Help me understand the A320 position computation [flight sim Fenix A320]
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Joined: May 2022
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From: Michigan
Help me understand the A320 position computation [flight sim Fenix A320]
DISCLAIMER: I'm not a real world A320 pilot...... yet and also this is a long page of text so be prepared 
So as a nerd working my way towards becoming a ATP pilot I do a lot of hardcore flight simming. I love aircraft systems and the A320 is perfect for this, I got my hands on a full A320 FCOM and watched all the CBT on it. I fly it exactly by the FCOM/FCTM/QRH, the ones I have is from 2022/2022/2018 respectively. Yes I know flight sim is a game and it's not the real thing but this is the closest I'll get to flying the bus save the A220 or A320 NEO series (which NGL looks really good).
I have made myself "guides" or descriptions of the aircraft systems to keep my memory sharp, I REALLY get into it.
One of the systems I love the most is the FMGS namely the position computation portion, I spend months reading the FCOM and CBT and made myself this guide to use when I fly the Fenix A320. The Fenix A320 has the BEST FMGS simulation of any desktop sim since it comes with the most features and it even has dynamic satellite coverage simulation added along with other goodies, I can use the real FCOM to fly it.
What I need help with is from a real A320 pilot or instructor to "crosscheck" and make sure my guide is correct! I want to be using solid information. It really means a lot to me.
Before we begin the Fenix has the Thales FMS2 S7 version simulated.The A320 FMGS is designed to be a hybrid-type system which means it takes all available navigation data sources such as GPS, the other IRUs, and radio navigation receivers and combines them to maximize the availability and accuracy of all data. The design philosophy of combining as much data as possible makes the A320s FMGS EXTREMELY accurate and reliable compared to other planes.
The FMGCs compute their position known as the Flight Management (FM) position, this is our position in the world as computed by the FMGS, this is how we navigate.
The system navigates the plane by updating the FM position, there are three sources for this; GPS, RADIO, and IRS. Remember the A320 will mix these when possible. The system will always select the most accurate source for the FM position computation with a priority order of GPS > RADIO > IRS ONLY (if the components are functioning correctly).
First, each ADIRU in addition to calculating it’s inertial position is also programmed to create a hybrid IRS/GPS position called GPIRS which is a combined position from it’s IRU mixed with the GPS the ADIRU has selected. The ADIRU will select the most accurate of the two GPS positions available to compute it’s GPIRS position.
The A320 has three GPIRS positions given by each of the available ADIRUs (normally 3 if all are operative)
Second, the FMGC takes the positions from all 3 IRs and combines them into a MIX IRS position which is a average IRS position of all available IRs (normally 3). This is VERY important because the mix IRS position is combined with the selected GPIRS position chosen from only one of the ADIRUs (normally onside first) to compute FM position in GPS PRIMARY mode.
The MIX IRS position is also combined with the bias (the vector between the MIX IRS position with the selected GPIRS or computed RADIO position) to compute the FM position when flying without GPS and RADIO navaid coverage (oceanic or continental without GPS for example).
.
To ensure accurate computation of the FM position, the FMGS has three updating modes, regardless of the mode it will always be combined with the MIX IRS position.
The FM position updating mode currently in use by the FMGC is displayed on the POSITION MONITOR page underneath the FMGC 1 and FMGC 2 positions (the related FM positions computed by each FMGC). The number of available IR positions being used for the computation mode is also displayed; for example if 3IRS/GPS is displayed then 3 IR positions are available, if 2 IRS is displayed only 2 IR positions are available and so on.
The table describes the different modes used to update the FM position.
Method
Mode
Description
IRS / GPS
GPS PRIMARY
The FMGC selects ONE of the three ADIRU GPIRS positions and will use the selected GPIRS position to be combined with the MIX IRS position to update the FM position.
Normally the onside ADIRU GPIRS is used first and the selection follows a priority logic:
1st priority = FMGC 1 will use GPIRS 1 and FMGC 2 will use GPIRS 2 (onside GPIRS)
2nd priority = The FMGC will use GPIRS 3 (backup GPIRS)
3rd priority = FMGC 1 will use GPIRS 2 and FMGC 2 will use GPIRS 1 (offside GPIRS)
The MIX IRS position + the selected GPIRS position = FM position
The system will constantly select the most accurate of the three ADIRU GPIRS positions considering it’s figure of merit (FOM) and priority logic as described above. If the selected GPIRS does not comply with an integrity criterion based on a Horizontal Integrity Limit (HIL) and it loses the ability to detect failed satellites the FMS will reject the GPS PRIMARY mode and switch over to RADIO UPDATING.
