Date: 201410
Location: SCT.TRACON, CA
Flight conditions: Mixed
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class B LAX
ATC / advisory: TRACON SCT
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Contributing factors: Human Factors; Procedure
Primary problem: Human Factors

Narrative: We understood we were cleared to descend via the RIIVR [STAR] and also cleared for the Runway 24R ILS approach. Therefore; we complied.  After RIIVR; between 11;000 and 10;000; we were told to level at 10;000 and turn left to intercept the 25L ILS approach; which is what the Controller said we had been originally cleared for.  We complied. If memory serves me; this problem has occurred for quite some time. Often; Controllers speak very rapidly while issuing clearances; which also include requests to make speed and altitude changes different from the SID or STAR. These requests while easy to make; often require immediate computer; aircraft configuration; and control changes in order to comply. They are usually followed by frequency changes; aircraft noise; radio congestion; and other distractions. I respectfully suggest that maybe since this event has occurred before; a simple fix we might consider would be for example; to confirm the name of the first fix for the runway extension as a backup.Sometimes it can be easy to misunderstand a directive because of many factors; from rapid speaking on the part of the Controllers; flight deck noise; radio congestion; etc. Since my First Officer and I both were convinced we  heard 24R; as evidenced by the FMC being loaded accordingly and the fact that we turned after RIIVR toward the first 24R fix; maybe it would be best; since this has happened before to others as well; to have the Controllers and Pilots confirm the very first fix initiating the runway extension as a backup.


Second narrative: The Captain was the Pilot Flying. He briefed the RIIVR Arrival and ILS 24R approach and transition. He also briefed the possibility of receiving clearance for the ILS 25L approach from ATC. The LOC frequency for 25L was present on standby on the Captain's side. We received a clearance to slow to 270 knots during our initial descent and then a clearance to cross GRAMM at FL190 and increase the speed to 280 knots. Before reaching GRAMM; ATC switched us to SoCal Approach. SoCal cleared us to descend via the RIIVR 2 Arrival and cleared us for the ILS approach to a specific runway at the same time. The Captain verbally verified the last crossing restriction on the briefed approach of 2200 feet at JETSA and set 2200 feet in the MCP while in VNAV/LNAV. I agreed and repeated 2200 feet. At some point before RIIVR; SoCal slowed us to 250 knots. After crossing RIIVR and passing approximately 11;000 feet; SoCal told us to maintain 10;000 feet and cancelled the approach clearance. SoCal advised us we were joining the 24R LOC when we were cleared for the ILS 25L. SoCal gave us a heading to join the 25L LOC and cleared us for the ILS 25L.After completing the Parking Checklist at the gate; we debriefed the event. The Captain and I agreed that in our minds we both thought we were cleared for the ILS 24R at the time the clearance was received. However; neither one of us could rule out the fact nor the possibility that we may have been cleared for; and read back; the ILS 25L. If that is the case; neither one of us caught the discrepancy between the issued clearance and the approach programmed in the FMC.Although we may have been in the Yellow due to some task loading caused by receiving several altitude/speed changes from ATC in a short period of time; we did not return to the Green in that instance due to a failure on our part to verbalize/verify/monitor. I know this type of deviation is not infrequent at LAX. Would it be possible to have ATC ask Crews to verify and verbalize the first waypoint on the approach when clearance is received? This would force the Flight Crews to look at the LEGS page and verify that they are on the right transition. In other words; a similar concept to verifying and verbalizing the first waypoint on a RNAV departure by ATC and the Flight Crew when a takeoff clearance is issued.


Synopsis: B737-700 Flight Crew reports being cleared for the ILS 24R off the RIIVR 2 to LAX. Before intercepting the LOC for 24R; ATC cancels the clearance and tells the crew to turn left and intercept the LOC for the ILS 25L; and that the original clearance had been to 25L.