Date: 202105
Local time of day: 0001-0600
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Night
Aircraft: Regional Jet 900 (CRJ900)
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Window Ice/Rain System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Troubleshooting; Communication Breakdown; Situational Awareness
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Crew
When detected: In-flight
Contributing factors: Aircraft; Human Factors; Procedure
Primary problem: Aircraft

Narrative: I was the First Officer and Pilot Monitoring for our flight from ZZZ to ZZZ1. After an uneventful departure and climb we established our cruise at FL360. Approximately 15 minutes into our cruise the Captain noticed a bright light near the top of his side view window. He brought my attention to the window heat unit which was glowing. From my vantage point I initially thought the unit itself was aflame but after several seconds it became apparent that a short existed which was contained within the window panes. A glowing line similar to a light bulb's filament began snaking downwards from the heat unit between the window panes. The Captain tapped the window several times which managed to briefly extinguish the light. We both acknowledged the need to descend as soon as possible and I requested a lower altitude with ATC. ATC mentioned traffic directly in front of us and stated he would try to give us a descent after the traffic passed. During this time; around 10 seconds after the Captain tapped the glass; the short reignited and continued to grow. The Captain noted an acrid smell; unusually high heat emanating from his window; and again expressed the need to descend. I pointed out the traffic; now visible to us; at our 12 o'clock and stated that we can descend shorty after they passed underneath us. For a third time I requested a lower altitude with ATC. The window failed shortly thereafter. The Captain's side view window let out a loud bang and fractures spider-webbed its entirety. For the first time since this incident began we received a L WSHIELD HEAT EICAS message followed by another L WINDOW HEAT EICAS message.Considering the potential for smoke/fumes and loss of cabin pressure we both immediately donned our oxygen masks; ensured they were at 100%; and established communications between ourselves and ATC. As I was donning the mask I saw the previously mentioned traffic pass safely below us. I relayed this to the Captain who began an emergency descent and told me to notify ATC. I [advised ATC] and let the controller know we were beginning a descent. As the Captain was flying our descent I sent an ACARS to Dispatch notifying them of the L WINDOW failure and that we would need a diversion. The Captain noted that we were close to ZZZ2. I plugged ZZZ2 into the fixes page; identifying that we were 55 miles away and subsequently plugged ZZZ3 into the fixes page; identifying us to be 90 miles away. Our first thought was to proceed to ZZZ2 as it was so close. I requested a heading from ATC as the airplane was about to turn north towards ZZZ1 along our original route and that we would likely be diverting to ZZZ2 but were still weighing our options. ATC gave us a heading which we accepted. As we were established in our descent I told the Captain I'd notify the flight attendants. He continued to fly the airplane as I called the FAs. I provided them a brief note of what had happened; that we were diverting; and that we would make an announcement to the passengers when we knew where we were going. They were professional and calm throughout the notification. The Captain handed control of the airplane to me so he could run the QRC and QRH. I accepted the aircraft and he quickly and effectively ran both checklists to their completion. By the end of his checklists we received a message from dispatch identifying ZZZ2 and ZZZ3 as good diversions. We were closer to ZZZ2 but still in the flight levels. We discussed our options. Considering ZZZ3 was a crew base; maintenance base; and nearly perfect to descend to from our present position we elected to proceed to ZZZ3. After establishing ourselves at 10;000 feet we removed the oxygen masks and donned our normal headsets for easier communication. The Captain gave the flight attendants a full briefing and updated the passengers on what had happened and why we were going to ZZZ3. I flew the airplane with autopilot engaged; managed ATC communications; notified dispatch that we were an [priority] aircraft and going to ZZZ3; and proceeded to obtain ZZZ3 weather; performance numbers; and set up the approach in the FMS. By the time the Captain completed his duties we were at 5;000 feet and on delaying vectors for Runway XXL into ZZZ3. We discussed what both of us had done ensuring we had not missed any items seeking immediate attention. Satisfied; he briefed the approach; we ran the descent checklist; I transferred the aircraft controls back to him; and I notified ATC that we were ready to proceed to the airport. The rest of the flight continued uneventfully. I called Ops to ensure we had a gate; we shot the approach; several emergency vehicles followed us after landing; and we canceled [with ATC] while taxiing in.The Captain detected a light in his peripheral vision. He discovered a short working its way through the window pane.Repeated cycling of the windshield heat in a prior checklist revision likely led to the unit's failure and short. We knew it was likely a matter of time until the window failed. We immediately began planning our descent and diversion choices.I was surprised at how challenging it was to hear ATC and/or the FAs with the oxygen masks on. Our breathing was very loud over the intercom/speaker which made it very hard to hear other voices. It might be worth noting in sim sessions.




Synopsis: First Officer reported that the Captain's side window experienced a short in the window heat system; requiring an emergency descent and a diversion to a precautionary landing.