Performance Mismatch Airbus A320FAM + Taxi Fuel

SimBrief Dispatch
1

Dear Derek,

I would like to adress my concerns regarding your performance database in terms of altitude and fuel calculation for all Airbus A320FAM, using the FSL-series in P3D.

Flying currenty a testflight MUC-LHR on an Airbus A321-231 (D-AIDM) with a cruise level offset of M1000 and performance bias of P005 with the following mass & balance data:

ESTIMATED TOW 74218 LAW 69798 ZFW 65842

Taxi Fuel

First of all it came across that it would be way better to have the oportunity to select a Taxi Fuel in kg instead of minutes. With IAE engines and 15min taxi fuel, 150kg are assigned. From my experience with FSL, it takes already 80 kgs for APU and engine start which gives not too much room for taxi and those engines are quite hungry on ground. For our entire virtual Airline, taxi values do not match especially on CFM engines and A321-SL with high Take-Off Masses.
Conclusion: Apply a function to select a certain Taxi value instead of time?

Trip Fuel (Cruise)

Furthermore, climbing out, values on 4 different testflights showed a gap of around 400-500 kg between actual Fuel on Board and estimated Fuel on Board. This situation will resolve within 20-30 min in flight where those values start to match again.

Flightlevel

Simbrief assigned a Flightlevel of 300 and later on at 340 on AUTO mode where the A321 is capable of doing initially FL380 on this flight. Do you have any hint for me how to get better planning results?

Best Regards and thank you!
Alex

2

Hello, we are looking into the burn rates and we will take into consideration the suggestion of an open taxi fuel box to add KG or LBS. Also we are looking into different engine types and burn rates for the future. Thanks for the feedback

Regards,
Jason

3

Hi Alex,

As Jason said, we are looking into improving the fuel options down the road (including better taxi fuel options). That being said, to address your other concerns:

Taxi Fuel

I don’t share your experience in this regard, in my experience SimBrief’s A320 taxi fuels have matched up well with real world OFPs. Are you running the APU during the entire preflight? Of course, many operators do this for COVID now, but I believe many of them load extra fuel to cover this (instead of in the taxi fuel). I guess some operators might include it in the taxi fuel though.

In pre-COVID times, SOP would be to only start the APU about 10 minutes prior to pushback, which means it would only end up running for ~15 minutes, or ~30kg burn total.

As I said, eventually better fuel options might be added to give you more control over this. But in my experience SimBrief’s taxi fuel is very close to real OFPs, and also pretty close to the FSLabs (which I fly quite regularly myself). For example:

Real OFP Engines Taxi time Taxi fuel
AWE IAE 0:15 200 kg
BER CFM 0:12 138 kg
DLH CFM 0:14 168 kg
EIN CFM 0:24 276 kg
ETD IAE 0:23 307 kg
EZY CFM 0:13 150 kg
EZY CFM 0:15 173 kg
JBU IAE 0:17 156 kg

Of course, other operators may decide to plan more taxi fuel. Every airline has their own procedures I’m sure, but the “average” operator seems to be close to SimBrief’s numbers. Maybe SimBrief was about ~20-40 kg shy on average, I have adjusted this but it’s quite minor I feel. IAE does seem to burn slightly more, but in most cases it only seems to be by ~40 kg.

Trip Fuel

During climb, you unfortunately cannot reference the fuel burns on the OFP. They are not accurate, that’s a SimBrief limitation and unrelated to the A320 profile. However, the values observed at TOC, and during cruise thereafter, should be accurate. Since you mention that the numbers line up after 20-30 minutes, I assume the discrepancy you are seeing is during climb, which is to be expected at the moment.

EDIT: See my post below, I have made some improvements to this now.

Flightlevel

FL300 was initially assigned due to an outdated RAD restriction for the airway segment INPUD Y102 ALIBU. I have removed that restriction as it seems to be no longer valid.

