Idle-power descents under study in U.S. and abroad

 - October 9, 2006, 12:05 PM

At one time or another most pilots have thought about how neat it would be to chop the throttles at FL300 or above and not touch them again until the airplane was on final. At one time that seemed impossible, but not anymore. UPS and Qantas have flown more than 200 such procedures successfully.

Idle-descent procedures save fuel, reduce noise, cut engine emissions and ensure precise arrivals. Called continuous- descent approaches (CDAs) in the U.S. and tailored arrivals (TAs) in Australia, the technique will likely one day become the standard operating practice in high- and medium-altitude arrival streams.

CDAs do not rely on autothrottle flight-path management; they are true idle-power descents. Originally proposed in 2001 by Boeing’s Air Traffic Management Division–now part of Boeing’s Phantom Works–the concept links a continuous, steady descent path from the top of descent (TOD) point to the entry fix of the destination Star procedure.

During a two-week period in September last year Boeing 757s and 767s operated by overnight freight carrier UPS flew 125 CDAs into the company’s main cargo hub at Louisville, Ky. During the same period, Qantas Boeing 747s and Airbus A340s flew 80 TAs into the Sydney and Melbourne airports.

These approaches were precisely calculated procedures, using the aircraft’s flight management computers (FMC). But the procedures of the two evaluations were different, reflecting different aims. The UPS trials, flown in coordination with the Massachusetts Institute of Technology and the FAA, were aimed at assessing CDAs as a means of reducing noise and engine emissions, as well as fuel burn, during night arrivals over heavily populated areas. Qantas and Airservices Australia (the country’s privatized ATC provider) were interested in the TA concept’s traffic sequencing and ETA prediction accuracy as well as its associated fuel-saving potential.

Datalink Updates

For the UPS trials, two fixed inbound descent tracks for traffic arriving from the West Coast were tied to the respective initial fixes of the Stars serving Louisville’s Runways 35 and 17, and both were stored permanently in each aircraft’s FMC. Then, with foreknowledge of a given aircraft’s weight approaching TOD and the wind profile along the descent path, ATC advised the crew of the arriving aircraft of their cleared TOD point, which they then entered into the FMS.

In the Qantas trials, ATC used the aircraft’s weight and the wind profile and coordinated with the airline’s dispatchers to calculate the TOD and a specific descent path that would bring the aircraft over the initial Star fix at the required time. In this case, however, the TOD and flight path routing were sent directly to the aircraft’s FMS by datalink, without crew involvement, other than to accept or reject the clearance with an appropriate keystroke.

Each evaluation demonstrated the potential of the procedures. UPS data showed significant reductions in noise and engine emissions, along with fuel savings of between 118 pounds for the 757 and 364 pounds for the heavier 767. In addition,  the Louisville procedure knocked two minutes off the 757 descent times, and two-and-a-half minutes off the 767’s.

In the Qantas trials, where entry timing was a key requirement, every aircraft arrived over the terminal area entry fix within 30 seconds of the ETA calculated by its FMC 40 minutes earlier as the aircraft approached TOD. In one case, entry fix crossing was within two seconds, and in most other cases between 10 and 20 seconds. The A340s and 747s saved between 400 and 800 pounds of fuel each time, or potentially up to $100,000 per aircraft per year.

Fans Implementation

An interesting sidelight to the Australian trials was in the use of datalink–in their case, the future air navigation system (Fans) implementation. The system allowed ATC to send unique arrival paths, along with target altitudes and speeds, to each aircraft’s FMC without imposing the workload of a demanding reprogramming on the crew. It also lessened the need for additional FMC capacity, since only the destination Stars, to which the uplinked descent path was automatically connected, needed to be stored permanently.

Unfortunately, the lack of a U.S.-wide datalink system dictated that for crew workload considerations, the UPS trials were limited to just two descent tracks. While these were stored permanently in the FMCs, and thus called up easily, they offered much less flexibility under changing conditions.

Equally unfortunately, the FAA has postponed implementation of its controller/ pilot datalink communications program until after the arrival of the agency’s massive multibillion-dollar en route automation modernization project, currently forecast for completion in 2010.

In the meantime, the only ATC datalink services available are Fans over the Pacific, North Atlantic and parts of Europe, and the quite different–and non-compatible–aeronautical telecommunications network, which the Europeans are promoting aggressively. Fans, however, has been developed specifically for Boeing and Airbus aircraft and compatible equipment is simply not available for corporate aircraft, except for the BBJ and ACJ.

However, at ICAO’s Air Navigation Conference last year, NBAA representatives obtained an agreement that Fans corporate aircraft would not be handicapped unnecessarily in Pacific and North Atlantic airspace because they lack the equipment.

Nevertheless, the UPS and Qantas research underscores the fact that the growing application of ATC datalink procedures, particularly in Europe, will put increasing pressure on corporate flight departments to examine carefully their future equipment needs.