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Ray Proudfoot

Flying a SID with the FMC

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Hi PMDG and all,I have just finished a flight from Manchester (EGCC) down to London Gatwick (EGKK) and have some concerns about how well the FMC is able to control the vertical aspect of the flight-plan during the departure phase of what is admittedly quite a complex SID.Three images are attached. The first shows the FMC plan. As can be seen speed restraints have been assigned to the first few waypoints. The second shot shows the route as shown on the ND. The third shows the MCP settings (after VNav had been disconnected).http://forums.avsim.net/user_files/34293.jpghttp://forums.avsim.net/user_files/34294.jpghttp://forums.avsim.net/user_files/34295.jpgWhat happened in reality is that after setting LNav and VNav expecting the MCP to fully control the flight VNav was disengaged and caused the aircraft to descend. The "VNav disconnected" message appeared in the FMC. I had to control the altitude manually but the aircraft continued to fly the lateral aspect of the flight without problem.Can anyone please explain why VNav should disconnect when all waypoints (lateral and vertical) were all easily achievable given the constraints?The second problem concerns the ILS at Gatwick. I was landing on 26L and the frequency for that runway is 110.90 with a heading of 262. During the approach the FMC warned of an invalid frequency and the aircraft drifted off course. I hand-flew the the approach and landing.Given that the PMDG gets its info from the AIRAC data supplied by Richard Stefan I'm baffled as to why this problem occurred. Here's the data as supplied in WpNavAID.txt by Richard which is located in the correct FMCWP sub-folder under FS2004...EGKK 26L ILS/DME IWW ILSD 51.151188-000.168192110.90IComments appreciated.Cheers,

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As far as point #1:Your screen shot shows a constraint of 3000, yet your MCP altitude is commanding 5000?VNav is telling you to get bent as it can't do both ;)Set your MCP to 3000 and try it again. What should happen is once you clear the 3000 constraint, you will get a CHECK MCP message and VNAV will turn into ATL HOLD (at 3000). Reset your MCP to 5000 then press VNAV again. The current version does not understand a "cross 3000 or Above"On a side note, I am looking at Manchester and I can't see a sid that looks remotely like that, what sid is it?Ray(edit)Checked out 110.90 at EGKK for ILS 26L ... no issues here with FS9. Both Channels captured fine.http://forums.avsim.net/user_files/34343.jpg

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"On a side note, I am looking at Manchester and I can't see a sid that looks remotely like that, what sid is it?"It's a HON SID (either 1Y or 1R) which is the correct SID for jet aircraft departing EGCC 24L or R for an airfield in the LTMA. What's not correct is the showing of a constraint of at 3000ft at the various waypoints before SANBA. There is only one "restriction" before SANBA at 5000ft and that is HON D58 at 3000ft or above which I presume is the point SANBA-5.As you've already pointed out the FMC does not understand "3000ft or above" so the best way for Ray to fly this SID automatically would be to remove all the false 3000ft constraints and just leave the correct SANBA at 5000ft - any reasonable climb rate will ensure that SANBA-5 (HON D58)is crossed well above 3000ft . Then his setting of 5000ft in the MCP will be the correct one and VNAV will have no trouble understanding what is required.PeteP

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Hello RayI've just flown out of egcc on the HON1R, with no problems, vnav hit the levels spot on. Setting 5000 in the MCP is fine, you could set higher if cleared to that initial altitude. VNAV would just continue the climb after sanba to the initial cleared altitude.Simon

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Hi Ray and Pete,Many thanks for your replies. I tried the flight again and setting 5000 for the SANBA waypoint and setting 5000 on the MCP solved the problem. I'll amend the EGCC.TXT file to make the changes permanent.The SID is the HON1Y from 24L. It's unusual in that it has to deviate around the town of Knutsford, Cheshire. The local population had enough clout with the authorities to get their way :-)Cheers,

