MD-11 glide capture logic

[Orlaam 306]

I have only done a few flight, but I've have not seen this behavoir. Two flights from a transition point put me below the G/S and the MD did not climb to capture it. I wonder if something esle is going on?

[michal 44]

Jared,I think your problem based on the pics you provided is that you are **never** truly in altitude HOLD state while approaching the GS capture. You are not in altitude hold because your "3000" is in white, if you were truly in hold it would have been in magenta instead. In fact your altitude seems to be all over the place, always significantly below 3000, no wonder it is not in hold. I don't know what is going on but from the pics it looks like extremely messy approach. So if there is no valid vertical mode and then GS comes around - the aircraft may in fact climb since this is the only vertical mode known to it that it can 'grab onto'. I don't know what messes your approach so badly - I would try to repeat it with NO weather and see how it goes but it is entirely possible your procedures are not exactly textbook.

[VIPERGTSR01 22]

I have never had this problem either, my aircraft has never climbed to intercept the GS, and when I do select it above the GS it smoothly decents to intercept never have dive problem either.Jay V

[Guest cfgarrod]

Maybe it helps if we have a look at the colour coding of FMA modes:White: manual control or pilot entries.Magenta: FMS calculated commands/values.Green: fail operational autoland functions.Cyan: AP engagement.Amber: cautions and failures.Red: warnings.Your FMA shows HOLD 3000 in white so you selected this altitude yourself. In the 4 screen shots the aircraft is not holding this altitude, it's at 2420, 2170, 2350 then 2590ft. Are you inadvertently overriding the autopilot with your control yoke?Chris G

[Guest JaredA]

Nope when the autopilot is engaged I never touch my yoke plus I turned the sensitivity all the way down to the lowest setting to help prevent exactly that inadvertently overriding the autopilot and wouldn't the AP trip off if I were overriding it that much? I could tell it was trying to recapture 3,000, but it couldn't. In the second set of pictures it was in profile up till, as I stated in the other post, I deleted the 250 speed restriction then the autopilot couldn't capture the alt, it flew through it, but didn't capture as you see there so I switched to LVL change to see if it would recapture and as you can see it never did, I'm at a loss myself on the recapture deal. The only thing I saw a true difference in, as stated before, is the climb up into the glide didn't occur with auto tuning of the radios, but IT DID WITH MANUAL TUNING. Maybe, I should start another thread, but something else I saw, which it hasn't done till then, is the plane was on a 272 heading coming out of ATL to fly the PNUTT departure off 27R I preselected 250 as a heading and when the autopilot was engaged and I pulled the heading knob the autopilot commanded for a right turn to 250 instead of a left turn to 250 and after passing 10 in the climb, the FMC did not command the plane to accelerate above 250 knots, it did once it was past the second fix on the SID, but by then with the right turn instead made me fly over the airport and delayed the on course turn. Very strange indeed, I hope that the recapture thing and this wrong turn and not accelerating above 250 knots is FS just telling me it didn't want to work and needed a rest.

[Guest JaredA]

The heading and speed deal must have been FS complaining cause I'm in flight now and performed the same produce and this time the autopilot did both the turn and acceleration above ten correctly, FS can be odd sometimes.

[michal 44]

but didn't capture as you see there so I switched to LVL change to see if it would recapture and as you can see it never did, I'm at a loss myself on the recapture deal.

So here you go .. until you master the "capture" stuff your ILS approaches will be hit-and-miss. This is a common misconception among armchair pilots that you can punch numbers into autopilot and the rest will be taken care of, - autopilot can only capture course, altitude, GS, speed, etc. if conditions are right, it is responsibility of the pilot to make sure that capture conditions are satisfied. By the way Chris is correct about the colors - my mistake.

[Guest JaredA]

Yea, I know its the pilots job to monitor the system, but in this situation I wasn't in a high traffic area or there wasn't a lot of traffic so I made ever attempt to allow the system to correct itself, but it didn't thats why this thread was started in the first place, to see if it was just my learning curve to the MD-11 or a possible logic error. I haven't seen an FS add-on that is "perfect" upon release.

