If you take off and are climbing out and suddenly get UNABLE NEXT ALT on the FMC, is this merely an informational alert or does the FMC expect you to take action to correct the situation? There may be no time to change anything if it's close to the airport and you have several departure waypoints in fairly quick succession. Is this message supposed to turn off or de-activate VNAV? Would you just call up ATC and tell them you can't make the specified ALT? What happens next? Appreciate guidance, thanks.Jonathan

Informational, usually goes away once you are up to speed and getting altitude below you. I've not flown SID that even a heavy 747 couldn't make the constraints. VNAV is not deactivate by this message. If you were to have a problem getting above one of these, yes, you would advise ATC, they might do nothing or vector you for traffic.

I am no pro but I have found that if you are at say 10000 ft crossing xyz wpt and the next wpt in the fmc is fl270 but only say 10nm away, that means climbing 17000 feet in 10nm the fmc is going to say "no way, that won't happen" especially right after take off when you are full of fuel, hense the unable next alt! Someone else may be able to explain this better.

That's true, Rick, but your example doesn't sound like a published departure, which was referred to in the original.

That's true, Rick, but your example doesn't sound like a published departure, which was referred to in the original.

And after another read of the original post, I also found my response to be less than accurate as well! I had over looked the published departure part :( This is how we all learn eh? I get that "unable next alt" on decent all the time. As well as the "drag required" message on the fmc. Thanks for the heads up!

And after another read of the original post, I also found my response to be less than accurate as well! I had over looked the published departure part :( This is how we all learn eh? I get that "unable next alt" on decent all the time. As well as the "drag required" message on the fmc. Thanks for the heads up!

That's really interesting you get UNABLE NEXT ALT on descent--I've never seen that! I have had DRAG REQUIRED from time to time which is usually not a big deal as it comes up if you are at or very close to 10,000' at more than 10 knots over the speed restriction for 10,000' and below. I throw out spoilers for s few moments and the message goes away.Dan, thanks for your reply above. Jonathan

I have seen UNABLE NEXT ALT on descent. The ZIGGY arrival at KONT is a good candidate, try slowing down. Your decent angle will steepen significantly at the same rate of descent if you go slower. The MD11 will automatically slow down to 210-220 KIAS at TOD for ZIGGY.