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42
SECTION 8
Crucial Knowledge Gaps
Information gaps identified in this report are described in the impact of severe delays are not captured, nor is plane-to-
the following sections. plane variability. Any attempt to quantify the health risk
posed by acute effects of HAPs must accurately account for
situations in which emissions and concentrations are highest
8.1 Emissions-Related
(e.g., long delays, low mixing heights, etc).
Knowledge Gaps
These information gaps affect the accuracy of emission
8.1.3 Full Characterization of HAP
inventories (and any related health risk assessments).
Emissions Near Idle Thrust
Measurements of HAP emissions indices have been per-
8.1.1 Effect of Temperature on
formed at a limited number of power levels near idle (e.g., at
Aircraft HAP Emissions Near Idle
APEX1,2,3 the common engine thrust settings were 4%, 7%,
HAP emission rates can vary by well over a factor of 3 and 15%). A significant amount of interpolation is required
within the range of conditions encountered by many airports. to portray the effect of engine power on emission indices.
There are no measurements at subfreezing temperatures,
even though a significant portion of annual aviation activity
8.1.4 Single-Engine Taxiing
occurs during such conditions.
As discussed in Section 5.1.3, the prevalence of single-
engine taxiing is unclear, though anecdotally is thought to be
8.1.2 Characterization of True Idle Levels
rare. If actually prevalent, this could have a large impact on
and Times-in-Mode
emission inventories.
Since the gas-phase HAP emissions are dominated by the
idle phase, knowledge of actual idle times is necessary. Addi-
8.1.5 General Aviation
tionally, since the emission factor is a strong function of
Emissions Characterization
throttle setting, knowledge of the real-world power levels
used are necessary. Emission indices for true ground idle With the exception of lead emissions, which are well char-
(sometimes labeled 4%) can be a factor of 2+ greater than the acterized, emissions from the general aviation category of
7% ICAO certification point. The likely distribution of throt- piston engines are largely unknown.
tle settings used as the aircraft progresses from the terminal
to the runway (or the taxi-phase of an LTO) is unknown. This
8.1.6 Breadth of Measurements Across
time-in-mode problem is probably highly variable and
Engine Types
dependent on the frequency of flights. This problem does not
lend itself to a simple relationship expressed as total emis- The APEX campaigns increased the total number of com-
sions per LTO. mercial turbofan engines characterized to more than 10. This
Since most emission inventories (i.e., those made under is not necessarily a representative sample of aircraft engines.
NEPA requirements) are required to consider average condi- There are scant data on the variation of hydrocarbon emis-
tions and not worst-case scenarios, day-to-day variability or sions with engine age and maintenance history.
