737ngx How to navigate when IRS fail?

[kekstrom 1]

I have a question: How should I navigate if both of the IRS (L and R) fail? Because when I switch them off inflight my ND goes blank and all that I have left is the old-school magnetic compass. I understand that autoland doesnt work because of the loss of IRS data mut it should not affect the GPS? Shouldn't the GPS of the aircraft be sufficient to provide waypoints, distances etc to the ND? Of course the atc can give me vectors etc but I dont believe its realistic that all the aids for navigation are lost if both IRS fail? Only procedure I can find in the QRH is the one that tells me to put the IRS to att to get attitude information and I get that but what shoud I do to know where I am and where I am going??

 

Could someone with more knowledge about IRS/GPS systems of the 737-/800/900 explain this to me?

 

Thanks in advance!

[PHXMD-11 86]

Do not touch those switches in flight unless directed by the QRH. Align the IRS and move the selectors to NAV and leave them alone.

 

Fly the aircraft into a level attitude and hold that. Maintain speed. Switch the IRS selectors to ATT. I think you will also need to insert your position into the FMS. That will give you your instrumentation back. I also think you have to use the conventional VOR-VOR navigation. However, I don't think you have the capability after that to autoland the aircraft.

[sandpatch 9]

Yeah I too have wondered OP's question. It is also the same thing when aligning the IRS. I enter the position but I still have to wait 8+ minutes until the map shows up. In the MD11 (with GPS) it pops up immediately but without you have wait too.

 

Seems like a GPS would be quite nice to have during a communication loss.

[PHXMD-11 86]

Yeah I too have wondered OP's question. It is also the same thing when aligning the IRS. I enter the position but I still have to wait 8+ minutes until the map shows up. In the MD11 (with GPS) it pops up immediately but without you have wait too.

 

Seems like a GPS would be quite nice to have during a communication loss.

 

You can set the alignment time for the IRS via the FMS options menu. I think there is a GPS on the 737. It just isn't the default FSX one.

 

The only reason you would have three radios fail is if you have a complete electrical failure which is nearly impossible. The IRS ATT mode will provide heading information when it is entered and the ALIGN light is extinguished. Fly the headings you have on your flight plan (I think the default FSX planner does give these headings. So you aren't screwed if you're having a REALLY bad day.

[sandpatch 9]

You can set the alignment time for the IRS via the FMS options menu. I think there is a GPS on the 737. It just isn't the default FSX one.

 

The only reason you would have three radios fail is if you have a complete electrical failure which is nearly impossible. The IRS ATT mode will provide heading information when it is entered and the ALIGN light is extinguished. Fly the headings you have on your flight plan (I think the default FSX planner does give these headings. So you aren't screwed if you're having a REALLY bad day.

 

Yes Kenny, we all know how to read a QRH or adjust settings in the PMDG FMC. The whole was why you can't use GPS position information to display the map, not how to cheat in the simulation.

[ajacied551 1]

The Air Data Inertial Reference Unit (ADIRU) senses motion along the three axis's of the airplane Pitch, roll and yaw by three accelerometers Each accelerometer senses acceleration along one axis. The accelerometer can sense very large and very small accelerations along this axis.

The ADIRU uses accelerometers and gyros to sense the earth rotation rate and gravity. Earth rate and gravity are then used to calculate:

 

* Local Vertical

* True North

* Present Position Latitude

 

After the ADIRU has measured these values and present position (latitude and longitude) is entered, the ADIRU completes its alignment to true north and is then ready to navigate. ADIRS alignment time will vary based on local latitude.

 

If the ADIRU detects airplane movement durring aligment mode, the alignment process will stop. After the motion stops, a new alignment will start.

 

When the IRU fails in NAV mode during flight, switch the applicable IRU to the ATT mode and enter the correct heading ( use the heading from the Standby Magnetic Compass).

When the airplane is moving keep the IRU's in NAV mode. there no need to switch the IRS to OFF. otherwise you need to continue in the ATT mode for the rest of the flight. also the autopilot and other functions on the airplane system's is limited.

[kekstrom 1]

The Air Data Inertial Reference Unit (ADIRU) senses motion along the three axis's of the airplane Pitch, roll and yaw by three accelerometers Each accelerometer senses acceleration along one axis. The accelerometer can sense very large and very small accelerations along this axis.

