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aldox62

Much higher speed commanded by FMC on flying a SID than the expected one

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On flying a SID departing from Turin airport (LIMF - ITALY) RW36 I see that the commanded speed is far above the ones I see associated to SID wpts.

 

The commanded speed is 320kts while it should not exceed 220kts.

 

As you can see in the attached screenshots, while the legs page shows at the topmost wpts in the page a speed of 220kts, the vnav and progress pages show an econ spd equal to 320kts, which is the one the T7 tends to and reaches as it can be seen on the speed tape

 

The point is, I never entered such a speed while pre-flight configuring the T7 FMC and this speed does not appear to my knowledge in the SID definition.

 

So from where does this (I assume FMC-computed) speed come from ? I already searched the relevant part of the documentation and forum posts to no result. I also tried replacing by hand speed/altitude constraints with manual ones trying lower speeds but nothing changed, the commanded speed is always 320kts and there the airplane tends to.

 

Can someone help me understanding if I'm wrong somewhere - may well be - and why ?

 

Thanks a lot in advance for any contribution !

 

Regards

 

Aldo B 

 

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I see you have a cruise level of 6000ft in there?

That is not a normal cruise FL for the 777.

Not saying that it cant cruise at 6000ft......but it is unusual.

 

What happens if you set the cruise FL to FL330 or so (before take off).

This avoids the FMC going into cruise mode once it levels off according the SID in 6000ft.

If you watch the FMC closely, its display title should change from a CLB title to a cruise title automatically as the airplane levels off at cruise FL. For example ACT ECON CLB. At which point the aircraft will fly cruise speed (ECON, LRC, a fixed mach number or whatever you entered as cruise speed. If you entered nothing then ECON)

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Well, the cruise level is strange because this is a test flight I saved by just connecting a sid and a star from/to my home airport, a 17 minutes ride to take confidence with the T7.

 

However I tried your suggestion and it worked perfectly: setting FL100 as CRZ ALT the precomputed ECON SPD is never commanded as the sid/star legs' speeds prevail.

 

Thanks a lot for your kind insight !

 

Regards,

 

Aldo

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