IRS / RADIO
RADIO UPDATING
The FMGC uses the onside (or offside if onside receivers or onside FMGC is failed) radio navigation receivers to automatically tune to various NAVAIDs such as VORs or VOR/DMEs or LOC/DMEs or ILS throughout the flight.
By using PLACE - BEARING / PLACE - BEARING (between two DMEs) or PLACE - BEARING - DISTANCE (with a VOR or a LOC) the FMGC can triangulate it’s position and calculate a radio position to update the FM position from the radio signals.
During flight the FMGC will automatically tune the VORs to various stations as we fly, this position serves as a backup to the GPS PRIMARY mode in the event it’s lost (GPS signal lost or invalid).
IRS
IRS ONLY
The FMGC uses the MIX IRS position + the bias to compute FM position, inertial reference is the sole method of navigation.
The bias is the vector between the MIX IRS position to the GPIRS or RADIO position, this vector is constantly updated as long as the GPS PRIMARY is operative and/or RADIO positions are available.
The IRs will drift with time and after a while they will drift so much it will become useless, when the aircraft is in range of navaids the FMGC will automatically switch to RADIO UPDATING to update it’s FM position and help correct the IRU positions.
The FMGC calculates it’s navigation accuracy, this is displayed on the MCDU PROG page.
Regardless of which method is used to update the FM position each FMGC computes an Estimated Position Error (EPE), the EPE which is measured in nautical miles is the radius of which the FMGC will correctly guess it’s position with 95% accuracy. So for example a EPE of 0.03 nm means the FMGC will with 95% certainty correctly guess it’s position within 0.03 nm radius, and so on.
The Required Navigation Performance (RNP) is a value also in nautical miles in which the EPE must be less than for the FMGC to estimate it’s navigational accuracy to be HIGH. The FMGC has two estimated levels of accuracy; high accuracy or HIGH and low accuracy or LOW, you can check this on the MCDU PROG page.
When the EPE is less than RNP the FMGC will predict it’s accuracy as HIGH, when EPE is higher than RNP then the FMGC will predict it’s accuracy as LOW. However it is important to keep in mind that HIGH/LOW accuracy messages are based on ESTIMATED data, so the only way to confirm the true navigation accuracy is by doing the navigation accuracy check.
Even if LOW accuracy is displayed if the navigation accuracy check is positive, the FM position is considered reliable and managed guidance can be used.
POSITION MONITOR PAGE
This page shows the different FM positions calculated by the onside and opposite FMGC as well as the methods (IRS/GPS, IRS/RADIO, IRS) used to perform the calculation. Here is an example;
POSITION MONITOR
FMGC 1 = 4408.9N / 00754.9E <===== This is the FM position of FMGC 1
3IRS/GPS <===== 3 IRs are available and being combined with the selected GPIRS to compute the FM position, this is GPS PRIMARY
FMGC 2 = 4408.9N / 00754.9E <===== This is the FM position of FMGC 2
3IRS/GPS <===== 3 IRs are available and being mixed with the selected GPIRS to compute the FM position, this is GPS PRIMARY
GPIRS = 3308.9N / 00754.9E <===== The current selected ADIRU GPIRS that is being used by the FMGC to be mixed with the MIX IRS position)
MIX IRS = 4408.9N / 00754.9E. <===== The computed position from all three or operative IRs
IRS1. IRS2. IRS3 <===== The navigation mode (OFF, NAV, or ATT) and the IR drift from the FM position in nautical miles
NAV 0.4 NAV 0.1. NAV0.3
However in the event of failures or degradation the sources may change for example;
2IRS/GPS <==== 2 IR positions are still available to compute MIX IRS position combined with the operative GPIRS to compute FM position
3IRS/RADIO. <==== 3 IR positions are available, the MIX IRS position is being combined with the computed RADIO position to compute FM position
And etc.
