Reference waypoint for "unable next alt" message?

[Hardy Heinlin]

Hello,

on the CLB page, I've assumed the "next altitude" referring to the UNABLE message is not necessarily the constraint displayed in 1R but the first constraint that has an AT type or an AT OR ABOVE type. That may sit a few legs after the active constraint if that is an AT OR BELOW. -- It's nice to be warned as soon as the unability is known, and not only when it becomes active -- which may happen just a mile or two in advance if it's a short leg.

The FMC predicts the entire flight profile, so it's able to provide a warning well in advance. On the other hand, it may happen that the reference of the warning isn't shown on the CLB page if it isn't referring to the constraint in 1R. And that might confuse the crew?

Any opinions?


Regards,

|-|ardy

[United744]

I think the alert refers to literally the next (first) hard altitude constraint in the flight plan, and not that necessarily displayed on the VNAV page.

Let's say the first altitude constraint is AT OR BELOW. It says 2000 ft, but because we're heavy and it is a hot day, we will only make 1000 ft, then no alert will be generated.

If the same altitude constraint is AT, or AT OR ABOVE, but we will end up too low (predicted) at that point, then the alert will be generated.

What I'm unsure of is if the MCP selected altitude is below the first hard altitude constraint, whether any alerts are generated, or "RESET MCP ALT" is displayed.

O/T: If you enter a takeoff runway, and an approach, let's say the first altitude of the approach is 2500 ft, but you put in a cruise altitude of 3000 ft, despite it being many miles away and easily achieved, the FMC will still alert "UNABLE NEXT ALT".

Is this correct?

It seems that it should compute an unconstrained climb, and based on distance (even direct distance) it should not generate this alert as there is sufficient distance to reach the altitude.

[Hardy Heinlin]

"RESET MCP ALT" is for the descent preparation only.

It doesn't matter whether you are unable to climb above the MCP ALT when in ACT CLB. In climb, the MCP ALT is an AT OR BELOW. The MCP ALT has no lat/lon point, obviously (unlike a waypoint "AT"). So the MCP ALT cannot be a hard constraint, nor an ABOVE constraint as you can't climb above it.

Even if the FMC were to trigger a "RESET MCP ALT" message because you're unable to climb above the MCP ALT, the message wouldn't make sense: If you wind it down you would level off even lower, and if you wind it up your climb rate wouldn't improve either :-)

As for the approach constraint: The FMC first determines the CLB and CRZ segments by using the entered CRZ ALT and other performance data. Every constraint that sits in the CRZ segment or thereafter will be considered a DES constraint. So if your CRZ ALT is low enough there will be no CLB/DES constraint conflict.


|-|ardy