From unknown reason sometimes FMC does not want to calculate V speeds.

Whole FMC is set: route, runways, SID/STAR, flaps, trim..., but FMC still give message "V speeds unavailable"

What is the reason ?, why sometimes FMC can sometimes cannot calculate V speeds ? 

Irs fully aligned?

From unknown reason sometimes FMC does not want to calculate V speeds.

Whole FMC is set: route, runways, SID/STAR, flaps, trim..., but FMC still give message "V speeds unavailable"

What is the reason ?, why sometimes FMC can sometimes cannot calculate V speeds ? 

 

 The msg is due to Vmca/Vmcg

 

Simple answer - Select a derate or increase the aircraft's weight by adding fuel

 

 

 

V–SPEEDS UNAVAILABLE – For certain high thrust/low gross weight takeoff conditions, FMC V–speeds are not calculated. Adjust gross weight and/or takeoff thrust limit to enable V–speed calculations. 

 

The FMC will protect for minimum control speeds by increasing V1, VR and V2 as required. 

 

However, the FMC will not compute takeoff speeds for weights where the required speed increase exceeds the maximum certified speed increase. 

 

This typically occurs at full rated thrust and light weights. 

 

In this case, the message "V SPEEDS UNAVAILABLE" will appear on the FMC scratchpad and the takeoff speed entries will be blank. 

 

Takeoff is not permitted in this condition as certified limits have been exceeded. 

 

The options are to select a smaller flap setting, select derate thrust and/or add weight (fuel). 

 

Selecting derate thrust is the preferred method as this will reduce the minimum control speeds. 

 

Note that the assumed temperature method may not help this condition as the minimum control speeds are determined at the actual temperature and therefore are not reduced. 

 

Incase of 777-200 LR: The FMC will not compute takeoff speeds for weights below those shown in the Minimum Weight for FMC Takeoff Speeds Calculation table. 

 

In those cases where the required speed increase exceeds the maximum speed increase built into the FMC, the message "V SPEEDS UNAVAILABLE" will appear on the FMC scratchpad and the takeoff speed entries will be blank. This typically occurs at high rated thrust, high flaps setting and light weights. 

 

To obtain speeds for the takeoff in these situations, the options are to use available performance software, or select a smaller flaps setting, or use reduced takeoff thrust and/or add weight.

Do you have your take off gross weight entered?   Also if you have max thrust selected and a low gross weight takeoff in some conditions the V speeds wont be calculated/displayed.

Rob - thanks for explanation.

My 777 was almost fully loaded with fuel and pax, plane was really heavy, and I chose D-TO 1.

 

 

My 777 was almost fully loaded with fuel and pax, plane was really heavy, and I chose D-TO 1.

 

If I am not mistaken, doesn't the FMC not calculate V speeds if the runway entered is too short for a safe takeoff with the given gross weight?  I believe this is influenced by flap setting and selected derate.  I recall resolving this problem by selecting a higher flap setting.

 

Mike

We really need TOPCAT for the 777

Agree