Most aircraft seats rely on a simple lap belt to keep passengers safe during the flight. But in some premium cabins, particularly those where the seat is not facing directly forward or aft, passengers sometimes discover a three-point harness awaiting their use. Why is there this difference?
The aircraft seatbelt
When we’re traveling in our cars, there’s no question about using a three-point harness. However, in aircraft cabins, most often we’re presented with a lap belt. In the economy cabin, premium economy and some forward or aft-facing business class seats, this is the only option.
But in some premium seats, particularly those which are angled away from the forward/aft trajectory, such as the ever-popular herringbone, we’ll often find a three-point harness, just like we have in our cars. Why is this the case?
The explanation in IATA’s Cabin Operations Safety Best Practices Guide (as found on Skybrary) is as follows:
“Three-point diagonal fixing seatbelts are occasionally installed in premium seats, including those positioned at an angle away from the forward/aft axis of the aircraft. The intent of the three-point harness is to protect the occupant from contact with the forward side of the seat shell.”
Essentially, when a passenger is sitting in a seat that is not angled towards the front or the rear of the plane, their body will move differently in the event of sudden deceleration than that of someone who is facing forward or aft. Rather than lurching forwards, passengers could be thrown sideways, so the three-point harness helps to stop this from happening.
Sometimes, even forward-facing premium seats will have three-point seatbelts. This is because it allows passengers to remain reclined during taxi, takeoff and landing.
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The 16g seat
In 2005, the FAA published an amendment to the rules governing the standard of seats required in passenger transport aircraft. This built on a regulation that dates back to 1988, which is based around calculating something called the Head Injury Criterion (HIC).
The HIC is a very complicated sum that was originally used in the automotive industry to calculate the risk to occupants of a serious head injury. It takes into account time, acceleration rate and the proximity of objects that the head could strike to produce a number. In aviation, the 1988 FAA ruling stated that the HIC must be below 1,000 for the aircraft to be safe.
In economy class seats, the proximity of the seat in front is such that the head doesn’t have time to reach a huge acceleration rate before impacting the seat in front. This, it was found, was enough to be HIC compliant. The ‘brace, brace’ instruction requires passengers to place their head already on the seatback in front, which lowers the HIC even further.
The FAA’s rule amendment in 2005 (FAR/CS/25.562) upped the safety requirements of all seats, creating a problem for airlines and seat manufacturers. Previously, the rating had been based around ‘9g seats’, which would keep the HIC low when a force of nine Gs was applied to the occupant. The 2005 update now required this force to be 16 Gs.
In economy, airlines discovered they could remain compliant with the 16g seat rule simply by padding the headrest of the seat in front. But in business class, airlines were required to identify what objects the passengers’ head could hit if they swung around in an arc from the lap belt. With bulkheads, IFE screens, tray tables and other hardware in business class seats, being compliant started to become a problem, particularly in angled seats.
The solution to this proved to be simpler than many might have thought. The three-point harness gives passengers the ultimate level of protection, keeping their upper body stationary and preventing the head from swinging around and hitting anything in the cabin. While they haven’t been adopted in every passenger cabin, they are a common sight in aircraft with angled seats.
Have you used a three-point harness on a flight? Let us know in the comments.