I’m not sure I agree with your comment that the A321 can do FL380 on this flight. Assuming your cruise weight on this flight is around 72000 kg, Optimum altitude for an A321 CEO (non-sharklet) is ~FL360 (Max altitude is a hair under FL380, but no flight planning system would ever plan a flight at the max altitude. Both because it’s inefficient, and because it leaves no margin).

So at this weight, SimBrief will target FL360. But since you’ve set a cruise level offset of M1000, it will actually target FL350, resulting in FL340 on this flight due to direction of flight.

Best regards,

Derek

4

Update: I’ve improved the accuracy of the navlog’s fuel and time values during climb. Let me know if it’s any better for your future flights!

Best regards,

5

would it be possible to adjust taxi fuel consumption as per aircraft?

A319 10kg/min
A320 11.5 kg/min
A321 13.5 kg/min

For now I believe simbrief calculates 11.3 kg/min for all A320 family

Thanks

6

Thanks, I have updated the taxi fuels accordingly.

Best regards,

7

Good Afternoon Derek,

thank you for your detailed feedback. Initially I have to excuse myself - preparing this flight I have initialized the MCDU INIT B Page automatically which led to a wrong ZFM and ZFMCG which was the clue regarding the wrong OPT altitude. So that one goes up to me!

A recent testflight from LHR to MUC and ongoing MUC to LIS shows that the increased taxi fuel consumptions match the requirements way better. As you have already told, I know about SOP’s for APU operation and usually I do switch the APU on directly before goign off-block while using external air condition. Nevertheless, I would appreciate the function of selection a certain Taxi Fuel value (which incorporates the APU consumption prior departure by the way acc. AMC1 CAT OP.MPA.150) to have a better overview over my calculated figures.

1233×462 17.3 KB

So far, the climb calculation seems way improved and matching the figures but one remaining issue is a constant discrepancy between the PROG page (OPT Flightlevel) and Simbrief’s planned Flightlevels.

1155×466 27 KB

Flying currently MUC-LIS, (having 200kg additionally on top of the fuel figures + a shortcut during departure plus cont. climb and not staying on FL240 as per OFP req.) the fuel figures seems proper as well, but the same here: Discrepancy between planned and optimum flightlevel. Overflying MINSO with +400kg on board on the EFOB, the OPT is shown with FL335 whereas FL350 is planned.

Is there anything I’ve setup wrong? I am currently getting my hands on the Flight Planning and Performance Manual (FPPM) as well as on the Inflight Performance Chapter of the OM-B of A320ceo, A321ceo (+SL) to verify to compare the OPT of FSL shown and verify if FSL’s data are correct. But you might have those data already, Derek?

Best Regards and thank you for your constant improvements!

8

Hi Alex,

I don’t think you’re doing anything wrong in this case, but I am not sure how accurate the FSLabs MCDU OPT altitude is meant to be. There can sometimes be discrepancies between the charted OPT and the MCDU OPT, and the reason isn’t always obvious. Many factors can affect it I believe, including CI, ISA DEV, and flight plan wind data/uplinks (which normally use Active Sky in the FSLabs as I recall. Active Sky is usually close, but may not exactly match SimBrief’s winds forecast).

Also remember that SimBrief will normally straddle the optimum altitude. So if the charted OPT is FL340 for the current weight, SimBrief will normally climb to FL350 (assuming enough margin exists between the MAX altitude as well). Then it will stay there until the charted OPT reaches FL360, at which point it will climb to FL370, etc. If you want it to never climb above the OPT, then a cruise level offset of M1000 can be used.

Finally, especially at the initial cruising level, SimBrief can sometimes overshoot the OPT by a bit more than 1000 ft. This is often more efficient (and easier to manage for the pilots and ATC) than levelling off for a brief period only to step climb soon after. So in cases where, at initial level off, the charted OPT will be FL338, it is possible for it to climb straight to FL350 instead of levelling at FL330.

In your specific example you were also slightly heavier than flight planned (by 400 kg); had you been 400 kg lighter the MCDU OPT might have read FL337 or FL338, which might be why SimBrief planned FL350 right away. Another reason it might choose to do this is if the winds were more favourable at FL350 vs FL330.

Best regards,