Green Dot in the Airbus A320: The Deep Dive Vol. 1

Ask your average A320 pilot to define Green Dot speed and you’ll often get some interesting, generally vague and mostly incorrect responses.
While this lack of understanding reflects the general nature of “modern” airline training, it’s good news for you, because today, we’re going to show you how to be the exception.
How is Green Dot defined in the A320?

Airbus defines Green Dot as the engine-out operating speed in the clean configuration.

Operationally it corresponds to the speed that allows the highest climb gradient with one engine inoperative when in the clean configuration.

Green Dot equals the Best L/D speed.

Remember that the total drag on an aircraft consists of both Induced Drag and Parasite drag.

Green Dot lies at the intersection of these two drag curves.

Green Dot has been calculated to provide the best L/D for a given altitude, temperature, mach and weight, in the clean configuration with one engine out.

This definition explains why GREEN DOT is used for an engine failure in cruise, when the obstacle strategy (drift-down) is required.

In these cases, altitude must be preserved for as long as possible due to high terrain.

To achieve this, we need to use a speed that provides the lowest rate and angle of descent.

GREEN DOT is this speed.

While Green Dot is technically defined with reference to engine out cases, when all engines are operating normally, the Green Dot speed will still give you a very close approximation of the Best Lift-to-Drag ratio speed.

So don’t worry, you can still use it with both engines turning and get a very close approximation of the Best Lift-to-Drag ratio speed.

Seeing just how GREEN DOT relates to the aircraft’s thrust and drag can help us to better understand why things can get unpleasant if you get too slow.

Let’s take a closer look…

Alright, there’s a lot going on here, so let’s break this graph down together.

(You can click on this graph to open it up in another window, so you can switch between reading the text and looking at the image).

First, check out those drag curves, and once again, note that the total drag consists of two drag subtypes, induced and parasite.

Induced drag (orangey-yellow) increases at slower speeds.

Parasite drag (red) increases at higher speeds.

Remember, Green Dot = Best L/D speed!

Start at Green Dot and move your finger along the drag curve in either direction.

You’ll see that flight at any speed that isn’t Green Dot will cause either parasite or induced drag to increase.

This means that drag increases for any speed that isn’t GREEN DOT.

Flight at any speed that isn’t Green Dot will cause either Induced or Parasite drag to increase.

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Thrust, Drag and Speed Stability

Let’s start by looking at Points A and B in the graph below (Figure 1.0).

These two points show where the thrust in use is exactly equal to the drag.

There are some interesting things going on around these points.

AT POINT A:

If we leave the thrust where it is, and disturb the aircraft so that the airspeed begins to increase, you’ll see that for any speed increase, the parasite drag must also increase.

Since we kept the thrust constant, the increase in drag will cause the aircraft to decelerate back to POINT A, right back where we started, which is good.

Green Dot, Induced Drag, Parasite Drag and Available thrust
Figure 1.0

Now let’s look at another case.

Again, let’s look at Figure 1.0.

We’ll start at Point A, and keep the thrust constant.

If the aircraft is disturbed so that the airspeed decreases, the parasite drag will also decrease.

Since we kept the thrust constant, this decrease in drag will eventually cause the aircraft to accelerate back towards POINT A.

Again, we’re back where we started, so everything is still good.

From this little thought experiment, we can see that an aircraft at Point A:

  • Will tend to return to Point A.
  • Is stable with regard to speed and that’s a good place to be!
Thrust, Drag and the Region of Reversed Command
Let’s once again take a look at Figure 1.0.
Green Dot, Induced Drag, Parasite Drag and Available thrust
Figure 1.0

Anyway, now let’s look at POINT B:

If we once again keep the thrust constant and disturb the aircraft so that the airspeed increases, we can see that as airspeed increases, the induced drag decreases.

This decreased drag will make the aircraft accelerate, all the way up to POINT A, at which point the drag will be equal to that at POINT B.

However, if the airspeed decreases from POINT B, notice that the induced drag increases.

This increase in drag will make the aircraft decelerate even more!

Welcome to the Region of Reversed Command.

This can easily occur in certain cases, which is why it’s important to understand just what operational choices we have at our disposal.

So, if you find yourself back here, you have two options:

If the speed is between POINTS B and C, you can see that there is still some thrust available (Thrust Margin) and you can use this excess thrust to accelerate the aircraft.

If you’ve let things go such that you’re below POINT C, then you’ve got a bit of a problem, because there is no excess thrust available.

Your only way out is to descend, trading altitude for airspeed.

Better start asking for lower.

