Unable to calculate V-Speeds? (FMC Message)

[Aamir 2,907]

Last night I was operating ACA15 to Hong Kong from Toronto. As I was prepping the FMC, I used my usual reduced thrust setting (TO-1) and proceeded to enter the CG and such so the FMC can calculate the v-speeds for me. Weird thing happened, it didn't! So after 5 minutes of fiddling, I went back and changed the thrust setting to TO, and the FMC was able to calculate the v-speed just fine. 

 

Can anyone explain the mechanics here? Is there a specific condition under which the FMC doesn't allow a reduced thrust takeoff/can't calculate the v-speed?

 

It was a CYYZ takeoff with ASN, so the visibility was about 2-4 miles, and the wind was calm. Precipitation in the form of rain was present. AC was a few (~5000?) kgs short of MTOW I believe. (Will check the PFPX doc again).

 

Thanks!

 

[77west 139]

Generally this happens if the selected derate will not provide a safe takeoff. Try a lesser derate. Or, you had another issue in the perf section.

[jetsflier 87]

Generally this happens if the selected derate will not provide a safe takeoff. Try a lesser derate. Or, you had another issue in the perf section.

 

I would agree with that statement. This may be a dumb addition or it might not but always make sure you have your departure runway selected somewhere... I've had my fair share of headaches over that error!

[77west 139]

Yup Jeremy, that's a prerequisite for calculating a safe departure!!! ha ha!

[Aamir 2,907]

 

 

This may be a dumb addition or it might not but always make sure you have your departure runway selected somewhere... I've had my fair share of headaches over that error!

 

Haha, that would seem like a fairly important addition

It was selected this time around though, I think Wes is right, seeing as changing the derate from TO-1 to TO fixed it. 

 

Thanks!

[tcas_climb 45]

You can do a search, this was discussed already.

[Aamir 2,907]

 

 

You can do a search, this was discussed already.

 

The problem was already solved, but thank you for your constructive contribution!

[tcas_climb 45]

The problem was already solved, but thank you for your constructive contribution!

You're most welcomed