When do we do Improved Climb!!

[Morse Code]

:o :o
Guys....Please explain in detail.
Thanks

[Tinstaafl]

Unlike piston engine a/c, jet a/c have an initial take off/climb out speed (V2) significantly lower than their best angle of climb speed (Vx)ie somewhere in the area behind the power/thrust curves.

By delaying the the rotation until a faster speed then the aircraft will have an initial climb speed that is closer to the lowest drag speed. This results in an increased climb performance due to the greater excess thrust available.

The penalty is that more runway length is required to reach this speed. Useful if there is an excess of runway length and an obstacle somewhere along the flight path that would not be cleared at the lower climb gradient achieved at a lower initial climb out speed.

The principle can also be used to trade off some of this 'extra' available climb performance into additional take-off weight, until minimim allowed climb performance occurs ie sufficient to meet 1 inop or obstacle clearance requirements.

[ 08 August 2001: Message edited by: Tinstaafl ]

[Morse Code]

Tinstaaf,

Thanks for the reply. I got the picture. If i remember , i have read someplace that improved climb is not done when an obstacle is present. Is it true.

MC

[wysiwyg]

Usually done when not field length limited but an obstacle exists that's significant but not too close to the airfield (4th segment?).

regards
wizzy

[sky9]

wysiwyg

Sorry to correct on a small point; improved climb is never done when field length limited as it uses the excess field length to increase the climb out speed.

In the early days on the 737 we had to calculate our own improved climb figures. This we did by subtracting the climb from the field length RTOW then go into an improved climb chart for the allowable increase in RTOW and the increase in V2 speed required.


Improvements in computerisation allowed improved climb calculations when the T/O was limited by obstacles. These days you just read it off the charts.

[ 09 August 2001: Message edited by: sky9 ]

[CaptainSandL]

Improved Climb Technique aka Increased V2 Technique

This technique uses any excess ASDA to accelerate to a higher V2 to improve the 2nd segment climb gradient, thereby increasing the climb and possibly obstacle RTOW limits.

Because of the higher runway speeds used, this technique may not be used when the runway is slippery or contaminated or if the antiskid is off. Also, you may not use an improved climb if it is possible to take-off with normal speeds, for example by using engine bleeds off.

Example 1 – Increasing the Climb Limit
Excess ASDA is used to accelerate to higher speeds to increase the climb gradient, thereby improving a climb limited RTOW.



Example 2 – Increasing the Obstacle Limit
Excess ASDA is again used to increase the climb gradient, in this case for obstacle clearance. This will not always help because the rotation point will get closer to the obstacle as Vr increases, the technique is more effective the further away the obstacle is from the runway. Also, its effective height will change if there is any runway slope.


The above should have had diagrams to show the two examples but they didn't paste.

S & L

[john_tullamarine]

A couple of observations, if I may.

(a) overspeed V2 takeoffs are entirely optional provided that the spare runway is available and the data is scheduled in the AFM. That is, the procedure is not related to whether the takeoff could have been performed using min V2 speed schedules.

(b) the procedure is not much use if you have close in obstacles. However, with a significant obstacle in the latter part of the second segment, one might obtain a better RTOW. It comes down to a geometric consideration.

[lets go nads]

One must also be wary of Vmbe limits. ;)

[mutt]

Captainsandl,

Have a look in an Airbus AFM or Performance Manual. You will find that they do improved climb on every takeoff. Admittedly, they call it Optimum V-speeds.

Mutt.

[411A]

mutt--
Are these done with slats only (flaps retracted)?

[john_tullamarine]

An overspeed takeoff can be scheduled with any flap setting for which the data is available.

For mid-flap setting, this might be of use on mid-length runways with spare distance and obstacles

The more obvious use is for (comparatively) very long runways where the WAT limit can be improved for a better RTOW at the minimum flap setting. This is similar to the standard min V2 use of lower flap settings to get better weights.

[ 10 August 2001: Message edited by: john_tullamarine ]

[CaptainSandL]

Mutt

True improved climb is only permitted when you can’t get enough weight with normal climb speeds (unless the runway is contaminated and the antiskid is u/s) because of the increased risk associated with the higher runway speeds. I have seen figures for a Vr of 175kts in a 737-300 !!! Do Airbus not consider these speeds an increased risk?

As an aside, UK CAA operators of the 737 always use “Alternate performance” when using normal speeds. This means using a slightly higher (5kts) V2 on the assumption that the autoslats will not work when you need them. This effectively means that every take-off is done as a mini improved climb. Since the 737 is usually climb limited, this has the net effect of improving almost all of our data (ie all except field limiting cases). In every other aircraft I have flown, allowing for unservicabilities worsens your performance, not the 737. Confused yet? I am.

