Corporate Jet Flight Crew reported receiving a GPWS terrain alert and a low altitude warning from ATC on approach to DWH airport following unintended autopilot disconnect.
Synopsis
Corporate Jet Flight Crew reported receiving a GPWS terrain alert and a low altitude warning from ATC on approach to DWH airport following unintended autopilot disconnect.
Narrative
I was PIC (Pilot In Command) flying to DWH. We were vectored to IAF; TUFFS and cleared for the RNAV GPS approach to Runway 17R at DWH. Weather was IFR at the field and was reported as 900 ft. broken; visibility 5 miles; and gusty cross winds. Approach progress was going smoothly with the autopilot engaged and SIC (Second In Command) flying the airplane. We were configuring and slowing to cross the final approach fix at Vref+10 kts and 1;800 ft. according to the approach plate.We crossed OILER intersection at 2;000 ft. and descended to cross the FAF CUBIR at 1;800 ft. After 1;800 ft. was captured by autopilot; I set in 1;000 ft. in the altitude window for our next crossing restriction.At some point after OILER; the autopilot disconnected on its own; either the control yoke was bumped hard enough to cause a disconnect; which occasionally happens caused by flight crew; or I gripped the control yoke too hard when using the Mic transmit button on the yoke and it disconnected; or autopilot sensed a problem and disconnected itself; I don't know. Aircraft starting descending to 1;000 ft. about 3 miles prior to FAF. I think at the same time; ATC was telling me to contact Tower. I never switched to Tower because of the situation in the cockpit.I realized what had happened to the autopilot; and SIC was trying to regain control of aircraft. The terrain warning went off and about at the same time ATC gave us a low altitude alert; asked us to check our altitude immediately; and climb to 2000 ft. We followed ATC instructions. We were in VFR conditions at that point and I could see the airport. I told ATC we had the airport and could continue visually. ATC said the field was still reporting IFR conditions and he could not give us a visual approach.ATC then vectored us for another RNAV Runway 17R approach that terminated with a full stop landing. No issues with autopilot during second approach.In hindsight; I should have been more proactive with monitoring the approach progress since I was the Monitoring Pilot and called for a go around immediately after the terrain warning; but everything happened so quickly and before I could do anything; I was responding to ATC request for an identify on the transponder and complying with a climb to 2;000 ft.In conclusion; I failed to properly monitor aircraft situation and SIC flying.I have a toolbox full of resources from 30 years of CFIT training and flying with [air carriers] and should not have allowed the aircraft and SIC to get into that situation. I'm not pointing fingers except at myself.I described the events of our flight as best as I could recall. ATC did a great job with responding to our terrain alert situation.
Second reporter narrative
Me and my Captain were flying a trip to David Wayne Hooks Airport. The whole trip was mostly IMC with scattered thunderstorms throughout the whole area. We were cleared for the RNAV Runway 17R approach at DWH. We flew over the IAF of TUFFS and then headed for the IF(OILER) to cross at 2;000 ft. and 210 kts max. Due to the weather; I started early to configure and prior to OILER; I was at flaps 7 and making the turn at 180 kts. In making the turn toward the FAF (CUBIR); I called for flaps 20 and was slowing to 150 kts and called for gear down. We started descending from 2;000 ft. to cross CUBIR at 1;800 ft. when the autopilot disconnected and my attention was diverted to what was happening. With the auto pilot malfunctioning; the Captain was troubleshooting the problem. I made the mistake of doing what I knew I shouldn't be doing by letting my attention be diverted from strictly flying the airplane. I descended below 1;800 ft. to 1;000 ft. and was stabilized with the runway in sight when ATC directed us to go around and climb to 2;000 ft. which we did. We received vectors and came back and executed the approach without any incidents. In conclusion; after [many] years of flying for [an air carrier] when flying the airplane; you must always fly the airplane and never allow yourself to be distracted when things malfunction. This is another lesson learned in my career not to be repeated.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.