Dynamic Aircraft Wake Separation Based on Velocity Change
Round 1
Reviewer 1 Report
The paper titled "Dynamic Aircraft Wake Separation Based on Velocity Changing" can be recommended for publication, by me, if the authors change or reflect upon the following:
The title is not grammatically correct.
The central premise of the paper is:
The effect of dynamic change of aircraft's flight state on wake separation reduction.
Therefore, the study brings the knowledge of flight's dynamic state, which has been neglected.
However, it is not clear why the paper uses speed variation that is limited in range. Why only variation of 10 m/s is considered ?
Many of results due to inherent small variation in velocity will overlap. Please explain why a greater range of variation not selected. ?
Small Improvements:
- Please edit sentence structures and make sure tenses are coherent within the same sentence.
- Make sure the units are mentioned from table 2 onwards.
- Improve figure quality, the labels are two small. Labels should be the size of text. Furthermore, increase the linewidth. Legends are hard to read.
- In think it would be better to plot figure 4 in semilogx, or the x-axis in log scale.
Regards
Author Response
Please see the attachment
Author Response File: 
Author Response.pdf
Reviewer 2 Report
- The authors uses the panel method to estimate the induced roll moment and ICAO induced-roll moment thresholds as criterion for determining "safe" separation. This could be considered as an extension of the Tatnall-Bowles model (1998) as alluded to in Ref. 6. Furthermore, the authors proposed using this threshold to determine a "dynamic" separation for wake.
- Given that time-based separation study conducted at London Heathrow airport could lead to dynamic separation distance without having to communicate changing distance threshold at different time, it would be interesting to see what the different separation distance would be if following the time-based separation versus the proposed method.
- Also, given that most aviation regulations/publications use nautical miles and knots, it might be a good idea to label all plots in those units, or at least give a conversion in the caption.
- Since we're talking about landing aircraft pairs, the lateral & vertical offset allowed for the follower aircraft would be very limited, making the section "Delineation of simplified hazard area" not applicable to wake separation. I would recommend removing this section all-together.
- Figures 9 & 10, and the associated text section, wasn't very clear about what the different lines are for. The text and captions should clarify that the velocities and x-axis are for the leading aircraft and the y-axis is the separation threshold for the follower aircraft.
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Note to authors: The updated manuscript did not include the reference section with the PDF. I am unable to cross-check any addition/deletion of references for relevance.
Specific response to author's replies:
1. I agree that some of the previous studies were not conducted with the flight-experiment derived roll-coefficient in mind. However, simply changing the threshold/parameter used does not make this a novel research. The novelty of this paper is on the temporal change in wake-based safe follow distance as approach sequence progresses, so we'll keep our focus on that.
2. I'm glad that you've included some analysis with the TBS. Figure 4 might be sufficient as a table showing maximum induced rolling moment for each velocity pairing, while highlighting the pair with maximum rolling moment for each aircraft pairing (i.e. merge with Table 7.) The reasoning is that the distance from vortex core is not really relevant for the purpose of induced-roll tolerance, while a single plot showing the variability of induced-roll with changing horizontal distance might be more appropriate in the "Following Aircraft Response Model" section.
3. Appreciate the inclusion of unit conversion. This made reading it a lot easier to comprehend and compare to existing regulations.
4. Acknowledged that the authors removed the section.
5. Most paper writing guide during my time in university asks for self-contained and self-explanatory captions to figures. This might not be the case any more. Anyway, I believe the ICAO wake separation distance is static thought the approach sequence (in exact NM distances, so 4 NM and 6 NM respectively), so shouldn't that be a flat line in Fig. 7 & 8 (assuming that it's a composite plot with absolute distance for ICAO sep. and calculated sep. while showing sep. diff. for different initial sep. distance)? Also, the captions and text mentioned "separation reduction" while the vertical axis was labeled as "aircraft separation."
Additional Comments:
- Figure 5 and the associated text never made clear what the difference in acceleration/deceleration is between the leader and follower aircraft. It appears that the model has an acceleration/deceleration limit as seen be the identical slope in Fig. 7 & 8 for sep. distance reduction.
- I think there might be some misunderstanding on how the separation minima is implemented. This is not an "initial separation" to be enforced at some point before the approach sequence began, but to be observed throughout the flight regardless of the difference in relative speed. That is to say, if the following aircraft is at a speed that would breach the sep. min., it would be asked to slow down or be instructed to go-around (in the case of approach).
- The conclusion chapter should note the limitation of this analysis, including the simplified rolling moment analysis might not capture induced 3D stall or other more complex aerodynamic interaction between wake and aircraft; the non-inclusion of ambient turbulence level, cross wind, ground effect, etc.
Author Response
Response to Reviewer 2 Comments
I'm really sorry about the last manuscript I submitted did not include the reference section, and I have dealt with the problem. Please forgive the inconvenience caused to you.
