Air carrier flight crew reported having the incorrect nav mode selected on departure resulting in a turn towards terrain below the ATC MVA. ATC issued an alert; vectors and climb to a safe altitude.
Synopsis
Air carrier flight crew reported having the incorrect nav mode selected on departure resulting in a turn towards terrain below the ATC MVA. ATC issued an alert; vectors and climb to a safe altitude.
Narrative
The first officer and I completed our briefings; pre flight procedures; and checklists. We briefed the ZZZZZ departure to the ZZZZZ transition and I read the plate to him and we agreed everything checked out and we were both aware of the departure and all other aspects of the flight.When we received out takeoff clearance for runway XXR I pressed LNAV instead of Heading mode. Upon departure I followed the FMS as it lead us into a left turn at 400 feet altitude. I failed to realized that we should have been flying the SID assigned heading of 126. We realized our error when ATC informed us we were too low to the MSA in the sector. ATC instructed us to fly to an altitude of 5000 feet and turn left to a heading of 180.We detected the deviation when ATC advised us we were too low in the sector of flight and at that time we were too late to make a turn back onto course.Cause: The cause of this deviation was my misinterpretation of the departure to fly heading 126 until given a vector to the first fix and following the FMS on the departure.We followed ATC instructions to climb and turn to a heading until we were given a direct to the fix ZZZZZ on the departure.Suggestions: This deviation could have been avoided if I took a closer look at the departure and the verbiage it states to fly a heading until assigned a vector by ATC and selecting a lateral guidance mode that will not cause a deviation. In the future if there is something I do not fully understand on a departure I will brief it with my co pilot.
Second reporter narrative
On departure we did not have the correct nav mode entered. We were in LNAV instead of HDG. After rotation and climb sequence; that led to us turning on the SID prematurely toward the first waypoint instead of getting the vectors off of runway heading first. The early turn caused us to be temporarily below the min vectoring altitude during our climbing turn. ATC gave us vectors that continued the spiraling climb-turn that we were in until we were clear of the MVA alert. Initially ATC leveled us off on a vector; but then issued higher altitudes and a new clearance to the first waypoint of the SID. We never lost sight of the ground and terrain around us as it was VFR with high ceilings. There was no other traffic threat of note in the area. The flight proceeded normally.ATC gave us an MVA alert; a turn and a level off altitude. Then the vectored us in a spiral climb and then cleared us on route. It was not until we were safely at cruise altitude that we were able to go back and look at things and figure out what the problem was. Cause: After a lot of preflight discussion between the pilots internally; and then Operations; and then with the owners it was decided that we would change the destination to ZZZ1. This was due to operational and safety concerns with changing weather conditions as reported by the replacement PIC on the ground there and the METAR and forecast. These changes happened while passengers arrived and were being loaded on the plane by the PIC. I went through all the processes of getting a new clearance; changing the flight plan setup and navigation ; etc. This was; I think; the third change to departure transition routing since filing; but certainly at least the second SID transition change. The checklist had been done previously; but when the Capt got in the seat he did a checklist recap and briefed the changes and highlights. As pilot monitoring I was letting myself accomplish other actions while the Capt was talking; including his departure brief. Because of that I was not as focused on the words and the departure plate as I should have been. If I had been more attentive I might have re-read the inital SID instructions and emphasized the nav mode for takeoff; but also I know that is addressed in the takeoff brief and checklist so I wasn't focused on it. After normal taxi out and checklist completion we were cleared onto the runway into position and hold. Here there was one distraction. Tower had called out a helicopter beside the field to previous landing traffic. As we were taking the runway the tower called the helicopter out to us wanting us to positively ID the traffic position. I acknowledged traffic to tower and after a couple of attempts I talked the Capt's eyes onto the helicopter offset from the field. We finished the below the line checklist items with clearance for takeoff. The PF called LNAV in response to that item. For the previous 25 minutes we had talked about the mission changes so much; and how the departure routing now mixed into the ZZZ1 arrival fixes; that my mind was thinking ahead to the actual FMS routing of the departure and not the first SID instruction of 'runway heading until vectored'. Added to normal communication and takeoff actions; I did not catch the error and back up my PF. Takeoff was otherwise normal until the early turn and alerts. One other thing of note. I later loaded the departure back into the FMS to see what it looked like in the FMS and there was no 'Discontinuity' between the vector portion of the SID and the first waypoint. I am not brand new; but neither am I 'experienced' with this FMS yet; but I seem to recall that most vectors are represented by said 'discontinuity'? On examination I cannot tell you why some are and some are not. We focused on compliance with ATC directions. Once leveled off and safely enroute we went back and analyzed what went wrong. Suggestions: Despite the comfort level of flying with the same crew for many legs over 6 days I need to remember that each brief is specific in its threats and directions. That means a sterile environment for digesting the info and listening to each brief more deliberately is required.Also I would be interested in having an instructor talk to me about the 'Discontinuity' function in the FMS.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.