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This may be an efficient and convenient approach. There are certain requirements for NEPA that
are not required under conformity. For example, NEPA requires the development of reasonable
alternative actions, whereas conformity does not (conformity only requires analysis of the
proposed alternative). In this case, it may be a more realistic approach to perform a conformity
analysis for only the one alternative selected instead of for all alternatives. At a minimum, when
the specific alternative is selected in the NEPA process, the conformity air quality analyses
should be performed as appropriate. A joint notification and public participation process also is
possible, as long as the requirements for each regulation are met.
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5.4 References/Sources
Additional guidance on EPA’s interpretation of the General Conformity Rule and answers to
common general conformity questions is provided in EPA’s General Conformity Guidance:
Questions and Answers (Reference 73) and EPA’s New General Conformity Q’s & A’s
(Reference 77). Guidance also is provided in the Policy and Guidance section of Title I and the
CAAA bulletin board of the EPA Office of Air Quality Planning and Standards (OAQPS)
Technology Transfer Network (TTN) bulletin board system (Reference 85). Guidance documents
also can be obtained from the appropriate EPA Regional Office.
 
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6. MITIGATION/CONTROL MEASURES
There are various measures that airlines and airports can take to improve the environmental
performance of their operations. This section briefly describes several possible emission
reduction measures that can be applied to aircraft, GSE, and APU operations. These measures are
identified so that they may be factored into an air quality analysis when used by an airline(s) or
the airport. Currently, there are some airports and airlines that are implementing these measures
not only for emission reductions but for non-environmental benefits like cost reductions.
However, each measure has certain constraints that limit its full implementation. This section
also describes the factors that should be considered when evaluating and implementing a specific
measure.
6.1 Aircraft
This section describes modified operating procedures that can be used to reduce aircraft engine
emissions. In general these procedures do not require additional equipment or aircraft
modifications. Since these procedures may require changes to an airline’s standard operating
practices and may not be feasible in all weather conditions or at all airports, they should always
be implemented at the discretion of the pilot in command.
6.1.1 Single/Reduced Engine Taxiing
Most aircraft are able to taxi and idle with less than all engines running. Operating less engines
during taxi and idle reduces the associated emissions substantially. The engine(s) in use operates
at a higher power than it would otherwise, but this is at a somewhat more efficient point on its
power curve. The remaining engines must run for about two minutes prior to takeoff power to
achieve thermal stability, as well as two minutes prior to shutdown after landing to cool down.
Despite the operating time required for thermal stability, most of the taxi and idle time would be
with a reduced number of engines operating. Many airlines have employed this measure where
feasible since the early 1970s. Because reduced engine taxi and idle is feasible for certain aircraft
at most airports from a safety standpoint, it is currently employed extensively for fuel
conservation and economic reasons. For those aircraft that are amenable to reduced engine taxi
and idle, this measure provides the safety and control needed while still reducing HC and CO
emissions.
However, the implementation of reduced engine taxi/idle varies and is not always feasible. The
number of engines that can be reduced during reduced engine taxi and idle varies by aircraft type
due to the location of the engines, aircraft weight, and aircraft size. For some aircraft reduced
engine taxi and idle is not feasible at all due to control and safety concerns. Directional control
problems could occur in these aircraft because of the adverse, unbalanced thrust that may be
created by using less than all engines. Safety concerns include ground personnel and equipment
hazards that may be created when the operating engine(s) is brought to the power level necessary
to initiate movement of these aircraft. Other factors such as weather, taxi surface, taxi slope,
ramp congestion, and individual airline practices also affect safe reduced engine taxi/idle and
require on-the-spot judgment of the pilot in command. Under Federal Aviation Regulations
(FAR), the pilot in command of the aircraft is responsible for the safety of the passengers, crew
　
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