Relation between VLS and compressibility effect (A320)

[pineteam]

Hello everyone,

Something is bugging me for a while now and I hope to find an explanation with you guys.

FCOM: "Above 20 000 feet VLS is corrected for mach effet to maintain a buffet of 0.2g"

Please correct me if I'm wrong, I'm really trying hard to undestand:

From what I undestand, VLS increases above FL 200 due to compressibilty effect on the wing and on the pitot tube; For a constant IAS, the stall speed will increase as mach number increases the more we climb with decreasing temperature. So makes sense that VLS increases. But since compressibility also affects pitot tube and the higher and faster we fly, the IAS will overrread. And now I'm confused cause the IAS displayed on the PFD is not corrected for compressibilty, so assuming we are flying at FL330 at 270 IAS. Does it mean the aircraft is actually flying at like 262kt EAS and the IAS is overreading by 8kt if you remove the compressibility effect? We just accept that small pitot reading error? Does the FMS knows exactly by how many knots the compressibility affects The IAS?

Please bring some lights with basic explanations, I'm no scientist.:}

Thank you.

[KayPam]

Hello,

For your question related to speed computations, all effects are taken into account in order to yield the most accurate TAS possible.
The computer in question will rather be the ADC (the name can vary depending on the aircraft model)

At higher mach numbers, your max coefficient of lift will decrease. As well as your stall angle, by the way (alpha prot and alpha max decrease dramatically).
Hence, your VLS will increase accordingly, in EAS.
The CAS increasing above the EAS will moreover increase the VLS value read in CAS.

Yes, since your PFD indicates a CAS (the PFD indicates a IAS by definition of the word indicated, but the speed that is shown is actually correct so it's also the best estimate of the theoretical CAS), it will be a different value than the EAS.

[vilas]

pineteam
Airbus Safety First magazine issue No. 18, 21 and 24 has articles on Control your speed at Take off, in Cruise, Descent, approach and landing. The answer to your question may not be there but all you need to know and more about speed you will find in these three issues of Safety First.
http://www.ukfsc.co.uk/wp-content/uploads/2016/10/Airbus-Safety-First-Mag-July-2014.pdf

http://www.ukfsc.co.uk/wp-content/uploads/2016/10/Airbus-Safety-First-Mag-January-2016.pdf

https://flightsafety.org/wp-content/uploads/2017/07/Safety-first-magazine-issue-24.pdf

[pineteam]

Hello guys,

Thank you for your valuable time to answer my questions.
It’s more clear in my head now. I need to brush up my ATP notes on those things.

Vilas, thank you for the links, I love these magasines. I will check those one more in details.