Takeoff Perf Calculation Issues

[Deer-in-Motion]

Almost stalled on takeoff because the perf calc gave me a V2 of 123 kts. At the same takeoff weight in the Fenix it was about 20 kts higher.

[richboy2307]

Hi could you please provide a screenshot of your payload and t/o calc pages on the efb? Also of the INIT B and PERF TO pages on the MCDU.

To cross check if the weight/weather and other taken by the EFB/Plane are matching what was actually in loaded into the plane. Thanks!

[Deer-in-Motion]

Here you go. Took these this morning. I've included a screenshot of the Fenix EFB for comparison. Similar takeoff weight.

[Deer-in-Motion]

And I just realized I didn't get the other screenshots you wanted. Sorry. In a bit of a hurry this morning.

[Deer-in-Motion]

Flight tonight the v speeds looked more reasonable. Except taking off on RW8 at KDEN took about 10k feet of a 12k runway. Which feels odd.

[richboy2307]

On 5/25/2024 at 11:04 AM, Deer-in-Motion said:

Flight tonight the v speeds looked more reasonable. Except taking off on RW8 at KDEN took about 10k feet of a 12k runway. Which feels odd.

Performance calculations are a complex subject, with a lot of variables at play, but comparing performance numbers of a CFM LEAP powered A320NEO to CFM56/IAE V2500 powered A320 is not very valid or useful.

The takeoff speeds are optimised for maximum flex for the given restriction, meaning on a dry runway 50ft elevation at the end of it, and 35ft for a wet one. Most of the time on the line you are not taking the full length you use an intersection a few up and then if you take full length you have more margin.   But as said before it's aiming to flex as much as possible while giving as much climb gradient for the 2nd segment which is often why you see 2nd segment limited message, and it will try to use up as much runway as it can if it means it can flex the engines more.

Just for illustrative purposes, this is below from a real perf calculator for this type.
Between this and 56,000kgs speed changes 3kts less. So it's more than normal to see these higher speeds too. 

[fbasili]

I noticed that this aircraft has poor takeoff performance compared to Flybywire or Fenix. It always consumes the entire runway on takeoff and is always very close to stalling afterwards. I don't know if it depends on incorrect derate calculation or incorrect engine power management. We hope you resolve

[NebraskaFarmer]

There is definitely an issue with performance calculations on short runways. As shown in the first example by Deer-in-Motion above with a 2009m takeoff run. For such short runways it always gives impossible speeds. It is impossible to take off at those speeds without stalling. All the speeds are likely at least 10knots to low.

To reproduce just calculate perf for any runway shorter than approx 2300m with a gross weight of at least 65000kg. It seems as if the perf tool adjusts the speeds down to a speed than can be achieved on the available TORA instead of saying TO not possible or TOGA required, higher flap setting, etc. Longer runways work fine as seen in the second example by Deer-in-Motion.

[NebraskaFarmer]

Perhaps this can be more clearly demonstrated with different take off runs on the same runway. Let’s say runway 25C at EDDF, a 4002m runway.

Those are my settings:

Gross weight: 64872kg
Conf 1+F
Air Cond: ON
Anti-Ice: OFF
Force TOGA: ON
Surface: Dry
Wind: 250/10
OAT: 19C
Altimeter: 1015

Now for the full length of 4002m I get the following speeds:

V1: 150
VR: 150
V2: 152

Now I shorten the TORA in the calculator to 2000m and get the following speeds:

V1: 130
VR: 130
V2: 134

So the calculator is telling me that while all other parameters remain the same the aircraft can take off with speeds that are 20knots slower on a shorter runway.

The perf calculator is just not working as it should.

 

[richboy2307]

Hi @NebraskaFarmer

Shared your findings with the team, which includes real world pilots of the type, with access to real tools for this same type of stuff. The results seem as expected, the speeds shown have margin built-in and are way above the minimum take off speeds anyways, and are valid as long as they are not below VMCA & VMCG.

Also remember, these are "optimal" speeds, as opposed to "absolute minimum" speeds for the aircraft to takeoff (or not stall in takeoff  configuration). 

So coming back to your EDDF Rwy 25C scenario, I've run it through the A20N EFB, see screenshots below (click to expand):

TORA 4368m:

TORA 2000m:

You'll note the difference in the limiting factor for the speeds on the yellow band.

• TORA 4368m - the speeds are limited by '2nd Segment' climb gradient primarily.
• TORA 2000m - the speeds are limited by 'Runway Length' primarily.

With a longer runway, with nothing else "limiting" (aka deciding) the calculation, it will normally optimize for a better (than bare minimum 2.4%) 2nd Segment climb gradient, taking advantage of the extra runway till it achieves the best speed for it to do, whilst still maintaining proper stop margins etc.

Having a 20 kts difference in this case is totally normal then as it starts to bring the speed back to accommodate the shorter TORA, so it will sacrifice the climb gradient after takeoff instead (target the bare minimum 2.4% one instead). Hence the runway length becomes the "limiting" (aka deciding) factor, as opposed to the 2nd segment climb gradient. If the computed values end up being less than the "minimum" speeds, then it will return 'UNABLE TO FIND SAFE PERFORMANCE DATA' alert instead.

So assuming proper technique is used, the aircraft shouldn't be stalling even on shorter runways, as even the bare minimum values it will push out has some margin built-in to them, and if it couldn't retain that margin, it wouldn't calculate any speeds for you in the first place.

So if you're noticing issues, please record a video clip of your takeoff and initial climb, and share here so I can show the team to understand if something else might be the issue.

Thanks!

[Azlog]

To add to what @richboy2307 has said, different airlines use different calculations which they have worked out depending on what the airline requires. Some will be looking for minimum noise and engine wear (low Vr) while others might be looking for maximum climb performance (high Vr) but it doesn't mean the aircraft couldn't get airborne at lower speeds. So some might use the whole of a 3000m runway just to get off the ground at a relatively low speed. Others might decide that they want maximum climb performance, even from short runways and use low FLEX temps to get extra thrust. The calculator on the v2 EFB seems to like to use the whole runway when compared to the FBW version.

Only potential flaw I've encountered in it is that it doesn't like to use low V1 speeds compared to the Vr speed which makes short runways return an UNABLE message. For example, you would never see on this EFB calculator a V1 of 104kt and a Vr of 134kt which I have seen and used on other calculators