I am confused about the following issues. I would appreciate if experts can help me with these: Question 1:Generally in any modern jet, my understanding is that the FMC has the capability of auto tuning the navigation radios in accordance with the entered flight route, for the purpose of continuous updating the IRS. The classing 737s which did not have a GPS, had a VHF NAV panel that looked more straight forward for me than the NG panel. In the classic 737, the panel had a switch to go between AUTO and MANUAL tuning. There was separate displays for the active frequency that show precisely what is the active mode, and what is the active frequency. The 737NG does not have any sings of auto-tuning the radios. My question is: Does this mean that the IRS relies only on the GPS for continuous updating? And if so, why do we bother making sure that our flight plans have enough ground aids along the route? Isn't this necessary anymore? And if the FMC does auto-tune the radios, how is this indicated on the displays, and how can I switch between the manual and the auto-tuning? Question2:My understanding about the VNAV, is that it can manage airplane speed and vertical path by assigning an altitude and speed for each fix along the route. LNAV drives the airplane laterally along the route. My question is: How can the VNAV stay active if I disengage the LNAV? I tried this several times with the NGX, and the VNAV does not disengage when I switch from LNAV to HDG SEL while following radar victory for the approach. How can VNAV still manages the airplane while it is not following the route fixes?

HiThe answer to your first question is in FCOMv2 11.20.9 under Distance Measuring Equipment.

I am confused about the following issues. I would appreciate if experts can help me with these: Question 1:Does this mean that the IRS relies only on the GPS for continuous updating? And if so, why do we bother making sure that our flight plans have enough ground aids along the route? Isn't this necessary anymore? And if the FMC does auto-tune the radios, how is this indicated on the displays, and how can I switch between the manual and the auto-tuning?

The FMC uses GPS, IRS, and DME. I think it can grab LOCs even. I think it's on the NAV STATUS page where you can toggle the sources of the updates. To see the sources - the actual frequencies - is also available, I think it's somewhere on the INIT REF page. Maybe NAV UPDATES. You can actually see the different VORs queried. Question 2, I dunno. I know it can stay in VNAV, but I don't know how it starts to control its path if it's not on a path. I always switch out to LVL CHG or V/S. Great question.

You are right, the aircraft has to be in LNAV in order to VNAV. In the G-5 you get a "couple data invalid" warning if you change your mode from LNAV while using VNAV. The aircraft will hold what ever descent and speed it had when disconnected.

The aircraft continually up

I am confused about the following issues. I would appreciate if experts can help me with these: Question 1:Generally in any modern jet, my understanding is that the FMC has the capability of auto tuning the navigation radios in accordance with the entered flight route, for the purpose of continuous updating the IRS. The classing 737s which did not have a GPS, had a VHF NAV panel that looked more straight forward for me than the NG panel. In the classic 737, the panel had a switch to go between AUTO and MANUAL tuning. There was separate displays for the active frequency that show precisely what is the active mode, and what is the active frequency. The 737NG does not have any sings of auto-tuning the radios. My question is: Does this mean that the IRS relies only on the GPS for continuous updating? And if so, why do we bother making sure that our flight plans have enough ground aids along the route? Isn't this necessary anymore? And if the FMC does auto-tune the radios, how is this indicated on the displays, and how can I switch between the manual and the auto-tuning?

