UNABLE NEXT ALT? (and a question on fuel consumption)

[Guest kelvind80]

Hi,While setting up the FMC for a flight from London Heathrow (EGLL) to San Francisco (KSFO), I worked out the optimum altitude and put this into the Cruise Alt section of the FMC, only to receive the message "UNABLE NEXT ALT". Just wondered why I received this?Also, how accurate is the fuel consumption on the PMDG 747? I was watching the ITVV 747-400 on the same flight (same route and similar load) and they only had 122.2 (x 1,000) kgs of fuel, I don't get anywhere near that low when I calculate. Just wondered why?Thanks a lot,Kelvin

[FLEX1978 373]

Hi Kelvin,"UNABLE NEXT ALT" have a look at the VNAV page you should see next to LSKR1 the amount of feet you will be below your altitude restriction (sorry this is from memory I'm at work) It basically means your not going to meet your first altitude restriction on the SID Either Select a higher climb thrust level or clear the restriction.As far as fuel use, there is no point trying to compare figures to a video, it will be roughly the same but Crz alt, Tempreture winds ZFW engine type etc etc all make a difference.CheersRob

[Vauban 0]

Hi Kelvin,if you're new to the aircraft, the most common cause of UNABLE NEXT ALTITUDE is simply that the altitude setting on the MCP is not high enough. This setting puts a limit on how far you can ascend or descend using the autopilot.CheersGeoff

[FLEX1978 373]

Geoff, your thinking 'CHECK ALT TARGET'Rob

[Vauban 0]

OK Rob, thanks for that. I'm flying a different Boeing aircraft and assumed the FMC message would be the same.CheersGeoff

[downscc 1,327]

I often see the UNABLE NEXT ALT during a departure that has AT OR ABOVE constraints, but instead of clearing the constraint from the LEGS page as suggested I let VNAV take me to flaps up and the initial climb speed then switch to FL CH. In this mode, she will use pitch control to give you max climb at a given thrust and speed and has always climbed above the constraint in my experience. Once past the constraint, it's back to VNAV for the remainder of the climb.You might also get UNABLE NEXT ALT if the cruise altitude you have selected for a long flight is above the maximum attainable until you've stepped up. The cruise altitude needs to be set to your initial cruise altitude before the first step, not the final cruise altitude. The step feature will guide you up when the time comes.

[FLEX1978 373]

The VNAV page will tell you exactly how many feet it expects the aircraft to undershoot the restriction which is quite handy as quick reference. normaly an increase in your take off perfomance will cure the problem. The problem I find is that unless you 'dont' select VNAV until right before Take off you will get keep getting the message on the FMC causing you to have to delete it every time you want to make a entry.Rob

[downscc 1,327]

Yeah, she is persistent. There would be times I'd rather hand fly the takeoff until safe for the FL CH mode (don't use it before establish on climb, bad things happen).