Analytical Models for CO2 Emissions and Travel Time for Short-to-Medium-Haul Flights Considering Available Seats
Round 1
Reviewer 1 Report
I consider this paper to be an interesting contribution to the field of sustainable aviation. It provides a novel methodology for the consideration of aviation alternatives - which is an area of growing importance for the transport industry. The two applications shown are relevant and demonstrate how the methodology could be employed by industry stakeholders.
Overall, the paper is well written and clear in its arguments. I have suggested some grammatical changes and minor areas for further discussion which I think would strengthen the paper further (in the attached Word doc).
Comments for author File: 
Comments.pdf
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 2 Report
Dear authors,
First of all, you wrote a very interesting article which provides essential contributions to the global discussion on solutions on climate change.
I’d like to share some comments with you.
You use the term “ASK” (line 50 and 54). But you explain it later in line 224.
You should explain the difference between CO2 and gCO2 explicitly. (line 50)
Further, you use the term “validation” e.g. in headlines 3.4 or 3.5.3 – the problem is, what you do is no validation. You just compare different models. In this term you calculate the uncertainties of your model. But a validation includes much more essential mathematical procedures that have not been performed or described in the paper.
Uncertainties are one part of each analysis and represent the base for a validation. Please describe the systematic and random errors (and the error propagation in your model). You should add the error bars in the figures.
The same is for the comparison of flights and rail connections. Uncertainties due to rail connections are not provided. Without error bars in figure 13, that figure has no expressiveness.
The travel times (rail) are not really comprehensible. e.g. there are three high speed train connections from Berlin to Munich. Which connection has been chosen?
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
OVERALL COMMENTS
This study presents models for calculating gate-to-gate CO2 emissions for short-haul and medium-haul flights. It is proposed that these models, which account for the number of available seats, are suitable for identifying transport routes where rail would be a viable alternative to aircraft in terms of emissions and total travel time. The study therefore addresses an important issue in the planning of sustainable transport systems. The following comments are suggestions for improving the paper.
AIRCRAFT TYPES
The categories shown in Table 1 are incorrect. CAT D should be A321, A320, etc. CAT E should be CRJ7/9, E195, E190, etc.
=> Correct the information in Table 1.
The mistake in Table 1 is worrying. Was this mistake confined to Table 1? Or was it repeated, for example in the calculations for Figures 5-7?
=> Check that the correct categories were used in the subsequent calculations.
It is not clear how you selected the 13 aircraft types shown in Table 1.
=> Explain the process for selecting the aircraft types.
=> Why did you not select other aircraft types in CAT D and CAT E? (Eg: B739, AT72.)
LIMITATIONS OF THE STUDY
The methodology is based on a selection of CAT D and CAT E aircraft types. In the validation (Section 3.4), the mean value of emissions was calculated for the CAT D aircraft that were selected. If you had selected different CAT D aircraft (eg: B739), the mean would have been different.
=> Explain that the mean is based on a limited selection of aircraft.
The comparison with rail transport requires an adjustment to allow for transport time to and from the airport or train station (Section 4.2.1). This adjustment is important, but the methodology outlined in lines 532-543 does not explain clearly how the additional times were determined. Nor does this section include any references to previous studies.
=> It would strengthen the paper if this section was re-written to explain clearly how the additional times were decided.

ACRONYMS & ABBREVIATIONS
This study uses a number of acronyms and abbreviations, and a list is provided on pages 19-20.
For some acronyms, the meaning is not given when the acronym first appears.
=> It would be helpful if the meaning was added to the following: ASK (line 50), KPI (line 51), PAX (line 53), pkm (line 54), SESAR (line 94), EMEP (line 194), GCD (line 195), NM (line 204), PM (line 308).
Some acronyms appear in the body of the paper but are not in the list on pages 19-20.
=> Add these to the list: SESAR (Single European Sky ATM Research) and UNFCC (United Nations Framework Convention on Climate Change).
Also, I think that Marketing Information Data Tapes should be MIDT.
=> Change “MDTI” to “MIDT” in lines 482 & 642
REFERENCES
The url for reference 39 does not work.
=> Check the url and change as necessary.
PROOF-READING
Overall, the paper is well written. However, there are numerous English language errors which could be easily corrected.
=> Have a native proof-reader check the entire paper.
Here are some examples of the types of errors:
Word choice: in line 15 change “with only punctual exceptions” to “punctuated only by exceptional events”
Single/plural: in line 20 change “While other industry” to “While other industries”
Single/plural: in line 35 change “considering destination” to “considering destinations”
Single/plural: in line 47 change “This type of operations are” to “These types of operations are”
Single/plural: in line 70 change “two cases studies” to “two case studies”
Word choice: in lines 99, 125 & 131 change “reduction on” to “reduction in”
Noncountable noun: in lines 136-137 change “Some evidences indicate” to “Some evidence indicates”
Spelling: in line 179 change “dependant” to “dependent”
Capital letter: in line 227 change “Flight level” to “flight level”
Unnecessary repetition: in lines 345-346 change “are more proportionally more important” to “are proportionally more important”
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
Thank you for making the revisions to your paper. I think that this study addresses an important issue in the planning of sustainable transport systems.