The NG´s navigation suite continually uses the GPS signal for position updating alongside IRS position and Radio position. In other words, the first source for FMC position updating will always be the GPS, unless the signal becomes degraded, unavailable, or GPS updating is switched off by the crew using the NAV STATUS pages in the CDU. Regarding radios, the following priority order is used for position updating:1 two or more DME stations2 one VOR with a collocated DME3 one localizer and collocated DME4 one localizer. Now, from FCOM vol. 2 (11.31.5) The station identifiers and frequencies of the selected radio navigation aids are displayed on the NAV STATUS page 1/2.FMC logic selects the GPS position as the primary update to the FMC position. If all GPS data becomes unavailable, the FMC reverts to radio or IRS updating.The dual frequency–scanning DME radios are automatically tuned by the FMC. The stations to be tuned are selected based upon the best available signals (in terms of geometry and strength) for updating the FMC position, unless a specific station is required by the flight plan. Radio position is determined by the intersection of two DME arcs.If the DME radios fail, or if suitable DME stations are not available, FMC navigation is based on IRS position information only. The two VHF Nav radios are used by the FMC for localizer updating during an ILS approach and by the crew for navigation monitoring. Now comes my bit os speculation: as you probably know, DMEs and VORs are not the same thing, They are different pieces of equipment using different frequencies. What happens thw vast majority of times is that a VOR station might be collocated with a DME station, and when you tune the VOR you automatically tune the DME. The FCOM makes it clear that the NG has a DME scanner which will continually choose and tune two DMEs for position updating purposes. This makes it clear that the NG in fact, does have autotune capability. What is unclear is whether these are separate radios within the aircraft that cannot receive any input by the crew or whether these are the DME receivers associated with the NAV1 and NAV2 radios used by the crew. In this instance, should the crew tune a VOR or VOR/DME into either of their nav radios, then this would supercede the station which was previsouly autotuned by the FMC. A second possibility which makes less sense to me is that the NG has two "ocult" nav radios that are used solely by the FMC to provide adequate and continual position updates and which indeed are autotuned. Thus they would work independently of what is tuned by the crew, essentially making the airplane have 4, rather than 2 nav radios. I wonder which is the case. I would bet on the former. edit: I´m betting most real world pilots would become confused as well if they stopped to think about it and went to the FCOM for answers. One way to try and solve this would be to have PMDG ask Boeing directly

Now, from FCOM vol. 2 (11.31.5) The dual frequency–scanning DME radios are automatically tuned by the FMC.

I read this that each Nav Radio is capable of tuning to TWO frequencies at the same time, Think of it as TWO seperate Receiver Tuners, in each Radio unit ( Like some DVR have DUAL TUNERS, and can tune to two TV channels at the same time), If this is so, then it would explain how the FMC can be tuning one, while the Crew are using the other. ie Maybe the FCOM is saying: Each of the (2) dual frequency–scanning DME radios (have one of their 2 dual frequencies) automatically tuned by the FMC. This would seem to make sense, but may not actually be true -- maybe I am reading too much "between the lines" of the FCOM text. -----Notice also the word SCANNING. If the FMC is going to use the STRONGEST signal, it will have to spend some of it;s time, SCANNING the frequencies, to determine the one with the strongest signal.I am guessing it does this "occasionally", and spends most of it;s time tuned to the one it has determined is the one it wants.

Sorry, in the end I didn´t answer your questions. I´m not a RW NG pilot but I have read the FCOM extensively regardign these issues because I have had the same questions in the past. So I´ll try to give you my take on these matters. "Does this mean that the IRS relies only on the GPS for continuous updating?" - First off, it´s not the IRS that relies on the GPS for updating. It´s the FMC. The FMC relies primarily on GPS for position updating, unless something happens to the signal. The airplane always has another system to fall back on if the GPS fails or is inhibited. IRS and GPS are independent systems which update the FMC position. But GPS does not update IRS position. From what I gather IRS position starts drifting once airborne and can only be "updated" once the acft parks and is realigned. In this instance, you end up using GPS position to update the IRS. But not while in flight. "And if so, why do we bother making sure that our flight plans have enough ground aids along the route? Isn't this necessary anymore?" - Years ago it was necessary to navigate form station to station. Nowadays it´s for redundancy. But you are correct that this is becoming less necessary in modern days and the airspace is changing to reflect this. One example is Brazil. Many new Rnav airways were created in the last 5-10 years where you can navigate sometimes 500-1000 nm without relying on a single navaid, shortening your trips´ segments. Basically the acft relies on GPS and IRS position to navigate in these instances, like in transoceanic flights. This doesn´t happen in the US where the ground is littered with thousands of navaids, but in the dense amazon forest it´s really hard to service a radio station. "And if the FMC does auto-tune the radios, how is this indicated on the displays, and how can I switch between the manual and the auto-tuning?" -For the answer to that, read my first post. It certainly is available, and you can´t mess with it, unless you manually tune a radio. To see this, goto NAV STAUS pages in the CDU and/or select pos update on the EFIS.

I read this that each Nav Radio is capable of tuning to TWO frequencies at the same time, Think of it as TWO seperate Receiver Tuners, in each Radio unit ( Like some DVR have DUAL TUNERS, and can tune to two TV channels at the same time), If this is so, then it would explain how the FMC can be tuning one, while the Crew are using the other. ie Maybe the FCOM is saying: Each of the (2) dual frequency–scanning DME radios (have one of their 2 daul frequencies) automatically tuned by the FMC. This would seem to make sense, but may not actually be true -- maybe I am reading too much "between the lines" of the FCOM text.

