777 EFB Speeds

[wilco77]

Could someone explain to me the relationship between the EFB speeds on 777 vs the FMC.

Is the FMC speeds based on balanced field length?

Thanks

[Wizofoz]

AFAIK no 777 FMC actually calculates take-off speeds- they need to be externally calculated and entered.

[Three Wire]

Not quite correct Wiz. Some airlines calculate and uplink them as part of the Route/Performance/Winds uplink. I believe that those airlines that do, use Balanced Field Length calculations.

Airlines that tailor their performance to improved climb (V2) tend to use EFBs.

As always refer to your company docs for their correct answer.

[STBYRUD]

I think wilco77 means the FMC Ref-speeds (which are a customer option I believe), not uplinked speeds. The FMC reference speeds are derived from runway elevation, weight, temperature, entered wind, aso just like the balanced takeoff V-Speeds in the performance inflight section of the FCOM. However, the EFB OPT will calculate improved climb speeds if possible, then you might see a large difference to the FMC calculated speeds.

[Wizofoz]

TW,

Well, yes, I was aware of that- speeds calculated by dispatch and entered by uplink- as I described.

STBY,

I know some FMCs have that capability- I didn't know if any 777 operator used it. I'd be interested to know if any do and, if wilcos outfit is one, why they ALSO get figures from the OPT.

[Skyjob]

Word of caution on ANY model Boeing using FMC speeds:

Obstacles are NOT taken into account as they do not exist in the FMC!
You ONLY have balanced field length figures available to you.

[sleeve of wizard]

The FMS calculates and displays recommended VSPEEDS based on gross weight, runway condition, derates, altitude, temperature, and other performance factors. REF is displayed in the header line for each speed when the value displayed is not pilot-entered or prompt-selected, as shown in Figure 4-38. VSPEEDS are displayed in small font until pilot-entered or prompt-selected.
FMS-recommended VSPEED computations are inhibited from display when the Takeoff Speed Inhibit OPC option is enabled. V1 computations are inhibited from display when the V1 Inhibit OPC option is enabled. Manual VSPEED entries are still permitted with either OPC option enabled. Valid entry range is between 80 and 300 inclusive.
Pilot-entered or uplinked takeoff speeds are limited to no less than the FMS-computed minimum V1, VR, and V2 speeds. When a pilot-entered or uplinked takeoff speed is less than the corresponding minimum value, the respective MIN V1, MIN VR, or MIN V2 header is displayed along with the associated minimum value. :ok:

[777AV8R]

To go back to this questions: EFB speeds on 777 vs the FMC.



The FMC speeds provide only raw balanced field speeds. For any airport having obstacles or one requires 'improved climb' amongst other things, these speeds are incorrect.

If you are referring to OPT calculations that are coming off the installed EFB, depending on the operator's requirements, these 'should' be the actual usable V speeds that are to be used for the operation.

OPT speeds come as a result of the usual environmental, weight input but also flight operations engineering input of obstacles, slopes, contaminated runways and improved climb requirements. This list is not exhaustive.

If a crew uses the OPT speeds, it is these V speeds that are 'overwritten' on the takeoff performance page in place of the FMC generated speeds. On the 787, the OPT speeds can be pushed to the FMC and accepted after crosschecks have been made on the data.

Hope it helps.

[wilco77]

Thanks guys for the reply. It does clear the air some what. But as I have been reading in other sites as well as what u guys have posted it is more clear to me now that the FMC speeds is based on balanced field length. As obstacles,runway slope, and condition are no taken into account.

[Yonosoy Marinero]

As obstacles,runway slope, and condition are no taken into account.

Obstacles aren't, but the runway slope, wind and temperature can be input manually into the FMC T/O pages for it to recalculate Vspeeds.
I'm not sure what it does with ambient pressure. It might take it from the Air Data boxes...