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Sorux123

A few questions regarding the Autopilot

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Hello folks, I did my first Radar Contact flight today and i ran into some wiered stuff with the MCP. Now the ATC started vectoring me away from my FMC route about 5nm from my ToD. After a while i get to contact approach and chose an IAP approach, to try and get back on my route. Question 1: When i chose a heading to fly so i could intercept my route again(because LNAV wouldnt let me engage it, my guess is i was to far away from the FMC route) the actual heading that the plane would fly was 5 degrees of from what i set in the MCP. How come? Question 2: Later on i canseled the IAP approach(got to messy trying to get back on the FMC route) and requested a vectored approach. So ATC eventually assigned an altitude for me, which i set into my MCP and hit the "ALT hold" button. The plane wouldn't descend to this altitude for some reason. I tried to reset the FD's, retap the AP switch, but no luck. So in the end i just had to use vertical speed to get to my desired altitude. Why wouldnt the plane descend in "ALT mode" ? Question 3: After i got my landing clerance and my final vector from ATC(10-15nm from the airport), i turned on LOC mode on the MCP. The button lit up but the plane wouldnt follow the localizer diamond on the CDU. Just a bit later i set my FMC for an ILS to my assigned runway, and only then was i able to make the plane line me up on the localizer using the "LOC" mode. Is this intended? i mean, can i only use the LOC mode if the ILS approach im going to fly is programmed into the FMC? Thanks for your time! - Arthur

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Question 1: When i chose a heading to fly so i could intercept my route again(because LNAV wouldnt let me engage it, my guess is i was to far away from the FMC route) the actual heading that the plane would fly was 5 degrees of from what i set in the MCP. How come? You are correct about the FMC not being able to get you back on the route because you are too far away from the flight plan path line, so you either need to choose to go direct to the next waypoint in your route if you want to do it automatically with the FMC, otherwise you can just manually drive nearer to the line using heading hold or flying by hand, and then line up with the flight path a bit (within about 30 degrees should be okay, but to be honest it's better to get closer to it than that if you want it to capture the line quickly), whereupon you should be able to use LNAV again. Getting off the route can cause things to freak out a bit in the NGX I've noticed (as it can in many FS aircraft), notably if you try to use accelerated time on high settings, but why you were getting a five degree offset could be a lot of things, probably some NAV mode or whatever you went into on the FMC when faffing about. Question 2: Later on i canseled the IAP approach(got to messy trying to get back on the FMC route) and requested a vectored approach. So ATC eventually assigned an altitude for me, which i set into my MCP and hit the "ALT hold" button. The plane wouldn't descend to this altitude for some reason. I tried to reset the FD's, retap the AP switch, but no luck. So in the end i just had to use vertical speed to get to my desired altitude. Why wouldnt the plane descend in "ALT mode" ? The aircraft will hold altitude in ALT mode, what you need to use is LEVEL CHANGE to actually get it to leave your present altitude and start up or down if you are controlling descents via the MCP (which is in fact quite common in the real world, where pilots will use the altitude setting in the MCP window as both a visual aid memoir and as a 'hard deck' for the level they have been cleared to, because the FMC will not override that setting and cause a level bust, so it's more useful for following ATC instructions in many respects, and certainly when using RC, since the controllers chastise you for busting levels and if you do it a few times, you'll get a crappy end of flight evaluation from RC). Question 3: After i got my landing clerance and my final vector from ATC(10-15nm from the airport), i turned on LOC mode on the MCP. The button lit up but the plane wouldnt follow the localizer diamond on the CDU. Just a bit later i set my FMC for an ILS to my assigned runway, and only then was i able to make the plane line me up on the localizer using the "LOC" mode. Is this intended? i mean, can i only use the LOC mode if the ILS approach im going to fly is programmed into the FMC? Nope, the COURSE settings on the MCP and the NAV 1 and 2 radio frequencies determine whether you will track a localiser overriding the FMC, which is in fact usually what the FMC is doing when you choose to set up an approach using the FMC, but you can simply dial in the correct runway magnetic heading in BOTH course windows on the MCP and tune BOTH nav radios to the ILS frequency and that will line you up if you go to VOR/LOC mode on the MCP regardless of what you have in the FMC, assuming you are in range of the ILS localiser transmitter, which is in fact often how many pilots will do it, since you rarely get to stay on an FMC plan all the way from take off to touchdown, and this is especially true with RC. Al

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1. No sure on this one. LNAV might not have engaged because the controls were not centred. 2. Alt Hold is exactly that. The aircraft won't descend to that altitude unless you tell it to, either by using V/S or LVL CHG. Once there, the a/c will hold that altitude. 3. Both Flight Directors on? RWY selected in FMC as an ILS approach? Both NAV radios tuned to the freq? Approach course selected?

