I don't disagree with you that crosswind has an effect on VMCG. All I'm stating is there is no correction in the AFM for crosswind on VMCG. It is simply a function of OAT, PA and thrust in the performance chart. As such, there is no adjustment for VMCG when AeroData computes our takeoff performance. Why there isn't a correction, I don't know.

 

My understanding is first, not computing the crosswind effect means sveryone’s take-off numbers look better, both in the marketing brochures and in the computations. Second, the FAA was convinced by the manufacturers that the chances any engine failure near a minimum speed scheduled take-off (V1 or Vref near Vmcg) AND having a crosswind strong enough to be a factor AND on the upwind was pretty low. Which then brings up the question why do we spend so much sim time doing V1 cuts when statistically they are low probability hazard?

GF

 

So the C-5 never does a reduced thrust takeoff?

 

No, we used reduced most of the time, but just like the FAA rules, Vmcg and Vmca were based on full rated power, which is why full thrust is available on a reduced thrust take-off.


GF

 

Oh for the old days when things were simple. That is, push those throttles right to the stops, no matter the conditions.

 

Because even though the risk is low, the probability of kill if the maneuver is performed poorly is extremely high.

On the 777 yesterday, we had driving rain and crosswinds for takeoff. The software computed a 20% reduction in thrust (TO2) but did not allow for a reduction by the assumed temperature method. My mind started thinking about this thread since we had a significant split between V1 and VR with a stop margin of less than 500 feet. I am glad FDX calculates stop margins.

That is better than previous company. There the only data points we were given for takeoff was power setting, V1, V2, VR and maximum crosswind. Smoke and mirrors to me.

 

When we use derated (10% or 20%) power as determined by our performance software by itself or in combination with an assumed temperature method (reduced thrust takeoff), Vmcg and Vmca are computed using maximum derated thrust. So, in those situations (such as the one Purpletoolbox described above), we don’t have assurance of full control authority should we select full rated thrust. The flight manual cautions that a thrust increase during a derated takeoff following an engine failure could result in loss of control. In fact, Boeing recommends not advancing the throttles during an engine out when using the combined derate and assumed temp because the derated max limit the Vmcg and Vmca is based on is not displayed.

 

Sneaky sim IPs would give a V1 cut, wait until you are all trimmed up but have not yet retarded the throttle, then suddenly restore the engine. Nobody expected that, because it’s a checkride and an engine-out approach is always next, right? Flailing & wailing could ensue when the “bad” engine came roaring back at full power.

 

No disagreement here. By “full rated” it means at the selected rating, not the maximum without the derate.


GF

 

The stop margin of <500' is likely an example of Derated thrust (TO2) being the only viable solution for takeoff as the TO or TO1 VMCG V1 would result in an Accel-Stop beyond the runway available. Assumed Temperature Method is not allowed for contaminated runways, but is available if the runway is wet.

In my view, on dry or wet runways, TO should be used with an assumed temperature. There is diminishing returns on engine wear below approx 20% thrust reduction. This would allow for full thrust (firewall) if needed throughout the takeoff. Also, assumed temperature is inherently conservative. Due to the differences in TAS, the accel-stop and accel-go distances will be less than what is calculated. Further, the most fuel efficient takeoff profile is maximum thrust, full climb thrust to altitude. At some given thrust setting there is an ideal intersection of fuel economy and engine wear.

TO1 and TO2 should be reserved for contaminated runways for the reasons addressed previously.