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On the A320 PROG page and in our limitations the REC MAX/ MAX FL is listed as FL398. What is the reason behind this ?

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Green Dot Speed (GD) is the aircrafts best lift to drag ratio speed. As altitude increases, the maximum thrust available decreases.

Looking at the graph beneath, if we increase our altitude and get closer to REC MAX then points 3 and GD move closer together because of the smaller thrust margin of the aircraft.

Having these points closer together means we could easily slip into a quick deceleration if speed is reduced. The REC MAX is calculated based on the service ceiling, aerodynamic ceiling and max certified ceiling and is considered as the safe upper limit.

Here is a nice article which covers this information in more detail:

https://www.airbus.com/content/dam/corporate-topics/publications/safety-first/Airbus_Safety_first_magazine_21.pdf

enter image description here

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  • $\begingroup$ I've never heard of the phrase Green Dot. I guess it's an Airbus thing? In RJ land we just referred to it as Min Drag. $\endgroup$ – John K Aug 8 at 15:12
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    $\begingroup$ Yes it's a specific term used for the airbus. As the name would suggest it's just a simple green dot which appears on the PFD. $\endgroup$ – Ben Aug 8 at 17:03
  • $\begingroup$ Is Single Engine Driftdown Speed on the AB based on GD or GD+ some margin? $\endgroup$ – John K Aug 8 at 17:34
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    $\begingroup$ If an engine fails on the airbus then the GD speed is recalculated as the drift down speed. It's handy how airbus simplify these things. $\endgroup$ – Ben Aug 8 at 18:07
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I just came across the answer in the AFM it states

"THIS IS THE MAXIMUM ALTITUDE AT WHICH IT IS POSSIBLE TO MAINTAIN CABIN PRESSURE ALTITUDE BELOW 8000 FT"

AFM LIMITATIONS

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