Performance Impact Assessment of Reducing Separation Minima for En-Route Operations

Round 1

Reviewer 1 Report

On page 6 (line 211) authors state that “…traffic data comes from the Eurocontrol DDR2 (Demand Data Repository) database”. On page 8 (lines 256-259) they add an explanation of the approach aimed to determine the traffic sample to be used in their simulations. Referring to ICAO Doc 8991 and considering the prevention of a “traffic bias due to the pandemic” they finally select the 7th of July 2018. According to Eurocontrol’s DDR2 (data derived from Historical Traffic website) the 7th of July 2018 is the least busy day of that month (ranked 31 on an ECAC wide level). Authors could explain in more detail how they applied the ICAO Doc 8891 methodology and which geographical subarea they considered for their calculations (traffic in the LECMZMU sector only?).

On pages 6 and 7 (lines 213-217) authors state that “…data corresponds to the last filed flight plans from the airlines. The exported database contains information on coordinates of departure and arrival airports, departure time, arrival time and waypoints. Aircraft performance data for every aircraft model comes from BADA 3 (Base of aircraft data from Eurocontrol).” The term 'last filed flight plan' refers to the so6_m1 data format. Those files also include timestamps for segments (and thus for waypoints as the beginning or the end of a segment). Did authors use this information too? Or is the time-wise behavior purely based on the performance data of the simulator once an aircraft has entered the simulation?

On page 7 (lines 225-227) authors add information on flights they generated for the growth scenarios: “The new entry time is restricted because no conflicts can be allocated at the entrance to the sector, i.e., the initial conditions no longer imply a conflict.” Do flights enter the simulation at their origin or at a specific location (sector boundaries of e.g. LECMZMU)?

On page 7 (lines 229-230) authors state: “The analysis of the influence of the SM on ATM performance is performed statistically using Monte Carlo Simulations to find patterns in a non-deterministic way.” Besides the generation of traffic growth applying randomized cloning to the flight data there is no mention of ‘repeated random sampling’ what is the typical feature of that type of simulation. Did authors run their scenarios multiple times? Which parameters did the vary (beyond traffic sample)? Did authors make use of the survey data (esp. variability) they collected for specific ATCO sub-tasks (listed in Table 1 on page 5)?

Typos / Misc.

p. 8 (line 256) “IATA” to be replaced by “ICAO”?

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

This is an interesting and contemporary paper on the implications of reducing aircraft separation minima from 5nm to 3nm. It is good to see that multiple impacts on safety, efficiency, environment etc are considered. The overall intention of the paper is very good. The paper is on the path to publication but requires the authors to consider the points listed below.

Here are some specific comments:

1)     It must be noted that the paper presents results that show improvements for all metrics when implementing the lowest separation minima of 1NM. However, for aircraft at cruise speed, the safety risk will increase dramatically, as the time to react and deconflict traffic is less for both the flight crew and air traffic controllers. This needs to be made clear in the abstract so as to not mislead the reader.

2)     Lines 13-17 - The forecast of growth does not account for the negative impact of the Covid-19 pandemic. Please include more recent forecasts for aviation traffic growth.

3)     Line 84 – The word ‘bushed’ should be ‘brushed’.

4)     Line 177 – The word ‘spent’ should be ‘spend’.

5)     Line 179 – The authors state the ‘…several Spanish ATCOs’ were surveyed. Please be specific and state the actual number of ATCOs surveyed. Ideally the authors should present the population of ATCOs, margin of error and confidence level.

6)     Lines 216-217 – Please provide a reference for BADA3 database published by Eurocontrol.

7)     Figure 3b – This is not a very clear map. Maybe a higher resolution image should be included for clarity.

8)     Line 261 – +50% of 47 flights is 71 flights when rounded up.

9)     Line 265 – The units, ‘NN’ should be ‘NM’.

10)  Figures 4a , 4c, 4d, 4e, 4f - The vertical axes need to include a detailed label rather than just the units.

11)  Figure 4a and 4b – It seems a little strange that as the minimum separation distance increases (aircraft spaced further apart) that the number of conflicts and conflicting flights increases. For greater separation you would expect less conflicts. It would be useful for the authors to explain in more detail in the text as to why the results are showing this trend.

12)  Line 271 – Avoid writing in the first and second person. For example, the sentence, ‘If we look at the fuel burn…’ should be reworded in the third person.

13)  Figure 4c and 4d – These two figures could be combined into a single graph by including a second vertical axes for CO2.

14)  Table 4 – For comparison purposes, it would be useful to include the fuel burn and co2 saving per flight.

15)  Line 340-352 – It is good to see that the authors acknowledge some of the challenges of reducing separation minima. Note that below 10,000ft altitude, the international speed limit is set at 250kts which means aircraft are slower. In response, in some airspace sectors below 10,000ft the separation minima is reduced to 3NM particularly around airports. Thus separation minima standards are already set to account for slower aircraft speeds (time-based). It would be useful for the authors to include this in this section.

16)  Section 4.2 – More simulations details required. For example, how many simulation runs were conducted as part of the Monte Carlo method? What was the mix of aircraft types simulated? Across what altitude range were the flights considered?

Author Response

Please see the attachment

Author Response File: Author Response.pdf