G280 First Officer aircraft reported an altitude deviation in cruise flight.
Synopsis
G280 First Officer aircraft reported an altitude deviation in cruise flight.
Narrative
Shortly after passing Waypoint; left front outer pane windscreen failed/shattered. Pilot in command (PIC)/pilot not flying (PNF) executed checklist; manual pressurization to lower differential pressure to 7.0 psi and slow to .72M/250kts. We requested descent via CPDLC to FL390 to aid in minimizing pressure differential. At FL390 cabin momentarily exceeded 10000 ft. from pressure 'bump' due to auto throttle movement and or turbulence. Consequently; this activated EDM (emergency descent mode). PIC stated we still had cabin pressure so I disconnected the Autopilot and manually climbed back to FL390; returned to course and re-engaged the Autopilot. We lost approximately 1000 ft. and were off heading approximately 60 degree. We did not encounter a TCAS event or loss of separation. Once stabilized we requested further descent via CPDLC to FL350. This gave us a larger buffer of cabin alt so EDM would not activate. We consulted AFM and base Maintenance and continued to destination at FL350; 250 kts; manual pressurization; and a very shallow descent. Situation increased crew workload and mandated good crew coordination for thrust changes and outflow valve manipulation to maintain appropriate cabin altitude pressure differential.
Second reporter narrative
We experienced a cracked windshield at cruise in ZZZ airspace; approximate location of Waypoint. Per the checklist; we needed to reduce the cabin pressure differential. We obtained a descent clearance to descend from FL430 to F390; we also manually increased the cabin altitude to approximately 9;500 ft. While level at FL390 the cabin altitude increased above 10;000 ft.; probably due to us encountering light turbulence causing the auto throttles to adjust; which had an effect on our cabin altitude because of us utilizing manual pressurization. The cabin altitude increase above 10;000 ft. caused the automatic descent mode of the aircraft to engage. The aircraft automatically reduced power to idle; commenced a left turn and started a descent. I was able to manually get the cabin altitude to below 10;000; and then the pilot flying hand flew the aircraft back to altitude while returning to our course. The aircraft descended approximately 1;000 ft. I notified ZZZ via voice of the deviation and once again when we were back on proper course and altitude.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.