Bonjour,

 

Does anybody know why it is impossible to engage LNAV on ground if the first leg's course is more than 5°away from the runway heading ? What is the technical reason hidden behind this.

 

 

Thanks

Best regards

Claude

 

 

Because the path prediction may or may not lie within a safe corridor as defined by local departure procedures.  The pilot is responsible for obstruction clearance, not the autopilot.  The only assured safe departure route extends from the runway centerline at a default or defined rate of climb, unless otherwise charted, to the minimum safe altitude.  Along the same lines, the autopilot will not accept roll control until above 400 feet for similar reasons; i.e., if a departure turn is determined to clear obstacles the TERPS criteria require 400 AGL prior to starting turn.

Hi Dan,

 

Thank you very much for your answer.

When  the first leg's course is more than 5°away from the runway heading LNAV cannot be engaged on ground, but as soon as the plane reach 400 feet, you are allowed to engage it.... then the autopilot with CMD, that's why I do not understand.

Why can't we do it on ground as the autopilot will not accept roll control until above 400 feet, as you said.

 

I am going to rack my brain...

Regards

Claude

Don't confuse autopilot with FD roll mode. LNAV can be active at 50 feet if armed on the ground, but you can't turn on the autopilot until a min of 400'. HDG SEL works below 400' as well, but will only command 8 deg bank.

 

 

Claude, if you can do the takeoff safely with the limits of the system, your doing great. Some of these "why" questions require a visit to Seattle.

 

If a departure is giving you a hard time, post some pics and we can offer suggestions. Sometimes forcing an LNAV course can fix your problem. Unless you're doing some serious RNP, I wouldn't worry.

You assume that it's always going to be safe above 400 AGL but not true, there are obstacle departures that require higher rates of climb and altitudes.  It's up to the pilot to fly the plane safely past these hazards before engaging AP. Just because you can engage at 400 AGL doesn't mean it's always the right thing to do.

Thank you for the answers.

 

If lnav is engaged on ground, it becomes active at 50 feet, and if not engaged on ground, we have to do it after 400 feet AGL and then it becomes active...  In both case above 400 feet  when everything is safe we can engage AP.

 

That's my understanding, and I if the first leg's course is more than 5°away from the runway heading we can't engage LNAV  on ground to avoid it being active at 50 feet.

Thanks

Claude