That is the fundamental question... I´m beginning to think you´r right. The NG has "4" nav radios. Two used by the crew for navigation. two used by the FMC for pos update

As far as 4 radios are concerbned, I researched, and found this additional information.

Source: http://www.b737.org....on.htm#Position Radio Position: This is computed automatically by the FMC. Best results are achieved with both Nav boxes selected to AUTO (happens automatically on NG), thus allowing the FMC to select the optimum DME or VOR stations required for the position fix. Series 500 aircraft have an extra dedicated DME interogator (hidden) for this purpose and NG's have two. Radio position is found from either a pair of DME stations that have the best range and geometry or from DME/VOR or even DME/LOC.

ie The NG has TWO "HIDDEN" extra dedicated DME interogators for automated tuning use by the FMC

Guess that settles it! Thanks for looking into it Geoff

I wonder if the hidden Radios have seperate Breakers ? Needle in a haystack ? http://www.b737.org....it_breakers.pdf

The NG only has 2 DME breakers. That -500 CB list has three. I'd say each is one box with the ability to do two things at once (multi-mode).

Thank you guys for the comprehensive responses. I did consult the FCOM before I get here, but was not very successful in extracting precisely the information I need. Regarding the two hidden DME radios, aren't those supposed to be displayed on the NAV STATUS page? This page shows six values arranged in two columns. The two values in the 1st row are always showing the frequencies tuned on the NAV1 and NAV2 radios. The other four values keep changing all the time, and show frequencies for different stations along the route. What does this mean? Are those FOUR hidden DMEs that keep automatically tuned all the time? If so, then everything is nearly clear now. If not, then probably I need to explore more FCOM to see an explanation for the structure of the NAV STATUS page. Special thanks for Victor.

The NG only has 2 DME breakers. That -500 CB list has three. I'd say each is one box with the ability to do two things at once (multi-mode).

Would be interesting to see an Electrical Schematic ... but with 35+ miles of wiring, thats going to be a very big PDF !! Interesting stuff (at least to me it is), but not really necessary info to fly the plane. I wonder thing like : What happens if you try to turn the landing lights on when on battery power, and the battery is low.Do they FLASH on and off ???Why would they Flash ON/OFF ??

Thank you guys for the comprehensive responses. I did consult the FCOM before I get here, but was not very successful in extracting precisely the information I need. Regarding the two hidden DME radios, aren't those supposed to be displayed on the NAV STATUS page? This page shows six values arranged in two columns. The two values in the 1st row are always showing the frequencies tuned on the NAV1 and NAV2 radios. The other four values keep changing all the time, and show frequencies for different stations along the route. What does this mean? Are those FOUR hidden DMEs that keep automatically tuned all the time? If so, then everything is nearly clear now. If not, then probably I need to explore more FCOM to see an explanation for the structure of the NAV STATUS page. Special thanks for Victor.

4 Hidden DMEs ? They seem to be BREEDING !!. I thought be were talking about 2 normal visible "Pilot operated" DMEs (on Nav1 & Nav 2), and then two HIDDEN DMEs that the FMC controls and tunes automatically, to help determine position.

Two DMEs. Each is multi-mode thus can track your favorite DME and the FMC's favorite. Because sharing is nice. That's how I read it, anyway.

Two DMEs. Each is multi-mode thus can track your favorite DME and the FMC's favorite. Because sharing is nice. That's how I read it, anyway.

Hahaha. It´s been a long time since I last delved deep into the CDU´s more occult pages such as NAV STATUS. I don´t remember seeing four stations being autotuned, and I´ll give it a go when I get home tomorrow evening (on call tonight unfortunately). I do remember fidgeting with the 777´s FMC (in this case the real bird) and things are way simpler there. That specific airplane doesn´t have nav radio panels in the pedestal anymore. You just use a dedicated page in the FMC for nav radios where you can select your favorite radio station either by freq or id (makes things a lot easier). If you leave it blank the plane goes on autotuning all the way home. I now wonder though whether it has hidden, breeding DME receivers somewhere in the electronics bay. No need to thank Islam, this has been gnawing at my brain for some time too and I never had the pacience to post about it. You forced my hand hehe. And to be honesy I think I´m way over my head here, talking to an NG driver and an engineer on the same thread. If you guys want to know something about medicine I might be more of a help!