Date: 201411
Local time of day: 0601-1200
Location: C90.TRACON, IL
Flight conditions: IMC
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight phase: Descent
Airspace: Class B MDW
ATC / advisory: TRACON C90
Aircraft component: FMS/FMC
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Human-Machine Interface; Confusion
Anomaly: ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Contributing factors: Procedure; Human Factors; Aircraft
Primary problem: Ambiguous

Narrative: We were in full compliance of both Company procedures and the alert for ENDEE3 22L RNAV Y.  We had selected; verified; executed; and briefed MIING transition since it was closest and afforded less time to reprogram and re-brief. We had discontinuity as discussed after WADLL.  Approaching MIING; we agreed it looks like we should re- select STERE [RNAV Y] transition since we would not be getting MIING [RNAV X]. PM re-selected 22L RNAV Y and then STERE transition. We both verified it on LEGS page. We agreed and executed it; with it showing MIING as active waypoint; next was WADLL; then the discontinuity followed by STERE. At this point; we were crossing MIING and got an LNAV disconnect. Within two seconds; recognized and put WADLL up top and executed to go direct thus correcting the discontinuity caused lateral deviation. ATC informed us in the turn we turned the wrong way and issued a heading which we complied with (same as direct WADLL which we were doing). We followed all guidance issued and were fully aware of the recent issues Pilots are having with this arrival. I see two separate issues with the same arrival. One is masking the other. Fortunately there is a very simple corrective action needed to rectify the latter.ATC needs to issue a transition to the RNAV Y 22L. Leaving Pilots guessing; especially in IMC conditions; on which to set up and brief is unacceptable. There simply is not enough time after crossing MINGG at 4000 feet in congested airspace to rebuild and re-brief an approach.

Callback: The reporter clarified that the transitions he is referring to are actually two different RNAV approaches to Runway 22L. The RNAV X connects directly to the ENDEE3 arrival at MIING and the RNAV Y requires radar vectors after passing WADLL on the ENDEE3. Changing from the X to the Y confuses the FMC if the change is executed near MIING and also restrings the entire ENDEE3 arrival. The reporter believes that ATC has agreed in the future to not change between the X and the Y late in the arrival.



Synopsis: B737-700 Captain describes FMC anomalies that occur when changing from the RNAV X RWY 22L to the RNAV Y RWY 22L approaching MIING on the ENDEE3 arrival to MDW. This change results in the entire ENDEE3 arrival being entered into the FMC and creates a route discontinuity after MIING.