Ok you've reached the end, please feel free to correct if it's wrong! Thank you for your patience and helping a bus pilot wannabe
So as a nerd working my way towards becoming a ATP pilot I do a lot of hardcore flight simming. I love aircraft systems and the A320 is perfect for this, I got my hands on a full A320 FCOM and watched all the CBT on it. I fly it exactly by the FCOM/FCTM/QRH, the ones I have is from 2022/2022/2018 respectively. Yes I know flight sim is a game and it's not the real thing but this is the closest I'll get to flying the bus save the A220 or A320 NEO series (which NGL looks really good).
I have made myself "guides" or descriptions of the aircraft systems to keep my memory sharp, I REALLY get into it.
One of the systems I love the most is the FMGS namely the position computation portion, I spend months reading the FCOM and CBT and made myself this guide to use when I fly the Fenix A320. The Fenix A320 has the BEST FMGS simulation of any desktop sim since it comes with the most features and it even has dynamic satellite coverage simulation added along with other goodies, I can use the real FCOM to fly it.
What I need help with is from a real A320 pilot or instructor to "crosscheck" and make sure my guide is correct! I want to be using solid information. It really means a lot to me.
Before we begin the Fenix has the Thales FMS2 S7 version simulated.The A320 FMGS is designed to be a hybrid-type system which means it takes all available navigation data sources such as GPS, the other IRUs, and radio navigation receivers and combines them to maximize the availability and accuracy of all data. The design philosophy of combining as much data as possible makes the A320s FMGS EXTREMELY accurate and reliable compared to other planes.
The FMGCs compute their position known as the Flight Management (FM) position, this is our position in the world as computed by the FMGS, this is how we navigate.
The system navigates the plane by updating the FM position, there are three sources for this; GPS, RADIO, and IRS. Remember the A320 will mix these when possible. The system will always select the most accurate source for the FM position computation with a priority order of GPS > RADIO > IRS ONLY (if the components are functioning correctly).
First, each ADIRU in addition to calculating it’s inertial position is also programmed to create a hybrid IRS/GPS position called GPIRS which is a combined position from it’s IRU mixed with the GPS the ADIRU has selected. The ADIRU will select the most accurate of the two GPS positions available to compute it’s GPIRS position.
The A320 has three GPIRS positions given by each of the available ADIRUs (normally 3 if all are operative)
Second, the FMGC takes the positions from all 3 IRs and combines them into a MIX IRS position which is a average IRS position of all available IRs (normally 3). This is VERY important because the mix IRS position is combined with the selected GPIRS position chosen from only one of the ADIRUs (normally onside first) to compute FM position in GPS PRIMARY mode.
The MIX IRS position is also combined with the bias (the vector between the MIX IRS position with the selected GPIRS or computed RADIO position) to compute the FM position when flying without GPS and RADIO navaid coverage (oceanic or continental without GPS for example).
.
To ensure accurate computation of the FM position, the FMGS has three updating modes, regardless of the mode it will always be combined with the MIX IRS position.
The FM position updating mode currently in use by the FMGC is displayed on the POSITION MONITOR page underneath the FMGC 1 and FMGC 2 positions (the related FM positions computed by each FMGC). The number of available IR positions being used for the computation mode is also displayed; for example if 3IRS/GPS is displayed then 3 IR positions are available, if 2 IRS is displayed only 2 IR positions are available and so on.
The table describes the different modes used to update the FM position.
Method
Mode
Description
IRS / GPS
GPS PRIMARY
The FMGC selects ONE of the three ADIRU GPIRS positions and will use the selected GPIRS position to be combined with the MIX IRS position to update the FM position.
Normally the onside ADIRU GPIRS is used first and the selection follows a priority logic:
1st priority = FMGC 1 will use GPIRS 1 and FMGC 2 will use GPIRS 2 (onside GPIRS)
2nd priority = The FMGC will use GPIRS 3 (backup GPIRS)
3rd priority = FMGC 1 will use GPIRS 2 and FMGC 2 will use GPIRS 1 (offside GPIRS)
The MIX IRS position + the selected GPIRS position = FM position
The system will constantly select the most accurate of the three ADIRU GPIRS positions considering it’s figure of merit (FOM) and priority logic as described above. If the selected GPIRS does not comply with an integrity criterion based on a Horizontal Integrity Limit (HIL) and it loses the ability to detect failed satellites the FMS will reject the GPS PRIMARY mode and switch over to RADIO UPDATING.
