Of the various use cases of the Pratt & Whitney JT8D (the most-produced low-bypass turbofan in history, and the second-most-produced turbofan of any description1), those installed on the 737-100 appear to use a set of auxiliary suck-in doors, similar to those so familiar from the earlier JT3D:

JT8D on 737-100; note suck-in doors

(Image by Steve Fitzgerald at Airliners.net, via Fæ at Wikimedia Commons, modified by Fæ and Marc Lacoste at Wikimedia Commons.)

In contrast, the JT8Ds installed on other types of aircraft, so far as I can tell, do not have suck-in doors:


JT8D on DC-9

(Image by Cory W. Watts at flickr, via Josve05a at Wikimedia Commons.)


JT8D on 727

(Image by Pablo Andrés Ortega Chávez at flickr, via Wikimedia Commons.)


JT8D on 737-200

(Image by Andre Gustavo Stumpf Filho at flickr, via Wikimedia Commons.)

Caravelle 10

JT8D on Caravelle 10

(Image by duch.seb at Wikimedia Commons.)


JT8D on Mercure

(Image by Michel Gilliand at Airliners.net, via Fæ [again] at Wikimedia Commons, modified by Fæ at Wikimedia Commons.)

Why did the JT8D need suck-in doors on the 737-100, and not on any of the many other aircraft it was attached to?

1: Behind only the General Electric/SNECMA CFM56 high-bypass turbofan, used on (among many, many other aircraft) the 737-300 through -900.


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