Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Workload; Time Pressure; Human-Machine Interface; Situational Awareness
ATC Issue All Types; Conflict Ground Conflict; Less Severe; Deviation - Altitude Overshoot; Deviation - Altitude Undershoot; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Executed Go Around / Missed Approach
Aircraft; Procedure; Weather
Around 300' on final; tower issued instruction; 'go around; aircraft on runway'. They assigned an immediate turnout to a north westerly heading and a level off of 2000'. I hand flew to 2000'; called for VNAV and engaged the autopilot. Being that we were given an early turnout; with VNAV engaged the airplane started an immediate descent to the published restriction of 1100' that we had not crossed due to the early turnout. Upon seeing this; I immediately clicked off the autopilot and returned to 2000' after about a 250' loss of altitude. After calling for LVL CHNG; straight and level; I reengaged the AP; end of event. On arrival into DFW with approaching front; the airport was switched from a south to a north operation. On vectors with approach ATC was having difficulty with a new controller who I believe was in training. It seemed like every other instruction he was giving was immediately changed by a supervisor. In his defense; there was a passing front and a good amount of traffic saturation. With strong tailwinds until short final; this was probably the first time this controller had seen this type of weather event. On the missed approach; I should have called for level change; not VNAV as we were taken off the published missed procedure which had an 1100' restriction. In my opinion; the controller in training should have been removed and the supervising controller should have taken over after the 3rd correction they had to make in short time. Aircraft were being vectored too close together with the winds the way they were. It seemed like every other instruction the controller in training was giving had to be corrected. On our 2nd approach; we had to query them on what speed they would like because we saw the same issue happening. The controller said 'maintain 190k' to which the supervisor immediately came on and said 'no; maintain 160k.' I immediately configured for final approach and slowed further to 150k. Had we not queried ATC when we did; we would have no doubt had to go around again as traffic ahead was barely clear of the runway as we landed. In a weather event such as this; with DFW congestion; ATC should not have been training or at least should have removed controller in training after a few corrections had to be made. Also; I do not know why the tower had us make an early turnout and a level off lower than the published missed approach altitude. A straight ahead missed or published missed to published missed altitude before vectors to get back in line would have been fine.
ATC directed a go around below 500 feet due to runway 36L being occupied; and instructed us to fly a heading of 320 and climb to 2000. The First Officer; as the pilot flying; engaged the TOGA function and went around. As the aircraft approached 2000 feet the altitude hold function automatically engaged and the speed window opened. There was an overshoot of approximately 150 feet above 2000. As the aircraft was returning to 2000 the First Officer engaged Command B; and shortly after engaged VNAV. It seemed the aircraft was about to level off at 2000 feet until VNAV was engaged; at which time it began an aggressive descent. I immediately brought it to the First Officers attention and he overrode the automation by disengaging the autopilot and auto-throttle; and correcting the pitch and power; as the 'Don't Sink' alert was heard. There were many events occurring simultaneously as we sought to correct the situation by regaining the lost altitude smoothly; in order to avoid another overshoot; respond to ATC; and maneuver around a rain shower. A few seconds later we returned to 2000 feet and reengaged the autopilot on altitude hold uneventfully. The lowest altitude I observed was 1750 feet; which was 250 feet below the assigned 2000 feet. The published missed approach has a conditional instruction of a climbing left turn after reaching 1100 feet. During the event I quickly looked at the legs page to analyze VNAV performance and saw the 1100 foot altitude on the legs page. I do not know if that was the reason for the descent; as it appears to be illogical for VNAV to respond that way; given that the published missed approach altitude is 3000 and 2000 was in the altitude window. It seems in this type of situation allowing the aircraft to remain in Altitude Hold Mode is the safer method.
A Flight crew reported conducting a go around and exceeding the assigned altitude then while correcting the auto pilot descended the aircraft below their assigned altitude.