Is the separation distance between two aircraft approaching an airport the same under CDA and Non-CDA scenario?

Question

I'm working on a project that investigates Continuous Descent Approach (CDA), the procedure by which an aircraft approaches an airport for landing by descending continuously from cruise altitude with idle or near-idle thrust setting. As a Continuous Descent Operation (CDO), CDA provides operational benefits in terms of fuel burn reduction which reduces emissions as well as noise levels around airports.

In my project, I'm studying CDA operations during high traffic periods with the aim of developing key performance indicators (KPIs) for airports. However, I really need some expert and professional advice from pilots and air traffic controllers (ATC) on how CDA is being coordinated, managed, and conducted between pilots and ATCs. The following questions underscore my point:

1. If a pair of jet aircraft is approaching an airport, is the separation distance between them smaller, the same, or larger under CDA and Non-CDA scenarios?

2. Just before touchdown on runway, is the aircraft's approach speed the same under CDA and Non-CDA scenarios?

3. In general, how many track miles could be used for approaching traffic at a major airport?

4. In general, is the airspeed during descent different between CDA and Non-CDA scenarios?

5. Just before touchdown, will the pilot utilize thrust to increase speed or will the aircraft be on its own volition, under CDA and Non-CDA scenarios?

Answer 1

Continuous descent operations do not warrant a change in separation minima. The same separation minima (wake turbulence, radar minima, lateral, horizontal and vertical minima) apply at CDA airports as they do at all other airports.

To answer 2, 4 and 5 at the same time: continuous descent approach means that an aicraft descending from cruise level (typically 10-11 km.) to the final approach procedure (typically about 1000 metres) will do so in one continuous motion - e.g. without levelling out and maintaining a certain intermediate level along the way. Although it is possible, and widely utilised, crews are not required to use idle engine thrust for the descend. It may well be the case that an aircraft is required by air traffic control to maintain a certain speed during descend, in order to establish and maintain the separation requirements mentioned above. As such, you can see CDA operations where aircraft apply thrust during descend, but where the descend is still conducted as one continuous motion. On the final approach, usually starting at an altitude of about 1000 metres, 20 or-so km. from the airport, there is no difference between CDA and non-CDA operations. Aircraft will fly completely normally, using engine thrust and lift augmentation devices (flaps, slats) to maintain an appropriate airspeed (actually, angle of attack) so that the aircraft descends at the correct rate to hit the runway. For a normal jet aircraft, it is not possible to fly the final 20-or-so km. of the approach with idle thrust - it simply wouldn't make it to the runway.

When talking about track miles, I will assume you are talking about the distane from the top of descend (TOD) until the runway threshold (THR). For an average jet airlines, descending from a level of 10-11 km., it will take somewhere between 100 and 120 nautical miles of distance to get from TOD to runway THR. For non-CDA airports, this distance will generally be longer, because the descend has to be initiated sooner, but you will also find examples where there is almost no difference.

Answer 2

If a pair of jet aircraft is approaching an airport, does the separation distance between them is the same or larger, under CDA and Non-CDA scenario? Ans: Same. U.S. separation starts are in FAA Order 7110.65. It has one set of standards

Just before touchdown on runway, does the aircraft approach speed is the same, under CDA and Non-CDA scenario? Ans: Generally the same.

In general, how much track miles could be used for approaching traffic at a major airport? Ans: Many instrument approaches are defined for 15 NM (the Initial Approach Fix). However, often aircraft are cleared to the Intermediate Fix, about 10 NM from the runway.

In general, is the airspeed during descent differs CDA and Non-CDA scenario? Ans: I don't know.

Just before touchdown on runway, will the pilot utilize thrust to increase speed or aircraft will be on its own volition, under CDA and Non-CDA scenario? Ans: Just before touchdown, the pilot will generally initiate a flare maneuver to reduce the impact.