Really this should not be an issue at 13,000ft. The topic of climb performance came up in a different thread recently so I did some testing at that time. I found that, in spite of the G36 service ceiling ostensibly being 18,500 feet, with the “regular” (not turbo) G36 mod, I could happily climb to over 20,000 feet. You can read about it here:
https://forums.flightsimulator.com/t/climbing-with-autopilot/443823/5?u=slabsfly
TL,DR: at 12,000ft. the aircraft was climbing at 800fpm and even at 20,000 ft. it was still climbing at an unrealistic 200fpm! This at full throttle, auto mixture, 2,700RPM, FLC mode and IAS 100kts. Because I thought the normally aspirated mod improved performance I repeated the test with the stock G36. This did even better: climbing at 950fpm @ 12,000 and 250fpm @ 20,000 ft. Noted that this was with 2 people and half tanks but still: according to the POH that is not realistic. The linked post includes a copy of the POH climb data for the aircraft.