A bit of digging into the 7Z9363 accident investigation has brought up some interesting information regarding the DC-9-80 ("MD-80")'s flap system:
Flaps may be positioned in any of the six permanent detents in a 0 to 40-degree range by movement of the Flap/Slat Handle. The authorized flap settings for takeoff in the MD-83 were 4, 6, 11, 15, 17, 20, and 24.73 A “Flap T.O. Sel Wheel” on the center pedestal would allow the pilot to further position the flaps from the position selected by the Flap/Slat Handle position anywhere in the 0- to 13- and 15- to 24-degree range.74 This was used to set the final takeoff flap setting, as determined by the runway analysis review for the airport/runway.
74The Ameristar Air Cargo MD-83 COM, Limitations, page 1-24, stated “Flaps (AFM): Do not use flap settings between 13 and 15 degrees.” [Operations Group Chairman's Factual Report, NTSB docket DCA17FA076, page 32, my emphasis.]
18.104.22.168.1 System Description
The aircraft has an inboard and an outboard flap on each wing. The flaps are mechanically controlled and hydraulically actuated. The crew moves the flap lever to the appropriate detent depending on the flight condition (i.e. take-off, landing). The flap lever has fixed detents at UP, 0, 11, 15, 28, and 40. The lever can also be positioned in one-degree increments for takeoff settings using the Dial-A-Flap from 0 to 24, except for a small do-not-use range at 14. Through a series of cables and pulleys, control valves are exercised and provide hydraulic pressure to the flap actuators to extend or retract the flaps. [Systems Group Chairman's Factual Report, NTSB docket DCA17FA076, page 59, section headings bolded in original, other emphasis mine.]
Reinforcing this prohibition, the DC-9-80's takeoff-flap selector wheel has a yellow "DO NOT USE" label covering the space between the 13-degree and 15-degree markings:
(Image originally by the National Transportation Safety Board [it's on page 32];1 cropped and red-ovalled by me.)
Why does the DC-9-80's flap system come with a prohibition against using flap settings between 13 and 15 degrees for takeoff?
1: The underlying photograph was taken by NTSB personnel, and the red arrow was added by the NTSB to point out the selector wheel in question.