IRS / RADIO
RADIO UPDATING
The FMGC uses the onside (or offside if onside receivers or onside FMGC is failed) radio navigation receivers to automatically tune to various NAVAIDs such as VORs or VOR/DMEs or LOC/DMEs or ILS throughout the flight.
By using PLACE - BEARING / PLACE - BEARING (between two DMEs) or PLACE - BEARING - DISTANCE (with a VOR or a LOC) the FMGC can triangulate it’s position and calculate a radio position to update the FM position from the radio signals.
During flight the FMGC will automatically tune the VORs to various stations as we fly, this position serves as a backup to the GPS PRIMARY mode in the event it’s lost (GPS signal lost or invalid).
IRS
IRS ONLY
The FMGC uses the MIX IRS position + the bias to compute FM position, inertial reference is the sole method of navigation.
The bias is the vector between the MIX IRS position to the GPIRS or RADIO position, this vector is constantly updated as long as the GPS PRIMARY is operative and/or RADIO positions are available.
The IRs will drift with time and after a while they will drift so much it will become useless, when the aircraft is in range of navaids the FMGC will automatically switch to RADIO UPDATING to update it’s FM position and help correct the IRU positions.
The FMGC calculates it’s navigation accuracy, this is displayed on the MCDU PROG page.
Regardless of which method is used to update the FM position each FMGC computes an Estimated Position Error (EPE), the EPE which is measured in nautical miles is the radius of which the FMGC will correctly guess it’s position with 95% accuracy. So for example a EPE of 0.03 nm means the FMGC will with 95% certainty correctly guess it’s position within 0.03 nm radius, and so on.
The Required Navigation Performance (RNP) is a value also in nautical miles in which the EPE must be less than for the FMGC to estimate it’s navigational accuracy to be HIGH. The FMGC has two estimated levels of accuracy; high accuracy or HIGH and low accuracy or LOW, you can check this on the MCDU PROG page.
When the EPE is less than RNP the FMGC will predict it’s accuracy as HIGH, when EPE is higher than RNP then the FMGC will predict it’s accuracy as LOW. However it is important to keep in mind that HIGH/LOW accuracy messages are based on ESTIMATED data, so the only way to confirm the true navigation accuracy is by doing the navigation accuracy check.
Even if LOW accuracy is displayed if the navigation accuracy check is positive, the FM position is considered reliable and managed guidance can be used.
POSITION MONITOR PAGE
This page shows the different FM positions calculated by the onside and opposite FMGC as well as the methods (IRS/GPS, IRS/RADIO, IRS) used to perform the calculation. Here is an example;
POSITION MONITOR
FMGC 1 = 4408.9N / 00754.9E <===== This is the FM position of FMGC 1
3IRS/GPS <===== 3 IRs are available and being combined with the selected GPIRS to compute the FM position, this is GPS PRIMARY
FMGC 2 = 4408.9N / 00754.9E <===== This is the FM position of FMGC 2
3IRS/GPS <===== 3 IRs are available and being mixed with the selected GPIRS to compute the FM position, this is GPS PRIMARY
GPIRS = 3308.9N / 00754.9E <===== The current selected ADIRU GPIRS that is being used by the FMGC to be mixed with the MIX IRS position)
MIX IRS = 4408.9N / 00754.9E. <===== The computed position from all three or operative IRs
IRS1. IRS2. IRS3 <===== The navigation mode (OFF, NAV, or ATT) and the IR drift from the FM position in nautical miles
NAV 0.4 NAV 0.1. NAV0.3
However in the event of failures or degradation the sources may change for example;
2IRS/GPS <==== 2 IR positions are still available to compute MIX IRS position combined with the operative GPIRS to compute FM position
3IRS/RADIO. <==== 3 IR positions are available, the MIX IRS position is being combined with the computed RADIO position to compute FM position
And etc.
Ok you've reached the end, please feel free to correct if it's wrong! Thank you for your patience and helping a bus pilot wannabe
Joined: Jul 2006
Posts: 1,100
Likes: 111
From: Somewhere over the rainbow
Hello.
Very detailed summary, thank you!
