A320
Flight Crew Last 90 Days 240; Flight Crew Total 16000; Flight Crew Type 4100
ATC Issue All Types; Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Other / Unknown
Air Traffic Control Issued New Clearance; Flight Crew FLC Overrode Automation; Flight Crew Requested ATC Assistance / Clarification
Aircraft; Procedure
This is simply to inform of a design flaw in the HKORY 1 RNAV arrival into IAH. No deviation occurred; but the design may lead to a violation from an arriving aircraft. BEDLM is supposed to be crossed at or above 10;000 FT at 280 KTS. With the aircraft on autopilot and in managed speed and on VNAV profile; the profile is followed to cross BEDLM at 10;200 FT; the problem occurs in that the FMGC is planning a descent below 10;000 FT just after BEDLM to cross CHKEN at or below 7;000 FT. To do this; the FMGC disregards the 280 restriction and attempts to slow to 250 before reaching BEDLM; thus having the aircraft attempt to arrive below 280 KTS at BEDLM. We recognized the commanded speed being low; and manually selected 280 KTS to cross BEDLM on speed; but then we had to leave the planned vertical profile and level at 10;000 FT manually to slow to 250 to descend below 10;000 FT. Left to its' own plan; the FMGC would be too slow at BEDLM; or if you stay on vertical profile at speed select 280 for the BEDLM restriction; it will attempt to fly below 10;000 FT at 280 KTS unless you take it off of it's planned vertical profile and slow above 10;000 FT manually. We thoroughly reviewed all the loaded crossing restrictions well prior to starting the arrival and they were all correct. The possibility of errors occurring by flight crews is high at that point due to the managed descent profile planned by the FMGC. As another side bar with the new arrivals; they are runway direction specific. We were filed for a west arrival and landing; but the airport changed direction; to the east; while we were still well out from the arrival start point; but we were still on a filed westerly arrival. We asked ATC after 20 minutes since the ATIS change to opposite direction traffic; as we approached the arrival entrance point; and their answer was a terse response that we were filed on a westerly arrival for a transition to a west runway. We said that the operation had turned around but we would continue to fly as filed; but we really needed a new arrival. ATC was quiet for a bit; and then started giving aircraft the HKORY arrival to land east. The point of this is that there is an apparent disconnect; or lack of procedure; to communicate runway configuration changes with Center; which leaves aircrews in a no mans land until ATC catches up. It could lead to confusion between ATC and aircrews.
A320 Captain describes Airbus FMGC idiosyncrasies crossing BEDLM on the HKORY 1 arrival to IAH. The autopilot will attempt to violate the crossing restriction at BEDLM unless the crew intervenes.