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Optimised(Improved Climb) Take Off Performance Changes..or Why are my V speeds different?

Darren Howie

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Darren Howie

Greetings all!

Having seen several people questioning the speeds being generated by the miraculous new FSlabs performance system i figure id spend some of my many hours of waiting to go back to flying to explain what is an incredibly complex topic in just a few paragraphs(yeah right!).

For those who dont know me i was lucky to fly the 320/321 for 16 years with 5 years on the 330 in there as well these days on the 787.

I also ground instruct teaching stuff like performance and all the other topics initial Airbus guys learn before they hit the fleet so in short i know a little or just enough to trick people into thinking i do know about all this rocket science stuff.

What has changed or more appropriately what is this optimised perf stuff all about?

In short in the past and anyone who has done perf on a 727 for an ATP exam will know that V speeds came from a weight that was the either the maximum uplift or the current weight based off the runway surface available the Take off run available or TORA.

So by definition speeds where tied to weight and weight was tied to runway length so only runway length determined speeds.

Longer runway higher speeds shorter runway lower speeds.

First restriction Runway Length

As time went on regulators imposed a climb restriction on single engine perf (one engine out) to ensure all the punters would survive an engine failure at V1 not to then only be killed hitting the hill at the end of the runway. They came up with the requirement that the aircraft on one engine must be able to climb at 2.5% minimum until single engine acceleration altitude.

Second Restriction Obstacle based Climb Performance.

As time went on airport designed and runway design changed incorporating overrun areas and in many countries fly over areas known as clearways. Aircraft could use these to meet the runway length calculation as long as they could be airborne by the end of the physical surface and reach 35'(15' in the wet) by the end of the "clearway". This distance is called the take off distance available or TODA.

Likewise they could use the stopway to stop in the event of a V1 rejected take off this distance including this stopway area is called accelerate stop distance or ASDA.

Third change clearways and stopways.

So in short performance calculations got a lot more complex and generally only big airlines with dedicated performance departments could benefit. Time goes on and computing power etc becomes cheaper and as is does more airlines want to take advantage of the extra weight you can use.

At the same time manufacturers realize that taking off at minimum V2's from very long runways using only a fraction of the length increase risks significantly so the concept of overspeeding is developed.

What is overspeeding?

Well simply it means you use that huge runway to get the aircraft to a much higher speed than you would use worked out off a balanced field length.

It does two two things. One is ensure if you have a failure at V1 the aircraft is in the best energy state(highest speed) so that is climbs better AND by needing less rudder handles better has less drag and CLIMBS BETTER.

All this stuff requires huge computer power to work out the tables used by airlines to work out the numbers they see day to day.

By now being able to use ASDA instead of TORA far higher weights can be uplifted and aircraft can operate far more efficiently read make more money by carrying more punters..

So how does this all work in the sim world?

Well up until now many performance calculators you would of seen and used are "balanced field" calculators and none of them incorporate obstacles.

Somehow the FSlabs team have built a new perf calculator that incorporates ALL of these things for the first time yes it even looks at P3D terrain obstacles in determining weights.

The flip side of this is that in the past you saw nice consistent speeds with them going up as weights increased and down as flap settings increased.

Welcome to airline operations 2021 style where speeds, flaps and most values are no longer consistent.

By using extra runway to improve energy state ie higher V speeds expect to see speeds at Cfg1 and 2 the same, occasionally 2 will be higher occasionally 1 will be higher.

If you photograph airliners ever wonder why they all take off in around the same area even though the weights vary massively?

Simple performance work backwards from the critical obstacle at 2.5% gradient to work out where on the runway the aircraft MUST be airborne by to ensure clearance. Then from that point the calculators derive flex values and V speeds to ensure on reaching that point you are airborne.

By using all available runway on every take off means weight makes little difference in speed calculation. The aircraft is held on the ground to a speed FAR in excess of what would be used using balanced field speeds. Again ensuring energy, control etc.

Rule 1 there is NO CONSISTENCY! So check your data every time ie take off weight, config, flaps.

