Narrative:

Our flight deviated from the lateral track depicted on the RNAV arrival we were assigned; we went off course. Company policy has us brief the arrival routing; approach; missed approach; post landing taxi plan to include hot spots and expected hold short points exhaustively. That briefing is then acknowledged off by the approach checklist normally executed before descending through 18;000 feet. We had done our jobs and properly briefed the assigned RNAV arrival and expected runway. The destination ATIS advertised ILS and visual approaches to 16L & 16R. We had briefed the anticipated arrival runway (16R). Weather was VFR; 6 miles visibility and 6;000 feet overcast ceiling (nighttime). Late in the arrival phase (at or below 10;000 feet) the approach controller issued us a completely unexpected clearance; we were issued the RNAV (rnp) Z to runway 16C. I believe that 16C had been listed as the departure runway [on the ATIS]; not an arrival runway; and is not the usual runway we are normally assigned. This chart has an effective date of 13 nov of this year; it has only been in effect [for a week or so]. The ATIS had said nothing about RNAV rnp approaches to the destination. There were no navigation fixes out of service and the ILS's were serviceable. To complicate matters only one pilot's ipad had chart 22-21 RNAV (rnp) Z to 16C; the other pilot's ipad did not have this chart (we believe we both updated our ipads prior to our first departure the previous day and were current). Neither of us had seen or flown this approach before. There is a safety reason company procedure has us brief the instrument approach well in advance; and a sterile cockpit below 10;000 feet... Below 10;000 feet things get busy and both external and internal threats are compounded by the time compression and the relative closeness of the underlying terrain. The very late runway approach clearance forces us to reselect the approach from the aircraft's FMC (the approach was available in the aircraft FMC) and review the various points. We also need to tie the newly assigned approach to the RNAV arrival. This requires briefing; verifying and cross checking. In the middle of all of this briefing; verifying and executing activity the LNAV function disconnected and the jet wandered off the lateral course of the RNAV arrival. We discovered our LNAV disconnect and took immediate action to rectify our off course error. By the time ATC queried us about our lateral deviation we were rejoining the arrival (I estimate we got 2 miles off course). Interestingly enough the same RNAV (rnp) Z rwy 16C was issued to a competitor's jet immediately behind us. They sounded a little surprised by the instrument approach assignment. A few moments later that same competitor's jet told the approach controller that they were 'unable' the RNAV (rnp) Z to 16C approach. The approach controller told them to expect the ILS to 16C. In retrospect I should have done the same thing as I expect the captain of that flight is not sitting up at a computer filling out their equivalent of this form; as they probably didn't make the mistake I did. I asked the controller if he had time for a question; to which he answered affirmative. I asked him what was driving the RNAV (rnp) Z to rwy 16 approach we had been assigned. He answered me: 'expect the ILS to 16C.' except the ILS to 16C didn't answer the question asked. In fact he is now changing the runway approach assignment that forces me to start the whole briefing-selecting-verifying-executing exercise again. I'm not sure that controller has any idea what he does to our work load when he changes an approach from an ILS to a RNAV (rnp) back to an ILS (now on a base turn descending to 3;000 feet). I understand managing my work load is not his job. I can always just say 'unable' and go around. I can usually keep up; and I'm interested in cooperating; but tonight I missed a LNAV disconnect and got off course trying to make it work.we are VMC now and the safety of the aircraft is assured so I start the process of selecting the ILS both from the jet's FMC and my ipads. Halfway thorough this exercise the controller asks if we have the destination airport in sight to which we answer in the affirmative. He clears us for a visual approach. I am familiar with RNAV (rnp) approaches and am comfortable flying them. The timing of this never previously issued or flown unexpected approach type clearance below 10;000 feet; not listed on the ATIS (that's why we listen to the ATIS right?) placed a heavy load of keypad pushing; briefing; cross checking and verifying that is normally done 30 to 40 miles behind our position; and at least 10;000 feet higher. Tonight I committed an automation error; the autopilot reverted to heading select after the LNAV disconnected and we drifted off course. This event was caused by my inability to follow a set of instructions correctly; the task was unexpected; assigned late and required multiple steps. In the midst of trying to get the steps correct I committed an automation error that caused the LNAV to disconnect resulting in a lateral course deviation.

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Original NASA ASRS Text

Title: B737-NG flight crew experiences a runway change during arrival to SEA on the HAWKZ 4 approaching COFAY. The ATIS had said to expect the ILS 16R and the runway and approach were changed to RNAV RNP Z Rwy 16C. During the change to this new procedure; a discontinuity is introduced and a track deviation occurs.

