Gulfstream 150 flight crew reported receiving a low altitude alert from ATC during circle-to-land in IMC. The flight crew corrected their altitude and continued the approach to a safe landing.
Synopsis
Gulfstream 150 flight crew reported receiving a low altitude alert from ATC during circle-to-land in IMC. The flight crew corrected their altitude and continued the approach to a safe landing.
Narrative
While descending on the RNAV XX Circle to XY; I was the PIC; and Pilot flying. The aircraft was configured for circling; and we were about 1NM from ZZZZZ. The aircraft was setup with autopilot on; NAV Mode was selected and activated for Lateral navigation. VNAV was selected for the aircraft for vertical navigation; and was in VVS for the selected decent mode; with 500fpm was depicted on the vertical speed tape. a magenta 2300 ft. was selected to indicate the aircraft would level at 2300 ft. to cross ZZZZZ at that 2300 ft. before continuing decent to the Cyan (blue) Cat C circling Minima. The aircraft instead descended below 2300; myself; and the Pilot not flying (PNF) caught it as it went below 2300. I was able to level the aircraft at 1800 ft momentarily. ZZZ Tower Issued a Low Altitude alert to us which we acknowledged immediately. While Leveling the airplane we came into visual conditions; and continued the decent to circling minima as we were now past ZZZZZ and in visual conditions. we continued to the airport and circled; and safely landed Runway XY without incident. We did not receive anything further from ATC about the incident.Following the Incident; the PNF and I agreed our response was acceptable; and further wondered the reason the aircraft did not level at the selected VNAV altitude at ZZZZZ. We are submitting this report as evidence of our experience to hopefully help fix any issues that may come up with other crews or aircraft with this same issue.
Second reporter narrative
We were established on the RNAV XX planning to circle to Runway XY. I had already loaded the RNAV XX; checked the waypoints and altitudes; and loaded landing performance in the FMS. We had also briefed the approach and circle to land while still being vectored; prior to starting the approach. After checking in with tower who instructed us to continue the approach and advise starting our circle I verified that a magenta 2300 was displayed above the altitude tape; indicating that the aircraft would level at 2300 ft. at ZZZZZ before descending to minimums (1200). I then continued with the before landing checklist to verify the aircraft was in proper configuration for landing. Just prior to ZZZZZ both the Captain/PF (Pilot Flying) and I noticed the aircraft was below 2300 ft. and descending to minimums. At the same time ZZZ Tower issued us a low altitude alert. I called to correct and PF simultaneously disengaged autopilot and stopped the descent/corrected our altitude. The correction was at approximately 1800 ft. MSL. We crossed ZZZZZ and continued with the approach; transitioning from IMC to VMC just prior to reaching minimums. The remainder of the approach along with the circle to land was uneventful. PF and I de-briefed the event after our flight and agreed we responded appropriately. We were both unsure as to why VNAV did not capture 2300 at ZZZZZ but rather descended right to minimums. I will ensure to pay closer attention to altitudes/automation when on final approach in IMC. We also agreed documenting the altitude deviation/incident with a report would be appropriate.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.