So, many teach, incorrectly, that V2 is your best angle of climb speed with one engine inoperative. This is incorrect. The truth is that V2 is the MINIMUM speed at which the aircraft will achieve the certification-required climb gradient with one engine inoperative.
In reality, a speed slightly higher than V2 will provide a slightly better climb gradient with one engine inoperative. "Slightly" depends on a number of factors...I do not have the expertise to precisely delineate those factors....suffice it to say, it depends on wing design, etc.
The aircraft, when certified, must meet certain performance criteria with one engine inoperative. Ultimately, the aircraft, in order to meet these requirements, must adjust takeoff weight. Thus, for a given runway length, existing obstacles, etc., the aircarft has a maximum weight (temperature, pressure, etc.) to achieve these requirements.
You are to be at this minimum speed (V2) by 35 feet, gear retracted, after losing an engine at V1 and continuing the takeoff. You need "X" amount of runway to achieve this....given weight, temperature, pressure. (15 feet, instead of 35 feet, if using a 'wet' runway.)
But, let's say that you have a really long runway, but you have obstacles in the departure path. In this hypothetical example, runway limits are not a problem (accelerate stop, accelerate go).....since you have much more runway than needed....but, need a really good second-segment climb gradient to miss the obstacles. Or, to put it another way, your take off is second-segment-climb limited. (In this example....)
So, why not allow the aircraft accelerate to a slightly faster speed for V1, for Vr, and for V2? Why not! You have plenty of runway in this example. No need to worry about stopping in the case of a rejected takeoff, since you have much more runway than needed. Use a higher V2 to get a slightly better second-segment climb gradient, taking advantage of the excess runway you have.
So, this is what you do....you increase the speeds....so that, after takeoff, your 'new' V2 speed gives you a slightly better climb gradient.
So, typically, as a general rule, you use this 'improved climb' when you are second-segment climb limited, but have excess runway for your takeoff run.
If I may add a couple of observations to PantLoad's commentary...
(a) if one looks at the climb performance, say, gradient against speed, then you end up with something approximating an upturned teacup section. That is, at the low speed end (assuming you are not truncating the graph due min speed considerations) you will have a relatively low gradient. Then, as you increase speed, the gradient increases. Further increases will get you to a plateau region of the graph for which significant speed variation doesn't result in much change but, somewhere in the scatter, will be the maximum gradient. Beyond this, increases in speed see a reduction in gradient.
The routine certification V2 is somewhere down at the lower speed (and lower climb gradient) end of the graph for a bunch of reasons.
If you have sufficient runway etc., then you have the option of increasing the speed (within a reasonable range, perhaps 20-30 kt) to pick up some extra gradient capability at the expense of a quite significant runway distance penalty (keep in mind that distance needed considers speed squared as a first approximation).
There is no useful purpose served in going further up the speed scale as the distance penalties become rather horrendous and you rapidly start seeing the gradient benefit fall away .. ie a simple case of cost/benefit.
As PantLoad suggests, if the runway/obstacle geometry suits (and it doesn't always conveniently do so) a takeoff scheduled with a higher V2 (overspeed takeoff or improved performance takeoff) you might be able to get a better TOW as limited by obstacles.
(b) a combination of lower flap and overspeed schedules may be needed to obtain the high end of the MTOW capability for an aircraft .. ie you have to have very long runways to use every last kilo capability.
...a combination of lower flap and overspeed schedules may be needed to obtain the high end of the MTOW capability for an aircraft .. ie you have to have very long runways to use every last kilo capability.
Yup, sure do. An example is with an L1011 at JNB...flaps 4 required to obtain the max weight possible, and every bit of the 14,495 feet of runway is used, with an improved climb speed schedule. Tires? Better have good ones, as chucking off bits at high speeds ain't good.
John, Pantload, It also has the added benefit of giving you higher takeoff weights for any given temp. This has an economic benefit of allowing a higher Flex Temp for takeoff. It also can allow a better takeoff weight when you have a takeoff penalty - e.g. Anti-skid unit or similar u/s.
Forgive my dumbness but can someone explain in simple terms how extra speed gives you a better gradient of climb? I would have thought the problem was that extra speed would get you closer to the theoretical critical obstacle faster?
I cannot believe you, Cent, of all, are asking this? There has to be a trap here. However, since you are, may I ask what you think the gradient of climb is at Vs?
I'm sure you recall that best angle is (approx) Vmd? So, as long as Vmd is more than V2(Imp), angle of climb will increase with speed. Once you exceed Vmd, you start to lose. My dusty brain cell tells me that best R o C is 1.32 Vmd? All to do with the curvy drag/speed graph and power/thrust maths. I'm sure a boffin will be along soon to cover the thread with calculus.
My dusty brain cell tells me that best R o C is 1.32 Vmd? All to do with the curvy drag/speed graph and power/thrust maths.
Some months ago the question was extensively discussed on another thread. It took me a little while to figure it out, and I finally posted this graph: Ratio of best RoC speed to Vmd . It assumes a parabolic drag polar, and thrust constant with speed. As you can see, the ratio increases with thrust/weight ratio, and is equal to one (1) for the T/W that provides level flight at Vmd. I don't think I should cover the thread with the underlying calculus.