Narrative: Our flight deviated from the lateral track depicted on the RNAV arrival we were assigned; we went off course. Company policy has us brief the arrival routing; approach; missed approach; post landing taxi plan to include hot spots and expected hold short points exhaustively. That briefing is then acknowledged off by the approach checklist normally executed before descending through 18;000 feet. We had done our jobs and properly briefed the assigned RNAV arrival and expected runway. The destination ATIS advertised ILS and visual approaches to 16L & 16R. We had briefed the anticipated arrival runway (16R). Weather was VFR; 6 miles visibility and 6;000 feet overcast ceiling (nighttime). Late in the arrival phase (at or below 10;000 feet) the approach controller issued us a completely unexpected clearance; we were issued the RNAV (RNP) Z to runway 16C. I believe that 16C had been listed as the departure runway [on the ATIS]; not an arrival runway; and is not the usual runway we are normally assigned. This chart has an effective date of 13 Nov of this year; it has only been in effect [for a week or so]. The ATIS had said nothing about RNAV RNP approaches to the destination. There were no navigation fixes out of service and the ILS's were serviceable. To complicate matters only one pilot's iPad had chart 22-21 RNAV (RNP) Z to 16C; the other pilot's iPad did not have this chart (we believe we both updated our iPads prior to our first departure the previous day and were current). Neither of us had seen or flown this approach before. There is a safety reason company procedure has us brief the instrument approach well in advance; and a sterile cockpit below 10;000 feet... Below 10;000 feet things get busy and both external and internal threats are compounded by the time compression and the relative closeness of the underlying terrain. The very late runway approach clearance forces us to reselect the approach from the aircraft's FMC (The approach was available in the aircraft FMC) and review the various points. We also need to tie the newly assigned approach to the RNAV arrival. This requires briefing; verifying and cross checking. In the middle of all of this briefing; verifying and executing activity the LNAV function disconnected and the jet wandered off the lateral course of the RNAV arrival. We discovered our LNAV disconnect and took immediate action to rectify our off course error. By the time ATC queried us about our lateral deviation we were rejoining the arrival (I estimate we got 2 miles off course). Interestingly enough the same RNAV (RNP) Z Rwy 16C was issued to a competitor's jet immediately behind us. They sounded a little surprised by the instrument approach assignment. A few moments later that same competitor's jet told the approach controller that they were 'unable' the RNAV (RNP) Z to 16C approach. The approach controller told them to expect the ILS to 16C. In retrospect I should have done the same thing as I expect the Captain of that flight is not sitting up at a computer filling out their equivalent of this form; as they probably didn't make the mistake I did. I asked the controller if he had time for a question; to which he answered affirmative. I asked him what was driving the RNAV (RNP) Z to Rwy 16 approach we had been assigned. He answered me: 'expect the ILS to 16C.' Except the ILS to 16C didn't answer the question asked. In fact he is now changing the runway approach assignment that forces me to start the whole briefing-selecting-verifying-executing exercise again. I'm not sure that controller has any idea what he does to our work load when he changes an approach from an ILS to a RNAV (RNP) back to an ILS (now on a base turn descending to 3;000 feet). I understand managing my work load is not his job. I can always just say 'unable' and go around. I can usually keep up; and I'm interested in cooperating; but tonight I missed a LNAV disconnect and got off course trying to make it work.We are VMC now and the safety of the aircraft is assured so I start the process of selecting the ILS both from the jet's FMC and my iPads. Halfway thorough this exercise the controller asks if we have the destination airport in sight to which we answer in the affirmative. He clears us for a visual approach. I am familiar with RNAV (RNP) approaches and am comfortable flying them. The timing of this never previously issued or flown unexpected approach type clearance below 10;000 feet; not listed on the ATIS (that's why we listen to the ATIS right?) placed a heavy load of keypad pushing; briefing; cross checking and verifying that is normally done 30 to 40 miles behind our position; and at least 10;000 feet higher. Tonight I committed an automation error; the autopilot reverted to heading select after the LNAV disconnected and we drifted off course. This event was caused by my inability to follow a set of instructions correctly; the task was unexpected; assigned late and required multiple steps. In the midst of trying to get the steps correct I committed an automation error that caused the LNAV to disconnect resulting in a lateral course deviation.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.