Date: 201208
Local time of day: 1201-1800
Location: MDW.Airport, IL
Flight conditions: Mixed
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E C90
ATC / advisory: TRACON C90
Aircraft component: FMS/FMC
Component problem: Improperly Operated; Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Human factors: Distraction; Confusion; Situational Awareness; Human-Machine Interface
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew FLC Overrode Automation
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: While at 10;000 MSL on descent in MDW; we were cleared to cross at HALIE at 6;000. Earlier in the descent; the Captain and I were (ironically enough) talking about VNAV and altitude restrictions and the programming and proper use of the FMC; VNAV etc. A few miles from HALIE; ATC asked us in a sarcastic tone of voice: 'Do you remember that crossing restriction at HALIE!?' I looked at the FMA and we were in VNAV PATH. I looked at the Descent page on my FMC/CDU and saw HALIE at 6;000 FT. I looked at the MCP and saw 6;000. I looked at the Cruise page and saw 10;000 FT. I cannot explain why the jet did not initiate a descent earlier and on proper descent profile.I disengaged the autothrottles (I believe) and deployed the speedbrakes; and at the same time; both the Captain and I knew we would not make HALIE at 6;000; so I told the Captain to ask for relief. He then asked Approach for a turn; apologized; and Approach quickly gave us a vector and a descent to 4;000. The remainder of the approach and landing was uneventful. What bothers me so much is that we verified the programming; verbalized the callouts; and monitored until we were distracted with our own discussion of VNAV crossing restrictions. All the while this jet was not about to make the restriction even though; I believe; all programming and VNAV functions were done correctly. I'll be the first to admit my mistakes; but I need to know what they were so I (and others) can learn from this! If the jet was not going to make HALIE at 6;000 why did it stay in VNAV PATH and not switch to VNAV SPD like we see a lot of times when we are not going to make the next restriction and further action is required? This is what puzzles me so much!I realize and understand we need to monitor the airplane; and as we get more automation and functionality; it is that much more important. However; it has always bothered me that when the jet drops out of VNAV PATH and switches to VNAV SPD; there needs to be more than just an FMA change; a light; an FMC annunciation; or something. For an action as critical as not making an altitude restriction; we need a better warning system in place for the jet to tell us that it is not going to make an altitude restriction unless action is taken.




Synopsis: Although they believe they programmed the FMS properly to comply with ATC's clearance to cross HALIE at 6;000 MSL the flight crew was surprised to find they had never left 10;000 MSL when ATC queried them about making the restriction. An acknowledged contributing factor was the flight crew's failure to monitor their flight path; anticipating that the FMS would do what they thought they had programmed.