ASRS 2076486

Reporter 1

Cleared for the RNAV [Runway] XX cross ZZZZZ (IAF) at 2000 ft. Captain set the altitude bug to 100 ft. and descended in green FLCH with GP armed. We both thought that VNAV was armed and missed the fact that the vertical mode was green. We descended below 2000 ft. to 1700 ft. and quickly corrected back up to 2000 ft. The Captain began correcting when Tower gave us a low altitude warning. Landed uneventfully afterwards. At the time I was focused on ensuring we had the correct approach loaded, as ATC gave us a different IAF that made us reprogram the approach. I should have verified the FMA was set correctly once the FMS was fixed. Being tired after a 12 hour 4 leg day with thunderstorms and a delay was a factor in why this slip up occurred.

Reporter 2

Cleared for RNAV [Runway] XX. LNAV engaged, APP armed. Altitude selected to minimums (first mistake). We were in green FLCH when I thought I had engaged VNAV which would have caught our final altitude of 2000 ft. (second mistake). Since we weren’t in VNAV the aircraft descended below 2000 ft. We arrested the descent at 1700 ft. and immediately corrected back to 2000 ft. about the time we received a low altitude warning from Tower. After correcting we landed without incident. These mistakes on my part were due entirely to the fact that I was tired after almost 12 hours of duty, four legs of flying, dealing with thunderstorms and other issues. I was not as sharp as I needed to be.