Descent Terminology

There are many terms used in conjunction with quiet descent technology…

They are often used interchangeably, but they do not have the same meaning, and so some of the arguments regarding them are no more than misunderstandings.

So to set the record straight:

Spoiler:
– The key to a quiet descent is being an IPD. OPD technology is designed to make sure that the approach is truly IPD.
– SERFR was supposed to be OPD, but design mistakes caused it to be broken, and that’s why it is noisy.
Wait, what?!    Read below about the meanings of all the acronyms!

Continuous Descent Approach (CDA): Any approach in which the altitude and speed vary continuously all the way from top-of-descent to landing.  The old BSUR arrival was not a CDA, as seen in the picture below, since speed was constant along most legs, and the profile had “flat spots” every time the planes wanted to slow down. Flat spots flown at idle-power are not a major issue, since they don’t require engine thrust, so didn’t cause BSUR to become noisy.

BIG SUR:
Not a CDA, but still quiet.
Has flat spots, but no level flight with power.

Idle-power descent (IPD): Any approach in which the engines remain in flight-idle power setting throughout the descent. This is the most important property of a quiet and efficient descent. The old BSUR arrival, if flown by a competent pilot, was very close to being an IPD, but it had hard-wired altitude and speed constrains (e.g. at BOLDR) and required the pilots to gauge factors such as wind conditions, type of plane, load on the plane, etc.  Old-hands sometimes refer to any idle-power descent as an OPD, but this is not the terminology that the FAA uses.

Tailored Arrivals (TA): This is a property of CDAs which means that every airplane will execute the descent in a slightly different manner – tailor made for it.

Optimized Profile Descent (OPD): The name of a specific technology implemented by the FAA for quiet and efficient descents. It makes use of all of the above concepts, so an OPD approach is an IPD, a CDA, and a TA…  The technology goes beyond just describing the above concepts. It includes the actual code in the airplane’s Flight Management System (FMS) that executes the OPD, it includes the ability of ATC to deal with planes that are performing CDAs, etc. When the FMS flies an OPD arrival, it can gauge all the parameters much better the average pilot can, and can start doing so from further out.

SERFR was supposed to be an OPD arrival, but because of external factors (such as the Class B conflict) it didn’t work well. Further, the placement of the waypoints was such that once it didn’t work well, it worked very badly, with segments of level flight with power.

Because an OPD is programmed to figure out an optimized descent per each flight, the FAA does not specify altitudes or speeds at most waypoints.  The most they can do is specify min-max vertical “gates” that the airplane has to thread, but if all goes to plan, the plane will never touch the min-max limits of the gates. However, the placement of the gates is important in case the pilots for some reason have to fly the arrival manually, since they prevent the pilots from deviating too much from an IPD.

Long story short: The key to a quiet descent is being an IPD. OPD technology is designed to make sure that the approach is truly IPD.
SERFR was supposed to be OPD, but design mistakes caused it to be broken, and that’s why it is noisy.

3-degree glide slope: Airplanes are designed so that under flight-idle power setting, they can glide at a rate of 3 nautical miles per each 1000′ of altitude loss (which works out to 3 degrees) while maintaining a constant velocity. That last bit is important! If a plane descends at a shallower angle while at flight-idle power, it will start slowing down.

2.8-degree glide slope:  If we look at a pre-NextGen route like BSUR, we see a serious of 3-degree constant-velocity glide slopes, separated by short sections of level flight, where the planes reduce their velocity.  If we look at the end-to-end angle, including both types of segments, that angle is approximately 2.7-2.8 degrees.  Not surprisingly, OPD arrivals (which fly along a continuous diagonal) end up flying at about that same angle.

There is little doubt that if a plane were to try to fly at a 2.8-degree slope while maintaining constant velocity, it will require thrust and be noisy. However, OPD does not do that, since it simply does not have constant-velocity segments.

Flat Spot: A small (~3 mile) stretch of horizontal flight that’s done at flight-idle power setting. It is used to slow down the airplane, since the power setting is already at a minimum. BSUR had those, and they are not a major problem.

Level flight at constant speed: The big devil. Level flight at constant speed requires about twice the thrust as flight-idle (~50-60% vs. 25-30%) and can be several times as loud. Level flight with thrust occurs when the airplane has too far to go relative to its altitude, which is common when being vectored. It should not occur during procedured descent, and the whole concept of OPD is designed around eliminating it to the greatest possible degree. Such segments can be seen in the SERFR image below.

SERFR:
A Broken CDA, Noisy.
These long level flight segments at 10,000′ and 8000′ indicate horizontal powered flight.

Dive and Drive, Staircase Descent: Terms used to describe descending to an altitude and then flying level at that altitude. In some places FAA messaging is over-eager to describe the advantages of NextGen and subsequently paints pre-NextGen practices in an almost derogatory manner. “Dive and Drive” was never a good practice, and both ATC and pilots attempted to keep airplanes as high as possible as long as possible, so that they can practice IPD towards the airports. Ironically, the best example of “Dive and Drive” that I can find is post-NextGen BRIXX, as shown below.

BRIXX:
No CDA, very noisy.
Ouch. Very long “drive” sections, about 20 nm long, only 2500′ above ground level.
Similar to what happens on SJC reverse-flow arrivals over Sunnyvale, Mountain View, and Palo Alto.

“You can either go down or slow down”: A meme taught at flight school… It means pilots should either lose altitude or lose speed, but not do both at the same time.  The reason has nothing to do with what the airplane can do… It’s just that pilots are instructed to keep one parameter constant (velocity or altitude) while varying the other because it’s how the human brain best handles things.  There are also no controls in the cockpit that make is natural to slowly vary both speed and altitude.

A Flight Management System (FMS), on the other hand, is not human. It has no problem controlling two simultaneously varying parameters such as altitude and speed, and that’s exactly what it does in OPD.

Vectoring: Vectoring is a directive by Air Traffic Control (ATC) to a pilot to go in a certain direction. At first glance, it has nothing to do with altitude or descents…  But once a flight is vectored, the distance it has to go until it lands is no longer known… So the rate of descent is unknown… and so OPD and IPD are both out the window, and the pilots will have to use thrust (if the path is longer) or speed brakes (if the path is shorter) or sometimes both (if given more than one directive).

“Maintain 8000”: ATC speak for “make noise”. See Level flight at constant speed above.


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