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IRS Auto Updating While Airborne

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IRS Auto Updating While Airborne

Old 6th Apr 2022, 01:10
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IRS Auto Updating While Airborne

B737 -300 / 400,.. NON GPS, Just DME/DME is the equipment I am asking about. After initialize IRS on ground, then the "update at TOGA" and reaching line of sight, the auto tune starts picking up various DME's from in range stations and gives a valid navigable lateral path. What specifically is driving the map or needle? is the FMC processing this DME information and driving the nav solution? or are the DME stations constantly updating the IRS/IRU's, then the IRS feeds this to the FMC's for navigation? The crux of the question is this, is the IRS once initialized on the ground constantly updating it's position with DME or will it only navigate when all other sources are failed?
This picture taken 350 miles from nearest station.

barrow is offline  
Old 6th Apr 2022, 03:10
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It was a while ago, but…

If I recall the IRSs do not update, they are a source of input to the FMC. TOGA updates the FMC position to the selected Runway or intersection. DME/DME or Nav updating only updates the FMC. In the absence of that, you will have IRS drift, which would be outside RNP tolerances.

The CDU displays a Radio position, IRS or FMC (best guess) position. Dual FMC option is certified as sole source of navigation.
awair is offline  
Old 6th Apr 2022, 05:40
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Love you picture, an airplane at last!

Somewhere inside the FMC is a prompt to display IR positions on-screen (green rings). Use it to investigate and see what is going on the next time you are returning inside NAVAID coverage. [I am definitely not saying to switch-off RADNAV update to test this, and never done it myself ]

Sure, the IRs are stand-alone and receive no outside inputs. Other than that you're asking the right questions. Check The Boeing 737 Technical Site - Home Page which is a pillar of wisdom that started well in the pre-NG days (the book is a must have).
FlightDetent is offline  
Old 6th Apr 2022, 06:38
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As others have said and AFAIK the IRSs do not realign in flight.

The FMC present position is probably the core variable for most navigation and that is derived by a cunning mix of IRS, GPS, radio aids and occasional other inputs such as the press of the TOGA switch on take off.
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Old 6th Apr 2022, 07:10
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Funny thing on the -3/4/500 without GPS and only 2 IRs it is simple enough to understand and even visualize.

Last edited by FlightDetent; 6th Apr 2022 at 18:32.
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Old 6th Apr 2022, 13:11
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As others have said, once the IRSs have aligned, that’s it. I think even the runway update only affects the FMC position, not the IRS. The FMC position will be based on:

LOC/DME (during approach)

in that order, as far as I can remember.

During my FO initial line training on the 737-300 in 1987 (pre-GPS), we were on a LGW-TFS sector. Some time after coasting out near Faro, we got the “IRS NAV ONLY” message. My trainer asked me, “so, what’s navigating the aircraft now?” “IRS”, I replied confidently. “Actually, it’s still the FMC, but the FMC position will gradually drift towards the IRS position, until we get radio updates again” came the wise response.

Fast forward a few years and this time I was in the LHS, again approaching Faro, our destination. Suddenly the aircraft turned left. Inspection of the EHSI showed that the pink string was a few miles to our left and as we were in LNAV, the autopilot was doing its best to keep up. “Ah, we must have lost radio updating and the FMC is using the IRSs now”, I informed the co-pilot. Imagine my surprise when he said, “actually, we’re still using DME/DME!”

POS INIT page showed that the radio (and FMC) position and the IRS position differed by about 5nm, which was a bit excessive after only 2.40 flight time. We reverted to “raw data” and confirmed that in fact the DME/DME position was in error. Staying with old-fashioned methods for the reminder of the descent, approach and landing, proved uneventful (day, VMC).

On the northbound home sector, we were prepared and when the same thing happened, we inhibited the DMEs that the FMC was agility-tuning until we were well clear of the area. It turned out that the FMC was using some TACAN equipment which had been moved temporarily on the back of a lorry for a military exercise!

Great photo, by the way! Brings back happy memories…..
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Old 6th Apr 2022, 18:31
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Flying about 25 mins without RAD NAV coverage towards Baku, first visit ever, night IMC at 4 a.m. body clock. A bit later than expected update came alive and all looked nicely throughout the RNAV arrival.

Attempting to join the ILS nothing matched well, especially DME and GP. Turns out the map shift was almost 3 miles (closer to the field). Seems the radio introduced the error, probably based on scalloping single-source Baku VOR or Yerevan across the mountain range.
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Old 6th Apr 2022, 21:20
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Its been a while so some of the numbers maybe incorrect. It is a pilot explanation not a detailed technical explanation.

The IRS does not update position but the FMC using IRS and external inputs does update its position. The FMC navigates the track.

Without GPS the FMC updates position usually by DME/DME/IRU. After 12 minutes without FMC DME/DME/IRU updating the message IRS NAV ONLY appears.

The FMC begins to apply a 'worst case' IRS drift allowance of 0.833 NM/hour to the ANP, so after about 2.3 hours without a DME/DME/IRU update (or LOC/DME, VOR/DME if enabled in your aircraft) you will see the message 'unable reqd nav perf RNP'. This does not actually mean the aircraft is actually off track, it just means that the confidence of the nav solution is outside calculated statistical probability of position. Note this is based on the worst case drift allowance (I was told that it was a rate based of errors in an iron gyro system applied to a ring [email protected] gyro for certification but don't have any documentation to confirm). Ring [email protected] gyro usually performs much better than that which is why you only see small map shifts when the FMC is eventually updated with a good DME/DME update. The ANP will rapidly decrease once a good position update is obtained.

Single navaid destinations were always problematic as the position update was never very accurate, and I recall that there was an altitude and distance limit before an update occurred (15,000 feet and 25 NM are numbers I recall but it may be in error after many years out of the B737).
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