EMB-120 flight crew reported a main and auxiliary generators failed in cruise and the APU could not be started resulting in a diversion and landing using battery power.
Synopsis
EMB-120 flight crew reported a main and auxiliary generators failed in cruise and the APU could not be started resulting in a diversion and landing using battery power.
Narrative
Prior to this flight; we (my FO and I had an equipment deferral). The deferral required the continuous use of the APU in flight to offset the deferred Left Main Generator). The previous flight; which was ZZZ1 to ZZZ2 was performed by my FO and I was the PNF (pilot not flying). As we were sitting at the end of the runway waiting to get our clearance I reminded him (just in case I missed it in our pre-takeoff briefing at the ramp) that; ' I will not be turning off the APU in flight because we are supposed to have it running.' We took off from ZZZ1; I ran the after takeoff checklist; and left the APU on for entire flight to ZZZ2.That concluded the flight from ZZZ1 to ZZZ2.Thenceforth; on this (the flight for the report that this pertain to); was ZZZ2 to ZZZ3. We picked up our single pallet of cargo; got the plane topped off; then went over our weather; destination; enroute; destination METARs TAFs and the Engine Out Departure Procedure (EODP) for ZZZ2. We went over it there (at the ramp) and again at the hold short area of the of the runway because I wanted to be extra sure; with the mountains in front of us; that there was no question of procedure. The EODP would require if needed a turn to left and the takeoff instructions from tower would likely be (and were) a climbing turn to the right. I thought he understood at this point about running the APU. Because I had just reminded him of it just 3 hours earlier on the previous flight.However after takeoff; he turned off the APU. It is normal (99% of the time) to turn off the APU after takeoff. It is VERY difficult to not turn it off as part of the after takeoff 'flow.'I didn't know he turned it off.Moving ahead to approximately 3 hours later we had an electrical emergency. Cabin started to climb (because in an electrical emergency the packs that provide pressurization don't work). The procedure requires a descent to below 20;000 ft to start the APU and we asked center for an immediate descent to FL190. The APU did not start and we ended up diverting to ZZZ.We took off from ZZZ2 with me; thinking that; because of my comment about running the APU the previous flight; WE both had briefed the deferral procedure.I was wrong. This ultimately; caused the electrical emergency?
Second reporter narrative
summary: Crew forgot / Failed to brief and check the requirements of a MEL.Leg ZZZ1 - ZZZ2 diversion to ZZZMEL was for a Deferred Main Generator MEL states that APU GEN must be run Continuously.The Crew was unaware of the requirement of the MEL and the Crew did not brief the MEL before departure. The Crew agreed to turn the APU off after takeoff.At FL280 west of ZZZ. The aircraft lost its other main generator and both aux generators.The crew donned masks and descended below FL200 to attempt to start the APU. The crew was unable to start APU. The crew followed the QRH precisely and put the plane in electric emergency mode. The crew then had 30 minutes of battery remaining. The crew also had a control fault of flaps due to no electricity and had to do a no-flap landing. The crew followed the correct checklists. The crew continued emergency descent down to 10;000 MSL the crew was vectored by ATC and was able to successfully land at ZZZ. The aircraft's main battery died 15 seconds after touchdown and the crew was instructed with a light gun.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.