2
$\begingroup$

I'm wondering, whether the Green Dot- speed calculation in the Airbus PFD does account for the actual bank angle of the aircraft.

Green Dot Speed is the speed for the best lift to drag ratio as it is stated in Safety first SPECIAL EDITION: Control Your Speed Series.

In that edition, it is stated, that the Green Dot-speed is a function of aircraft weight, which is quite obvious.

I wonder however, whether the current roll angle is also accounted for as the hodographs like given in Avrenli, Kivanc, und Barry Dempsey. „Is “Green Dot” Always the Optimum Engines-Out Glide Speed on the Airbus A320 Aircraft?“ The Journal of Aviation/Aerospace Education and Research, 2015. suggests:

Hodograph for A320: Glide Ratio over Speed and Banking Angle

Any ideas how to get those numbers for Airbus aircraft?

$\endgroup$
3
  • $\begingroup$ You can find a table from the QRH in this answer to What is an airliner's “green dot speed”, and how does it vary with weight, and why?. The table is only as a function of weight though. It does not say if the actual code in the aircraft takes bank angle into account. $\endgroup$
    – Bianfable
    Sep 17 '20 at 10:56
  • $\begingroup$ @Bianfable: I have seen the referenced QRH, but honestly I need some advice how to read it: For which aircraft are the tables valid? E. g. on p. 82 there is a tableand a very similar one on p. 84. (There are plenty more.) For which aircraft are these tables valid? Maybe you can tell me the trick on reading the QRH. (I'm not a pilot, but a programmer:-( ) $\endgroup$
    – opt12
    Sep 18 '20 at 12:42
  • $\begingroup$ That is probably for the different A320 variants (A318/319/320/321). Details will depend on the exact QRH you have (they are usually somewhat different for each operator). Does the page maybe list the variant or engine somehwere? $\endgroup$
    – Bianfable
    Sep 18 '20 at 13:13
2
$\begingroup$

The Green Dot speed is very stable on the PFD, unlike for example VLS which is a function of G (bank included), mach number and speed brake position.

According to Flight Crew Ops Manual(s) green dot speed varies only as a function of aircraft weight and altitude. For A320 a very crude formula is given: below 20000ft 2 x weight in tons + 85kts, above 20000ft add 1kt/1000ft.

Based on experience I would say FCOM is accurate on this matter. It wouldn't be the first time the whole truth is not revealed to the pilots, though :)

$\endgroup$
3
  • 3
    $\begingroup$ The PFD is very stable on PFD What did you mean to say there? $\endgroup$
    – TomMcW
    Sep 17 '20 at 14:31
  • $\begingroup$ I meant that it does not change as the aircraft is maneuvered, ie it fixed on short term. (it takes a while for weight and/or altitude to change) $\endgroup$
    – busdriver
    Sep 17 '20 at 16:02
  • $\begingroup$ @busdriver The first sentence has "PDF" for both nouns. That is what we don't understand. Did you mean for the first noun to be something other than "PDF?" $\endgroup$ Sep 18 '20 at 1:58
1
$\begingroup$

In theory, GD is 1.3 times the stall speed. The formula is: $$ V_\text{GD} = \sqrt{\frac{2 m g}{C_L r S}} \times 1.3 $$

Where: $m$ is mass, $r$ is density, $g$ is gravity, $C_L$ is max coefficient of lift and $S$ surface of wing

In level flight $g = 1$, but when banking $g$ should increase, therefore the GD should (in theory) go higher in the speed tape. At least that is what happens in the E-Jet. In this aircraft GD = max climb angle and best L/D. Also GD in the E-Jet accounts for ice accreation (mass increment in the formula). I hope this helps.

$\endgroup$
2
  • $\begingroup$ I just watched a video of Embraer as they obviously are somewhat more transparent on their calculations than airbus is, but there they explicitly state, that bank angle is not taken into account for the Green Dotcalculation. youtube.com/watch?v=EphvGccIwgA ~ Minute 0:50. So I wonder whether your described behavior is "what it should be" or "what it actually is". $\endgroup$
    – opt12
    Sep 18 '20 at 14:12
  • $\begingroup$ @opt12, yes you are right. That's in one of the many videos we saw in the company long ago. It's really not updated. With the new software loads (I think we are now in software load 27. something) the calculation has changed a little. For example, the GD now accounts for ice accreation and, if I'm not mistaken also g load. I will check it out next time I'm in the sim :) $\endgroup$
    – O'Terror
    Sep 18 '20 at 21:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.