I am leaning for an avionics exam and I encountered a question asking for the meaning of F, S and O . They appear to be "Characteristic speeds", which are calculated by the flight management system (FMS):

FMS Slide

  • 4
    $\begingroup$ What's the context? Could you quote the entire question, for example? $\endgroup$
    – user
    Commented Oct 4, 2017 at 11:57
  • $\begingroup$ I've never heard of the term in 20+ years in avionics. Closest thing I could relate it to would be the DME Free Scan Mode. Could they be referring to this as Free Scan Operation? $\endgroup$
    – Gerry
    Commented Oct 4, 2017 at 11:58
  • $\begingroup$ I found a slide in which $F, S, O$ appear. Maybe that help. It is right of $V1$ $VR$ and $V2$. $\endgroup$
    – MrYouMath
    Commented Oct 4, 2017 at 12:35
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    $\begingroup$ When asking this type of question you need to specify the context, ie the aircraft or manufacturer. In this case, you're talking about an Airbus. On the speed tape (on the PFD) there are various marking which refer to critical speeds that the pilot needs to know about - marked by the letters F, S and O. Airbus experts will chip in with the exact definitions of these speeds. $\endgroup$
    – RAC
    Commented Oct 4, 2017 at 14:27

1 Answer 1


F speed is the minimum speed at which flaps should be retracted from CONF 3 or 2 to CONF 1+F.

S speed is the minimum slats retraction speed, i.e. the minimum speed at which a clean configuration should be selected.

GD speed is the engine-out operating speed in clean configuration. In other words, it corresponds to the speed that allows the highest climb gradient with one engine inoperative in clean configuration.

source: https://www.icao.int/WACAF/Documents/Meetings/2015/ICAO-BEA/SafetyFirstissue20.pdf


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