A couple of examples using the real performance calculator to show we are not just making this stuff up lol.

Firstly a take off at Sydney Runway 16R one aircraft at 77000Kg the second at 70000Kg note the lighter aircraft has higher V speeds completely against balanced field length calculations. Why?

Well its lighter so accelerates faster and in the physical distance to the rotation point which ensures you can climb over obstacles climbing at a 2.5% gradient it can reach a higher speed! Simple..lol.

So in standard conditions 77000Kg Cfg 1 V speeds are V1 150/VR 151/V2 152 flex of 54

Compared to at 70000Kg note that at a lighter weight V speeds are greater with V1 154/VR 154/ V2 155 but Flex Temp is far higher at 67.

Now another example from Brisbane runway 01R this time showing how cfg at max weight makes little difference to speeds.

Both these are again at standard conditions, both 77000Kg and the only difference is Cfg1 vs cfg2

Cfg 1 V1 151/VR 151/V2 153 flex 54

And cfg 2 note very similar speeds at V1 150/VR 151/V2 152 flex 54

To see now what happens in the FSlabs calculator here are two examples one thing to keep in mind is the data used in the professional calculator ie obstacles, towers etc and primarily flight patch computations will never be to the same degree of accuracy that millions of dollars funds to survey terrain obstacles and the original Airbus perf data. In fact our paper charts which we still carry for a flysmart issue which pop up from time to time vary significantly from Flysmart be very conservative figures.

Cfg 1


And cfg 2 again note higher V speeds are consistent with use of optimised take off routines.


Finally lets have a look at the Boeing program numbers for a 787 out of Sydney same runway same conditions at 225000Kg.

Flap 5 versus Flap 15 same weight same runway etc.

Flap 5 V1 165/Vr 170/V2 176 flex 46 versus

 Flap 15 again not most speeds at the higher flap setting are greater where on traditional methods they would be less  we get V1 171/ VR 177/ V2 177 flex 45

Multiple examples that no longer are speeds related to flap or runway length so do not assume anything.

To summarize then expect to see speeds you have not seen before in many cases contradicting what you are used to seeing from more traditional flight sim perf calculators almost all of which use obsolete methods to determine speed information and none have any obstacle information.

Performance for airliners is an endless complex topic going FAR above my level of understanding but hopefully this explains part way as to why you may see speeds your not used to in the FSlabs and how it really does "all make sense".



See you in the virtual skies!

PS CHECK YOUR DATA!! PS Sorry due copyright stuff i had to pull the shots from the performance calculators used.




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  • Darren Howie changed the title to Optimised(Improved Climb) Take Off Performance Changes..or Why are my V speeds different?
Koen Meier

Take off performance but equally so Landing performance has gotten really complex over the years. It is important as simmers to understand to a degree how complex the system is and what it wants to achieve.

Also rather surprised that the TOPL values are rather close to each other in this scenario between FLysmart MTOW(PERF) and the fslabs TOPL.

however we have to respect the copyrights so no more nice images.

Edited by Koen Meier
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Stu Antonio

The most interesting thing about aviation I've read in a while.
Thanks for sharing!

Looking forward to the next lesson ;):)


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Ken Knowles

Almost makes me want to break out my performance atpl textbooks again... almost :-)

Insightful post thanks!

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Chris Kreuzbichler
11 minutes ago, Ken Knowles said:

Almost makes me want to break out my performance atpl textbooks again... almost :-)

Insightful post thanks!

I got Performance flashbacks from that. At least it's not GNAV haha (which I in the end then was fairly good at)

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Ken Knowles
11 minutes ago, Chris Kreuzbichler said:

I got Performance flashbacks from that. At least it's not GNAV haha (which I in the end then was fairly good at)

Ah the old crap five - who needs an FMGS!

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Darren Howie
On 4/14/2021 at 5:40 AM, Ken Knowles said:

Almost makes me want to break out my performance atpl textbooks again... almost :-)

Insightful post thanks!

Dont do it Ken dont do it lol...save yourself!!

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