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Hi Simon,The fact that you have flown this SID without encountering the problems I did is strange especially given the advice from Pete and Ray.Are you saying that you set 5000 on the MCP and engaged VNav after take-off and the aircraft didn't try to descend after passing any of the waypoints that had 3000 as an altitude constraint?I notice you flew a HON1R SID. Can you depart from 24L and try the HON1Y departure please?Cheers,

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Hi Ray,This problem with the FMC displaying a message stating the ILS frequency is wrong on 26L at Gatwick (EGKK). Do both Nav radios have to be tuned to the ILS frequency? On my two flights only Nav1 was tuned. Would that explain the problem?Cheers,

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Hello Ray, your right, I'm unable to get vnav to engage on the HONIY dep, after several attempts. But I have flown the HON1R sid again without any problem with the mcp alt set at 5000.thanks Simon

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http://forums.avsim.net/user_files/34428.jpgOk, trying to follow your error, I have put both in this picture.One on the left is "SINGLE CH" indicating that you forgot to dial the ILS frequency into the second nav radio and engage the second autopilot.The other on the right side is a little more evil in nature. It is caused because I am set to VOR mode on the ND, not APP mode, and the system is smart enough to tell that the radios are tuned to something other then a VOR (in this case ILS). Not absolutely sure, but I think you would get this same error if you reversed rolls (set APP on the dial, but tuned a VOR instead).Since you say you only tuned one radio, is it possible the other happened to be tracking a VOR?Left Radio = Left CMD A button, Right Radio = Right CMD A buttonThe other possible error would be an FMC message "NAV INVALID - TUNE 110.90" because you attempted to engage the second autopilot yet it's radio was set to something other then 110.90.Ray

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Hi Ray, others,VNAV will disconnect if it encounters a constraint with speed only (no constrained alt). This is stated in the manuals, and is modelled.So if you remove your speed constraints (by the way VNAV will limit speed according to flaps retraction schedule) it will be fine.anthonyAnthony MertonPrecision Manuals Developmenthttp://www.precisionmanuals.com

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Thanks for that information Anthony, I knew that I flown the sids before with vnav without a problem but got caught out with the speed constraints yesterday.I am part way through writing the egkk sid/stars now, so will get back to that.Simon

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Hi Ray (I'm going to have to call you something else I think!),You've got this spot on. I did indeed have the 2nd Nav radio tuned to a frequency other than the ILS. I think it was tuned to Mayfield - one of the waypoints on my flight.This would also explain the NAV-INVALID message on the FMC. These things must have changed in one of the patches. I'll fly it again tonight and see how things go with all my new-found knowledge. The only thing I would like is for Simon to amend the EGCC.TXT file to remove the 3000 restrictions from the relevant waypoints.Simon, any chance you could change this please? I'm not yet up to speed on the syntax and don't want to mess things up.Cheers,

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Hi Anthony,Thanks for this info. In this case each of the waypoints had an altitude constraint but because my MCP altitude was set to 5000 and not the altitude constraint for the waypoints in the SID the logic got somewhat confused.I'd already spotted that speed doesn't increase beyond flap settings. Nice touch!Cheers,

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How about Ray #1 and Ray #2.So much to learn and so little time. I am at work and I cut and paste this stuff and e-mail it homw for further study and inflight work.Tony

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Simon,Many thanks for creating these files. I'll fly back to Manchester and see how the aircraft copes with that 180 turn from 26L.Whilst I have your ear could you modify your EGCC files to remove the altitude constraints up to SANBA please (plus any other SIDs out of EGCC with similar constraints)?As Pete pointed out the initial clearance is up to 5,000ft so if you could set that for SANBA the earlier waypoints should have their altitudes set by the FMC automatically.Thanks,

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Hello RayI'll modify both files over the next couple of days, to sort the altitude constraints, back to work tomorrow:(Simon

