Date: 201412
Local time of day: 1801-2400
Location: D10.TRACON, TX
Aircraft: Boeing Company Undifferentiated or Other Model
Operator: Air Carrier
Flight plan: IFR
Flight phase: Descent
ATC / advisory: TRACON D10
Aircraft component: Autoflight System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Human-Machine Interface
Second reporter function: Captain; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Clearance
Contributing factors: Aircraft; Human Factors; Procedure
Primary problem: Ambiguous

Narrative: We were cleared to 'descend via' on the BOOVE one arrival; into DFW. The plane was descending in VNAV on the arrival with 8;000 set in the altitude window for LREEE. VNAV worked properly making all the altitude windows and airspeed reductions through MOWWW during the descent. The autopilot leveled the plane at 11;000 feet and slowed to 220 knots approaching DELMO intersection. Approximately 3 miles prior to DELMO; the plane began a descent; violating the clearance/crossing altitude. The autopilot was disengaged and we climbed back up to 11;000 feet. The FMC had 11;000 feet and 220 knots displayed as restrictions at DELMO. This came up when we selected the approach and was not edited by the flight crew. There seems to be a problem with the FMS database or autopilot interface. Reading F4 messages; it is obvious that there are problems with the arrivals into DFW. While the BOOVE is not listed; others are with the same problem; seemingly always descending improperly from 11;000. The easy solution is to set 11;000 in the altitude window anytime VNAV is used on arrivals into DFW until passing the 11;000 foot waypoint. Then don't use VNAV anymore on the arrival. That message should be sent to all pilots; in a place we are likely to see and read it.


Second narrative: We were on the BOOVE 1 arrival I was the pilot not flying; ATC had given us a speed restriction of 300 kts or greater until BOOVE and then to comply with the arrival speeds after BOOVE. Autopilot was operated in LNAV and VNAV with speed intervention until BOOVE and no speed intervention after that. The FMS did a good job on the way down (it did ask for drag three or four times probably due to lightweight and a strong tailwind). We leveled off at 11;000 ft at MOWWW and began slowing to 220 kts for the DELMO restriction. I observed the level off; satisfied myself that we would be at 220 kts by DELMO; and began checking the approach. I heard the F.O. exclaim and looked up to see us in a descent about 3 miles short of DELMO which we were to cross at 11;000 ft. The F.O. immediately disconnected the autopilot and returned the aircraft to 11000 ft but we had bottomed out at about 10600 ft. The F.O. advised that as soon as the airspeed reached 220 kts the autopilot began a descent and did not increase power. He also advised that he was slow to respond because of surprise and he took time to look at the chart before he took over.I am extremely concerned about this as the 11;000 ft restriction is there because departures are going under us at 10;000 ft making the potential for disaster high. [I have been] informed that this is an ongoing problem. To ensure safety I am going to set 11;000 ft in the altitude window until passing the fix that defines the restriction on these DFW RNAV arrivals. Once we figure out what is wrong and correct it; I will revisit that decision. This may or may not have some bearing but the F.O. was a little slow to remove drag when the 'Add Drag' prompt went away on the FMS; this may be the reason we were level at 11;000 ft well before DELMO. I am suspicious that being level early set off the erroneous descent.


Synopsis: Air carrier flight crew reports descending below 11000 feet approaching DELMO on the BOOVE1 RNAV arrival to DFW. The aircraft had been descending in LNAV and VNAV with the DELMO restrictions in the FMC. After a 400 foot deviation the flying First Officer returned to 11;000 feet.