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pcubine

What Went Wrong?

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I have no screenshots so I will try my best to describe the issue. A flight KORD-COBS using LNAV and VNAV until I begin to descend at which time I switched from VNAV to V/SO and disarmed the A/T. Several times during the descent I armed the A/T to make sure the speed stayed in the 250-260 range during the descent and finally armed it for good at 11000 ft. No speedbrakes required during the descent. Wind at KBOS was from the northwest so I decided to land on 33L. However, the FMC does not show an ILS Approach even though there is one as can be seen from the attached chart. So I selected 33L which was for a VFR approach. I then went to the legs page of the FMC and entered the fixes and altitudes as show on the ILS chart and connected BENNN to the GARDNER THREE STAR to complete the route. I entered a 110.70 in both nav radios and a course of 331 in both course windows. Once the plane turns towards the ILS course I press VORLOC and after several seconds the localizer is captured. The plane is still in V/S as opposed to VNAV. I press the APP button but it will not engage. I can see the glideslope diamond moving down the PFD. I continue to press the APP button but to no avail. I am below the glideslope, thru it, then above it. I have been unable to figure out what the problem is. Does anyone have any ideas? Thank youMichael Cubine

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This happens if you try to fly an ILS with the radios correctly tuned but without the approach loaded in the FMC. I believe it is a bug.

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You had told the FMC that you wanted to do a visual approach so there was nothing the APP button could do for you.
The requirement is to have one of the NAV radios tuned to the ILS frequency. Pushing APP should then arm the autopilot / flight directors for localizer and glideslope intercept. The localizer intercept does work in the NGX, but not the glideslope intercept. Nowhere in the FCOMs can I find any mention that the approach must be loaded in the FMC. APP should work just like it does in any old steam-gauge airplane.

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If v/s is on, the aircraft is getting contrary information if you try to turn on the app. Also I think that you need to be flying level to grab the glideslope, but in the latter case, the app button would normally light on. Henri

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If the wind is from the northwest I think I will just stay away from Logan until ILS 33L shows up in the FMC as an approach to KBOS. Michael Cubine

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There is a G/S on/off prompt, on the Init Ref page, I believe. I guess it has to to with the IAN option being installed on the jet. If you select G/S on, it should work. There is no need to be in level flight before capturing the G/S, so V/S works fine. I use it quite often, actually - it makes for a smooth transition from a gentle descent to the G/S. Good luck.

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I have no screenshots so I will try my best to describe the issue. A flight KORD-COBS using LNAV and VNAV until I begin to descend at which time I switched from VNAV to V/SO and disarmed the A/T. Several times during the descent I armed the A/T to make sure the speed stayed in the 250-260 range during the descent and finally armed it for good at 11000 ft. Thank youMichael Cubine
Turning the A/T off and on isn't really a technique that most pilots would use. If you'd like to stay at 250-260 knots, then just use that as your descent speed in VNAV or LVL CHG.

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Turning the A/T off and on isn't really a technique that most pilots would use. If you'd like to stay at 250-260 knots, then just use that as your descent speed in VNAV or LVL CHG.
Matt That's the only time I tried a V/S descent. I always go with whatever the FMC comes up with and use VNAV for descent with the FMC adjusted for speed and altitude constraints which are on the current FAA STARS charts but may not be present in the Navigraph database. However, when I use VNAV for descent the a/c speed increases and I must use the speedbrakes 2 or 3 times during the descent to keep it from getting out of hand. Pax don't like the vibration and sound associated with speedbrakes. They think some kind of disaster is about to happen. The MD-11F only has a crew and they are aware of what is happening. No problem with VNAV and altitude. I started the descent into KBOS about 190-200 mile out and keep the V/S around -1500 fpm. One thing that stands out from that descent was that when I turned the A/T off N1 didn't drop unless I blipped the joystick throttle several times and then it would only go from about 41 to 37. Maybe the blades were just windmilling. Who knows? Michael Cubine

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Matt That's the only time I tried a V/S descent. I always go with whatever the FMC comes up with and use VNAV for descent with the FMC adjusted for speed and altitude constraints which are on the current FAA STARS charts but may not be present in the Navigraph database. However, when I use VNAV for descent the a/c speed increases and I must use the speedbrakes 2 or 3 times during the descent to keep it from getting out of hand. Pax don't like the vibration and sound associated with speedbrakes. They think some kind of disaster is about to happen. The MD-11F only has a crew and they are aware of what is happening. No problem with VNAV and altitude. I started the descent into KBOS about 190-200 mile out and keep the V/S around -1500 fpm. One thing that stands out from that descent was that when I turned the A/T off N1 didn't drop unless I blipped the joystick throttle several times and then it would only go from about 41 to 37. Maybe the blades were just windmilling. Who knows? Michael Cubine
The NG, above FL350 gets pretty close to Mmo in a VNAV descent. I'll typically start down 4nm per 1000' above FL350 early (before the T/D) using DESC NOW or ALT INT. Below FL350, it seems to do pretty well. Hopefully you don't have to use speedbrakes or the throttles in the descent. As far as A/T on and off and N1 in the decent, that's probably more of a sim-throttle hardware interface issue. I think (?) PMDG had to model the N2 idle speeds through the throttle, and it's noticeable from what you and others write. If you wanted to, you could program some tailwind in the DESC Forecast. I think that is modeled and that should help keep the speeds down. The idle speed should be the same in V/S, VNAV, or with the F/Ds off. I hope that helps.

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I'm testing this flight right now and I'll try to see what's going on... The G/S flag does default to OFF on the APPROACH REF page - already noticed that and it's definitely part of this, but what I can't figure out is why you didn't get an IAN G/P mode annunciation either. When G/S is OFF you should still get the FMC computed G/P for the pitch mode after pressing APP. Also for what it's worth, starting a descent 200nm out in V/S at 1500FPM is not realistic at all. What matters in a descent is not the absolute vertical speed but rather how quickly you acceleration downward into it. It *is* common to use DES NOW or V/S a few miles prior to T/D to ease into it and capture the path from a bit below it. Once you're at a constant 3000FPM or whatever, it feels the same as any other constant rate of descent. People sense accelerations, not raw speeds. I've been on tons of real world flights where the speed brakes are used too. The 738 pretty much requires them in many cases, it's a very slippery airplane. Did you have the winds programmed in? VNAV should never overspeed the plane as long as it has all the information it needs...

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Ryan Its been so long ago I don't remember the particulars of that flight, However I always enter enroute winds by waypoints of the route and enter winds on the Descent Forecast Page for FL300, FL180, and 9000 ASL. Thank youMichael Cubine

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The NG, above FL350 gets pretty close to Mmo in a VNAV descent. I'll typically start down 4nm per 1000' above FL350 early (before the T/D) using DESC NOW or ALT INT. Below FL350, it seems to do pretty well. Hopefully you don't have to use speedbrakes or the throttles in the descent. As far as A/T on and off and N1 in the decent, that's probably more of a sim-throttle hardware interface issue. I think (?) PMDG had to model the N2 idle speeds through the throttle, and it's noticeable from what you and others write. If you wanted to, you could program some tailwind in the DESC Forecast. I think that is modeled and that should help keep the speeds down. The idle speed should be the same in V/S, VNAV, or with the F/Ds off. I hope that helps.
Matt Right after I read your post I started entering tailwinds in the ACT DES FORECAST page. Prior to that I had entered a combination of the last waypoints and destination airport winds aloft for FL300, 18000, and 9000 which is probably the correct way to do it. But now I am using tailwinds and that really reduces the need for for the use of speedbrakes. ThanksMichael Cubine

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