title: Potential Environmental Benefits of Extending Aircraft Pushback Operations
Creators: name: O'Connell, Dennis R.
Publisher: name: Old Dominion University
publisherIdentifier: https://ror.org/04zjtrb98
publisherIdentifierScheme: ROR
Year: 2011

description: This study considered extending the operational use of pushback tractors to perform taxi-out airport ground operations for commercial aircraft as an alternative to using aircraft power plants for this operation. Pushback tractors are designed to reverse aircraft from the terminal gate to a position where the aircraft can taxi independently to the departure runway. In this investigation the process of pushback would be extended to pulling the aircraft to the departure runway then returning the pushback tractor to the terminal of origin, a new procedure known as extended pushback.

Data from major aircraft operators and engine manufacturers were collected to create an accurate database of aircraft/engine performance. The engine emissions databank provided by the International Civil Aviation Organization was used to approximate the aircraft fuel consumption and emissions rates for idling and taxiing aircraft. The Auxiliary Power Unit fuel rate and emissions were obtained from previous studies and manufacturer documents. Data for the performance of pushback tractors was provided by the TUG corporation and Duetz Diesel engines. The fuel consumption and emissions performance data was calculated using information from Duetz Diesel engine performance graphs. Observations of airport operations, testimonials of pilots and ground crew, and FAA ground airport procedures were considered to create an approximate model of taxi-out operations.

Aircraft were divided into four aircraft classifications of wide-body, heavy, narrow-body, and regional so as to be matched with representative pushback tractors from the TUG corporation that operate the listed category of aircraft. The aircraft and tractor combinations were compared in two taxi-out procedures known a the standard pushback and proposed extended pushback operations. An aircraft using all available engines to taxi-out to the runway is known as a standard pushback operation. An aircraft using only half of its available engines to perform taxi-out operations is known as half engine standard pushback procedure which was proposed by other studies and is currently implemented on a limited basis. The extended pushback operation is the combination of the aircraft on auxiliary power to maintain aircraft systems, the pushback tractor pulling the aircraft to the runway, decoupling at a designated end point, starting aircraft engines, and the tractor returning to the originating terminal gate. The term extended pushback operation, refers to the pushback tractor performing taxi-out procedures.

The new extended taxi procedure was evaluated against the current taxi procedure to assess potential fuel efficiency, along with financial, time, and environmental benefits. An analysis was performed for the approximated average taxi-out distance of 1.5 miles, which was determined after studying airport ground operations at several domestic international airports. The fuel consumption and emissions release rates of the extended pushback process were compared against the standard pushback procedure for full engines and half engines for a taxi cruise configuration. The results suggested that the extended pushback operation produced less emissions and was more fuel efficient with an increase in taxi-out time of about 3 minutes observed by the passengers. The comparison included procedures such as pushback from the gate, aircraft and tractor decoupling at the departure runway 1.5 miles away from the origin, and the fuel used for engine start up to thermal equilibrium.

An analysis of taxi-out idle delay was conducted to provide a benchmark for any idling delay due to take off queuing from traffic volume. One aircraft from each aircraft classification was considered for a total of four representative aircraft. The extended and standard pushback procedures were modeled at idle from 1 to 60 minutes in place. The results suggested that the extended pushback configuration was more fuel and emissions efficient when compared to a full engine standard pushback configuration. The 60 minute idle fuel and emissions results from a single aircraft engine in each aircraft category were compared to the corresponding APU and pushback tractor idle rate. The single aircraft engine required more fuel and generated more emissions than the APU and pushback tractor in idle for 60 minutes.

An implementation analysis was then performed for both Norfolk International Airport and Chicago O'Hare International Airport. The geographic layout of each airport was modeled and all flights for one day were recorded. The results from the airport were then summed and compared. The results from both airports suggest that the extended pushback taxi process requires an estimated five additional minutes of gate-to-gate travel time while realizing an estimated potential 60% reduction in fuel usage and emissions. The study results are sufficiently encouraging to suggest the concept be further vetted in physical testing and then potentially implemented on a system wide level.