Very light weight 777F Vref speed bump

[seagull967]

OK, reposting this as a separate question as I think it is getting missed as part of another thread:

wondering about that 777 approach speed bump. Vmca is approximately 136-137 KIAS and so, so depending on flap settings you'll hit the floor at weights something below 460,000 pounds . But something else happens at the lighter weights that I cannot find. I show Vref go from 136 at 340,000 pounds to 143 at 339,000 pounds and below. I have yet to find a satisfactory explanation for this.

777F. Here are flap 30 values. Notice the below 340k line. Below 340k the speed is the same for all flaps settings. It does decrease with altitude but why the increase below 340K?

Weight
(1000 lb)
Pressure Altitude (ft)
0 2000 4000 6000 8000 10000
780 184 184 184 185 185 185
740. 178 178 178 179 179 179
700 172 172 172 172 172 173
660 164 164 164 164 164 164
620 158 158 158 158 158 159
580 151 151 151 151 151 152
540 145 145 145 145 145 146
500 140 140 140 140 140 140
460 137 134 134 134 134 134
420 137 134 130 128 128 128
380 137 134 130 126 121 121
340 137 134 130 126 121 116
Below 340 143 140 137 134 132 128

[awair]

VREF Limitation (777-200LR/Freighter)

At lighter weights, FMC approach VREF speeds are limited to a minimum value to ensure controllability in the event of an engine failure. These minimum speeds vary with engine maximum thrust rating and destination airport elevation.

Does this help?

[awair]

And also this…

Landing

Do the normal landing procedure with the following considerations:
• The airplane has a tendency to float
• Reference speeds for Flaps 20, 25, and 30 will be the same
• Pitch attitude is lower than normal. Flaps 25 is recommended to increase the pitch attitude

[seagull967]

Thank you, but no. That is why the speeds are constant for all flap settings (at a given density altitude) below 460,000 pounds (Vmca). It does NOT explain why the speeds INCREASE below 340,000 pounds!

[mbcxharm]

But it is basically for a similar reason, although the technicalities behind the 340,000 lb (154,221 kg) weight I’m not sure about. I have been trying to look for the old bulletin which explained it all in greater detail than before it was incorporated into the FCOM/FCTM. My only justification for saying this now is this note from the FCTM which is appended to the chart for touchdown pitch attitude vs weight, which reads:-

“Note: When airplane gross weight decreases below 154,221 kilograms, the VREF and flap maneuver speeds increase by up to 11 knots. The increased speeds at lighter weights will protect directional control of the airplane and result in a lower pitch attitude.”

My search for the old bulletin continues…

[seagull967]

But it is basically for a similar reason, although the technicalities behind the 340,000 lb (154,221 kg) weight I’m not sure about. I have been trying to look for the old bulletin which explained it all in greater detail than before it was incorporated into the FCOM/FCTM. My only justification for saying this now is this note from the FCTM which is appended to the chart for touchdown pitch attitude vs weight, which reads:-

“Note: When airplane gross weight decreases below 154,221 kilograms, the VREF and flap maneuver speeds increase by up to 11 knots. The increased speeds at lighter weights will protect directional control of the airplane and result in a lower pitch attitude.”

My search for the old bulletin continues…

Thank you! Curious as to why that would improve directional control, hoping someone can find that old bulletin!

[john_tullamarine]

Curious as to why that would improve directional control,

I know very little about the triple 7.

However, based on some fun training tricks I used to use on the 737 sim endorsement details, perhaps in playtime at the end of your next sim, you might have a look at four failure scenarios ...

(a) aft CG, high weight, failure at min speed, during the rotation

(b) aft CG, mid weight, failure at min speed, during the rotation

(c) aft CG, min weight, failure at min speed plus 10kt or so, during the rotation

(d) aft CG, min weight, failure at min speed, during the rotation

and then come back and relate your observations .....

[alf5071h]

seagull, jt; for directional interest. https://arc.aiaa.org/doi/pdf/10.2514/1.C035222

And the more we rely on (training) simulation, the less realistic the results.

[BraceBrace]

Thank you! Curious as to why that would improve directional control, hoping someone can find that old bulletin!

I still have it in an old FCOM, it doesn't really explain anything. It states it is done for controllability reasons as there is excess power, and flap maneuvering speeds.

My only guess is a testing costs money issue. They might have discovered a reduced maneuvering capability at some flap setting anywere during the go-around with an engine failure (either Vref with landing flaps or any speed during the acceleration process with the selected flap at that point) below that specific weight. It then boils down to testing and that costs money. To save money they came up with one set of speeds below the weight, that works for all landing flap settings and incorporated these speeds in the manuals and FMC.

[john_tullamarine]

And the more we rely on (training) simulation, the less realistic the results.

Indeed, good sir, but the average pilot probably is best advised to steer clear of areas where there be ferocious dragons. I started using the exercise when I was doing some contract training work for an airline which routinely used overspeed schedules but had one regular ferry flight sequence where they routinely just used the min weight speed schedule without much (any ?) thought about what hazards they might be exposing their crews to by so doing ..... in the absence of any preparatory sim exposure training.

The sim sequence (presuming the sim is half realistic) is an eye-opener for most folks who have never been stretched a tad .... Both a useful stick and rudder exercise (regardless of fidelity) and generally gives folks the thought that Vmcg is a test point which one would be well advised to keep away from in the real world .... I found the sequence a very useful I/F exercise for command upgrade pilots. With a bit of practice, most could get to the point where they could accommodate a 0/0 takeoff raw data with the failure and track out on the LLZ. Generally made for a swelled head once they got on top of the pushing and pulling required

Looks to be a useful paper, thanks for citing it ... now, I shall make a coffee and have a detailed read ....

[seagull967]

I still have it in an old FCOM, it doesn't really explain anything. It states it is done for controllability reasons as there is excess power, and flap maneuvering speeds.

My only guess is a testing costs money issue. They might have discovered a reduced maneuvering capability at some flap setting anywere during the go-around with an engine failure (either Vref with landing flaps or any speed during the acceleration process with the selected flap at that point) below that specific weight. It then boils down to testing and that costs money. To save money they came up with one set of speeds below the weight, that works for all landing flap settings and incorporated these speeds in the manuals and FMC.

This is my suspicion, actually. Lots of great replies, thank you!

[seagull967]

seagull, jt; for directional interest. https://arc.aiaa.org/doi/pdf/10.2514/1.C035222

And the more we rely on (training) simulation, the less realistic the results.

Great paper, thanks and I agree!