CLB / CRZ Speed Constraint

[alpha12125]

I just noticed something and was wondering if this is intended behaviour of the plane.

I was doing a normal flight from Baltimore to Austin planned FL 360.  Everything went normal nothing unusal. During the climb i made an error however and set the cruising alt in mcp at 35400. This went unnoticed to me.

So i am cruising along and then i noticed that i am flying a little bit slow (.72) i look on the display and CRZ is displayed. So i go into the CRZ page and see that the crz speed should be .75, i change the value the fmc accepts it but nothing happens. Now i am really wondering and try to figure out what is wrong, i change the CI and the plane accelerates albeit not to the set speed. So now i am really looking what is wrong, i then click on the CLB page and see the speed constraint active. But when i look at the display above the engines it say CRZ.  So i quickly realize that my ALT is not correctly, i change it the plane climbs to 36k and everything works perfect.

Now i know this was my error, but i found it confusing that the plane is adhering to the CLB page, allthough it displays the CRZ mode.  Is this intentional ?

[Litjan]

The N1 limit indicator does not show the "phase" at all, it only shows the N1 limit. So whenever the plane is at the altitude you commanded through the MCP (in your case 35400) and in level flight, the limit will go to CRZ - this means that the maximum power that the autothrust can apply is cruise power. Again, this does not mean that you are in your cruise phase.

Another example. You take off, and your initial level-off altitude is 3000 feet (so you set 3000 on the MCP), even though your intended cruise flightlevel is 350 (and that is set in the FMS). You will see the N1 limit go to CRZ again when the plane levels off at 3.000, even though you are not at your "cruising altitude".

Cheers, Jan

 

Edited August 7, 2017 by Litjan

[alpha12125]

ahh thank you, so i guess my confusion was caused by a wrong assumption

but just to understand what you wrote in your example. if you level off at 3000, would the plane accelarte to cruising speed  or use the constraint set in CLB page ?

I guess that was my question in my case because i was on the assumption that when you level off the plane accelerates to cruising speed (.75)  and not use the 280/.72 CLB constraint it had set. 

Edited August 7, 2017 by alpha12125

[alpha12125]

After some more searching i think, this is intended in the 733.  I would assume the ALT INTV in the NGX does exactly that and in the 733 i would have had to reset the alt in the FMC.  
If you think about it more it kinda makes sense because if the plane needs to climb further it would need to pitch up significantly to reduce the speed to the CLB constraint and then accelerate again. in that regard it does make sense. 

[Litjan]

If you leveled off at 3000 due to the MCP altitude window commanding so while in a VNAV climb, the N1 limit indicator would change to CRZ and the autothrottle would use up to maximum cruise N1 to accelerate to VNAV climb speed. You would, however, most likely be already at this speed (since you where in the climb before) and therefore the autothrottle would not need to apply "full power" to accelerate.

Cheers, Jan

 

[alpha12125]

thanks alot for the explanation