Rate vs Angle of climb
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Rate vs Angle of climb
We were told to maintain a certain altitude by a certain waypoint. We were at managed speed (I'm on the A320) and we weren't able to do it. So I selected the speed and reduced it progressively by 5 knots each time to increase the rate of climb. My Captain suggested that we should go 275/.76M to achieve the best ROC. My idea was to reduce the speed progressively towards green dot to keep the rate of climb going so we get closer to the best angle of climb which gives us the best altitude within the least distance.
Which is correct way to go about this? Can't believe I'm asking such a basic question but yes I'm trying to get my head around it!
Thanks guys.
Which is correct way to go about this? Can't believe I'm asking such a basic question but yes I'm trying to get my head around it!
Thanks guys.
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Rate and Angle
Best RoC occurs at the speed where greatest excess thrust (thrust available/thrust required graph plots) exists. You would be correct in selecting a slower airspeed to result in a higher ANGLE of climb, however the range of usable airspeeds (between where excess thrust=0 at the low and high velocity values) at altitude is relatively small. Vy speed is about in the middle of the range, and remains relatively constant, 275 transitioning to .76 as your Captain mentioned.
Slowing beyond Vy will only increase climb rate temporarily, but stabilizes to both a slower airspeed and climb rate. Given the very small ratio of climb rate reduction to reduced groundspeed, I would guess the resulting angle increase is negligible (at altitude).
Slowing beyond Vy will only increase climb rate temporarily, but stabilizes to both a slower airspeed and climb rate. Given the very small ratio of climb rate reduction to reduced groundspeed, I would guess the resulting angle increase is negligible (at altitude).
Last edited by Armchair Astronaut; 15th Jun 2015 at 06:17.
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You didn't mention distance to waypoint and your altitude delta, which would affect best climb speed selection if it's a tight transition. Generally, the problem with using Vx beyond obstacle clearance is reduced forward visibility.
Have you had a look at the Getting to Grips with Performance booklet? (GTG with Cost Index might also be helpful)
Have you had a look at the Getting to Grips with Performance booklet? (GTG with Cost Index might also be helpful)
I find there is a lot of confusion about this and some pilots do not appreciate the difference between best rate of climb and best angle in a jet. What you were asked for was to make a constraint by a specific point. Therefore, you needed to achieve a good angle of climb. This means you need to climb at Vx where you have the maximum excess thrust and the minimum speed over the ground. If you had been asked to purely expedite the climb, the suggestion is that you would need to achieve the best rate of climb and this would be at Vy - the speed for which the difference between engine power and the power required to overcome the aircraft's drag is the greatest - which is maximum excess power.
If you have problems understanding the theory, imagine you were flying a F15. If you need to achieve the best angle, you would pitch to 90 degrees and apply full power. As the F15 has a greater than 1 to 1 power ratio, you would climb with no distance used and this would be at your best angle. But if ATC asked you for best rate, that wouldn't be achieved with a vertical climb which would slow you right down, you would opt for a 45 degree climb which would allow you to maintain airspeed and therefore a faster rate of climb.
I find much of this misunderstanding comes from people thinking along prop aircraft lines where the difference is much smaller.
In the A320, best rate can be achieved at 275/.76, but your best angle is going to be at green dot. In fact, your best angle will actually be at V Alpha Max, where the GPWS escape manoeuvre will have you back pressure on the side stick is maintained - but for normal ops, of course, we don't go there.
You were correct.
If you have problems understanding the theory, imagine you were flying a F15. If you need to achieve the best angle, you would pitch to 90 degrees and apply full power. As the F15 has a greater than 1 to 1 power ratio, you would climb with no distance used and this would be at your best angle. But if ATC asked you for best rate, that wouldn't be achieved with a vertical climb which would slow you right down, you would opt for a 45 degree climb which would allow you to maintain airspeed and therefore a faster rate of climb.
I find much of this misunderstanding comes from people thinking along prop aircraft lines where the difference is much smaller.
In the A320, best rate can be achieved at 275/.76, but your best angle is going to be at green dot. In fact, your best angle will actually be at V Alpha Max, where the GPWS escape manoeuvre will have you back pressure on the side stick is maintained - but for normal ops, of course, we don't go there.
You were correct.
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Dan there is formula for calculating best ROC IAS for the 320/321 B300 and involves GW. You wouldn't happen to know what that is perchance? I use one I garnered from experience (including AoA from the AIDS Param callup) but of course that could result in minor errors.
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ATC will specify what they want:
be level by intersection zzzzz, that means it is a climb gradient issue.
by level in x minutes, that is a rate issue, although in the end it is actually a gradient requirement, because the time constraint is based on a distance with a crossing traffic.
climb to xxx, 1,500 fpm or greater. Don't think too much, just comply with that.
Now, as for the way of achieving best rate and best angle:
best angle is green dot, but beware! green dot is an IAS and it increases with mach number, so don't select green dot if you are climbing to a high level. you can calculate the green dot at the target level, it is the actual green dot plus one knot per each level above FL 200. There is also a rule of thumb for calculating green dot based on weight.
