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Slick9

Struggling w/ glide slope intercept

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Ok, I've had the 777 for about a month now, after reading the docs, and perusing the forums,I decided to get out and take her for a few test flights. I've got the automated flight down and have been able to do several fully automated CAT 3 landings, and a few where I've taken over from the autopilot after intercepting the glide slope. So no issues there, the problem I am having is when i am manually flying. What I am doing is descending with LNAV/VNAV PTH, with the auto throttle engaged. After intercepting the localizer, I am doing about 180knots flaps 5, once the glide slope comes alive, I select speed intervene and reduce the MCP speed to 160 knots, set the altitude to the MDA, then I lower the gear, and select flaps 15 or 20. This is where I'm having a problem, once I intercept the glide slope, I am having to force the yoke forward and essentially dive to maintain position on the glide slope.

 

Any suggestions for how I can get the auto throttle to reduce power at the glide slope so I can essentially release back pressure on the yoke and have the aircraft begin to descend on the glide slope? Or would the correct procedure be to use the auto throttle disconnect switch on the throttles to take over manual control of the throttles prior to g/s intercept?

 

Hope the question is clear, it's almost midnight and I need to be in bed!!!!! Anyway, any help will really be appreciated.

 

Richard Bansa

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VNAV isnt going to be of help unless your doing an RNAV approach. Nor is LNAV in use at this point. I assume you mean an ILS approach of which you should have been in approach mode after intercepting the localizer. Its the same whether the AP is on or you are flying manually. 

 

Always anticipate anticipate. Looking at the charts you will see at what point the GS starts to descend. Gear down usually just before so you already have drag. add flaps as your speed drops. If your not ahead of the aircraft, you will always have to play catch up and that is never a good thing, particularly in a heavy. 

Never had a problem slowing down the 777 on the GS. 

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If you want go from maintaining altitude to descending at ~800ft/min, you're going to have to do something.

 

If you are flying manually, the plane isn't going to automatically do anything to intercept glideslope. You are going to have to set a lower nose attitude, or reduce speed, or both.

 

If you have the autothrottle on, and commanded to maintain 160kts, it isn't going to "know" you are "just about to" descend. It is just maintaining 160kts (within certain tolerances, obviously it won't maintain 160kts if you suddenly yank the yoke back to full aft position, give it a chance).

 

So, if you want to keep 160 knots in your 'flat' segment maintaining altitude, then start to descend while still maintaining 160 knots, you are going to have to push that nose down.

 

The Autothrottle should then react to the speed increasing by reducing throttle to maintain 160 knots. The speed increase will happen because you are falling a little bit with gravity when you descend.

 

So again.

  • Manual flight = The autopilot won't initiate descent (Because it's not turned on)
  • Autothrottle will adjust throttle position to maintain a speed (Obviously it can't reduce thrust below idle thrust, or increase above max)
  • Autothrottle will not 'pre-empt' your descent and adjust throttle position to initiate descent (Because it doesn't know you want to descend).

Just push the yoke forward, establish a rate of descent that is desirable, and let the Autothrottle maintain the speed you have set in the MCP. Use the MCP speed window to slow down (and remember that as you slow down you may need to adjust trim to maintain attitude and/or rate of descent.)

 

If you want to do the ol' Secondary effects of Controls (Power for descent rate, pitch for speed) you are going to have to disconnect the Autothrottle too.

 

The Auto Throttle does one thing and one thing only. It maintains a set speed by adjusting the throttle/engine power up to go faster, and down to go slower. It reacts to descents only in the fact that it is reacting to the Knots indicated airspeed deviating and correcting the knots indicated airspeed by adding or removing thrust.

 

Keep in mind:
The Autothrottle can only REACT to what you do. It cannot pre-empt a descent.

 

Be smooth, Don't go from -0ft/sec to -800ft/sec in half a second, as this will let the speed increase (inertia+gravity will out-do the reaction speed of the autothrottle) and at the same time make half your passengers visit lunch a second time and wonder where all the grated carrots came from.

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In addation to the above, you are mixing things up or not describing your problem properly.

 

You probably realy should have been in bed :-)

 

Vnav/Lnav have NOTHING to do with a Glideslope!

 

You either follow a Vnav path, in Vnav Path mode with speed intervention selected and the MDA selected.

After the aircraft has stabilized on the approach you can change the the altitude window MDA to the missed approach altitude.

Vnav will still continue the descend!

 

or

 

You fly an ILS with LOC/GS selected (first armed then engaged) and you dont set the MDA in the altitude window for this. Upon GS capture (it changes from white to green) you set the missed approach altitude in the Altitude window!

 

Also, at the moment the Fly By Wire system is not programmed correctly.

The airplane is constantly auto trimming, fighting you basically.

This is not fixed untill SP1 comes out which will be when the -300 is released next year!

So forget realistic manual flying for now :-(

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Thanks for the responses!! 

 

Be smooth, Don't go from -0ft/sec to -800ft/sec in half a second, as this will let the speed increase (inertia+gravity will out-do the reaction speed of the autothrottle) and at the same time make half your passengers visit lunch a second time and wonder where all the grated carrots came from.  :lol:
In addation to the above, you are mixing things up or not describing your problem properly.

You probably realy should have been in bed :-) :P

 

Ok, now that I am fully awake, at least, I am supposed to be awake, I can describe my issue more accurately.  I think Hopskip confirmed what I suspected and have to get used to doing (push the nose down to start the descent at the glide slope intercept).

 

I think a large part of my problem comes from the way I'm used to flying the NGX, and I'm just having transition difficulties.  What I do with the NGX on manual approach is; auto throttle off, vnav path, and MCP speed intervene (not sure if this is the right nomenclature - I'm inputting the speed on the MCP), a little back pressure on the yoke to maintain altitude, flaps 10, once the glide slope goes active I drop the gear and reduce power - once I've intercepted the glide slope, I simply reduce the back pressure on the yoke, trim down a notch and the nose drops and we follow the glide slope home.  This works because I am controlling the throttle, and I can reduce power at the optimum moment to cause the nose to drop and begin descent.

 

So now I'm flying the 777 and with the autothrottle engaged and controlling power I've been using the same procedures as above, the difference the comes at the intercept of the glide slope.  Once the glide slope goes active I drop the gear, select flaps 20, reduce the MCP speed from 180 to 160kts, set the MCP altitude to MDH(A), but the autothrottle, as Hopskip mentioned above will not automatically reduce thrust - so at this point i have to push the yoke forward to "force" the nose down to follow the glide slope.  This is different from what I'm used to and it's taking some time to adjust to this method.

 

Hightowers mentioned being ahead of the plane, I feel like I've been flying her from the lavatory, i've moved up to the forward galley, one of these days I'll actually be flying from the flight deck.

 

Again, thanks for the answers!!

 

Richard Bansa

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Hightowers mentioned being ahead of the plane, I feel like I've been flying her from the lavatory, i've moved up to the forward galley, one of these days I'll actually be flying from the flight deck.

 

hahahahahah - that made my day.  Thanks!

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