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Corresponding uncertainty

What the reports say


What this means in terms of ‘confidence’

Aircraft engines will become much cleaner; use of ‘average’ not ‘charac-teristic’ values for aircraft emissions

The models make assumptions about future aircraft technology, in some cases for entire engine ‘families’ that do not yet exist.  These predictions use the results of ‘certification tests’ where they exist.  

Actual aircraft emissions can be different from those in aircraft engine certification tests (‘average values’).  This margin is up to 13% where few engines have been tested (as is the case for many aircraft that will be used in 2030).  

“The engine mix on a given aircraft type was allowed to evolve as individual aircraft are retired and as engines are replaced, assuming the replacement engine has an emissions performance appropriate to its date of introduction; new airframes introduce new engines into the mix.  The emissions performance of new engines was judged by QinetiQ on the basis of manufacturer’s targets and technology expectations”.

The ‘Revised Emissions Methodology’ report suggests that predicted NOx emissions were increased to reflect engine deterioration and uncertainty, but is unclear about whether this was definitely done and how much effect it had:  “At any particular time the engines in the fleet operating at an airport will be, on average, part-way through the maintenance cycle; in addition, there will be some longer-term degradation not restored by maintenance that will be restored only at refurbishment.  Thus, there may be a systematic bias in emissions estimated based on certification data...  The available data are also not detailed enough to make a distinction amongst the various phases of the LTO cycle so, in applying these values to the LHR emissions inventory, the percentage NOx increase noted above was applied equally to the NOx emissions from all phases. “

AEA Energy & Environment, Emissions Methodology for Future LHR Scenarios, S.12

AEA Revised Emissions Methodology, 2.30, 2.32

Use of average values  / certification tests could significantly underestimate future aircraft emissions.   

To uncertainties in predicting NOx emissions from aircraft must be added the political and commercial pressures which dictate the technology capability of engines offered to the airlines.

Also, ahead of any proposal from the EU on how to address NOx emissions, or an ICAO discussion on future stringency, coupled with the absence or any information on the political acceptability of both, will measures like the EU ETS make manufacturers prioritise carbon and fuel-efficiency at the expense of fully  minimising NOx?

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