The ADIRU uses accelerometers and gyros to sense the earth rotation rate and gravity. Earth rate and gravity are then used to calculate:

 

* Local Vertical

* True North

* Present Position Latitude

 

After the ADIRU has measured these values and present position (latitude and longitude) is entered, the ADIRU completes its alignment to true north and is then ready to navigate. ADIRS alignment time will vary based on local latitude.

 

If the ADIRU detects airplane movement durring aligment mode, the alignment process will stop. After the motion stops, a new alignment will start.

 

When the IRU fails in NAV mode during flight, switch the applicable IRU to the ATT mode and enter the correct heading ( use the heading from the Standby Magnetic Compass).

When the airplane is moving keep the IRU's in NAV mode. there no need to switch the IRS to OFF. otherwise you need to continue in the ATT mode for the rest of the flight. also the autopilot and other functions on the airplane system's is limited.

 

Thanks for the info-filled answer. Although everything mentioned above is something I already knew. If you read my original question you can see I was wondering why the ND doesn't show positional data when both of the IRS systems fail. As I said, you'd think the onboard GPS could provide sufficient positional information.

 

Of course I wouldn't normally switch off both of the IRS systems inflight. I did that just to test what would happen if they both somehow failed. Thanks anyway!

[Paul Deemer 13]

Check this out. Scroll down to 737 series, but all models are discussed and what happens in the event of complete loss of IRUs.

 

 

http://www.boeing.co...v_by_model.html

 

In a quote from the 737 Section says this.

 

 

In the event of dual IRU failure, the left GPS true track is displayed on page 3 of the PROGRESS pages on the CDU. This serves as a tertiary backup for the airplane heading or track and an update to IRS ATT mode.

[davierosoft 13]

Just to add things:

First of all, the aircraft is equipped with stby instruments wich will let you fly if both IRS fails.

An IRS could fail in different ways, one of them could only refer to the accelerometers, and not the laser gyros, in this case using the stby instrument you could use the ATT function to align in attitude only mode recovering the artificial horizon. You will need to mantain the aircraft leveled and with a costant speed, to do it use the stby instruments and the other avaiable.

You need also to tell to the IRS the heading as it cannot detect it if it does not perform a full alignment. (it needs to detect earth movements)

when in ATT mode and leveled it will recover attitude informations in around 30 seconds, but the time depends on how stable you are, or in what condition IRS is (if it is going to be faulty probably will not align)

 

Also remember that GPS will not give you any attitude information, GPS will only give you a position, a computer could draw a line between coordinates giving you a speed, a direction, but not the attitude, GPS is useless in this case.

Remember also that there is no connection between GPS and IRS, they are independent and only FMS uses both for position update, however, if IRS is lost, navigation function of the FMC will be completely lost.

[captdini 3]

Well Kekstrom,

 

If you knew all that already, then you would not be asking this question. In a simple expalnation, The CDS (Common Display System) supplies information to the 6 LCD Display Units (DU's) which it gets from the two DEU's (Displays Electronic Units). The DEU's in turn, get their information from various aircraft sensors and systems, the ADIRU being one of them, albeit , an important one.

 

The GPS is primarily used by the FMC for position updating.

 

Regards,

 

Dinshaw Parakh

[kekstrom 1]

Well Kekstrom,

 

If you knew all that already, then you would not be asking this question. In a simple expalnation, The CDS (Common Display System) supplies information to the 6 LCD Display Units (DU's) which it gets from the two DEU's (Displays Electronic Units). The DEU's in turn, get their information from various aircraft sensors and systems, the ADIRU being one of them, albeit , an important one.

 

The GPS is primarily used by the FMC for position updating.

 

Regards,

 

Dinshaw Parakh

 

 

OK, so the GPS on a state-of-the-art modern jet does nothing but gives you your present coordinates and heading? If thats the case, so be it. I just could not believe it but it seems so? Someone posted earlier that when u lose both IRS then you lose attitude information and that makes perfect sense to me. Of course the GPS only gives position (hence the name global position system) and speed calculated from that. And thats no problem since there is an standby artficial horizon on the panel that continues to work independetly of IRS. But that the GPS cant provide info on an ND about planes position and for example airfields around you or whatever is programmed in the GPS just baffles me. That if you lose sync in your two spinning gyroscopes or whatever you are completely back to basic VOR-VOR/VFR/ATC-vectored navigation?

