I was wondering , when I use the MCDU to calculate the v speeds for take off they differ from the speeds that I get if I use the “Wabpro” software sometimes up to 20 K difference. Is it that the MCDU is still not accurately modeled. Of course I am using the same T. O. Weight in both cases. Maybe I am missing something, it would not be the first time!
LP
Well Wabpro is calculating the A320CEO with different engines. The A320NEO has a highly different performance profile.
Our “Vspeed-maker” is a very pernickety guy ( 
). The takeoff and landing speeds were calculated and checked with real A320Neo pilots for several times. So they should be accurate.
If not, feel free to file an issue on our github page. I’m looking forward to the discussion with him (and I don’t mean that in any cynical way!! He’s a nice guy.)
I tend to put my trust and respect to the FlyByWire team and all the supporting people involved. I trust the product has been vetted and approved by people that are far more knowledgeable than me. So whatever they give us, I accept that everything is what it’s suppose to be doing.
If it is an issue. Someone more knowledgeable than me would have already reported it and I trust that they’re working with everything they have at their disposal to make it work the way it’s suppose to.
Thanks for your replies. I am following the “Real Airbus Pilot” videos on Utube and I can see that he is more often than not praising the mod team for their accuracy and it’s obvious they are improving the neo towards study level. Just one thing, I realise that “Wabpro” is not calibrated for the neo but it gives the elevator trim for take off. As far as I can see there is nowhere in the MCDU which provides that trim figure. Or is there?
LP
There is. It’s in the PERF page under Take off, there should be FLAPS/THS. You can input the trim level here. But note that this doesn’t really do anything other than display. And to be honest. I never really set elevator trim for take off in the A320. This is because the experimental version already has Rotate Law implemented. 320SimPilot on Youtube has a video about that. Basically, all you need is to have your hand on the sidestick. Pull the sidestick to rotate on take off, and just release it when you reach 15 degrees. And the airbus will maintain that by automatically adjusting the trim, even without autopilot.
Basically, you don’t really need to set trim pretty much the whole flight. I know I never touch the trim wheel ever because the aircraft automatically trims it for whichever phase of the flight you’re in.
How do you calculate V speed at MCDU? I use a site to obtain the values according to some characteristics of the flight
You don’t need to do any calculation externally since the MCDU can do all the calculation for you.
Here’s a video of me setting mine up. It’s pretty short and straightforward:
The THS field on the Takeoff Perf just serves as part of the Takeoff Config warning system. Normally this field would be set to the weight and balance calculations from the load manifest. The pilot then sometime after engine start and before takeoff would then set the correct trim setting using the trim wheel. When prompted, the takeoff config button would be pressed and the system compares the actual trim setting to what is set in the Perf page. If the difference is 1.0 off, it flags the config warning.
Does setting Flex to Temp made a difference in V speed calculations? I noticed in prior builds its was set very low and in latest builds its not set at all. So maybe setting that is also a factor. I don’t know much about that value, how should you set it and is it F or C?
Always Celcius. Airbus mostly deals with metric (except for altitude, distance, and speed).
I think I saw somewhere that it does affect the V speed calculation. If you use an external calculator, I think that you can put all the conditions in and it’ll calculate the FLX Temp for you if you are using FLX thrust. And with that in mind, it also calculates the V speed as well. But I might be wrong and a real pilot or engineer need to confirm this.
But I always set it to 58 degree C by default regardless of the conditions. I don’t want to be too bothered with actually setting the right FLX temp.
Currently, the flex temp setting is pretty much just eye candy. It doesn’t result in the correct thrust level, nor is it reflected in the MCDU takeoff speeds. The MCDU takeoff speeds are for a balanced field length condition where you are not limited by tire speeds, brake energy, takeoff climb, obstacle clearance, or departure path gradients. With that in mind, the takeoff speeds are very reflective of A320NEO/LEAP-26A actual takeoff speeds.
Developing a full takeoff performance calculator, including flex temperature reduced thrust performance, is a bit problematic at the moment because Airbus does not provide generalized takeoff performance data like in the past. Takeoff performance for real world operations for the NEO is generated by computer software that is tailored to the specific operator’s departures and routes. For the most part, data from the CEO is not going to be directly applicable to the NEO. There are ways to work around this, at least to some extent, but it will take some time.
Persnickety, hah!
I also wrote “nice guy”. 
Not a pro but what I normally do in the A32NX is that I take the ZFWCG from the INIT FUEL PREDICTION page and then look at the scale next to the trim wheel to see what THS i need to set. For example ZFWCG of 29, means THS of DN0.1 roughly. So then I set 1/DN0.1 on the take-off page under flap/THS. Don’t actually know if that is correct?
No. ZFW is zero fuel weight. ZFWCG is the center of gravity for that value. Fuel itself also affects the CG. If you want the complete CG of the aircraft, you use the CG from the gross weight.
Also the THS field in the Perf page is just a checking value for the takeoff config warning. The jet will fly just fine without anything input into that field.
Yeah, don’t read the rest of the thread about why those speeds will be inaccurate for the A320 NEO.
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