The limitation does only apply if you are using HDG SEL during an Engine Failure or during the Cruise close to the Opt/Max Altitude. LNAV is AoB protected tough and will use whatever it needs even if you have it in 15 degrees AoB
It is a protection that couples with the F/D as part of the AFDS.
Hope you find it useful
Last edited by olmedo; 27th November 2015 at 15:01.