B737 Take off V speeds versus weight
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Apologies for the delay in answering....... Needed to pop up to the local pub for a Guinness.....
Hudson,
With the introduction of our new fleet aircraft a couple of years ago, we had two schools of thought in relation to the V-speeds at actual temperature. Some wanted to use the convoluted method that I described above. They justified that with the argument that if you satisfy all of the takeoff requirements at 70T, the surely if you use the same takeoff thrust at the lower weight of 45T with the higher V-speeds, you will reach the V1 point sooner with a greater distance to stop and with less mass to stop.
Is this safe?? We couldn’t find any regulation that said that it wasn’t.
Is it prudent?? We operate in an environment of excessively long runways with severe Temperature inversions (+15°C) during the summers. Some felt happier taking the extra speed into the air. Using the higher V-speeds introduced the Boeing philosophy of Improved Climb (Airbus Over-speed.). The extra runway length was traded for greater climb performance.
Your situation is totally different; I seem to remember that you or someone else mentioned using a takeoff flap setting of 25 for takeoff from those atolls. If this was the case then keeping the aircraft on the ground for that extra 15 kts totally defeated the purpose of selecting the higher flap setting!
Some may argue that you should never have a higher V1 than absolutely necessary, but considering that we are using Balanced field V1s, Optimized V-speeds, Improved Climb or even just rounding V1 up to Vmcg. You may find that its an everyday part of our lives.
Mutt.
Hudson,
With the introduction of our new fleet aircraft a couple of years ago, we had two schools of thought in relation to the V-speeds at actual temperature. Some wanted to use the convoluted method that I described above. They justified that with the argument that if you satisfy all of the takeoff requirements at 70T, the surely if you use the same takeoff thrust at the lower weight of 45T with the higher V-speeds, you will reach the V1 point sooner with a greater distance to stop and with less mass to stop.
Is this safe?? We couldn’t find any regulation that said that it wasn’t.
Is it prudent?? We operate in an environment of excessively long runways with severe Temperature inversions (+15°C) during the summers. Some felt happier taking the extra speed into the air. Using the higher V-speeds introduced the Boeing philosophy of Improved Climb (Airbus Over-speed.). The extra runway length was traded for greater climb performance.
Your situation is totally different; I seem to remember that you or someone else mentioned using a takeoff flap setting of 25 for takeoff from those atolls. If this was the case then keeping the aircraft on the ground for that extra 15 kts totally defeated the purpose of selecting the higher flap setting!
Some may argue that you should never have a higher V1 than absolutely necessary, but considering that we are using Balanced field V1s, Optimized V-speeds, Improved Climb or even just rounding V1 up to Vmcg. You may find that its an everyday part of our lives.
Mutt.
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Sorry to blow the dust off this after so long, but it's an interesting thread.
Some speeds (V1, Vref etc) are related to runway length in its various guises.
Others (V2, VMU, etc) are related to aircraft weight.
To mix and match assumed temperature V1s and V2s is mathematically invalid, which would be fine if it had been proved by flight test, but it hasn't.
So, we should all be using actual speeds for actual weights.
Following an engine failure, do you want to use a 'wrong' V2, and thus invalidate your performance calculations? (V2 has ONLY to do with AUW).
Some speeds (V1, Vref etc) are related to runway length in its various guises.
Others (V2, VMU, etc) are related to aircraft weight.
To mix and match assumed temperature V1s and V2s is mathematically invalid, which would be fine if it had been proved by flight test, but it hasn't.
So, we should all be using actual speeds for actual weights.
Following an engine failure, do you want to use a 'wrong' V2, and thus invalidate your performance calculations? (V2 has ONLY to do with AUW).
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Some speeds (V1, Vref etc) are related to runway length in its various guises.
Not sure if i agree with this......... look at the simple Boeing V-speeds, I think that you will find that V1,Vref are weight dependent REGARDLESS of the runway length.
Mutt.
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Yep - interesting thread!
Mutt - my understanding is that the Boeing V1s in the (simplified) QRH are for the old 'balanced field', and that there should ALWAYS be a correction available if the field/obstacles are worse than this?