S & L

[wysiwyg]

Sky9 - apologies, my wording was not up to scratch. I should have added 'you are' so it would have read as follows:
'Usually done when (you are) not field length limited but...'

regards
wizzy

[mutt]

CaptainSandl

I was purposely vague as we only operate one model of Airbus, the A300, I was hoping that someone would jump in and explain the real logic. Anyway this is from the A300 FCOM. (Unfortunately my message will be without diagrams.)

Explanation of Takeoff Performance Calculation.

The performance calculation is optimized. This means that considering the runway, the obstacles (if any) and the environmental conditions, the maximum takeoff weight, associated with the optimum flap setting is the resulting parameter. Conversely, a max TO temperature will result if the chart is entered with the actual takeoff weight.

Considering a given runway and wind, given temperature and pressure conditions, given flap setting, the max weight that can be lifted varies with the V2/Vs ratio.

(Mutt) Draw a graph, weight increasing on the left side. V2/Vsmin and V2/Vsmax along the bottom. Within the graph you have two lines, an increasing “2nd segment limitation” as the speed increases and a decreasing “Runway Limitation” as the speed increases. The intersection of these two lines gives you the “Optimum V2/Vs” on the bottom and “Max takeoff weight” on the left. Still with me? (end Mutt)

As a general rule, the optimization leads to balance ASD and TOD, which means having a single V1. However, on very long runways, the airplane being limited by the 2nd segment climb gradient, it is possible to have a wide range of of V1’s, V1min being limited by TOD and V1max by ASD.

So what does this mean to a crewmember? Well all that he sees is a takeoff chart that looks something like this…

************************************************************ *******************
* S15/F00 * Default * 36 *
************************************************************ *******************
:ELEVATION= 0.0(FT) CODES LIMITATIONS : A300-620/AA/PW-7R4H1 :
:T.O.R.A. = 3000.0(M ) 1-STRUCTURE 5-TYRE SPEED : FAR BB77 11-Aug-01 :
:A.S.D.A. = 3000.0(M ) 2-2ND-SEGMENT 6-BRAKE ENERGY : :
:T.O.D.A. = 3000.0(M ) 3-RUNWAY 7-RWY 2 ENGINES: :
:SLOPE = 0.00( %) 4-OBSTACLE 8-FINAL T.O. : QNH=1013.25 (MB):
:-----------------------------------------------------: :
: : AIR COND. OFF :
: : ANTI-ICING OFF :
:----------------:------------------------------------: :
:TREF = 30 DC : WIND (KT) : WITHOUT REVERSE :
:TMAX = 55 DC : MTOW (KG) CODES : :
:-------: : MEAN V1 - VR - V2 (IAS.KT) : DRY RUNWAY :
: O A T :--------:------------------------------------:-----------------------:
: DC : 0 :
:-------:-------------:-----------:
: 30.0 : 165000 1-1 :
: : 158 164 167 :
:-------:-------------:----
: 35.0 : 165000 1-1 :
: : 159 165 167 :
:-------:-------------:----
: 40.0 : 163574 3-3 :
: : 159 164 167 :
:-------:-------------:--------

This was created using the normal Optimum defaults for Airbus. If I were to use a fixed V1/VR ratio as Boeing does, we would get this:

************************************************************ *******************
* S15/F00 * Default * 36 *
************************************************************ *******************
:----------------:------------------------------------: :
:TREF = 30 DC : WIND (KT) : WITHOUT REVERSE :
:TMAX = 55 DC : MTOW (KG) CODES : :
:-------: : MEAN V1 - VR - V2 (IAS.KT) : DRY RUNWAY :
: O A T :--------:------------------------------------:-----------------------:
: DC : 0 :
:-------:-------------:-------------:
: 30.0 : 163532 3-3 :
: : 150 164 167 :
:-------:-------------:-------------:
: 35.0 : 159295 3-3 :
: : 149 162 165 :
:-------:-------------:-------------:-
: 40.0 : 155152 3-3 :
: : 147 160 163 :
:-------:-------------:-------------:

Apologies for the formatting. Look at the 35° temperature. You will see that using optimum V-speeds allows you to takeoff with 165,000 kgs, where using a fixed V1/VR ratio does not.

So from this long winded answer, you will hopefully see that Airbus allows you the option to do improved climb all the time. Boeing gives you a fixed ratio with improved climb corrections.

Now I’m going off to hide before my good friend John_Tullamarine jumps on me for using more physical runway than technically required. :)

Finally, I don’t work with the Airbus, what’s written above is my understanding and may not be completely correct. If anyone is able to correct me, I would be much obliged. This data is also based on FAR’s

Mutt :)

[john_tullamarine]

God, but Mutt is good with figures ...

[Hudson]

Company I worked with 25 years ago had new 737-200's with -17 engines. Runway lengths at some of the Pacific island atolls varied from 4800 ft to 9000 ft. The Boeing performance engineer who designed our runway analyses suggested that we use one flap setting for take off instead of several that were available. He suggested flap 10. With improved climb we could get an additional 2500 kgs which meant that we could get off at 53,000kgs bleeds off, which was max structural.

