Flight Crew Air Transport Pilot (ATP)
Workload; Situational Awareness; Time Pressure; Human-Machine Interface; Distraction
ATC Issue All Types; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Aircraft; Airport; Chart Or Publication; Human Factors; Procedure
On arrival into DFW we were assigned DEBBB 1 RNAV arrival.  Then ATC modified our clearance to cross BELFER intersection at FL 240; and a speed restriction at BELFER. ( I believe it was at least 290 kts). Then we were cleared to descend via the DEBBB arrival; and given another speed restriction. ( I believe it was 270 kts). Then we were assigned another speed restriction (I believe it was 290 kts). Finally we were cleared to comply with the speeds on the arrival. We did make all of the altitudes on the arrival; but not at all of the speeds assigned. Because the VNAV is not 'on approach mode'(ie: flaps out); speed intervene will not cause VNAV to stay ON PATH; ON SPEED. Therefore each modification to the DEBBB 1 must be programmed into the FMC manually. Each modification requires one pilot to reprogram; and the other pilot to verify the change. Then it can be executed. Then the FMC must recalculate the entire VNAV PATH for the entire arrival; meantime VNAV PATH drops off to VNAV SPEED. Each modification to the arrival requires this same sequence of events. This puts a very high workload on the flight crew; heads down; on a descent into a busy terminal area. And in most cases even if this reprogramming is done promptly and accurately; the FMC cannot recalculate the new path quick enough to keep the jet on the correct vertical path. I do believe we can fly these complex new arrivals as published. However I know we cannot fly them safely if ATC continues to modify them on the fly.
Coming into DFW after 14 hour plus flight from HKG. Assigned DEBBB 1 arrival into DFW; descend via....then ATC said cross BELFR at FL 240; 290 knots. Cleared again to 'descend via' with 270 knot restriction. Now increase to 290 knots. Then cleared to comply with speeds on arrival. Made all the altitude crossing restrictions; but had to increase speed to make target altitudes. VNAV was not on approach logic; speed intervention and constant airspeed changes made for a busy; heads down experience while in the terminal area. It's a difficult situational awareness event; keeping up with the airspeed changes; modifying the FMS; and complying with ATC. It seems the approaches are being partly flown in VNAV; and partly given alt/airspeed restrictions as in the past. A difficult hybrid to manage; especially with the length of duty day. Do VNAV descents; or don't. The mixing of the two can lead to a high; heads down cockpit; in the terminal area.
A B777-300 flight crew; nearing the end of a 14 hour flight; discussed the difficulties and distractions of reprogramming and complying with ATC alterations to RNAV STAR 'descend via' clearances.