Cruise calculations

[tknudsen]

Dear team.

Much as I appreciate using Simbrief daily I have an issue with it.
When narrowing down the problem, I can honestly say that I do not fully understand how it calculates Flight Level. I do believe this calculation to be either random or wrong. Reason will be explalained in this example!

Route:
ENAL (AES) Ålesund Airport Vigra
ENGM(OSL) Oslo Airport Gardermoen

Now, Simbrief outputted FL370 for this flight. If not familiar with real world operaton based on the
airframe type, engines and weight balance, it would mean that end user take this height and predict it as is. Now Airbus has an OPT FL and MAX REC FL figures for any takeoff performance calculations. In this case the Optimal Flight Level, I belive this was FL368 with MAX REC F

The end point to this example is that the plane obviously struggled to reach this suggested Flight Level provided by Simbrief, as seen in this image where Top of Climb is depicted after Top of Decent.

Now this optimal suggested flight level was calculated by Simbrief, based on weather, cost index and these airframe data:

When turning to Simbrief Manual, one can read:

The desired cruising altitude, in feet. Leave this blank to let SimBrief choose one automatically. When setting a custom cruising altitude, make sure to keep your aircraft’s weight in mind. Most aircraft can only reach higher altitudes if they are light enough, and SimBrief will not prevent you from choosing an altitude that is too high for the planned weight. In most cases it is safer and easier to leave this blank and let SimBrief choose an altitude for you.

After extensive search, I found that in real life the most average occurance in Flight Level for this leg based on Scandinavian Airline Airbus A320 (which was used in above example) the frequent height clearance given as 31,000feet on ENAL to ENGM and 36,000 feet on leg ENGM - ENAL

ENAL - ENGM (data from Flight Aware)

and
ENGM - ENAL (data from Flight Aware)

Now variations will occure, that could be leg shorten by ATC, diversions due to weather etc.
Point being…
How does Simbrief calculates these flight level, does it take into considerations such things as weight, wind, cost index, leg distance, restrictions etc.

If one cannot rely on AUTO, or if this is random set or just an estimation, I would hope this will be provided as a hover description or if anything else to the end user. So that he/she can enter a manuel flight level contrary to what your manual says?

Cheers and again thanks for all your services
Tom Knudsen

[SimBrief]

Hi Tom,

@tknudsen

How does Simbrief calculates these flight level, does it take into considerations such things as weight, wind, cost index, leg distance, restrictions etc.

SimBrief takes into account weight, wind, ISA temperature, and leg distance. What it does not take into account is ATC or SID/STAR restrictions that may delay the climb or require an earlier descent. It also doesn’t fully consider the impact of Cost Index on the climb distance, but the error caused by this will only be apparent at very high cost indexes. For what it’s worth, many real world flight planners also do not consider these either.

It should be noted that based on the FlightAware tracking, the flights you are simulating are very often in cruise for up to 8 minutes. This would indicate that they could quite easily climb higher than FL310 if they wanted to (assuming ATC allows and no other restrictions exist).

It’s also possible that they are quite a bit heavier than you. The SAS A320neo can seat upwards of 174 passengers, but on your flight you had only 127 passengers and no baggage. The lighter weight could also explain why SimBrief planned you higher.

In real life, on short flights like this, pilots quite often just ask for a lower final altitude than planned if they think that the cruise segment will be too short. This isn’t unrealistic, the flight plan SimBrief provides isn’t set in stone, and the fuel/time difference between FL370 and FL310 on a short flight like this is negligible.

Another very big contributor is add-on accuracy. From SimBrief’s end, the data for the A320neo is a bit limited. It’s possible it is slightly over-estimating the climb performance. From the simulator’s side, it’s very possible that the aircraft is climbing slower in the simulator than in real life. The combination of these discrepancies can add up to produce larger inaccuracies, but this is just the reality of Flight Simulation. We and other add-on developers do our best with the data we have, but it will never be quite as accurate as the real thing.

With all that said, I will make some adjustments to SimBrief’s minimum cruise distance when selecting a flight level. Currently it’s set to 20nm, which is probably a bit too tight given the possible discrepancies described above.

Hope this helps, and best regards,

[tknudsen]

@SimBrief

In real life, on short flights like this, pilots quite often just ask for a lower final altitude than planned if they think that the cruise segment will be too short. This isn’t unrealistic, the flight plan SimBrief provides isn’t set in stone, and the fuel/time difference between FL370 and FL310 on a short flight like this is negligible.

Thank you sir. A small correction, according to “SeatMaps” Y174 yes, but that is not entierly true. According to SAS fleet list, they operate with Y180 and Y157 - Since the operator I normally uses Y180 and it fits with SAS, then I choose to use Y180

The weights listed in the image above is streight from Toliss Developer and should fit majority of the A320-251s.

I appreciate your feedback and I do fully understand that some airlines tend to choose a lower flight level than OFP has assigned. This comes with experience over time. Downside to this, it can be both very difficult and frustrating for end user to know “what is the correct altitude”, especially when flying online i.e Vatsim where pre-defined altitude is required for flight planning.

Other factors is that some airlines uses fuel saving procedures like inserting Cost Index 0 at FL100 and back to OFP Cost Index when entering cruise. This to save fuel (think about 30kg)

Appreciate your changes for sure

Cheers