are governed by FAR 25.785.
I've excerpted a few relevant bits of that regulation below.

Each seat, berth, safety belt, harness, and adjacent part of the airplane at each station designated as occupiable during takeoff and landing must be designed so that a person making proper use of these facilities will not suffer serious injury in an emergency landing as a result of the inertia forces specified in §§25.561 and 25.562.

Straightforward enough: "If we crash people sitting in the seats shouldn't suffer serious injury."
The loads the seats and seat belts must withstand are, in part, described in FAR 25.561 and 25.562 which deal with emergency landing conditions - among the requirements are for the seat and seat belt to handle a 9 G forward load with FAA-Standard 170-pound adults.

The 170-pound adult certification criteria is why you will find discussions about how larger/heavier individuals are "less safe" -- a heavier individual will impose more load on the seats, seat belts, and attachment points, and may exceed the ultimate limit load of an approved seat/seat belt design.

Each occupant of a seat … must be protected from head injury by a safety belt and an energy absorbing rest that will support the arms, shoulders, head, and spine, or by a safety belt and shoulder harness that will prevent the head from contacting any injurious object.

In plain English: "Either you need to be secured with a shoulder harness so you can't hit anything, or the seat in front of you needs to be soft and padded so it won't crack your skull open and kill you when you're in the brace position."
(This is the same logic used in school busses: Rather than strapping every passenger down with a 5-point restraint we make sure they'll stay in their seat and everything they can hit from a seated position is soft enough that it probably won't kill them.)

Each seat or berth, and its supporting structure, and each safety belt or harness and its anchorage must be designed for an occupant weight of 170 pounds, considering the maximum load factors, inertia forces, and reactions among the occupant, seat, safety belt, and harness for each relevant flight and ground load condition

i.e. "In a worst-case crash a FAA-Standard 170 pound adult shouldn't break the seat or rip the seat belt out of the attachment points." as I described earlier. In practice the regulation sets a minimum standard, and most seats will withstand more force than that.

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    $\begingroup$ Sadly, in America, at least, there aren't too many FAA-Standard 170 lb passengers. I wasn't 170lbs as an average height, healthy, active college student, and it's gone downhill since. $\endgroup$
    – FreeMan
    Oct 7, 2015 at 19:22
  • $\begingroup$ To add to @FreeMan's concerns, the average or taller person will physically not be able to bend down to the brace position. When I sit in a standard economy seat with 31" pitch and bow down, my face will hit the seat in front just above the foldable tablet (which isn't soft by any measure). It will not go any further unless I get some more joints in my spine. $\endgroup$
    – Peter Kämpf
    Oct 7, 2015 at 20:20
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    $\begingroup$ @FreeMan One day the certification standards will be updated (the "standard passenger weight" in AC 120-27E is 190 pounds now), but that day is not today :) $\endgroup$
    – voretaq7
    Oct 8, 2015 at 2:49
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Aircrafts' seatbelts are for security reason. And yes, security while flying is "just to keep the passengers in seat" in case of a void or, on ground, in case of a bump or the plane leaning.

Apart from that cases, there is no need for a complicate 2/5/10k point system in an impact against a mountain: you will be just plain dead anyway.

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    $\begingroup$ Security? Usually, if I'm not mistaken, "security" in English means protection against intentional harm, whereas "safety" is protection against happenstance. $\endgroup$
    – hmakholm left over Monica
    Oct 7, 2015 at 16:14
  • $\begingroup$ @HenningMakholm: thanks for your feedback. Feel free to use the Edit button to correct the answer, then :-) $\endgroup$
    – motoDrizzt
    Oct 7, 2015 at 16:22
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    $\begingroup$ There is something in between a slow graceful stop and hitting a mountain. Rapid deceleration would clearly benefit from more than a 2-point harness. $\endgroup$
    – Jamiec ♦
    Oct 7, 2015 at 16:23

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