Date: 201706
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: MD-80 Series (DC-9-80) Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Generator Drive
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: We were climbing out when the L CSD (Constant Speed Drive) OIL PRESS LOW light came on. We checked the left CSD oil temperature and it was pegged off scale high. The left generator then clicked off and on several times before we switched it to OFF. FO flew the airplane and did the radios while I followed the L CSD OIL PRESS LOW checklist in the QRH to the end. After verifying with the FO; I pressed the L CSD disconnect for 3 seconds but left generator still showed VOLTS. I pressed the disconnect again for 3 seconds and the VOLTS showed zero and the oil temperature was decreasing. However; the LOOP B fire light came on and then went out after several seconds. We tested both FIRE LOOPS A and B and got a good test. But; with the possibility of a fire or overheat condition and the CSD not cleanly disconnected or sheared; we [advised ATC] and returned to the [departure airport] and requested ARFF to check the engine for fire before we taxied to the gate. I used the EMRGENCY LANDING and OVERWEIGHT LANDING checklists. I then called the flight attendants and [told them] to plan on a normal landing and that we plan NOT to evacuate. I made a PA informing the passengers that one of our generators went out but that we have two other generators working normally. But; out of an abundance of caution; we will return and have safety vehicles check our engine before taxi in. Please follow the flight attendants instructions and remain seated after landing. The FO made a normal landing at 134;000 lbs. with a descent rate of 700 fpm. I elected to have the FO land so that I could concentrate on all of the other things we had to do. I stopped on the runway and made a PA after ARFF gave me the all clear: This is the Captain; remain seated; remain seated; remain seated. I had ARFF follow me to the gate.Normal disconnect of CSD would show zero volts and frequency. Since there were still VOLTS showing; I had to disconnect a second time and then I got zero volts and frequency. Had it not been for the LOOP B light coming on; would I have gone on to [destination] as per the checklist? I don't know but I was very uncomfortable even before the LOOP B light came on so I think I would return to [departure airport]. It turned out that the CSD connection did shear. The mechanic could not touch the housing as it was so hot after shutdown at the gate. Could have been scary if I continued.


Second narrative: The QRH procedure does not say what to do if the CSD doesn't disconnect. I think Land at Nearest Suitable Airport should be added.


Synopsis: MD-80 flight crew reported returning to the departure airport after experiencing failure of the left generator drive with no clear disconnect; followed by a Fire Loop light.