I know the final app mode will not engage if the acft is more than 1.5NM lateral (From final track) or 150 ft high.But if I arm the APP NAV before the final altitude, that is not gonna prevent me to be at the right altitude before the FAF to have all the conditions for engagement of final app mode.If you have a look at TEL-AVIV RNAV(GNSS) APCH RWY 30 (you can google it and you'll have it from ISRAEL AIP) you will see that IF and FAF tracks have 30 degrees difference and the altitudes are also different. If you manage to reach 1500 feet after the IF, maybe your lateral deviation from the final track would be more than 1.5 NM so if you push the APPR pb right there you won't have the conditions for FINAL APP engagement and the arrow will still be white, instead of blue