TRP flex calculation faulty?
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Hello,
Using the flex calculation method from the FCOM, I get a different N1 FLEX than what the TRP calculates.
Example:
ENG AIR - OFF
ENG ANT-ICE- OFF
T_AMBIENT = 15C
T_FLEX = 40C
T_FLAT = 25C
Pressure Altitude = 30 ft
N1_REF = 94.2
N1_FULLRATE = 92.8
dN1_FLEX = 0.16 * (T_FLEX - T_AMBIENT)
N1 FLEX = N1 FULLRATE - dN1_FLEX = 88.8
The calcualted N1_FLEX is higher than 78%, not lower than N1_REF - 8 and T_FLEX is higher than both T_FLAT and T_AMBIENT and is therefore valid.In the JF RJ, the TRP calculates 90.1.
Am I misunderstanding the FCOM or is there a problem with the TRP?Best Regards
Johan -
The N1 values on the TRP have been simulated to be as accurate as possible based on the information we have in the FCOMs. The example you have listed isn't something I can find in any of our FCOMs, so without further information, the difference may be explained by another factor, such as engine variant.
Mark - Just Flight
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@Mark The method is found in FCOM Volume 3 Chapter 9 Topic 7.1 Page 8.
Also keep in mind that merely using the N1 REF tables as a temperature lookup for the flex temperature gives you a value called N1 FULLRATE, not N1 FLEX.
TO REDU is meant to reduce the thrust further than just looking up an N1 for a higher temperature.The data tables in the FCOM won't give you a lookup for what the TO REDU mode outputs because there is no table for that.
That value is calculated, not interpolated. -
Here's more information:
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Yeah seems to very confusing.
These data are used in the following example:
Field elevation: 0 ft
Ambient Temp.: 15°C
Flex Temp.: 35°CUsing the Table from the Bae 146, and following the description in the bottom right, I get this:
This is the same as what the calculator from Flightsim.to gives:
In a test at EKBI with the following Data:
Field elevation: 217 ft
Ambient Temp.: 10°C
Flex Temp.: 35The table gives 86.7
The calculator gives: 86.7
So both the table and the calculator say 86.7, but the TRP says 90.3
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I extracted some data from a flight video where you could get all the data required for the calculation.
I placed the aircraft at the exact same stand, set the weather to match the video and entered the flex temp.
The results are below:Real ACFT: E3244
Engine: LF507-1F
APT: ESSB
Elev: 47
OAT: 3
Flex: 41
QNH: 995
ENG ANT-ICE: OFFReal aircraft
N1 REF 92.9
N1 FLEX 86.5Calculation by me:
N1 REF 93.0
N1 FLEX 86.5Calculation by JF RJ:
N1 REF 92.3
N1 FLEX 88.6I also noticed that selecting all ENG AIR - ON and PACKS - ON had no effect on N1 REF.
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Hi all,
I passed the comments in this thread onto our systems coder who has left the following response:
"I think the OP is assuming TO REDU is the same as TO FLEX, when it is not in this case. TO REDU is calculated considering values of N1ref, Tredu and N1redu in figures 7.1.1 to 7.1.3.
AFAIK, the TO REDU value in the TRP is the optimum N1 value that can be set at the current OAT by simulating a higher temperature, which is Tredu from the tables, NOT TFlex.
TFlex can be used to obtain an even lower N1 value than the one calculated by the TRP and requires using information from RTOW table and figure 7.1.4 in the FCOM, so it must be manually calculated. Once the N1 Flex is obtained, the user can increase the TRP Temp reference until the desired N1 bug value is displayed in EICAS."
Hope that helps.
Mark - Just Flight
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@Mark Regarding this:
"I think the OP is assuming TO REDU is the same as TO FLEX, when it is not in this case."The FCOM says:
The "REDU values are just particular FLEX values".The mode TO REDU on the TRP carries out the calculation I posted multiple FCOM references to.
When you push the TO REDU button, it will set a default T FLEX, this temperature is called T REDU.
You can actually calculate the values in the N1 REDU table using the method I provided. The reason for this is simply that N1 REDU is the same as N1 FLEX, it's just been decided that a particular T FLEX will be called T REDU.If your TRP implementation isn't carrying out the calculation I provided from the FCOM, then it's not doing what the real aircraft does.
How can I be so sure about this?
I did the calculation using real data from 3 videos and my calculations matched the N1 set by the TRP for the particular temperature setting in the videos and pressure altitude (±0.1% N1 precision is acceptable).
If I was wrong about all of this, then the calculations should return non-matching values.When I do the exact same scenarios in the JF RJ, the values output by the TRP doesn't match at all.