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Thanks a lot for your answers guys! Theres just one more thing. Jeffhunter and Jasog, you guys both mention that i need to have the ILS rwy selected in my FMC. Chock on the other hand dosent mention this, and says that its the course settings and the NAV radios that determine which ILs you are tracking, regardless of whats in your FMC. Or perhaps i missinterperated what you were saying regarding that; Chock? Confuses me a bit, since as i wrote in my initial post i had a different runway programmed into my FMC than the one i was tracking via the COURSE settings and the NAV radios. And once i programmed the proper runway into the FMC, i was able to get established on the localizer via the "LOC/NAV" mode. But then again, at this point i was MUCH closer to the runway than i had been when i first tried to catch the localizer with the "LOC/NAV" mode. And this leaves me to think that i might have been just too far away to track the localizer the first time, like you mentioned Chock. How do i actually know if i am close enough to hit the "LOC/NAV" button? The first time i was trying(that's when it failed) i saw the Localizer diamond all the way to the left on the CDU, and it wasn't coloured on the inside. That probably dosent make sense lol, but what i mean is the diamond would only have purple grids, the inside of the diamond is black or whatever. Not purpler at least :( Once again, thank you! Cheers - Arthur

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Does seem real world when you try to re-intercept your route after ATC has given you different instructions.

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What I mean by by 'regardless of the FMC' in relation to what airport you have selected, is that an ILS system is of course a radio beacon (well, two radio beacons actually, one for the localiser and one for the glideslope), and so what tracks these signals is not the FMC in the truest sense, but the nav radios, which slave to the autopilot and fly the plane based on that, your MCP settings and the course you have selected (which should be the runway's magnetic heading), although it is true that the FMC will command the radios to tune and track radio beacons if you key in all the right info into it, and in fact 'under the hood' this is what the FMC is constantly doing in order to ensure it is where the IRS thinks the aircraft is, since it cross-references various signals from all kinds of sources in its database when it is doing its navigation thing. But regardless of that, the MCP and its settings are king when it comes to driving the aeroplane, and this means you can of course completely override the FMC settings and do an ILS approach via the radios and the course you enter on the MCP when you select VOR/LOC and then APP, since these are what will steer the aeroplane for the runway when you knock off LNAV and VNAV, although it would admittedly be a bit weird to be entering completely different runway info on the FMC from what you have up on the MCP. The average ground-based nav aid has a reliable range of roughly 50 miles or so (actually it can be a lot more than that depending on atmospherics, your altitude, and the beacon's location in the terrain, but for practical navigation purposes that fifty mile figure is a good ballpark figure to think of when low down, since it is the range at which you can be pretty sure you'll pick signals up and is generally what was being thought of when most country's aviation organisations placed those VORs in order to provide a network of suitable nav aids). However, ILS transmitter signals are not like a standard VOR/DME, they are pretty directional and don't fan out a lot off the runway heading, so you really want to be thinking about coming at them from roughly thirty degrees off the runway heading's extended centreline (or less) and probably at about twenty miles or so out from the threshhold (since this means you'll effectively fly up into the glideslope beam, which is the correct way to go about things), which any STAR or decent ATC guidance will set you up to do, even the default FS ATC (eventually LOL). There are exceptions to this at some airports, owing to terrain clearances or the need to have an offset signal (rare but not unknown), however, 20 miles out and thirty degrees off is a good general rule that will keep you out of trouble and line you up nice and early in most cases. Al

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Maybe the reason that inputting the approach into the fmc allowed the ap to capture the localizer is that there is an option that pmdg have implemented that automatically changes the course windows on the Mcp to the ils course in order to eliminate the possibility of you dialing a course that is different than the FSX airport course for the ils (you can have this option set to off which is how I leave it because I don't think the real plane does this, and I haven't experienced a discrepancy between FSX ils course and the ones the charts say). But if you do turn it off, you must remember to set the ils course in one course window for a single channel approach, or on both course windows for a full, dual autopilot approach.

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Hi Arthur, I'll defer to Alan on that one. I gave it a quick test, and found that inputting the destination rwy in the FMC is indeed not an essential requirement, and that I can achieve the same result by manual tuning. cheers,

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