 

Guess its for the best to pack a simple 50 dollars car-GPS with a charged battery with me on future flight. Just in case..)

[tcas_climb 45]

I know that on the 767 if you lose all 3 IRUs, then the FMC will revert to a sort of basic mode where it will lose the waypoint names in the route and they will be replaced by LATLON coordinates. I forgot what they call it.

[davierosoft 13]

OK, so the GPS on a state-of-the-art modern jet does nothing but gives you your present coordinates and heading? If thats the case, so be it. I just could not believe it but it seems so? Someone posted earlier that when u lose both IRS then you lose attitude information and that makes perfect sense to me. Of course the GPS only gives position (hence the name global position system) and speed calculated from that. And thats no problem since there is an standby artficial horizon on the panel that continues to work independetly of IRS. But that the GPS cant provide info on an ND about planes position and for example airfields around you or whatever is programmed in the GPS just baffles me. That if you lose sync in your two spinning gyroscopes or whatever you are completely back to basic VOR-VOR/VFR/ATC-vectored navigation?

 

Guess its for the best to pack a simple 50 dollars car-GPS with a charged battery with me on future flight. Just in case..)

The point is that FMC in the 737 needs a point that will be used for comparation, IRS will be that primary information source. GPS is a fully external source of info, it could also be hijacked or deactivated, but the aircraft must still be able to fly.

Nav radios are also used to detect aircraft position and have also a lot of precision. Completely hidden the FMC automatically tunes DMEs to different stations in order to detect the aircraft position wich will be compared with the others.

Now, if you loose the IRS the FMC loses the primary info for tuning DMEs, this losing a second way to detect position.

But remember that GPS is a feature of modern planes, on classics the GPS is not connected with FMC, the GPS is used only for gpws terrain computation.

So GPS is not a vital source.

[davidz 26]

OK, so the GPS on a state-of-the-art modern jet does nothing but gives you your present coordinates and heading?

 

Actually, the GPS only provides 4 pieces of information: latitude, longitude, geometric altitude and UTC time. All GPS units in the world only provide this information (except military units - they're probably hiding some special Easter egg feature in those :P)

 

The ND displayed position/heading/track is from the FMC. The FMC position is based on IRS position and updated by various sources. These other sources cannot be used by the FMC as the sole source of position information. Why? VOR/DME, DME/DME, LOC/DME, etc., are all incapable of providing an accurate enough position to avoid the FMC position jerking around. GPS doesn't update quickly enough to be smooth. Under normal conditions, it is the IRS that allows interpolation of the position as it knows what the net acceleration of the aircraft is at all times.

 

Yes - if you lose your two spinning gyros altogether, then you have a big problem. But ask yourself - when was the last time you heard an aircraft lose it's ADIRU/IRS? Last time for me, was the QF 744 that had a total electrical failure, in which case the crew had bigger problems on their hands than just navigation. ;) In any case, it is far more likely for the FMC to fail.

[sandpatch 9]

I am wondering why the GPS positions aren't enough. Spoofing is almost impossible, jamming is much easier but than it could just say "GPS jammed", "SNR = 0" or something like that in the FMC. It is enough for us in the maritime industry but not enough for an airplane? Which BTW has much more free space available...

 

Anyway, the VOR thing below the ISFD screen, is that also part of the backup instruments? I just tried entering a "IRS L FAIL" and a "IRS R FAIL" and I couldn't get the arrow to point in the right direction. All I could see was the DME distance on the ND. For those wondering, it was a VOR/DME station. SLU near ESPA. Can anyone reproduce that?

 

Actually, the GPS only provides 4 pieces of information: latitude, longitude, geometric altitude and UTC time. .

 

I know that I am nitpicking but the GPS provides GPS time, it is a bit little bit different (about 20 seconds) from UTC time http://www.leapsecond.com/java/gpsclock.htm. Dough I am pretty sure you knew that.