TechRep From , joined Dec 1969, posts, RR:
Reply 3, posted (13 years 5 months 2 weeks 1 day 13 hours ago) and read 1534 times:
These are the inlets for the CTC's (Cabin Turbo Compressors), there are four installed on the DC-8 models. They turn at about 35,000 RPM and are used for pressurization. The four switches are located on the Flight Engineers panel with a gauge to view RPM. Under each switch is an overspeed button to simulate an overspeed condition, without overspeed protection circuitry, the CTC would grenade the turbine.
Number 1/2 are paired and 3/4 are paired, meaning they intersect each other with a check valve. Sometimes the check valve would break and the other CTC could run backwards, a common logbook write up.
Those scoops are also heated and the scoops anti-ice switch, located on the overhead panel, energizes the system.