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] first officer; 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 first officer 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 first officer; 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 I asked him if anybody had reported a newer version of the ATIS yet than we had onboard; as one was expected; and then checked in descending via the hawkzrnav arrival.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.

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Original NASA ASRS Text

Title: 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.

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.

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.