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jjmuzz

error messag 'unable 250kt at fix' (start of a star)

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Yep - its because the restriction limit is set to 240 below or at 10000. You can change that your self on the descent page 250/ and execute.

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I was struggling with that yesterday as well and found two solutions to my problem. One of them is documented in the FCOMv2, page 1128: modify the flight plan.

 

Second one I found was that the error came up there because I started my descent in a wrong way. I was using AP and VS and not the descent (DES on the FMC). Also, while I'm not 100% sure about this, my speed brake wasn't armed, so I suppose AP couldn't use it (anyone - is my understanding of arming the spoiler correct?). Combined with these two, speed restriction at my descent waypoint couldn't be met.

 

Also, keep in mind that meeting 250 in the climb, depending on the weather conditions (too hot, headwind, etc) may also present a challenge to the FMC.

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Alternatively, go to that fix on the LEGS page, and either change the speed to 240, or alternatively 250B.

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my speed brake wasn't armed, so I suppose AP couldn't use it (anyone - is my understanding of arming the spoiler correct?)

 

Hey Icemark,

 

you should arm the speedbrakes just before takeoff (so the speedbrakes deploy when you rejet the takeoff) and landing (it´s also part of the "before landing" checklist), so they can deploy automatically after touchdown. There is no connection to the error message mentioned by the OP.

 

Best regards,

 

Arne Bergner

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you should arm the speedbrakes just before takeoff (so the speedbrakes deploy when you rejet the takeoff)

 

Unless you're looking for a date with the loud and slightly overbearing takeoff configuration warning horn, you should not arm the speedbrakes before takeoff.

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Unless you're looking for a date with the loud and slightly overbearing takeoff configuration alarm, you should not arm the speedbrakes before takeoff.

OOOps,

 

it´s late here... Mixed the planes...

 

 

Cheers,

 

Arne Bergner

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I was struggling with that yesterday as well and found two solutions to my problem. One of them is documented in the FCOMv2, page 1128: modify the flight plan.

 

Second one I found was that the error came up there because I started my descent in a wrong way. I was using AP and VS and not the descent (DES on the FMC). Also, while I'm not 100% sure about this, my speed brake wasn't armed, so I suppose AP couldn't use it (anyone - is my understanding of arming the spoiler correct?). Combined with these two, speed restriction at my descent waypoint couldn't be met.

 

Also, keep in mind that meeting 250 in the climb, depending on the weather conditions (too hot, headwind, etc) may also present a challenge to the FMC.

FMC will give you that error regardless of mode, so VNAV or VS, it makes no difference.

 

Michael Moe has it right. If you monkey with the LEGS, you might delete a restriction that should be there.

 

The A/P doesn't use the speedbrakes, so arming them doesn't help until touchdown.

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This is covered in multiple places in the Intro manual and both tutorials - it isn't really an "error" per se, it's a notice that you're asking VNAV to do something that it can't do.

 

The airplane will not violate the global 240/10000 restriction on the DES page - you need to either change the global restriction to 250/ or change the waypoint restriction itself to something that is 240 or below. The airplane will not violate what's called the "monotonic path" concept - it won't slow down to the 240 at 10000, then speed back up to 250 at a lower waypoint. Restrictions in descent must always be either be the same or lower than the previous one. The FMC will not speed up or climb after it has started a VNAV PTH descent.

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OOOps,
 
it´s late here... Mixed the planes...

 

Happens to the best of us :)

 

 


just before takeoff (so the speedbrakes deploy when you rejet the takeoff)

 

FWIW, if you use reverse, they will open anyway (IIRC there is a mechanical link that will pull the speedbreak handle to ARM position when reverse is activated, then WoW sensors will send a command to open)

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