A320
Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Engineer; Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor
Flight Crew Last 90 Days 180; Flight Crew Total 12000; Flight Crew Type 585
Human-Machine Interface; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Aircraft; Procedure; Human Factors
We were flying to IAH. I am an A320 Captain and was receiving right seat training (to become a LCA) from a Line Check Captain. I was the pilot monitoring. We were originally filed on the WHACK2 arrival (which is for a west operation in IAH). Since IAH was landing to the east I asked Houston Center if we could still expect this arrival. Houston re-cleared us via the TWSTD3 arrival.At some point we were cleared direct to WHACK and to cross WHACK at 13;000. As I recall we were then at FL220. The Pilot flying (PF) (Captain) properly selected 13;000 in the FCU and entered the crossing restriction altitude in the FMS. I verified that his entries were correct. Several miles prior to the TOD arrow I made a remark that it was almost time to start down. A couple of miles prior to the TOD arrow the PF initiated a managed descent to 13;000. We were below the path (later directly on the vertical path). Everything looked as we expected it to look.The published crossing altitudes for the WHACK fix are between FL200 & 17;000 @ 280 KTS. Again; we had been cleared to cross WHACK at 13000. The fix after WHACK (TWSTD) has a published crossing altitude of between 16;000 & 13;000 @ 280 KTS. Approximately 10 miles prior to WHACK both the PF and myself realized that the aircraft was not descending the way it should. The PF clicked off the autopilot; went into open descent; and extended the speed brakes (only 1/2 speed brakes are available with the autopilot connected). Several miles prior to WHACK ATC asked us to slow to 250 KTS. I informed him that we would not be able to make WHACK at 13000. He told us 'no problem' then gave us a revised clearance to descend via the TWSTD3. I also asked whether he wanted us to maintain 250 or comply with the published speeds on the STAR. He told us to maintain 250 KTS.The LCA I was flying with said that he had programmed the FMS correctly (I agree) and that this was a known anomaly with the Thales FMS. He said that when we were cleared to an altitude below what the published altitude for WHACK; with the corresponding other crossing altitudes at TWSTD; that the FMS gets confused as to what altitude to make. He told me he would be following up on this with the Fleet Captain and technical pilots for corrective action. Obviously we both could've done a better job of monitoring the ongoing descent. I was lulled into believing that since everything was loaded correctly and was descending on profile that it would make the restriction as entered.
The reporter states that his company has issued a bulletin covering this anomaly;  descending to a hard altitude with window altitude to follow; will cause the hard altitude to be missed under certain circumstances. The window altitude must be changed to a hard altitude at or below the earlier altitude to use managed descent.
A320 First Officer on the TWSTD3 RNAV arrival to IAH reports being cleared by ATC to cross WHACK at 13;000 feet and the Captain sets 13;000 in the FCU and in the FMGC WHACK. The waypoint after WHACK is TWSTD with a crossing restriction between 16;000 and 13;000 which remains displayed and the reporter states confuses the FMGC into starting a late descent for the WHACK restriction. The flying Captain states that this was a known anomaly with the Thales FMS.