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Ideally, approaches should be executed as a continuous descent approach (CDA), maintaining idle thrust and only levelling off (or reducing vertical speed to <500 fpm) briefly to reduce speed, while still keeping the thrust at idle.

While I cannot speak for every region, in Europe, executing a CDA is widely regarded as the most beneficial method of flying. It conserves fuel, as engines are at idle, and significantly reduces noise pollution. The majority of European airports support CDAs, although there are exceptions. For instance, Bergerac Airport in France necessitates maintaining an altitude of 2,500 feet for an approach to runway 27.

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As a Boeing 737 pilot, I am keen to learn about any rules of thumb or strategies that other pilots employ to effectively execute a Continuous Descent Approach (CDA), particularly when piloting a Boeing 737. I aim to stay in VNAV, but I am also prepared to utilise LVL CHG and V/S as required. Our company's Standard Operating Procedures recommend using a 10nm ring for the landing runway, and I make use of the altitude range arc (known as the “green banana”) to enhance situational awareness.

Let’s take London Gatwick as an example:

enter image description here ICAO Chart ILS/DME RWY 26L from UK AIP

In my flight planning I may work out what maximum altitude I need to be at 10nm out using this calculation:

Altitude at 10nm = Altitude at 5nm + 5 x (altitude loss per nm on glideslope) 
= 1840 + 5(1840-1520) 
= 3440 ft 

The chart gives the 5.5nm to intersect the glideslope at 2000', but I may also write down what altitude I would intersect the glideslope at 3000':

At 3000’, mileage required = (3000 - Aerodrome Elevation)/(altitude loss per nm on glideslope)
= (3000-203)/(1840-1520)
= 8.7nm

I would write these 3 altitude-distances down and refer back to them during the descent to make sure I am on profile. I would cross reference my progress with the "green banana" and 10nm range ring, and of course the VNAV profile (737 specific).


What effective rules of thumb or strategies can be employed to effectively execute a CDA?

  • When considering altitude at specific distances, such as 10nm out, do you rely on pre-calculated altitude-distances, or do you employ a more efficient, quicker mantra?
  • Are there any more reliable or simpler techniques to enhance situational awareness during a CDA, ensuring adherence to the desired glide path without deviating too high or low?
  • In scenarios such as encountering a stronger than anticipated tailwind, how do you adjust or preemptively manage being high on the approach? What indicators are you looking for to inform you that you are high on approach.

Personally, I find that the most critical point is from ~7000' to ~4000', trying to balance the best moment to decelerate and/or keep on the profile.

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  • $\begingroup$ When I was on a team that developed (what we called) Continuous Descent Arrivals in the US the team was also comprised of representative pilots from the major air carriers servicing that airport. As I recall, the FMS flew the arrival/approach automatically. Does your aircraft not do this? $\endgroup$
    – RetiredATC
    Commented Apr 13 at 2:29
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    $\begingroup$ @RetiredATC I’m not sure what it is like in States, but in Europe we rarely follow the actual STAR arrival. By radar vectoring, we will often expect (quite dramatic) shortcuts on the STAR or a vector to a 10nm final etc. So we find that we can’t accurately predict and therefore can’t program the true arrival in then FMS. Even if you fly on the arrival track exactly, a tailwind may get you high on profile (quite insidiously). So we can rely on the FMS to a certain extent, but situational awareness of vertical position is also incredibly important. $\endgroup$ Commented Apr 13 at 7:11

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I've seen the technique of putting the green xx mile arc around either the runway or the FAF waypoint, and I'm not a fan. There are too many variables at work to make any particular answer of being at some altitude or some airspeed when reaching that arc generally reliable. Simplest counter-example: being 30 miles from the airport at 10,000' and 250 knots is good gouge when you're on base; if straight-in, you have too much energy; if on downwind, you're below an ideal profile. Same sorts of limitations exist with an arc around the FAF.

What I've found works better is primarily considering altitude when on an RNAV approach or an arrival that will transition into an instrument approach with no or minimal level segments ahead.

Take as a starting point that being at 2,000' above the runway on a 3-degree path (so about 6 NM track distance to go), with flaps at 5, and speed below the clean maneuvering speed (not necessarily all the way slowed to the Flaps 5 min maneuvering speed, but somewhere between the -UP and the -5 markers)... for most reasonably normal days, if I'm at that state passing 2,000' I can lower the landing gear and configure normally and be easily stabilized by 1,000' above the runway. Strong tailwinds will mean backing that out a bit, but in general, on path at 2,000' with flaps 5 is a good place to be, dropping the gear at about that point.

So if you're on a procedure that keeps you roughly on a 3 degree path, what do you need to be doing above 2,000' to achieve that state there? Evaluate your winds and gross weight and existing airspeed and decide when to start extending flaps to get there. If you're 20 knots above the -UP marker with light winds, then maybe 3,000' or slightly below will be a comfortable point to make the initial flap extension. If you're assigned a speed right above the -UP marker, then you can probably delay extending the flaps until maybe a mile outside the 2,000' point and that can work comfortably. If you're already at Flaps 1 or 2 and stable at the maneuvering speed for that configuration, then you can hold that all the way to 2,000'.

On the other hand, if you're at 250 knots and on a 3 degree path, you probably need to be starting at 4,000' getting rid of energy -- and if you're much lower than that and still fast, then leading with the landing gear may be necessary. A clean 737 on glide path doesn't bleed energy very quickly, and with a tailwind, it may not slow at all without significant help... option 1 would be extending the speed brakes to get the speed down to 240 (i.e. 10 knots below the placard limit for initial flaps) then flaps 5; option 2 is extending the gear immediately, and option 3 is everything: gear+speedbrake+Flaps 5 until you can extend more flaps -- which is a "save" maneuver and not a recommended way to plan to slow and configure! (If you're having to do that, the best choice -- if your procedures allow it -- is to configure all the way to landing flaps in level flight before resuming the descent... this gets the most drag out soonest so it's working for you the longest.)

All of this is clearly a discussion of technique and requires some sense of what "looks about right" so as to achieve the landing configuration in a timely (i.e. stabilized) but not drastically early manner; until that sense of what looks right becomes somewhat developed, err on the side of configuring early... the extra gas burned in dragging the gear an extra minute or two is far less than the gas burned in a go-around & extra trip around the pattern. But, as that sense does start to develop, I find that watching altitude and comparing my state in that regard to the altitudes on a straight-in approach tends to be the best guide -- better than using distance-from-the-FAF in isolation, for instance.

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