B737 Undifferentiated or Other Model
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Flight Crew Last 90 Days 402
Communication Breakdown; Human-Machine Interface; Situational Awareness
Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Flight Deck / Cabin / Aircraft Event Other / Unknown; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Provided Assistance; Flight Crew Became Reoriented; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Requested ATC Assistance / Clarification
Human Factors; Weather
It was the last flight of two-day to MDW. Cleared 3000 FT at SAILZ; cleared 22L; Y. First Officer set lower altitude. I acknowledged. I looked down at ATIS for a second; looked up to find aircraft descending below 3000 FT before we hit SAILZ. LNAV was engaged but VNAV was not. I called out error. Pilot Flying disconnected autopilot and corrected flight path back to 3000 FT. We landed uneventfully.
We were cleared direct SAILZ at 3000 feet; cleared RNAV Y 22L; descending from 6000 feet to 3000 feet. I thought I had executed the new cruise altitude of 3000 feet and we had been cleared the approach and were in LNAV and VNAV path as we went into the WEATHER (clouds no rain or thunderstorm). I had set 1500 feet in the window as a lower altitude. VNAV had disconnected without us noticing it and the Captain caught the descent as we passed below 3000 feet. I immediately disconnected all automation and smoothly corrected back up to 3000 feet as we had gone several hundred feet below assigned altitude. We spoke with the Approach Controller and he said there was no traffic in the area and 'no problem'. We re-engaged all automation prior to SAILZ and continued the approach and landed uneventfully. Be extra vigilant when it is late at night; weather; running late; etc. Good communication with the automation is paramount and I failed to notice the change in automation that we had gone out of VNAV. The PM caught the altitude miss and we corrected but it should have been caught sooner. I had failed to 'verify'; a very important step to remember when fatigue is starting to set in.
B737 flight crew reported overshooting the cleared altitude during descent due to a failure to recognize a change in automation.