Flight director accuracy?

[pipper 0]

I have been noticing for a while now while performing ILS approaches that the flight director is not necessarily accurate. For instance, I frequently get a fly up command when I am below the glide slope-obviously this is wrong. I have noticed this during most of my ILS approaches. I occasionally select APP mode if I am busy or the viz is especially bad. Then the flight director seems to work better after I disconnect the A/P and manually fly the last five miles. Most times though, I hand fly the entire ILS after GS capture and thats when I notice the flight director bars (not the cross hair type-the V bar) don't do a good job of helping me on the glide slope. The localizer commands tend to be much better. I was wondering if anyone has similar results or if I am doing something wrong here. Cheers and tks in advance.......Joe Swier

[michal 44]

>For instance, I frequently get a fly up command when>I am below the glide slope-obviously this is wrong. Yes, if you are below the glide slope you should fly up (it is self evident) ... however if logic detects that your closure rate is too fast and overshoot is likely it may command the "down" direction even though you are below the GS.Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2

[Guest boxjockey99]

belive it or not this happens in the real thing too. The flight director doesn't cope too well with strong winds on the real aircraft and you need to watch it carefull or you will find yourself merrily flying off the localiser with the F/D giving no correction cues. This is why you should always back up and appraoch with raw data!You may find that the crosshair F/D is more accurate. I used to use a V-Bar flight director on the Fokker F27 and loved it so was a little sceptical of the crosshair variety when I transferred to the 737. Having flown it for about a year now I have found the crosshairs to be more accurate and easier to use but how much of that is just familiarity I don't know. Hope that helpsKris

[Guest frankathl]

Hi Kris,"you should always back up and appraoch with raw data!"I'm not sure I understand the meaning of "raw data"? Can you explain further, please?Thanks,Frank

[Guest lbb]

What he means is that you should have the real radio aids tuned. So in the case of an ILS you shouldn't rely on the FD only, you should keep your eye on the ILS pointers as well (probably even more).Leo Bakker

[pipper 0]

Jeez......I re read what I wrote and made not one but 2 errors.I meant to say that I hand fly the airplane after intercepting the LOC, and that I get fly UP commands when I am ABOVE the GS. You are supposed to get fly UP commands when BELOW the GS. Big difference!Aside from these, the other observations are correct. Thanks to all who chipped in....maybe I can try the cross hairs. I was really curious if anyone else who used the V command bar had these issues. Cheers......Joe Swier

[Guest frankathl]

Thanks, Leo.Follow up question:Do the FD and VOR/ILS displays receive data from different sources? I thought they were both controlled by the Flight Control Computers, which rely primarily on GPS position information, AFAIK?BR,Frank

[Guest Lenny Zaman]

be carefull with this, i recall that autoflight system use a combination of inertial navigation and ILS signals(DME... even better)mainly what happens is at larger distances from the threshold, the autoflight system will get most of it's input 'from the ILS signal'but once you go below a certain altitude(AGL) the 'inertial' path takes over. ...Cheers

[CF104 0]

>Thanks, Leo.>>Follow up question:>Do the FD and VOR/ILS displays receive data from different>sources? I thought they were both controlled by the Flight>Control Computers, which rely primarily on GPS position>information, AFAIK?>>BR,>FrankUmmm, no.The F/D and VOR/ILS gets all of their data through the VHF NAV receivers when selected in VOR or APP mode. The radio nav aids have no input from the GPS.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.precisionmanuals.com/images/forum/ng_driver.jpg

[michal 44]

Remember that FD is not a pointing device - it doesn't point where you are in relation to the beam. So it is not entirely impossible to see FD command that contradicts its oposition vis-a-vis the beam center. Like I said above it will happen if you are closely to overshooting.Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2

[marcom 139]

inertial navigation comes into play if you lose the ILS signal during an autolan below a specific radio height. It shouldn't play a role during CAT 1 approaches as below 200 ft you won't be looking at the F/D anyway.Regards,Mark

[Guest boxjockey99]

MArkI am not sure about the NG but the 300 series I fly on a day to day basis doesn't use IRS data directly if the ILS drops out. in essence it is always using IRS data as they are always running and supplying position information hence you get the MAP display. The NG is better as it has independant DME/DME fixing for FMC position updates whereas we loose that when we switch the nav radios to manual for the ILS frequency. THis means we usually get a significant map shift by the time we land. The scenario of losing the ILS signal on the approach doesn't seem to be covered in the tech manual but I haven't looked that carefully, I'll ask around though. I would suspect that the AFDS would revert to HDG SEL and V/S at the descent rate you had when you lost the ILS. Also I would be surprised if the A/P didn't drop out thus telling you something is amiss. I would think it unlikely for the AFDS to revert to LNAV /VNAV as these are FMC modes and not directly / quickly changeable by the pilot, ie realistically I would not want to be poking at the FMC when I was so close to the ground, I'd want to be using the MCP directly. Having said that if I saw that the ILS had gone then I would have initiated a go around hit the #### (TOGA button) and climbed away in N1 / TOGA and then get my chum to do battle with the FMC while I enjoy myself and pole the thing around a bit! As I say I will look into what actually happens if you loose the ILS and get back to you!Kris

[marcom 139]

Hi KrisI'm pretty sure that you're right - ie you'll have some sort of mode change or AP dropout should you lose the ILS raw data. What I meant was that if you lose ILS raw data close to the ground (lets say a few feet before FLARE engages during an autoland, the aircraft will not change the AP mode and will maintain V/S and current track as if the ILS was still being recieved.Note I have absolutely no idea at what altitude this is armed or if there is a max time period where the a/c can continue without actual raw data.Ian?Regards,Mark

[Guest frankathl]

Thanks, John.1. My query above was based upon the fact that it seems to me that the logic of cross-checking one navigation system(Flight Director) against another(VOR/ILS raw data) requires that the navigation systems in question should each receive their information from separate, INDEPENDENT data sources? Otherwise, if one is incorrect, then they both are.2. It is not clear to me in what sense the VOR/ILS data is "rawer" than any other data source?3. "The radio nav aids have no input from the GPS."No, I didn't think that. What I meant was that the aircraft determines its position, first and foremost, from the GPS signal(if available). For this information, I was relying on the following statement from the flight manual, but I may be misinterpreting its meaning:"In flight, the FMC position is continually updated from the GPS (as installed), navigation radios, and IRS. Updating priority is based on the availability of valid data from the supporting systems.FMC position updates from navigation sensor positions are used in thefollowing priority order:

[Guest boxjockey99]

MarkYes I think you are correct on that, this doesn't however rely on the IRS's at all other than attitude and V/S data ie not positional data. The autoland on the 737 is a pretty crude piece of kit as it was effectively a bolt on for the basic AFDS. This as far as I am aware has not been changed in the NG and it remains crude but very effective in what it does. In flare mode all the AFDS does it maintain its track as it was passing 50ft and pulls up to a set attitude to give it the correct angle to land at! at 27ft the A/THR retards the thrust levers and wallop you hit the ground! This is why autolands on the 737 are always ... firm! alas there is no whizzy technology chattering away making tiny adjustments all the way to the ground as in the airbus but it works, it gets us in and we live to fight another day!Happy trailsKris