9.10 Commercial Aircraft, Jet

9.10.1 Emissions

Introduction

Considered in these categories are criteria pollutant (particulate, organic, NOx, SOx, and CO) and greenhouse gas emissions (CO2, CH4, and N2O) from commercial jet aircrafts during their operations at the three major airports in the Bay Area, namely, San Francisco International (SFO), Oakland International (OAK), and San Jose International (SJC). A classification system for commercial aircraft was formulated consisting of major passenger, cargo, and commuter/air taxi aircraft. The major passenger aircrafts are further broken down into sub-groups of short-ranged, medium-ranged, long-ranged, and seasonal/chartered aircraft. Both the major passenger and cargo aircraft categories are primarily jet aircraft.

The basic types of gas turbine engines used for commercial jet aircraft propulsion are turbojet and turbofan engines:

  1. In a turbojet engine, large quantities of air enter the engine in the front and then compressed and squeezed by the compressor before passing into the combustion chamber. This resulting mixture of fuel and air is then burned to produce hot, expanding gases. These high velocity gases pass through a turbine that is used to drive the compressor. The remaining energy in the air stream is used for aircraft propulsion. The earlier centrifugal types of compressors used in turbojets were reliable and simple, but the amount of thrust produced was relatively low because the compression ratio is not very high. These engines were also noisy and had poor fuel economy. Therefore, the quieter and more fuel-efficient turbofan engines rapidly replaced these engines.

  2. Turbofan aircraft engines power the majority of airline transports in service today. The air entering the forward end of the engine is compressed and then heated by burning fuel in the combustion chamber. The turbofan engine uses its fan to accelerate additional air around the outside of the engine (called the bypass flow) to produce a larger, slower-moving exhaust mass for efficient high subsonic propulsion.

Methodology

The pollutants emitted by an aircraft during take-off and landing operations are dependent on the emission rates and the duration of these operations. The emission rates are dependent upon the type of engine and its size or power rating. An aircraft operational cycle includes the landing and takeoff, or LTO cycle. For criteria pollutant emission inventory, an LTO cycle includes all normal operational modes performed by an aircraft between its descent from an altitude of about 2300 feet on landing and subsequent takeoff to reach the 2300-foot altitude. The 2300-foot limit is a reasonable approximation to the meteorological mixing depth over the Bay Area metropolitan areas. The term “operation” is used by the Federal Aviation Administration to describe either a landing or a take-off cycle. Therefore, two operations make one LTO cycle.

For criteria pollutant emission calculations, the aircraft LTO cycle is divided into five segments or operational “modes” and categorized by:

  1. Landing approach (descent from about 2,300 ft. to touch down),
  2. Taxi/idle-in,
  3. Taxi/idle-out,
  4. Take-off,
  5. Climb out (ascent from lift-off to about 2,300 ft.).

The emissions are based on the time of operation in each mode and the emission rates of the engines. The time in the landing approach and climbout modes are assumed to be 3.02 minutes and 1.55 minutes, respectively. Take-off time of 0.95 minute (including 0.25 minute for reverse thrust) is fairly standard for commercial aircraft and represents the time for initial climb from ground level to about 500 feet. The time in taxi/idle mode usually varies with airports.

For greenhouse gas (GHG) emission inventory, in addition to LTO cycle explained above, the aircraft landing approach and climb out modes above 2,300 feet elevation and aircraft cruise mode in the District’s air space is also included.

The information on number of aircraft operations and fleet mix was obtained from the three major commercial airports in the Bay Area, the Federal Aviation Administration (FAA), and the Metropolitan Traffic Commission (MTC). Modal emission rates for aircraft engines in commercial use were obtained from the FAA’s Aircraft Engine Emission Database, the U.S. Environmental Protection Agency (EPA), International Civil Aviation Organization (ICAO), and Intergovernmental Panel on Climate Change (IPCC).

Emission rates vary according to engine type and operating mode. Emission factors for specific aircraft were estimated by the equation:

\[ \text{EMF} = \text{N} \times \sum{\left( v_e / v_t \right)_{m,p}} \times \text{TIM} \] where:

  • \(\text{EMF}\) = emission factor (lb/LTO);
  • \(\text{N}\) = number of engines;
  • \(\left( v_e / v_t \right)_{m,p}\) = engine emission rates (lb/hr) at mode \(m\), pollutant \(p\); and
  • \(\text{TIM}\) = time in mode \(m\) (hr).
Sample calculations

TOG emissions for B747-300 (long-range aircraft):

\[ 4,878\ \text{LTO/yr} \times 24.56\ \text{lb/LTO} \div 365\ \text{day/yr} \div \text{2000 lb/ton} = 0.164\ \text{ton/day}\ \text{TOG} \]

County Distribution

The county location of each airport was used to distribute emissions into each county, where SFO is in San Mateo County; OAK is in Alameda County, and SJC in Santa Clara County.

Table 9.18: County fractions.
category ALA CC MAR NAP SF SM SNC SOL SON
#1115 Major Passenger Aircraft (SFO) – – – – – 100.0% – – –
#1119 Major Passenger Aircraft (OAK) 100.0% – – – – – – – –
#1123 Major Passenger Aircraft (SJC) – – – – – – 100.0% – –
#1127 Cargo Aircraft (SFO) – – – – – 100.0% – – –
#1128 Cargo Aircraft (OAK) 100.0% – – – – – – – –
#1129 Cargo Aircraft (SJC) – – – – – – 100.0% – –

Monthly Variation

Monthly distribution was based on the average monthly number of operations at each airport.