Don't be brisk, select the speed little by little as you said, 5 by 5 kt is nice. Also, you can use mach instead of IAS. In this manner IAS will be reduced progressively.
Also, if you can meet the constraint without going slower than best rate speed, then don't go slower than that, as it would be less economical to do so. ECON speed at CI zero, or turbulence speed are considered close to best rate speed.
With this considerations you will avoid zooming up like a rocket and then ending up climbing at green dot speed and 100 fpm with two or three levels to go yet.
be level by intersection zzzzz, that means it is a climb gradient issue.
by level in x minutes, that is a rate issue, although in the end it is actually a gradient requirement, because the time constraint is based on a distance with a crossing traffic.
climb to xxx, 1,500 fpm or greater. Don't think too much, just comply with that.
Now, as for the way of achieving best rate and best angle:
best angle is green dot, but beware! green dot is an IAS and it increases with mach number, so don't select green dot if you are climbing to a high level. you can calculate the green dot at the target level, it is the actual green dot plus one knot per each level above FL 200. There is also a rule of thumb for calculating green dot based on weight.
Don't be brisk, select the speed little by little as you said, 5 by 5 kt is nice. Also, you can use mach instead of IAS. In this manner IAS will be reduced progressively.
Also, if you can meet the constraint without going slower than best rate speed, then don't go slower than that, as it would be less economical to do so. ECON speed at CI zero, or turbulence speed are considered close to best rate speed.
With this considerations you will avoid zooming up like a rocket and then ending up climbing at green dot speed and 100 fpm with two or three levels to go yet.
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best angle is green dot, but beware! green dot is an IAS and it increases with mach number, so don't select green dot if you are climbing to a high level. you can calculate the green dot at the target level, it is the actual green dot plus one knot per each level above FL 200. There is also a rule of thumb for calculating green dot based on weight.
Still miss being able to choose either max angle or max rate, the 737 offered that as standard...
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the EXPEDITE button is a horrible thing!
It will very sharply pitch up the airplane to achieve green dot, often overshooting it. Ugly! And like I said, that green dot can be good at the current level, but it is below high level green dot. Also, there might be no need at all of such a radical speed reduction.
If you want to go to green dot, do it with selected speed, nicely and progressively. EXPEDITE does no magic, it is a very silly option, and on top of that, the button is in the same place where the 340s have the ALT HOLD button, so imagine what happens in airlines with both types.
It will very sharply pitch up the airplane to achieve green dot, often overshooting it. Ugly! And like I said, that green dot can be good at the current level, but it is below high level green dot. Also, there might be no need at all of such a radical speed reduction.
If you want to go to green dot, do it with selected speed, nicely and progressively. EXPEDITE does no magic, it is a very silly option, and on top of that, the button is in the same place where the 340s have the ALT HOLD button, so imagine what happens in airlines with both types.
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We didn't have red or green dots in my day, in fact we didn't have too many computers. But we did know that if you had a crossing restriction based on time, we used best rate of climb speed, and if the crossing restriction was based on distance, use best angle...
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I've had the privilege to see the expedite function does it's thing twice, and yes it overshot green dot on both occasions and without our intervention, surely it would have gone right to VLS, not pretty at high altitudes.
Speaking of which, I've seen very few captains whom I fly with use the technique of reducing the speed progressively to the best ROC or AOC speed whichever the case may be. Most just select the speed they speed they want right away, leaving the aircraft increase the rate of climb and run out of energy shortly thereafter. I have no hard figures or proof for this, but surely reducing the speed progressively is more efficient?
Speaking of which, I've seen very few captains whom I fly with use the technique of reducing the speed progressively to the best ROC or AOC speed whichever the case may be. Most just select the speed they speed they want right away, leaving the aircraft increase the rate of climb and run out of energy shortly thereafter. I have no hard figures or proof for this, but surely reducing the speed progressively is more efficient?
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I have seen many embarrassing situations after an ATC request to reach a level by zzz point.
On one occasion, we were climbing to 370, our optimum altitude, and we got a "expedite through 360" ATC instruction.
the pilot reduced speed too much, and after the initial increase in rate due to the energy trade, the airplane got stuck at green dot and a very low rate of climb, and green dot increased as we climbed so we had to actually accelerate to maintain it, and rate became less and less, till it was zero… while crossing 360.
And it is not easy to get out of that situation! The airplane will not accelerate easily, specially 320s which are very limited in thrust. It was so embarrassing, we should have requested a descent to 340, but after a loooooong time, feet by feet, we managed to leave 360. I'm talking about minutes, here.
On one occasion, we were climbing to 370, our optimum altitude, and we got a "expedite through 360" ATC instruction.
the pilot reduced speed too much, and after the initial increase in rate due to the energy trade, the airplane got stuck at green dot and a very low rate of climb, and green dot increased as we climbed so we had to actually accelerate to maintain it, and rate became less and less, till it was zero… while crossing 360.
And it is not easy to get out of that situation! The airplane will not accelerate easily, specially 320s which are very limited in thrust. It was so embarrassing, we should have requested a descent to 340, but after a loooooong time, feet by feet, we managed to leave 360. I'm talking about minutes, here.