The crucial thing to understand is that an aircraft at POINT B is unstable with regard to speed.

Remember that flight at speeds less than Green Dot, can bring you closer to this point, especially at high altitude.

Where Can I Find Green Dot?

Look for the green dot on the PFD.

But remember, it’s only displayed when the FLAP LEVER is in the “0” position and the gear is not compressed.

You can also usually find a digital value on the MCDU TAKE OFF and APPR PERF pages.

How is Green Dot Calculated?

Green Dot is a “characteristic speed.”

Characteristic speeds are computed by the FACs.

With both FACs working, FAC 1 supplies characteristic speeds to PFD 1 and FAC 2 supplies PFD 2.

If one FAC fails, the other FAC computes all of the characteristic speeds.

If both FACs fail, all characteristic speeds, including GREEN DOT, are lost!

In other words, you’re FAC’d!

Here’s a subtle point.

The FAC computes the characteristic speeds displayed on the PFD and the FMS computes the characteristic speeds that are displayed on the MCDU.

These are both based on the weights that have been inserted into the MCDU, so make sure you enter them correctly!

How Can I Calculate Green Dot?
Here’s a handy rule of thumb.Below FL 200: GD = 1.5 x GW (tons) + 110 Above FL 200: Add 1 knot per 1000 ft to the figure obtained above.
Handy Green Dot Conversation Starters!
Try these out at your next party!
  • As per the QRH, Green Dot is the eventual speed target for a Dual Engine Failure, with or without fuel remaining.
  • In a Dual Engine Failure case, the average glide distance at Green Dot with no wind is 2.5nm/1000 ft. (ROD = -1700 fpm!).
  • With a FCTL SPLR FAULT ECAM, if one or more spoilers are fully extended, the speed target should be Green Dot +10. This minimizes fuel consumption.
  • The best speed and rate of climb for long-term situations is between Green Dot and the ECON speed.
  • When you see a “DECELERATE or T/D REACHED” message, the aircraft is suggesting that slowing down to Green Dot might be a good idea. This helps when getting high on profile.
  • When clean, with full speed brakes out, VLS may be higher than Green Dot! Watch out!
  • During a go-around, if, at the acceleration altitude, the speed target doesn’t increase to Green Dot, pull the ALT knob!
  • Through “S” speed, before selecting flaps zero, make sure that the speed target is at least equal Green Dot.
  • If the wing anti-ice is INOP and you’re picking up ice, the minimum speed is VLS +10/Green Dot.
  • The aircraft’s MAX ENDURANCE (most time aloft) speed is approximately equal to Green Dot.
  • Cycling the navigation database while airborne is not advised!
  • Among other things, It will delete the primary and secondary flight plans.
  • If the aircraft is in managed speed when this happens, then Green Dot will become the speed target.
  • During a go-around, the managed speed target is Green Dot.
The Takeaway
Green Dot:
  • Is the engine–out operating speed in the clean configuration.
  • Corresponds to the speed that allows the highest climb gradient with one engine out.
  • Is the target speed for a drift down using the obstacle strategy.
  • Provides a very close estimate of the speed for providing the aircraft’s Best Lift-to-Drag ratio (L/D MAX).
Let’s take another quick look at that graph:
Green Dot, Induced Drag, Parasite Drag and Available thrust
Figure 1.0

As an aircraft’s altitude increases, the maximum thrust available decreases.

This is essentially the same thing as moving the dark blue MAX THRUST AVAILABLE line closer to the lighter blue THRUST line.

When we do this, you can see that Point C will move closer to Point B.

This, essentially, means that the aircraft’s available thrust has decreased.

While that’s a nice little bit of knowledge, what you really need is to know how to apply this in an operational way.

Good thing you’re here.As we mentioned earlier, when flying level at high altitude, speeds below GREEN DOT can result in a rapid increase in the induced drag.

This increase can cause the aircraft’s speed to quickly decrease even further, beyond the point at which it can be recovered using the thrust available (Point C).

So remember, when you’re up at cruise, speeds below Green Dot can ruin your day.

Keep a close eye out for changing temperatures and winds.

These are a common culprit and when the changes are large enough, they can create havoc with your speed.

And when this happens, the only way to recover may be to descend.

Better start asking for lower!

Now, let’s check out our Staff Rapper, “Bus Driva” as he shows off his Green Dot Rap!

So that’s been our Deep Dive into Green Dot.

I hope you found it informative and maybe even a bit enjoyable.

By now, you should have a slightly better idea of the importance of Green Dot and its operational context.

Be kind, be smart, fly well.

Until next time…

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