Point 1: I agree that some of the previous studies were not conducted with the flight-experiment derived roll-coefficient in mind. However, simply changing the threshold/parameter used does not make this a novel research. The novelty of this paper is on the temporal change in wake-based safe follow distance as approach sequence progresses, so we'll keep our focus on that.
Response 1: Thank you for your thoughtful comments. The research in Reference 6 balances the roll moment received by the aircraft when it encounters the wake vortex through the controllable moment generated by the aileron. The threshold calculation in this research requires the parameters of the aileron. We could not obtain these aileron parameters, so we chose to use the roll moment coefficient as the threshold. The two thresholds are similar in principle. You mentioned above "The temporal change in wake-based safe follow distance as approach sequence progresses" is shown in Fig 5.
Point 2: I'm glad that you've included some analysis with the TBS. Fig 4 might be sufficient as a table showing maximum induced rolling moment for each velocity pairing, while highlighting the pair with maximum rolling moment for each aircraft pairing (i.e. merge with Tab 7.) The reasoning is that the distance from vortex core is not really relevant for the purpose of induced-roll tolerance, while a single plot showing the variability of induced-roll with changing horizontal distance might be more appropriate in the "Following Aircraft Response Model" section.
Response 2: Thank you for your valuable comments. We have changed Fig 4 to a table(Tab 7 of the updated manuscript) showing maximum induced rolling moment for each aircraft pairing, and merged it with Tab 7 of the previous manuscript. And we have put a single plot(Fig 3 of the updated manuscript) to show induced rolling moment for each velocity pairing in the "Following Aircraft Response Model" section.
Point 3: Most paper writing guide during my time in university asks for self-contained and self-explanatory captions to figures. This might not be the case any more. Anyway, I believe the ICAO wake separation distance is static thought the approach sequence (in exact NM distances, so 4 NM and 6 NM respectively), so shouldn't that be a flat line in Fig. 7 & 8 (assuming that it's a composite plot with absolute distance for ICAO sep. and calculated sep. while showing sep. diff. for different initial sep. distance)? Also, the captions and text mentioned "separation reduction" while the vertical axis was labeled as "aircraft separation."
Response 3: We are grateful to reviewer for pointing out this problem. The separation standard set by ICAO is the minimum separation to be maintained between aircraft. Since the flight velocity is always changing during approach, it is difficult to accurately maintain the separation stipulated by ICAO (a flat line in Fig. 7 & 8) in the actual control process. Therefore, the flight separation allocated in the actual control process are often greater than ICAO standard. As the flight velocity will change during the approach, the flight separation between the front and following aircraft will show a decrease as in Fig. 7 & 8 (curved curve part in Fig. 7 & 8). We need to ensure that the flight separation between the front and following aircraft is greater than the calculated minimum safe separation(a dotted line in Fig. 7 & 8) throughout the flight. And we have changed "separation reduction" mentioned in Fig. 7 & 8 to "separation change". When horizontal coordinate is 0, the value corresponding to the vertical coordinate of each curve in the figure shows the reduction of the separation.
Point 4: Fig 5 and the associated text never made clear what the difference in acceleration/deceleration is between the leader and follower aircraft. It appears that the model has an acceleration/deceleration limit as seen be the identical slope in Fig. 7 & 8 for sep. distance reduction.
Response 4: Thank you for your comments. We have added the specific acceleration/deceleration used in Fig 5 ((a)-(h) of Fig 5 of the updated manuscript)
Point 5: I think there might be some misunderstanding on how the separation minima is implemented. This is not an "initial separation" to be enforced at some point before the approach sequence began, but to be observed throughout the flight regardless of the difference in relative speed. That is to say, if the following aircraft is at a speed that would breach the sep. min., it would be asked to slow down or be instructed to go-around (in the case of approach).
Response 5: Thank you for your precious comments. This problem is similar to Point 3. As the flight velocity will change during the approach, the flight separation between the front and following aircraft will show a decrease as in Fig. 7 & 8 (curved curve part in Fig. 7 & 8). We need to ensure that the flight separation between the front and following aircraft is greater than the calculated minimum safe separation(a dotted line in Fig. 7 & 8) throughout the flight. You say "if the following aircraft is at a speed that would breach the sep. min., it would be asked to slow down or be instructed to go-around". We have modified Fig. 7 & 8 and the corresponding text content as you requested above. We choose to reduce the separation by adjusting the velocity of the following aircraft in the stage when velocities of the front and following aircraft are relatively stable(The first horizontal segment in Fig 6).
Point 6: The conclusion chapter should note the limitation of this analysis, including the simplified rolling moment analysis might not capture induced 3D stall or other more complex aerodynamic interaction between wake and aircraft; the non-inclusion of ambient turbulence level, cross wind, ground effect, etc
Response 6: Thank you for your valuable comments. We include the limitation analysis of this ressearch in the conclusion.(The last two sentences in the conclusion of the updated manuscript).
Author Response File: Author Response.pdf
Round 3
Reviewer 2 Report
The authors have adequately addressed all my comments.