[Guest keiron]

For those reporting that the MD-11 suddenly climbs to reach G/S, if you think about it critically you've almost solved your problem.Its better to already have your final approach alititude held with PROF mode, as Michal was mentioning earlier on, your ALT should show up on your ND in magneta The reason why your MD-11 choses to climb @ G/S live; I believe is no logic fault. It sounds to me as though you have your approach altitude held in manual AP mode (white on your ND) then you're hitting PROF once the G/S comes alive. If you're doing this then you're commanding the AP VNAV to follow vertical instructions in magneta. Depending on where the G/S vertical bug (coloured in magneta) is on your ND, is where PROF AP will follow.Obviously if the G/S comes alive and the magneta bug appears at the top of your ND then it's representing that the glide path is above you so AP will climb to lock on to it as you've rightfully commanded it to do so.If you prefer to use manual AP ALT HOLD on approach then you're better off waiting untill the G/S bug on your ND comes down to at least level with your FD. Then once you hit PROF, the AP will follow the G/S bug which will be level with you at your alitiude or slightly below depending on when your press it.I agree with Michal, it's down to misconception of the autopilot usage, no development bug involved here.I could be wrong but that's my personal understanding from reading the manual bit by bit.

[Guest JaredA]

Then why, assuming Micheal and Keiron are right, if you watch the Martinair video from ITVV on the demo of an CAT 3A approach wasn't the sim set up so the airplane was already coming in on the glide if the logic both talked about is true, look at the video at time 145:47 it was in PROF when APP/Land was pressed and the aircraft is on neither the LOC or glide and its holding 2,000 then it captures the LOC now why isn't it racing to catch the glide?????? based on what both of you are saying it should, but it doesn't and it shouldn't matter how the ILS was tuned manual or automatically and whether or not PROF was used till approach or not. No I press APP/Land not Prof on an approach, can't speak for others.Note: I didn't start this thread to start a war on who is right and who is wrong, I was hoping for folks to see my point of view on this.

[Guest Stepan]

Then why, assuming Micheal and Keiron are right, if you watch the Martinair video from ITVV on the demo of an CAT 3A approach wasn't the sim set up so the airplane was already coming in on the glide if the logic both talked about is true, look at the video at time 145:47 it was in PROF when APP/Land was pressed and the aircraft is on neither the LOC or glide and its holding 2,000 then it captures the LOC now why isn't it racing to catch the glide?????? based on what both of you are saying it should, but it doesn't and it shouldn't matter how the ILS was tuned manual or automatically and whether or not PROF was used till approach or not. No I press APP/Land not Prof on an approach, can't speak for others.Note: I didn't start this thread to start a war on who is right and who is wrong, I was hoping for folks to see my point of view on this.

Hi Jared,I read the whole thread through because I

[Guest Totalbeginner]

It seems (someone may want to correct me because I'm only guessing) that the data supplied by the FMS has higher priority, therfore if you arm the LAND mode while flying in the ALT hold mode, the plane races to intercept the GS as soon as it comes alive. It would do the same thing with the localizer should you arm the LAND mode while flying in HDG hold mode.So as others wrote: After managing you descend with ALT hold (according to ATC for example), you can either go back to PROF after reaching the initial approach altitude then arm the LAND mode, or you can stay in the ALT hold mode and arm the LAND mode in the moment you are near the GS.

No, this is incorrect.You can fly the vertical profile using any valid pitch mode (PROF, V/S, FPA, ALT HOLD etc) and G/S capture should only occur as the magenta diamond passes through the centre of the deviation scale (provided LAND is armed and LOC capture has already occured). The flight director should not command a climb, and nor should the A/P climb to intercept the G/S. Whether the vertical mode prior to G/S capture is WHITE (pilot selected) or MAGENTA (computer selected) should make absolutely no difference.Regards,Martin Neep

[Guest JaredA]

Exactly Martin!!! it shouldn't matter what vertical method you use to get to the approach altitude nor should it matter if the pilot manually selects the freq of the ILS to be used or its automatically tuned, thats why I'm wondering if there is a bug with manual tuning, because my first set of shots I manually tuned the radio, second I let it auto tune.

[michal 44]

I fully concur with Martin's reply. No autopilot in this world would leave the altitude hold mode to initiate climb towards the GS, or by the same token no autopilot would initiate a turn towards the localizer leaving the heading hold mode. Without the full video it is hard to tell what you guys above are doing wrong but the likely suspect is that you are never in any ALT HOLD mode, white or magenta (Jared's pics show he is not in ALT HOLD mode) and auto-tuning or manual tuning has absolutely nothing to do with it either.

[Guest JaredA]

I'll try it again both ways of tuning auto and manual, as we all agree it shouldn't make any difference, from below the glide with a loc capture first and see if it was just FS being dumb like the other cases or if it races to the slope either way of tuning. I'll take shots then after that I think this thread has run its course.