B737-900
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 185; Flight Crew Total 28800; Flight Crew Type 11326
Workload; Human-Machine Interface; Distraction
ATC Issue All Types; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Flight Crew Returned To Clearance
Aircraft; Human Factors; Weather
We were inbound to SJC on the SILCN4 arrival when we were cleared to FL240. We were told to slow to 250. A short time later we were told to maintain 250 until SILCN and then speed up to 280. The FL240 descent clearance had our altitude covered at SILCN so I used vertical speed to maintain the 250 kts and put 280 kts into the FMC to use when we crossed SILCN. Before that all happened we got a third speed change to go back to 250 kts. Eventually we got a descend via clearance. I can't remember exactly when or at what altitude we got the descent via clearance. Right before crossing VLLEY I noticed on the Prog page that we had crossed SILCN at 18;800 which could have been 1;200 feet low if we were descending via at that point. Things were changing very quickly so unfortunately I can't recall every detail.
While flying the SILCN 4 RNAV arrival into SJC in moderate turbulence; we were given several changes to the arrival well after reaching TOD and beginning our initial descent to FL 240. We were instructed to slow from 280 KIAS to maintain 250 KIAS for the remainder of the descent. At this time the CA (Captain) selected V/S; extended the speedbrake and began to slow while modifying the STAR speeds to comply. We were flying a 900 without Speed Intervention. After some confusion we were able to set-up the box to match our new cleared speed. At that time we were given a vector to fly for spacing. The CA elected to remain in V/S at this time. To the best of my recollection; we were next cleared direct to SILCN and told to maintain 250 till SILCN then resume published speeds (280 KIAS). I can't recall for sure but I believe we may also have been given a lower altitude at that time. At this time I became aware that the CA was having difficulty getting the FMC Legs Pgs to accept the new manual airspeed changes to comply with ATC's last speed clearance. So I began to pay close attention to his FMC inputs so that I might be able to help him get it set-up as he desired. While he continued that effort we were then cleared to descend via the SILCN 4 Arrival except maintain 250 KIAS. The CA asked me to set 4000' in the altitude window; which I did. Once gain he set about to change the speeds in the Descent and legs pages to comply. As the CA completed his speed inputs on the Legs pg to comply with the STAR; we crossed over the SILCN fix. As we did; he reselected VNAV mode on the MCP (Mode Control Panel) then we both checked our altitude to verify that we could comply with the next altitude restriction. It was at that time that we realized that we were approximately 1000' below the published lower limit for the SILCN fix (FL200). Immediately we questioned each other as to whether we were supposed to meet that restriction. In the FMC speed confusion; moderate turbulence; multiple speed changes; and vectors off then back on the arrival we both lost track. We got no calls from LA Center or NORCAL regarding a possible altitude deviation but decided it best to submit the [report] just in case it was. Despite windshear conditions the remainder of the descent; approach; and landing were normal.
B737 flight crew reported being below crossing restriction while reprogramming FMS.