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Present Position Updating. How?

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An interesting question has been brought up and it might be fun to get some feed back on this topic. This is all about how the airplane's Present Position is maintained for navigation. Exactly how does this occur? Gotta do a bit of setup first. The FMS provides the function of providing navigation calculations. The main piece of information it needs to provide this function is the airplane's present position. From this, all other information is derived. It doesn't know where London is. It has absolutely no idea. It only knows where London is relative to where the airplane is right now. "It's that-a-way from where I am right now." says the FMC. "6173 miles as the crow flys, nor-by-nor east." It's critical that the FMC has a precise understanding of where it is. It must know its Present Position (PP).So how does it tell? There are three systems that can provide it this information. The IRS, the GPS and the Navigation radios (VOR/DME). Any one of these has the potential to provide PP to the FMS, but which one (or ones) actually provide this function? For years, the INS was used to provide PP data to the older types of "FMS." They weren't call that back then. It was an

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"Then came IRUs (Inertial Reference Units, or as a system, IRS.). This is just an updated but down-graded INU (INS). It has fewer moving parts and is a lot more accurate and stable for maintaining PP . . . but they took out the FMS and put in a separate box. RNAV updates were still used to update PP to these even these more accurate IRSs, but crews say that RNAV updating is really unnecessary. So this

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>But to satisfy the needs of the a certification process, the>747-400 (and all modern Big Jets) still have all three of>these systems. Old habits are hard to break, but new habits>are almost impossible to certify in short order. GPS will be>the survivor, but its going to take decades. However right>now, the 3 systems are installed and intermingle in some way>to provide the FMS Present Position data.>I'm yet another avionics maintenance engineer checking in... (Gulfstream IV, specializing in the Honeywell SPZ-8400)GPS will be (is) the "top dog" in terms of position locating, but the IRU's are vital too, as they are the aircraft's sole source of attitude and heading information, irrespective of their navigation function. I don't see any way that they would go ever "go away" in the current fleet, as that would require a retrofit with a completely different AHRS system.I know that not all 747-400 aircraft are equipped with GPS... It wasn't available when the 744 first entered service, and I'm sure some of the older members of the fleet still don't have it.On the G-IV, using either the SPZ-8400, or the older SPZ-8000, the GPS will provide 100% of the navigational position data to the FMS under normal circumstances. (Assuming no RAIM errors). Next in order of importance in the NAV "mix" is DME-DME, then VOR-DME, and finally IRU position. GPS position is normally used on the ramp to initialize the the IRU's, rather than surveyed LAT/LON (i.e. published gate coordinates).Like the 747, the G-IV FMS will autotune VORs to help maximize the IRU's positional accuracy, but again, the GPS is "the" primary NAV source most of the time.On most G-IV's there is a standard Honeywell LazerRef control head in the center console. In normal ops, it doesn't get used - but in case of a complete failure of both primary and secondary FMS systems (Nav computers, CDU's or what have you), the crew can still perform INS navigation using the control head, the "old fashioned" way... by entering waypoints in LAT/LON format. (And, just like the old Delco INS in the 747-200, the LazerRef control head will only handle 9 waypoints at a time.)The control head will also give a digital readout of heading, cross-track error, ground speed and winds aloft etc.It's nice having that as a backup. Don't know if the 747-400 has a similar interface to directly control the 3 IRU's. I've never seen one in any 744 cockpit photos, but perhaps it's placed somewhere else on the flightdeck.Jim Barrett

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"Don't know if the 747-400 has a similar interface to directly control the 3 IRU's. I've never seen one in any 744 cockpit photos, but perhaps it's placed somewhere else on the flightdeck."Not on the 744 Jim ;) There's only a small panel for the mode selector switches."It wasn't available when the 744 first entered service, and I'm sure some of the older members of the fleet still don't have it."Retrofits are possible, but, yes, there are some 744's still without it (all our 744's have been updated)"Next in order of importance in the NAV "mix" is DME-DME, then VOR-DME,"Don't forget to squeeze LOC-DME inbetween GPS and DME-DME ;) LOC on the 744 has a higher priority than on aircraft such as the 737NG.Cheers.Q>

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Looks like the Gulfstream is still using the older INSs to provide attitude and present position. But it also seems, as usual, the G series has an even more functional (and cleverly implemented) navigation than Boeing. In this case older really is better. As I was observing before, the INS was an all-in-one system that had an quasi-FMS built in to it. It was called the

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> The FMS simply uses radio data as a basis of comparison so it > can "Update" the raw IRU present position to a more accurate > present position......>The FMS simply uses GPS data as a>basis of comparison so it can "Update" the raw IRU present>position to a more accurate present position.

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An observation suggesting that the FMC's do not up date the Inertial Reference Units at any time seems reasonable to me. Why would one even consider that the FMC would update an Inertial Reference Unit? . . .with what? A firmware upgrade? (I imagine that occurs at the factory.) With the old INSs, we update them every month with a new navigation data base. Just did one today. But that

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Sam...You'll find differences between the Boeing Manuals and operations and customer manuals all the time. It's very frustrating at times. The operations manuals seem to be more accurate as they appear to be updated more often.I wasn't sure if you were speaking metaphorically or not about the navigation module being removed from the IRU. I've never heard of a navigation module, but that doesn't mean it doesn't exist.I was once told by a guy from Honeywell that the FMC doesn't mix the Nav Radio position with the IRU position (despite what we read in the Boeing Maintenance Manul). Rather, the FMC will use a radio only solution... if the radio position is not too far from the IRU (triple mix) position, otherwise the radio position is rejected. Who to believe... that is the question. The Boeing Maintenance Manual is the engineer's "bible", but you'll find errors in it all the time. We are supposed to report errors, but there are just too many errors and too few hours in the day to do this kind of stuff. The last error I reported took 6 months to generate a change in the Boeing manuals.So what do we believe? Something some guy tells us... or something which is written down in a manual... Who knows.I understand what you say about the radio position not being able to be used alone, but strangely, with the IRU's switched off on the 744 and the aircraft being moved about, the FMC LAST POSITION changes with the GPS position. However, this data doesn't seem to be doing anything useful in the cockpit (as the map displays don't operate without the IRU's running.... neither does the autopilot, so you won't be able to use LNAV/VNAV and the other modes).Live and learn ;)Cheers.Q>

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