Please allow me to formulate some remarks on the CDA issue.
Concept
As the purpose of CDA is noise reduction, the non existence of level flight is not relevant. The focus should be on the avoidance of high thrust settings at lower altitudes. Consequently, level flight with the purpose of decelerating the aircraft with idle or near idle thrust meets the CDA purpose (the CDA acronym is therefore not an ideal term).
Regulatory framework
CDA execution is strongly aircraft type and aircraft weight related. The pilot is in the best position to judge the minimum noise CDA flight path. He will do that, not on the 3° flight path assumption, but rather on the more accurate FMS computation which is based on the energy concept. Therefore one single CDA profile is not feasible. The pilot should be given enough freedom to determine an ideal flight path in function of the circumstances.
Low power/low drag
Low power low drag approaches have two distinct purposes:
Low power = idle descent and deceleration, means following the ideal profile according to the aircraft weight, type and weather (wind).
Low drag = the use of clean optimum speeds (low speeds) as long as possible and extend drag devices as late as possible. (gear, flaps, speed brakes)
a. Keeping an aircraft high on profile will cause it to use drag devices such as speed brakes and gear (or high speed) which will dramatically increase the noise produced by the aircraft. An aircraft low on profile will need more than idle trust to recover the profile, also increasing the noise footprint.
b. In case of RNAV arrivals, (FRA type) even when RV will be issued, it is advantageous to assign an arrival route matching as closely as possible with the expected RV pattern as early as possible. DTG is very valuable information, but a correct route programmed in the FMS is even more accurate. The FMS is not able to calculate a profile solely based on DTG.
c. Phraseology to issue a CDA clearance is very important. Since CDA involves a descent, it should be clear to which extend the profile can be followed. It is my opinion that a normal descent clearance and altitude should be given. Is CDA an Approach or an Arrival? This very important to understand; an approach ends on the ground. An arrival ends at the start of an approach procedure. After receiving an approach clearance, a descent according to the procedure can be made without a specific descent clearance. This is not so with an arrival where specific descent clearances should be issued, whatever the altitude constraints might be. Confusion must be avoided because vertical separation and obstacle clearance is a safety critical issue. It is my opinion that CDA should be considered as an arrival. This would avoid confusion and avoid extensive review of ICAO material.
Aircraft operations
The energy concept used by the FMS to calculate the optimum profile is very important and needs to be understood by CDA procedure designers. Speed is as important as altitude and DTG. Controllers do sometimes issue impossible clearances because of the lack of understanding regarding the energy concept.
Final Approach
The moment of “gear down” is dependant on specific airline procedures. Aircraft manuals (at least for 737 and 320) recommend gear extension at 3000 ft AGL. This is not followed by most airlines which (regarding Low drag low noise policy) recommend gear extension in the range of 1500 to 2500 ft AGL. Low visibility procedures will require the standard 3000 ft as per aircraft manual. Tailwind might force the pilot to lower the gear early because deceleration is not possible with strong tailwind on a 3° flight path. The 160 kts restriction at the OM can not always be met, especially with heavy aircraft and tailwind. In case of non-precision approaches, approach configuration is required as from start IAP or 3000 ft AGL.
Base leg
Obviously, a downwind route is more sensitive regarding to changing DTG than a straight in route. Consequently, the issuance of a specific lateral route (or DTG) is more important in case of downwind routes or base leg routes.
Effect of the wind
This is a very important issue. It should be understood that flexibility in the profile is required in order to meet with the different wind situations. If not, increased noise because of drag devices or engines will be the result. Wind speed is usually much stronger at altitude than on the ground.
ATC system overview
Regarding speed control: once on a 3 degree profile, speed reduction is difficult without extra drag devices and is also much slower compared to a situation where descent slope can be (temporary) decreased or where a small portion of level flight can be included. Most Aircraft Operating manuals recommend Glide Slope interception from below. Interception from above is difficult, creates high flight deck workload and often leads to rushed or unstabilised approaches.
Manuals
See remark on the energy concept.