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Ray,I think that you may have misunderstood Anthony's explanation. What I think he was trying to say was that your problem has nothing to do with setting your MCP altitude to 5000 feet. The problem is that you have speed contraints for your first 3 waypoints (evidenced by the large font) with corresponding altitude constraints. The altitudes shown for those waypoints are FMC predicted altitudes (as evidenced by the small font), based on the 3000 ft contstraint at the fourth waypoint. Remove the speed constraints at the first three waypoints, and you should not have any trouble with 5000 (or 35000 for that matter) set in your MCP window. (Note: I don't know what the SID requires, so I can't comment on what changes might be necessary to comply with the SID.)In a climb, the FMC will honor the lower of the MCP altitude or the VNAV altitude constraints. It would unnecessarily add to pilot workload at a busy time if the pilot were required to match the MCP altitude with the altitude constraints. Normally, the MCP altitude would be set to the cleared altitude.Anthony, I couldn't find the reference in the manuals to the issue of VNAV disconnects on encountering a speed constraint without a matching altitude constraint.Don S.

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As a new user, and knowing two of the combatants :-) here I followed this thread with interest and mounting confusion, thanks for clarification Don ;-)I could not understand why VNAV would disconnect if the MCP had a higher altitude set than the FMC commanded, this going off my familiarity of the DF737 and PIC767 but your explanation cleared that up.However I guess that still leaves the question of speed constraints, I suspect Simon put those in so that the A/P could manage the sharp turns. If the speed/altitude constraints don't work how would you automatically limit speed, or would it be by manual intervention?So new and so much to learn :-)Dave Wild

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Hello DaveVNAV will disconnect if only a speed constraint is entered and no altitude constraint.I haven't entered any speed constraints in the sids I have done. I have entered altitude constraints, which seem to be causing problems. The 3000ft constraint on the HON1R/1Y sid from egcc, at HON58 or SANBA-5 (its the same point)is 3000 or above. I have set the constraint at 3000 here as the above feature isnt operational yet. And then later 5000 at SANBA.It should be possible to take off with an initial cleared altitude of say FL100 set in the MCP, select LNAV and VNAV and fly the sid until SANBA at which point VNAV should continue the climb to FL100.Important though dont set speed constraints against the waypoints that dont have alt constraints set.Flap settings will keep the speed down under VNAV and help around the initial turns.Hope this helpsSimon.

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> If the speed/altitude>constraints don't work how would you automatically limit>speed, or would it be by manual intervention?>Dave,I don't think the manual speed intervention in VNAV works yet in the PMDG FMC. I think I read somewhere that they will be providing it in a later update. If you want to stay in VNAV you have two choices: 1. Keep your flaps extended - VNAV will limit speed according to the flap retraction schedule, or 2. Insert the limit speed as either the speed restriction speed or the target climb speed in the VNAV climb page of the FMC.Don S.

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Hi Don,Thank you for the explanation. You are right - I had misunderstood Anthony's explanation. This message, together with replies from my other posted query last night, have given me a new insight into how the FMC works on the PMDG737. Previously I had assumed that it worked quite closely to the 767PIC. Clearly this is not the case.To sum the rules up. Inputting a speed OR altitude constraint without including the second parameter will cause the FMC/MCP to function incorrectly. Include both or rely on calculated values and all will be okay.I understand the significance of large and small fonts in the FMC. What I hadn't appreciated is the impact on the FMC. Elsewhere the point was made that speed is limited by flap setting. So 240/6000 for a given waypoint will not be achieved if flaps are extended.Quite a different beast to the 767PIC isn't it?Thanks,

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>Quite a different beast to the 767PIC isn't it?>Hi Ray,It certainly is! It's not quite to the same level of simulation yet, but they're getting there.As far as the speed/altitude constraint issue goes, it is not necessary to enter a speed constraint for every altitude constraint. However, as presently programmed by PMDG, it is necessary to enter an altitude if you have a speed constraint. As Ian Riddell pointed out in the other thread, in the real world 737NG FMC, this is not necessary.I also question whether the real 737NG FMC has a better way of dealing with your issue of selecting a cruise speed that is much higher than the target descent speed. I don't have an NG FMC manual handy to check right now, though.Don S.

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