Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Annex 16 standards are related to engine emissions during the LTO cycle; they do not
specifically address engine emission levels for altitudes above 900 m (cruise), although these
emissions are related to emissions during the LTO cycle.
In 1981, the ICAO Council adopted standards and recommended practices for aircraft engine
emissions by establishing regulatory emissions maxima for engines manufactured after 1985.
The regulatory conformance criteria for covered emissions are based on characteristic engine
performance during the LTO phases of flight, which represents on average of 15% of the total
flight segment. LTO cycle operating requirements are characterized by very low and very high
power settings, which are not representative of aircraft performance at cruise conditions. For
purposes of estimation, LTO cycle parameters are transformed into an emissions index (EI) for a
given airframe-engine combination to characterize emissions under cruise conditions.
On the recommendation of CAEP/2, in 1993 the ICAO Council amended the emissions
standards for NO x , reducing permitted levels by 20% at a representative engine pressure ratio of
30. The certification regime follows the LTO cycle basis established under the original stringency
standard. The CAEP/2 standard was adopted by ICAO member states with few, if any,
Figure 10-2: Engine certification to ICAO NO
exceptions as the universally recognized international standard for aviation. It is to be applied to
x
all engines produced after 2000, to all new and derivative engines for which certification has standards by engine type and engine.
been or is to be applied for after 1995, and, as a practical matter, to currently certified inproduction
engines that are to be altered to meet the standard. CAEP has since reviewed the
NOx stringency issue in light of local air quality and atmospheric concerns and in 1998 recommended that the standard be further reduced by 16% at an engine
pressure ratio of 30 (ICAO, 1998a). For pressure ratios above 30, the slope of the NO x maxima was returned to that of the 1981 standard to permit greater fuel
efficiency and therefore greater reductions in CO 2 , H 2 O, and oxides of sulfur (SO x O). Recognizing the need to protect the asset value of the existing fleet, the new
standard is to be applied only to new or derivative engines certified after 31 December 2003. In-production engines are thereby left unaffected by the stringency
requirement. The ICAO Council is scheduled to review the proposed standard and recommended practice (SARP) by the first quarter of 1999. Adoption by the Council
is expected, after which the SARP will be incorporated into ICAO Annex 16 for adoption by member states. Although there is a wide range of views concerning the
scale of costs and benefits resulting from increased NO x stringency, an analysis of the environmental benefits obtained to date under current ICAO engine stringency
standards has not been initiated.
The three ICAO SARPs are shown in Figure 10-2 (ICAO, 1995a). It suggests that ICAO stringency requirements have "pushed" engine emissions reduction technology
or, at a minimum, have ensured its incorporation into new and derivative engine designs. The relatively wide variation in technology levels among individual engine
types and whole engine families represented in Figure 10-2 raises three important issues. For purposes of regulating aviation engine emissions, the first policy matter
is whether use of the existing "best achievable" standard is preferable to the "best available" standard used for other industries. The second question concerns the
optimal level at which the standard should be set to promote sustained and rapid progress on further emissions reduction. The third policy matter pertains to the means
to evaluate tradeoffs concerning technical limitations in minimizing various emissions species and those pertaining to safety, performance, and environmental
objectives, in particular noise control.
ICAO practice has inferred that the "best technology" concept has a specific meaning for aviation. The airworthiness concern is that setting standards based on
unproved, anticipated, or nonexistent technology might result in untenable solutions, as might requirements that all engines meet a single, extremely low emissions
threshold. A matter yet to be addressed is that selection of a "best available" technology or technology level for aircraft engines has inherent competition and market
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