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The History of Dorsal Fins

samuel-dickinso
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Airplanes are wonderfully complex machines.  Fortunately for you, reading this article will ensure your flight-related vocabulary stays sharp and up-to-date.  For today, consider the tail end of the quintessential Southwest Airlines airplane­—the Boeing 737-300.  No doubt you have noticed the slight elongation at the base of our planes’ vertical stabilizers and wondered about its significance.  These additions are commonly referred to as “dorsal fins” and have emerged as the result of a fascinating history!

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Our story begins with the very airplane Southwest relied upon in its early days—the Boeing 737-200.  Note the steep descent from this plane’s vertical stabilizer as it joins the body.  This particular model was first powered by Pratt & Whitney JT8D-15 and JT8D-17 powerplants that offered 15,500 and 16,000 pounds of thrust respectively.  Though many Boeing 737-200 planes were subsequently equipped with more fuel efficient JT8D-9A powerplants, all three engine varieties were inefficient when compared to today’s modern engine standards and incapable of powering larger airframes.

The relatively small amount of thrust generated by these engines did allow pilots to maintain stability in the air without the dorsal fin addition of today.  As the 1978 Airline Deregulation Act opened up a world of opportunity for Southwest however, it became clear that the company would need to find a larger plane with a more powerful engine. Enter the Boeing 737-300.

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On December 17th, 1984, Southwest’s first ever Boeing 737-300 debuted with the name The Spirit of Kitty Hawk to honor the anniversary of the Wright Brothers’ first flight (and is now on permanent display at the Frontiers of Flight museum in Dallas).  This new plane featured high-efficiency CFM56 turbofan powerplants from CFM International—a collaborative effort between General Electric and France’s Snecma—rated between 20,007 and 22,008 pounds of thrust, depending on individual models.

The combination of a longer fuselage, higher engine thrust, and the need to maintain the directional control requirements both on the ground and in the air drove the need to increase the surface area of the vertical stabilizer.  The best way to do this without increasing the height of the tail (which was considered a limitation due to existing infrastructure limitations, i.e., hanger door and ceiling heights), was to add the dorsal fin!

4When it came time to design the NG, the increase in engine thrust levels required an even larger vertical stabilizer, and simply increasing the dorsal fin size would not suffice.

Boeing increased the height of the vertical stabilizer by ~5 feet to ensure adequate directional control capability with the higher thrust engines.

Are you curious about another feature of our airplanes?  Do you ever wonder how your bags make it into your aircraft?  Let us know in the comments below, and we’ll be sure to get the full scoop for you!

Photos by Richard West and Lari Pekurar