Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Workload; Communication Breakdown; Time Pressure
ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Procedure; Aircraft
There was some initial confusion on our cleared routing from clearance delivery. We were not cleared as filed; but rather via [Route 1]. Prior to engine start; we clarified with ATC that our routing was [Route 1 to LAX]. Initially after departure; Los Angeles Center cleared us direct to ZZZZZ. ZZZZZ was not one of the waypoints on our cleared route which we queried Center about. They seemed confused that ZZZZZ waypoint was not on our flight plan and said they would check with SoCal to find out what routing they would prefer for our arrival. Eventually they called us back and cleared us to LAX via SADDE8. This arrival is what we were originally filed to fly. Passing 11;000; ATC requested that we level off at 11;000 which we momentarily went above because we had been cleared higher and then descended back down to 11;000 complying with ATC's instructions. We entered the new arrival/routing information into the FMS and verified the accuracy. The waypoints on the arrival are the same fixes as the [Route 1] routing we had. A FMS discontinuity was where we expected it to be after SMO and confirmed. We continued to fly the assigned routing. Approach assigned us a 040 degree heading along with a continued descent into LAX. Approach queried us on what our heading was departing SMO. It appeared as if the FMS reverted to dead reckoning mode and we actually flew closer to 080 heading instead of what should've been a 070 heading departing SMO for that arrival procedure. The rest of the flight was continued as would be expected for our arrival into LAX runway 24R.ATC provided us with clearances normally associated with RNAV routing [for this flight leg]. It appeared as if the controllers were expecting us to arrive LAX via different routing (we were using non-RNAV routing). Additionally; our routing was changed repeatedly over the course of an already very short leg. Numerous changes/requests by LA Center over a very short period of time further compressed the crew's time and ability to manage tasks in the cockpit for an arrival into the highly saturated airspace of LAX. Reducing the amount of reroutes/changes in flight within the final 15 minutes from landing would have provided the crew better ability to manage their arrival/tasks into LAX.
While on the ground in ZZZ; we were issued a clearance for ZZZ via [Route 1 to LAX]. We clarified with ATC to be [Route 1]. After departure; Center cleared us direct to ZZZZZ. It wasn't in our route due to having an airplane with a deferred GPS; so I queried Center about it. He reassigned us direct to ZZZ1. I again clarified our routing after ZZZ1 to make sure we understood where we were expected to go next. Center said he would check if there was another routing SOCAL would prefer; and eventually returned to clear us to LAX via SADDE8 (ironically the route we'd been filed with to start with...). We accepted that clearance and were given an altitude assignment of 11;000 until further advised. I advised him we were crossing 11;000 and would go back down to it momentarily.I entered the new arrival; and I verified the waypoint accuracy. Unfortunately; what I missed was that the FMS added it after the existing routing we had to as far as the SMO VOR. It turns out the waypoints on that arrival are exactly the same fixes as the waypoints along [Route 1] we had previously used; so I didn't see the difference. Also; at that point; the pagination of the FMS had a discontinuity where I expected it to be after SMO. We continued on our course; then Approach called to make sure we were descending via the arrival. We had not been cleared for that; so approach assigned us an altitude. A few moments later; approach gave us a 040 degree heading and further descent. I read it back; then he asked what heading we'd left SMO with. I wasn't sure but said I thought it was about 070. I think the FMS had gone into dead reckoning mode and actually flew 080 due to the incorrect placement of the arrival. This is when I realized we had extra fixes in the FMS after the SMO with the FMS attempting to resequence us to the start of the arrival. Approach gave us some additional headings; descents; and an airspeed of 180 kts to get more accurately placed onto the downwind vector for 24R. Next; I got the FMS straightened out for the approach and we continued the approach and completed the flight.If this whole thing sounds confusing; it's because it was. I felt like ATC kept thinking we were on a different routing; altitude; and airspeed than we'd actually been assigned. A major contributor to the poor communication was the numerous changes to routing that we kept getting from ATC. We had several changes to the routing during a very short; task saturated flight. The routing changes were due to the plane having a deferred GPS; we were unable to accept any RNAV SIDs or STARs. The controllers seemed to be issuing instructions from habit; often consistent with typical clearances I've seen while flying the ZZZZZ arrival (hence the clearance to ZZZZZ and uncertainty why we hadn't descended as we neared SMO). It would have been helpful not to have a reroute in flight when we were only 15 minutes away from landing. Also; going through to the last page in the FMS would have helped me notice the re-sequence and extra discontinuity in the FMS. I didn't count pages; I just saw that it went to the right fixes; then had a discontinuity for vectors to the approach at the right place.
Air Carrier flight crew reported confusion with ATC and receiving multiple reroutes on short flight resulting in increased workload.