As you say, Vref is ALWAYS weight/flap dependent.
...and Hudson - I think your CP was nuts!
Mutt - my understanding is that the Boeing V1s in the (simplified) QRH are for the old 'balanced field', and that there should ALWAYS be a correction available if the field/obstacles are worse than this?
As you say, Vref is ALWAYS weight/flap dependent.
...and Hudson - I think your CP was nuts!
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BOAC,
You are right that the V1 in the QRH is based on balanced field, however what that means is that for a certain weight you need a specific amount of runway to satisfy the balanced field requirement.
For discussion sake lets say that a B737 needs a balanced field of 5000 ft at maximum takeoff weight. If you are operating from a 10,000 ft runway, do you adjust the V1 for the additional runway? (Forget about improved climb and derate). I would guess that the answer is no, you use the same V1.
The same goes for obstacles; neither the QRH v-speed chart nor the FMS provide you with the means to adjust speeds for obstacle clearance. You are required to reduce the takeoff weight and then get the associated V1.
So i believe that V1 is based on weight and not runway length.
Mutt.
You are right that the V1 in the QRH is based on balanced field, however what that means is that for a certain weight you need a specific amount of runway to satisfy the balanced field requirement.
For discussion sake lets say that a B737 needs a balanced field of 5000 ft at maximum takeoff weight. If you are operating from a 10,000 ft runway, do you adjust the V1 for the additional runway? (Forget about improved climb and derate). I would guess that the answer is no, you use the same V1.
The same goes for obstacles; neither the QRH v-speed chart nor the FMS provide you with the means to adjust speeds for obstacle clearance. You are required to reduce the takeoff weight and then get the associated V1.
So i believe that V1 is based on weight and not runway length.
Mutt.
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Boeing Laptop Tool
My company operates B737-700 with a Boeing Laptop Tool to derive performance calculations. This is a self dispatch tool that covers takeoff and landing performance as well as weight and balance.
With regards to V1, there are five selectable options:
-Optimum
-Balanced
-Min
-Max
-Balanced Plus
Our company uses Optimum because it allows the highest takeoff weight and also matches the FMC derived V1 (assuming no other factors such as MEL performance degradations).
The BLT also allows the option of Improved Climb as previously discussed in this thread. As a point of comparison a takeoff on a longish runway, using Improved Climb, gave an increase in weight of 9T and a V1 25 kts higher.
Effect of Obstacles
I plugged in a limiting obstacle that reduced the MTOW quite markedly (by 15T) and used a wet runway to force a larger V1/Vr split.
When I cleared the obstacle, the V1 at minimum thrust remained constant. Minimum thrust dropped by about 4% N1.
The obstacle reduced the MTOW but had no effect on V1 for the equivalent weight at minimum thrust. Interestingly enough, for a fixed thrust and weight, clearing the obstacle forced the V1 DOWN by 4 kts.
Effect of Thrust
A decrease in takeoff N1 using either derate method produces an increase in V1 and Vr. V2 remains the same regardless of takeoff thrust.
With regards to V1, there are five selectable options:
-Optimum
-Balanced
-Min
-Max
-Balanced Plus
Our company uses Optimum because it allows the highest takeoff weight and also matches the FMC derived V1 (assuming no other factors such as MEL performance degradations).
The BLT also allows the option of Improved Climb as previously discussed in this thread. As a point of comparison a takeoff on a longish runway, using Improved Climb, gave an increase in weight of 9T and a V1 25 kts higher.
Effect of Obstacles
I plugged in a limiting obstacle that reduced the MTOW quite markedly (by 15T) and used a wet runway to force a larger V1/Vr split.
When I cleared the obstacle, the V1 at minimum thrust remained constant. Minimum thrust dropped by about 4% N1.
The obstacle reduced the MTOW but had no effect on V1 for the equivalent weight at minimum thrust. Interestingly enough, for a fixed thrust and weight, clearing the obstacle forced the V1 DOWN by 4 kts.
Effect of Thrust
A decrease in takeoff N1 using either derate method produces an increase in V1 and Vr. V2 remains the same regardless of takeoff thrust.
