Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Workload; Confusion; Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Human Factors
While descending via on the DSNEE 4 RNAV arrival into SNA we were given a set of instructions approximately 5 miles from the DSNEE fix. I remember it as being; Aircraft X; after DSNEE; slow to 190 kts. and descend and maintain 6;300 ft. I repeated this instruction back to ATC and I believe my partner repeated it to me. He dialed in 6;300 ft. and we continued our descent to cross DSNEE at 8;000 ft. before descending further. As we crossed DSNEE; he switched from VNAV to vertical speed for the next descent as expected. He also hit the heading button and moved our heading bug to 190 which I did not expect. I questioned whether I had misunderstood ATC instructions. I immediately contacted ATC to query the heading and almost simultaneously a different controller jumped on the frequency to instruct us to climb back to 8;000 ft. and immediately make a right turn along with a low altitude alert as there was terrain nearby. We did so and continued normally to SNA. When we got on the ground and the aircraft was secured; my partner and I debriefed what had just happened. He was confused as to the series of events until I explained to him the instruction was for 190 kts.; not a heading of 190. He then fully understood what had happened. Great job by ATC to immediately notice we were off track and to issue new instructions. While this event happened visually and we had terrain in sight; it could have had bad consequences at night or in IMC without their quick response.We were in a fairly busy segment of flight and were given a fairly complex series of instructions. Somewhere in that quick 5 miles; we; as a crew did not communicate properly. While myself and ATC were on one page; my partner was on another. Things happened very fast leading up to the deviation; I had no reason to think he had misunderstood the instructions. I should have been listening more carefully when he repeated them back to me; but In this case I think he did repeat the right instructions back to me; but executed them differently. In the future; I think I will try to write down instructions when they are given in a situation like that. We have a lot going on; and sometimes it is possible to miss or misinterpret an instruction when it was given to be done at a point further down the line. I know I have missed a few and my partner has caught it in situations like this. Usually more along the lines of fly Heading xyz or descend to xyz after an intersection 5 minutes or so down the road.
We were established on DSNEEE4 arrival approaching DSNEE intersection. We had been given numerous vectors off STAR for aircraft separation then cleared back to STAR fixes. Airspeeds were also assigned different from STAR. Several minutes prior to DSNEE we were given a heading of 190 and altitude of 6;300 ft. to take after DSNEE. We were in absolutely clear weather with all terrain; traffic and SNA in sight. At DSNEE I turned to 190 degrees heading as assigned and commenced descent. Very shortly thereafter another controller intervened with a new heading of 280 degrees and to return to 8;000 ft. immediately. I complied immediately but at no time was there any EGPWS warning and the terrain was clearly in sight well below us. Shortly afterwards the previous controller came back on frequency with headings and altitude assignments. We continued uneventfully to landing at SNA.
Citation 560XL flight crew reported a confusion between an altitude and heading assignment that resulted in a clearance deviation and CFTT.