(CNN) — Passengers booked on the world's longest flight -- a 19-hour nonstop of more than 9,000 nautical miles between Singapore and Newark, New Jersey, set to commence this October -- needn't worry about numb butt syndrome.

The Singapore Airlines Airbus A350-900 ULR (an acronym denoting that it's modified for efficient Ultra Long Range flying) serving the marathon route won't be subjecting its fliers to economy class.

Travelers on this premier route will instead have a choice of just two premium classes: the 67-seat Business Class cabin, or the 94-seat cabin of Premium Economy seats so good they could almost pass for Business Class.

Now that the aircraft are only months away from delivery, it's crunch time for the airline and Zodiac Aerospace, manufacturer of the seats the airline has heavily customized.

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Inside the factory

To discover just how complex it is to construct an airplane seat, let alone one destined for the world's longest flight, CNN Travel headed behind the scenes at Zodiac's factory in Gainesville, Texas, as the last sets of the Premium Economy seats for the ULR aircraft came off the production line.

It's here that vacuum form-molding, plasma-cutting and skilled handiwork turn raw materials, like reams of leather and 50-foot-long sheets of structural aluminum, into neatly packaged feats of engineering that manage to relax, comfort and entertain as passengers fly from one side of the Earth to the other.

Although Singapore Airlines has been flying Premium Economy since 2015, the incredibly lengthy routes planned for the ULR aircraft, combined with their uniquely spacious layout, necessitated some updates, including shifting the seat-back pocket higher for improved knee and shin space, redesigning the bottle holders and cocktail table, and introducing a entirely new model: single "throne" seats with side storage bins that take the place of a seatmate.

Six of these solo seats are located at the rear of the Premium Economy cabin, where the layout goes from 2-4-2 to 1-4-1, and are already proving popular for bookings.

Comfort trials

There's no guarantee customers will approve of the seats, however. That is, until it's time for "comfort trials" testing, when the airline and Zodiac literally put butts into the seats for the first time.

"We run comfort trials with pressure maps that measure the weight distribution of seated passengers, which help us to determine if a seat is comfortable and for how long a seat will be comfortable," explains Sebastiaan Does, Zodiac's sales and marketing manager for southeast Asia.

"During these comfort trials we'll have people from different genders, ratios, weights, heights -- you name it -- and we'll let them sit in the seat for over eight hours."

The review process then moves from the factory to Singapore, as airline spokesperson James Boyd tells CNN Travel: "We want to make the ultra long-haul flight as comfortable as we can, and so we have a similar testing procedure in Singapore, whereby we invite our frequent fliers and staff to spend the night in the seat and report back."

The seats are tested in "comfort trials," when real-life people sit in them for more than eight hours. Cynthia Drescher

Amenities

Every inch has been considered and reconsidered, so that the Premium Economy seats and their 38 inches of legroom include calf and foot rests, a cocktail tray, over-shoulder reading lights, adjustable winged headrests, 13.3-inch screens, and three power points per seat (two USB ports and one universal AC outlet).

Amenities encompass noise-canceling headphones, pillows and blankets, 1,400 on-demand entertainment options, amenity kits, full-size bottles of water, Champagne and complimentary drinks (including Singapore Sling cocktails), and the ability to pre-order meals like seafood thermidor or Singaporean nasi lemak from the airline's previously Business- and First-Class-only "Book the Cook" menu.

Cradle motion

But can you really relax in the seats? One critical adjustment passengers won't see, but will surely feel, lies in the way the seats shift positions.

"Singapore Airlines evaluated the recline and asked to consider a different type of motion -- a cradle motion -- where the bottom moves down and forward with the back recline," explains Bob Funk, head of sales and marketing for Zodiac Seats US. "With a traditional recline, you basically hit the button and [the seat] goes back and the bottom doesn't move much.

"The cradle, on the other hand, balances forces, distributes your weight, and lets your upper body relax while at the same time managing the pressure on the seat pan a lot better. It creates comfort that's really beneficial when you're going to be sitting there for a long period of time."