V-speeds were increased around 15 knots at the time. It also meant that we could use lower Vspeeds when light weight, as Flap 10 was OK for really short runways.

My view was that it gave more flexibility to the pilot to have a whole range of available flap settings for take off ie Flap 1 right through to Flap 25. But the chief pilot of the day decided to stick with flap 10 and improved climb for all take offs.

[CaptainSandL]

Hudson,
A 737 take-off with flap 25? surely not.

Mutt,
I am still trying so understand your A300 tables, does the 1-1 & 3-3 after the RTOW mean structural & runway limiting respectively?

[Nick Figaretto]

Might be a stupid question, but here it goes:

As I understand, Improved climb is obtained by using a higher Vr, to obtain a climbout speed which is closer to the max gradient speed, thus giving a steeper climb gradient, but a flightpath which starts further down the runway, and thereby closer to the obstacles in the flightpath.

Would it be possible to use the normal Vr, but accelerate faster to a speed above V2 after liftoff, to obtain this steeper gradient? This way you wouldn't use any extra runway, but you'd still have the advantages that improved climb would give you.

...or?

Nick.

[john_tullamarine]

Hudson,

The FLUF, as both you and I only too well know, can takeoff with any of a host of flap settings, provided that the numbers fit the particular runway and environs for the weight of the day and so on.

As to a particular company's policy, that all depends on the moods and preferences of management.

The advantage of a single flap policy is simplicity, the disadvantage a probable reduction in best RTOW for many of the runways (to a greater or lesser extent depending on the numbers).

(We continue to have a lovely time here ... weather is great and the beer and wine more than acceptable .... Barry and Bill send their regards as well. We might never come home ....)

CaptainSandL,

Hudson has a gazillion hours (or thereabouts) on the bird and has flown just about the lot of the them.

Nick,

Doesn't work that way, unfortunately. You would end up worse off and the bumpy bits would be closer than before ....

[ 14 August 2001: Message edited by: john_tullamarine ]

[mutt]

CaptainSandL

YES!

Airbus charts show the balance between two limitations as a result of the optimization. So you could end up with these...

1.2 Structural Weight and 2nd Segment
2.2 2nd segment or 1st segment limitations for minimum V2/VS ratio
2.3 2nd segment or 1st segment limitations and runway limitation.
2.4 2nd segment or 1st segment limitations and obstacle limitation.
2.5 2nd segment or 1st segment limitations and tire speed limitation.
2.6 2nd segment or 1st segment limitations and brake energy limitation.
3.3 Runway limitation for the minimum V2/VS ratio.
4.4 Obstacle Limitation.
5.5 Tyre speed limitation for the maximum V2/VS ratio
6.6 Brake energy limitation for the maximum V2/VS ratio.

But the purpose of showing you the two charts was just to let you see the difference between using optimized vspeeds and a fixed ratio.

The increased weight capability with the higher V-speeds can also be converted into a greater thrust reduction.

As for your question,Do Airbus not consider these speeds an increased risk? Would any Airbus drivers care to answer this?

Mutt :)

[CaptainSandL]

Hi Mutt

I follow you now, as a Boeing driver (and not a perf engineer) I am not used to calculating speeds from tables like that. Our company 737 data has either normal speeds or the full blown max capability improved climb, with the associated huge increase in runway speeds - even if you only need an extra ton. A sledgehammer for a walnut maybe, but very easy to use. Your Airbus tables seem like a nightmare at first glance, but at least you only have to increase your runway speeds by the amount needed to meet your actual TOW, which must be safer.


John_t

I know Hudson well and have great respect for his experience, but I am sure that flap 25 is definitely for landing only. It may have been finger trouble or maybe he knows something else I don’t know ;-)

S & L

[john_tullamarine]

CaptainSandL,

Then we probably know each other as well - Hudson and I go back nearly 30 years.

Using the -200 as Hudson quoted,

takeoff 1/2/5/10/15/25
landing 15/30/40

[CaptainSandL]

Every day I learn something new about the aircraft I know most about.

S & L

[aluminum ovcst]

Figaretto,

I believe your idea of using the normal Vr and climbing out at the improved V2 doesn't hold up:

1. Imagine an engine failure at Vr - no room to accelerate here to achieve your 1 ENG INOP best angle speed (v2). You've just defeated the purpose of your augmented speeds.

2. I believe FAR 25 specifies V2 as equal or less than 1.2 Vr. Correct me please if I'm wrong. Don't have my perf books in front of me now.

Cheers,

AO

[quid]

ao-

V2 may not be less than 1.2 Vs. It has no relation to Vr.

P.S. - V2 is MUCH slower than the 1 Engine inop best ANGLE of climb speed.

[ 17 August 2001: Message edited by: quid ]

[Nick Figaretto]

Hmm. Yes, aluminum. I guess that really makes sense.

Nick.