What does LNAV engagement exactly mean?
Some old aircraft (737 Classic comes to mind) don't allow arming LNAV/NAV AFDS mode on the ground, as you normally have to be at least 400ft above ground (ICAO PANS OPS says 396ft) before making turns more than 15 degrees, at which point you would engage LNAV. I guess the regulation takes old aircraft into account.

On the newer aircraft, you can arm LNAV/NAV mode on the ground already and it normally becomes active at 50ft.