Even tho I have been flying for a while on Airbus, I’m still unclear about the difference between these 2 —>
3IRS/GPS: Is it combined with both GPS? If not then what is the difference with FMGC1 or FMGC2 position as my understanding each FMGC uses a mix of 3 IRS + GPS1 for FMGC1 and GPS 2 for FMGC2.
GPRIS: “Current selected ADIRU GPRIS”? Don’t they always use all 3 IRS and one GPS to compute FMGC position?
Very detailed summary, thank you!
Even tho I have been flying for a while on Airbus, I’m still unclear about the difference between these 2 —>
3IRS/GPS <===== 3 IRs are available and being mixed with the selected GPIRS to compute the FM position, this is GPS PRIMARY
GPIRS = 3308.9N / 00754.9E <===== The current selected ADIRU GPIRS that is being used by the FMGC to be mixed with the MIX IRS position)
GPIRS = 3308.9N / 00754.9E <===== The current selected ADIRU GPIRS that is being used by the FMGC to be mixed with the MIX IRS position)
GPRIS: “Current selected ADIRU GPRIS”? Don’t they always use all 3 IRS and one GPS to compute FMGC position?
Thread Starter
Joined: May 2022
Posts: 6
Likes: 0
From: Michigan
Hello.
Very detailed summary, thank you!
Even tho I have been flying for a while on Airbus, I’m still unclear about the difference between these 2 —>
3IRS/GPS: Is it combined with both GPS? If not then what is the difference with FMGC1 or FMGC2 position as my understanding each FMGC uses a mix of 3 IRS + GPS1 for FMGC1 and GPS 2 for FMGC2.
GPRIS: “Current selected ADIRU GPRIS”? Don’t they always use all 3 IRS and one GPS to compute FMGC position?
Very detailed summary, thank you!
Even tho I have been flying for a while on Airbus, I’m still unclear about the difference between these 2 —>
3IRS/GPS: Is it combined with both GPS? If not then what is the difference with FMGC1 or FMGC2 position as my understanding each FMGC uses a mix of 3 IRS + GPS1 for FMGC1 and GPS 2 for FMGC2.
GPRIS: “Current selected ADIRU GPRIS”? Don’t they always use all 3 IRS and one GPS to compute FMGC position?
"Current selected ADIRU GPIRS" ===> Normally in this case it's the onside GPIRS (following the priority logic).
Normally MCDU 1 is connnected to FMGC 1 and MCDU 2 is connected to FMGC 2. Again in normal ops the FMGCs use their onside ADIRU GPIRS (remember the system selects ONE of the three available ADIRU supplied GPIRS)
MCDU 1 POSITION MONITOR page shows GPIRS of ADIRU 1 (if the ADIRU is operative)
MCDU 2 POSITION MONITOR page shows GPIRS of ADIRU 2 (if the ADIRU is operative)
Normally each ADIRU (1 and 2) selects one of the two GPS receivers to compute their GPIRS position, this selection is based on which GPS receiver is the most "accurate" considering their FOM or HIL.
3IRS/GPS ====>. 3IRS means that three ADIRUs are operative, 3 inertial positions are being used (MIX IRS), GPS I believe is the onside ADIRU GPIRS (again GPIRS 1 for FMGC 1 supplied by ADIRU 1 and GPIRS 2 supplied by ADIRU 2 for FMGC 2)
The FMGCs use a priority logic to select the GPIRS position for FM computation in GPS PRIMARY. Normally first priority is given to it's onside ADIRU in this case ADIRU 1 supplies it's GPIRS position to FMGC 1 and ADIRU 2 supplies it to FMGC 2. If the onside ADIRU fails then the FMGCs switch to GPIRS 3 supplied by ADIRU 3. Final priority is given to the offside GPIRS, FMGC 1 will use GPIRS 2 and vice versa.
Now ADIRU 1 may for example select GPS MMR1 (or GPSSU) for that ADIRU to compute it's GPIRS but the next flight it may select GPS MMR2! The ADIRUs are VERY picky and they always choose the most accurate of the two installed GPS receiver positions to compute their GPIRS. Could be wrong so please correct if I'm mistaken.
The A320 is a nerd's aircraft, I love it! I hope I get to fly one here in the next 10 - 15 years.
