Date: 201208
Local time of day: 0601-1200
Location: ORD.Airport, IL
Flight conditions: VMC
Aircraft: B757-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Aircraft component: FMS/FMC
Component problem: Design
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Human-Machine Interface; Communication Breakdown; Situational Awareness
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Other / Unknown; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew FLC Overrode Automation; Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft; Chart Or Publication; Human Factors; Procedure
Primary problem: Ambiguous

Narrative: As I've previously reported on many occasions; the FAA insists upon creating and utilizing charted arrival procedures which have no 'end point.' Our FMC interprets this lack of an end point to add approximately 90 miles; 1;000 LBS of fuel burn and as much as a 30 minute longer ETA into the FMC arrival calculations. Once again; on my arrival at ORD this afternoon; the FMC displayed a 50 mile error; 1;000 LBS landing fuel discrepancy; and 20 minute arrival time error. Upon discovering and verifying this error in flight; I requested and received a clearance change eliminating the BULLZ 3 arrival and adding the Janesville 5 arrival; thus eliminating the multiple errors.

Callback: The reporter compared the BULLZ and JANESVILLE arrivals to illustrate the problem. The JANESVILLE STAR; for arrivals from the west; shows a last 36 NM leg from KRENA direct to ORD. A balloon note pointing at the airport advises flight crews to 'expect radar vectors; etc.' Until a runway is selected the FMC computes the route distance; fuel burn; etc. just to the airport via KRENA. The BULLZ STAR; on the other hand; also contains routes from the west but ends with fixes comprising downwind legs which take the aircraft some distance east of the airport before concluding with a continued downwind heading and the 'expect radar vectors...' balloon note. Reporter advises that; while en route and until a landing runway is selected; the FMS appears to 'assume roughly 90 NM of additional flight distance and associated fuel burn beyond the last downwind waypoint.The problem he associates with this predictive FMC methodology occurs when a flight is: 1. dispatched with minimum fuel reserves; and 2. en route weather or other delays unexpectedly increase the en route flight time. When those two factors co-exist the FMC computed arrival fuel will be reduced and; if the reduction is great enough; may force the flight crew to consider diverting for fuel prematurely.



Synopsis: A B757-200 Captain expressed his concern that the programing by which FMCs determine ETAs and arrival fuels is inordinately affected by the manner in which the selected STAR is constructed; some prescribing a direct route from the last fix direct to the airport and others ending at downwind legs from which the FMC assigns a conservative longer enroute time and applies the commensurate additional fuel burn to the predicted arrival fuel.