Cent, generally, improved climb is used to increase the takeoff weight, once you add this additional weight you are back at the same gradient, so from your point of view there isn't an improvement in the gradient, just an increase in the weight
once you add this additional weight you are back at the same gradient, so from your point of view there isn't an improvement in the gradient, just an increase in the weight
oh really ? unless you forget the extra runway to achieve the higher airspeed. So even if you have the same gradient it will start later, so Sent is right that
that extra speed would get you closer to the theoretical critical obstacle
You would be right if you accept "balancing" the extra runway length
(lower TO flap ) for extra speed (improved climb with lower flap), that gives you HIGHER GRADIENT, not the SAME, to be able to fly above Sent obstacle at the same hight.
It should work (and usually does) if you paste the link into your browser (works with Firefox). If it doesn't, don't bother, it's just an example anyway. The URL is: https:// immediately followed by: docs.google.com/leaf?id=0B0CqniOzW0rjYWJiNGY5MWYtZTZlNi00YjU0LTg4ZDUtY2M1Zjl iZTg4Mjhk&hl=en_GB&authkey=CPSbjdoG regards, HN39
Last edited by HazelNuts39; 27th Feb 2011 at 10:56.
Reason: another try to fool the PPRUNE editor
Excuse my butting in, but this one might help. It isn't exactly what you have been looking for, but it shows all the essential points. Values are fairly typical of an aircraft in the B737/A320 class. The relationship between best climb speed and Vmd depends a lot on what you assume for the variation of thrust with speed. The graphs show a typical variation, but if you assume instead that thrust is invariant with airspeed then you will indeed get the best climb speed above Vmd whereas this example has best RoC at virtually the same speed as Vmd. In this example V2min would be about 150 kts and Vs 124 kts. The increase in climb gradient potential as you go above V2min is clear, as is the fact that best RoC speed is well above the speed for best gradient. So far as the balance between runway length and obstacle clearance is concerned, of course you are nearer the obstacle at lift-off if you go for a higher V2, but surely a couple of hundred feet on TO distance is neither here nor there compared to the extra clearance that a better climb gradient will give over an obstacle 10 or more miles away?
(a) minimum V2 invariably will give a gradient less than maximum and is driven by the OEM's desire to present the aircraft in a favourable runway distance required light.
(b) if you increase V2 (a bit) above minimum V2 -
(i) the gradient will increase - initially comparatively nicely and, then, as the speed is increased further, less and, eventually, gradient will decrease. It follows that we are interested in the first section only. Looking at CliveL's graphs, we might start at a WAT limit of, say, V2min=140kt, or thereabouts, for 2.4% - increase to an overspeed V2=160kt and you improve the gradient (for the same weight) to around 3.5%.
(ii) the distance to get to the increased V2 increases - significantly - approximately (V2 overspeed/V2 min)^2.
(c) a little bit of extra speed may give you a better takeoff scenario, if the runway distance is there and the obstacles aren't too close to the runway. Clearly, though, it is a case of (rapidly) diminishing returns as the V2 is increased.
(d) end result is the usual compromise. IF you have some spare runway, AND the obstacles suit, you might be able to get a better weight off the runway using an overspeed takeoff schedule. Generally no good for close in obstacles but great for far out obstacles. This reflects the fact that the net flight path is lower until the TOD penalty catches up to the original minimum V2 gradient .. and thereafter it's plain sailing.
(e) whether you use overspeed schedules to get a higher takeoff weight or improved actual gradient is up to you (or the beancounters)
Location: In some hotel downroute or in some hotel doing union negotiations.
The boeing software we use for our 737s has as standard setting improved climb speeds activated. Given enough runway (MUC 4000m for example) it will try to use all runway and increase thrust reduction for any given weight. Usual speed increase above the non-improved climb speed is around 35 to 40kts additional to a quite noticeable reduction in take-off thrust. Many of our crew do not like to use improved climb for many reasons, one of them of course the prospect of rotating within the last 500m at 175kts instead of halfway down the runway at 133 or so, another one is the much longer (around 60s instead of 30s) take off roll which makes life for our ATCO-friends difficult especially at busy airports. And it feels kinda weird to take off with only 75% N1 which will lead to a thrust increase at climb thrust reduction even for climb 2.
J_T, the focus here appears to be on using Improved Climb to clear obstacles, but the same applies at hot and high airports without obstacles, the gradient remains constant at the certificated level, the TOR increases, the speed increases and the weight increases.
Denti, are you using Assumed Temperature Thrust reductions and IMPROVED CLIMB, or OPTIMIZED?
Location: In some hotel downroute or in some hotel doing union negotiations.
It is called optimum, but includes automatic selection of best flap setting, thrust setting, derates, assumed temperature, improved climb and alternate forward CG optimization. Everything is of course user selectable and as i said above many pilots deselect improved climb speeds. The program always uses tries to achieve the lowest possible take off thrust within given parameters.
Denti - 75% N1? Lowest values I can recall seeing is approx. 85 N1. KORD, rwy 28, 0C, 13000', 667' elevation, 120K 737, N1 is 82.6%.
To the original question - several excellent answers. Basically increase your speed with two engines running prior to a higher V1, and use that increased speed to allow better climb performance over terrain on the departure path.
Used when you have long runways(to allow increased V1 adjustment) with - 1. terrain clearance issues after departure