Date: 201604
Location: SEA.Airport, WA
Flight conditions: VMC
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class B SEA
ATC / advisory: TRACON S46
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Distraction
Second reporter function: Pilot Flying; First Officer
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: [Our flight to SEA] was a normal flight until cruise when we were cleared to descend to FL240. As pilot monitoring; out of an assigned cruise altitude of FL360; the aircraft went immediately out of VNAV; I temporarily saw an amber Control Wheel Steering PFD cue that was soon overridden [by] FO; first with a Vertical Speed selection (to get the nose down now!) followed soon thereafter with by Level Change selection; and immediately thereafter with a frequency change. Upon my checking in; we were immediately cleared to descend via the HAWKZ4 RNAV arrival landing south; which was exactly what we had briefed 100 miles before beginning the initial descent. As PNF I set the lowest altitude on that STAR at 6;000; saw it through to be verified; then accidentally abrogated my PM duties by not stating 'I'll set the next lowest altitude of FL220' as we approached HAWKZ in a Level Change pitch mode. Already high on the profile; well above crossing restrictions; it wasn't of IMMEDIATE concern but completely improper procedure on my part. Instead of correcting that; I passed the radios to the FO as I took to the PA to offer a good bye to our customers; making note of the (unusually) beautiful Seattle weather with splendid views of snow shaped volcanic mountains out the right window and the beautiful Pacific Ocean out of the left. Once [done] with the PA; I reported 'back on number 1 radio' to FO; who said he had switched us to Approach but had not yet checked in. I asked him if anybody had reported a newer version of the ATIS yet than we had onboard; as one was expected; and then checked in descending via the HAWKZRNAV arrival. I did not refer to the PFD to check what pitch mode we were in; but the Controller said 'Climb and maintain 10;000 feet'. Knowing we were on a Star and therefore this was such an unusual call; and somehow that made me unsure of what I had just heard; I said 'say again 'and the Controller unemotionally repeated; 'Climb and maintain 10;000 feet' which we immediately complied with as by that time I was seeing the bottom window of the next fix on my moving map display showed 10;000 feet at BREVE. The Controller then asked; 'why were you down at 6;000 feet?' and I said 'my bust'; as there was no excuse for this performance. I had been relying on the VNAV automation instead of the old fashioned; 'Set the next lowest altitude'; which forces both pilots situationally aware with respect to the profile. Today I think I was a lured by the pure beauty of a clear spring Seattle day and was obviously much less aware than I needed to be.


Second narrative: As we neared the top of descent on the HAWKZ 4 arrival into Seattle; ATC did not give us a lower altitude and we were high for the descent profile. I asked the Captain to request a lower altitude and ATC gave us FL240. We were approximately 2;000 feet high at this point and I announced a vertical speed descent and dialed in approximately 1;200 feet per minute descent. Once the aircraft was established in the descent I announced level change mode and extended the speed brake to help us descend as we were still above the descent path for the arrival. We were handed us off to the next controller as we began our descent. Captain checked in and we were told to descend via the HAWKZ 4 RNAV Arrival. The Captain set 6;000 feet into the MCP altitude window and we both verified it against the bottom altitude of the arrival. By this time I had stowed the speed brake. Captain said he'll talk to them and announces that we're descending via the HAWKZ 4 arrival. At this point I simply was not looking at our displays and a very short time later we were told to climb to 10;000 feet from our current altitude of 6;000 feet. This radio call immediately got our attention and the Captain set 10;000 feet into the MCP window and I selected level change and reduced our speed from approximately 280 knots to 250 knots. I knew right away that we never got back into VNAV path for protection. We were at 6;000 feet just North of PIKEZ intersection which had a restriction of between 12;000 feet and 15;000 feet with speed of 270 knots. We climbed back to 6;000 feet prior to BEREZ intersection and were given a telephone number to call for a possible pilot deviation. The rest of the flight continued without incident. I recall being in a bit of a daze during the descent; perhaps from finishing a 3 day trip the day prior and having a rather restless night of sleep last night.


Synopsis: B737 flight crew reported descending well below charted altitude on the HAWKZ 4 RNAV Arrival into SEA; citing lack of FMC mode awareness as a factor.