American Airlines Boeing 757. Flickr/Aero Icarus Twelve years after Boeing discontinued the 757, the long-time 'tweener in the company's lineup is still popular among U.S. airlines.

American, Delta, and United all operate large fleets of the airplane.

Even though the Renton, Washington-based plane maker booked more than 1,000 orders for the jets over its two decades in production, it was never a hot seller.

By the early 2000's, sales of the 757 had all but dried up, and Boeing put the plane out to pasture in 2004.

Although many of the planes are pushing 25 years of service, airlines are hanging onto their 757s. For instance, Delta, who has spent big bucks on a fleet of new jets, recently refurbished the cabins of its aging 757s instead of replacing them with newer planes.

So why are its customers so loyal? Is the 757 that good? To get some answers, Business Insider recently spoke with author and Boeing 757 pilot Patrick Smith about the plane.

"There's no denying the 757 is an old plane that was designed in the late 1970s, but the versatility of the plane is remarkable and unmatched," Smith, the author of the book Cockpit Confidential, told Business Insider in an interview."It's profitable on both short-haul domestic as well as trans-Atlantic routes."

Delta Boeing 757. REUTERS/Tami Chappell During it's production life, the 757 was always a bit of an oddity — a 'tweener of sorts. It's larger and offers greater range than the average narrow body jet, but smaller and cheaper to operate than a wide-body. Boeing and its customers, for many years, didn't quite know how to take full advantage of the plane's capabilities. However, when they did figure it out, it became an integral part of its operators' fleets.

Boeing is currently marketing a new stretched variant of its long-serving 737 — called the MAX 9 — as a potential replacement for the 757. Now, there's talk that Boeing may make the 737 even larger to get closer to the 757's capacity.

"Boeing is trying to push the 737 as a viable 757 replacement," Smith said, "In some respects it can, and in some respects it can't."

For instance, the 737 simply isn't capable of the same kind of engine performance as the 757.

According to Smith, the 737 falls way short of the 757 in terms of runway performance. The 737 requires a much higher takeoff speed and much more runway to get off the ground.

"The 757 can be off the ground in 4,000 ft. and under 140 knots," he told us. However, Smith recounts being in the cockpit jump seat of a 737 as two other pilots were flying. During takeoff, the author noticed that the plane didn't lift off until 160 knots and used up much more runway.

Furthermore, the 757 can comfortably climb straight to its cruising altitude. On the other hand, the 737 requires a step climb procedure that calls for the plane to climb to a certain altitude and burn off some fuel to lighten the load before continuing on to a higher altitude.

Runway performance matters. Especially when it comes to the type of flying the 757 and the 737 are asked to perform — trans-continental or oceanic flights with a full payload from smaller secondary airports.

Boeing 737 MAX 9. Boeing "The 737 falls short when you're trying fly it longer haul, coast to coast. It can do it, but not by much," Smith said. In addition, when it comes to flying across the Atlantic, "there's concern in the winter, when there are ferocious head winds, the 737 may need to stop for fuel."

The reality is that Smith believes the Boeing 737 is a good plane, but is being asked to take on a mission the design wasn't meant to perform.

"The Boeing 737 was essentially conceived as a small regional jet nearly five decades ago," Smith told us. "It's an extremely popular plane, but it's being asked to perform missions it wasn't designed for."

As a result, Boeing is simply not in a position to bolt bigger engines onto the 737. Since its introduction in the 1960s, Boeing has been installing larger and larger engines on the 737 as the size of the plane grew. Unfortunately, the amount of room underneath the wing hasn't changed. Thus, Boeing has all but maxed out on the size of the engines it can mount on the 737 without completely redesigning the plane's under carriage.

Instead of a stretched 737, Smith believes Boeing should have gone for an updated version of the 757 at some point during its production run.

"Somewhere along the way, had Being just re-engined the 757 with a new cockpit and new internal systems, it could have been a big hit," Smith told us.

In a statement to Business Insider, a Boeing spokesperson wrote:

"The 737 MAX will extend the Next-Generation 737 range advantage with the capability to fly more than 3,500 nautical miles. That’s an increase of 340-570 nmi over the 737NG which gives our customers the flexibility to open up new markets."

Icelandair Boeing 757. Flickr/Eric Salard With that said, the 757 isn't a perfect plane.

"It's a 6-across (seating) narrow body that's long and thin," Smith said. "And for passengers, it can take a long time to board and disembark."

In addition, the 757's cockpit is pretty old-fashioned when compared to modern airliners such as the 737 MAX. The controls of the 757 are also much heavier for pilots than its sibling — the Boeing 767. The 757 and 767 were developed at the same time and feature virtually identical cockpits.

Finally, the Boeing 757 generates a massive amount of wake turbulence as it flies — forcing air traffic controllers to keep other planes far away from passing 757s.

"No one really knows why, but the 757 has really strong wake turbulence," Smith told us. "The early 757s had worse wake turbulence than the 747."

Although Smith added that the addition of winglets on later 757s have greatly decreased the wake turbulence. The Boeing 757 is an aircraft that's come and gone. It served its tour of duty with honor and distinction. Now, it's time for Boeing to find a replacement worthy of stepping into its shoes.