About The Boeing 367-80 And That 5th Engine

By Spyros Georgilidakis | June 8, 2023

The 367-80 helped Boeing define the shape of commercial airliners for generations. But the plane was also a notable testbed – for many things.

The 367-80 during tests with a boom in the back of the fuselage. Photo: San Diego Air and Space Museum Archive

The story of Boeing’s “Dash-80”, as it was often called, could fill volumes. Yes, the de Havilland Comet was the world’s first jetliner. But the 707 and the Dash-80 that “spawned” it, effectively became the blueprints for all jetliner designs for well over half a century.

Petter recently made a video about the Boeing 707 and the 367-80 prototype on the Mentour NOW! channel, looking at their background and development. Obviously, Boeing used the Dash-80 to test many of the systems and features that would later make it to the KC-135 and the 707.

About The Boeing 367-80 And That 5th Engine
Flying with the 5th engine. Photo: San Diego Air and Space Museum Archive

But even after these jets entered service, Boeing had a few other projects in the pipeline. One of these was for a short and medium-haul airliner. Initially, Boeing planned to offer a version of the 707 with the narrower fuselage of the KC-135, called the 717.

That idea didn’t interest the airlines. So, Boeing instead offered a shorter version of the 707, which eventually entered service as the 720. But this was an interim solution. Boeing’s intention was to offer another aircraft for this role, which we now know as the 727.

Photo: San Diego Air and Space Museum Archive


Boeing 367-80 And Engine Testing

To make the 727 a reality, the manufacturer needed a platform to test tail-mounted engines. Enter the Boeing 367-80, once again. There are plenty of reports out there of Boeing testing this layout by using the 367-80. But finding pictures of it can be tricky.

About The Boeing 367-80 And That 5th Engine
Here the 5th engine looks different, compared to some of the other photographs. Photo: San Diego Air and Space Museum Archive

So, in the recent Mentour NOW! video, Petter asked viewers for pictures of the Dash-80 with this configuration. Several people responded, with pictures, links, and search tips! So in this article, we tried to collect as many of them as possible in one place.

Several of these pictures come from the San Diego Air and Space Museum Archive. Others come from current and former Boeing employees. They all show the Boeing 367-80 flying with a fifth engine on the left rear side of the fuselage.

Photo: San Diego Air and Space Museum Archive

We can see an S-duct mounted on the engine’s exhaust. Obviously, Boeing needed to keep the engine’s airflow away from the horizontal stabilizer. The 727 would have a T-tail configuration for this reason.

As for the identity of this engine, it very likely is a version of the Pratt & Whitney JT8D. But initially, Boeing planned on using an American-made version of the Rolls-Royce RB163 Spey for its 727. There is very little information on the extent of the Dash-80’s use in the process. But we do know that the JT8D would eventually win out.

About The Boeing 367-80 And That 5th Engine
A closer look at the 5th engine!


Details, Details

These photos of the 367-80 give us a few clues about how Boeing used this prototype for other tests. For example, none of the Dash-8’s four normal engines initially had turbocompressors on their pylons – which is still the case here.

About The Boeing 367-80 And That 5th Engine
There is more to see, beyond the extra engine

The wing of the Dash-8 has also gone through many changes. Initially, Boeing fitted a wing with a smaller chord on the 367-80. The top surface of the wing had a yellow color, while the leading edge and the underside were brown/red.

Later, Boeing would modify the Dash-80’s wing to increase its area. Along the way, a number of different high-lift devices (flaps, slats) were tested. In the picture above, we can see that the wing retains some of its original colors, but newer or modified surfaces are unpainted.

On its way to the museum, after restoration.

Today, the Dash-80 is on display at the Steven F. Udvar-Hazy annex of the Smithsonian, at Dulles International Airport. The jet made it there in 2003, after a decade-long restoration. But restoring it to a 100% original condition, after so many modifications over the years, was impossible.

Today it has different engines, a bare wing, fewer polished surfaces, and other minor differences. In any case, the Dash-80 is unquestionably a tribute to the origins of the jet age, and well worth a visit!

We would like to thank ALL Mentour fans who contributed pictures personally or shared links to them online!


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