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While I was reading the FCTM I came across this line mentioned below.

An increase in the Outside Air Temperature (OAT), that results in a decrease of the REC MAX FL.

I wanted to know if we enter a higher than normal temperature for a particular FL or we even forget to put the minus sign in front of the temperature value (our older A320s with Honeywell FMS accepts the temperature without the minus sign) how does this cause the FMGC to compute a lower REC MAX FL?

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  • $\begingroup$ Without having access to the documentation right now, I believe entering a higher temperature would cause the FMS to calculate the density altitude to be higher for each geometric altitude (it assumes a standard ISA temperature lapse). As the FMS thus assumes „the day to be hotter“, essentially, it would reduce its assumptions regarding aircraft performance capability and would therefore reduce its altitude predictions and recommendations. Just my speculation, hence not added as an answer. $\endgroup$
    – Cpt Reynolds
    Jan 28, 2018 at 19:39
  • $\begingroup$ @CptReynolds you are correct the FMGC assumes an ISA model until we modify the temperatures and tropopause level. I think you might be right as it may be linked is there any way to confirm your answer if right or wrong? $\endgroup$
    – Jai
    Jan 31, 2018 at 5:58

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FMS calculates REC MAX in cruise by calculating temperature from actual OAT and TROPO inserted in INIT A page. It assumes standard -2C/1000ft until inserted tropopause altitude which after it uses constant temperature.

During climb it calculates temperature profile from inserted tropo/temp. If your REC MAX seems to be off during climb check the inserted values. Also, when A/C reaches CRZ ALT entered to PROG page it starts to use measured OAT at once, so REC MAX can change suddenly if it was off in INIT A page.

Note that REC MAX is only function of A/C weight, TROPO and OAT. Cost Index does not affect REC MAX.

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  • $\begingroup$ Could you please expand some of those acronyms? I feel just as lost as when my son in the Army starts rattling stuff off... $\endgroup$
    – FreeMan
    May 29, 2019 at 19:02
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Yes it is the consequence of the predictive calculated air density, but once in flight the actual temperature is updated and the recommended max FL too

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  • $\begingroup$ are you confirming CptReynolds answer? $\endgroup$
    – Jai
    May 29, 2019 at 9:17
  • $\begingroup$ Yes, but more precisely bus driver answer $\endgroup$
    – user40476
    May 30, 2019 at 4:49

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