climb and maintain VNAV considerations

climb and maintain VNAV considerations

Postby Keith Smith » Fri Sep 21, 2018 9:39 am

Posted on our FB, mirroring here:

Tip of the Day: Climb and Maintain VNAV considerations.

It seems so simple, doesn't it? Climb and maintain ALWAYS means exactly that...go from your present altitude to the assigned altitude without any other restrictions.

Consider this sequence, "cross MORON at 9000, climb and maintain 13,000," followed later by "climb and maintain 15,000." (Extra points for hilarious fix name. Hey, it's real, it's near Palm Springs....MORON)

Do we still need to cross MORON at 9k on the way up to 15? We do NOT. If ATC still wanted the restriction, they'd have to restate it.

Similarly. if you're climbing via a SID with altitude restrictions and ATC says, "climb and maintain FL230," then all of the altitude restrictions go away. This is why, when flying out of LAS on the BOACH SID which tops out at FL190, you might still hear the controller say, "climb and maintain FL190." They're not doing it to remind you of the SID top altitude....they're doing it intentionally to remove the restrictions.

What does this mean from a VNAV standpoint for pilots who are using FMCs? It means that if you're given a climb and maintain instruction when climbing via a SID, you should be cautious about using VNAV unless you've removed all of the altitude constraints from the FMC first. A better move might be to abandon VNAV and use another climb mode that bypasses the FMC instructions, then swap to VNAV later on once you're in cruise. Alternatively, if your FMC has an "altitude intervention" mode which temporarily ignore altitude restrictions, that would be a great time to use it.

Safe flying.....MORON. Again, not you, just a fix near Palm Springs.
Keith Smith
 
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