tsentsan
Posts: 1921
Joined: Mon Jan 28, 2002 10:48 pm

Question On Descent In Vnav & ILS

Fri Sep 13, 2002 2:13 am

Hi guys,

Please have a look at the pic below....


I took this picture a while back while on a jumpseat landing into Singapore..

I noticed that the a/c was still on Autopilot with VNAV engaged. Also noticed that the Glide slope indicator was active though the 'APP' function was not engaged yet. This brings me to the question. Does an aircraft's VNAV function create a descent profile such that it would be aligned with glide slope or issit just coincidental?

Thanks,
Tsentsan

PS: Looks like we were speeding a bit  Smile/happy/getting dizzy
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Guest

RE: Question On Descent In Vnav & ILS

Fri Sep 13, 2002 3:56 am

While it actually depends on the individual aircraft's avionics setup, the G/S display is there to provide a visual reference for the pilot in the VNAV mode. The exact presentation and operational techniques will vary according to the FMS and avionics manufacturers; but generally speaking, the FMSes generate and fly not only a VNAV descent profile, but also a GPS generated pseudo-glideslope over nearly all of the various types of non-precision approaches. Additionally, they allow us to build "VFR" approaches to any non-IFR runway in the world. All of these approaches are displayed on the flight director exactly like an ILS approach and flown the same way. It makes absolutely no difference whether or not you're flying a VOR, GPS, NDB, Localizer, or Localizer Back Course - they are, for all intents and purposes, just another ILS. It really spoils you.

As to how they work, they simply use GPS and some magic to calculate a glidepath that will meet all of the minimum crossing restrictions published for the approach. From a safety point of view, they are excellent. You are able to set up a stabilized approach from the point that you are turned onto the final approach course. There is none of the "dive and drive" that can be so destabilizing. The only time we have to fly the various approaches the "old" way is during recurrent training and checkrides, but even then they're starting to ease up a bit.

To answer your specific question, most ILS approaches use a 3 degree glide slope. Three degrees also results in a comfortable descent rate (approximately 2500 fpm depending upon groundspeed) for enroute descents. Because of that, it's not uncommon for an enroute descent to match up with a 3 degree glideslope. I hope this answers your question.

Jetguy



 
mandala499
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RE: Question On Descent In Vnav & ILS

Fri Sep 13, 2002 4:14 am

Tsan,

AFAIK, The FMC will not select a descent profile along the glide slope unless you tell it to. This is in my opinion, a coincidence... the plane will dip below the g/s and will pick it again at a lower altitude (unless the ATC stepdown is lots of small ones).... where it'll be used for the final approach.

The ILS is tuned already anyway, hence it picks it up... but the A/P mode still shows it's following the descent profile.

You see the purple numbers there, 240 and 7000, the plane is still on it's descent profile to target 240K and hold 7000 feet. You see the plane is decelerating anyway. It's normal for planes to break through 10,000 above 250kts, and decelerate to 250K by the time they reach the assigned altitude.

That's my 1 cent's worth  Smile

Mandala499

When losing situational awareness, pray Cumulus Granitus isn't nearby !
 
tsentsan
Posts: 1921
Joined: Mon Jan 28, 2002 10:48 pm

RE: Question On Descent In Vnav & ILS

Fri Sep 13, 2002 10:57 am

Hmmm thanks Jetguy and Mr. Mandala  Smile/happy/getting dizzy

Jetguy - juz wondering.. would that "GS" indicator diamonds come out if you're on an VOR-DME approach? Or would it just be shown by the Flight Director?
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RE: Question On Descent In Vnav & ILS

Sat Sep 14, 2002 7:54 am

Tsentsan...
With all of the FMS boxes that I'm familiar with, a G/S is displayed anytime you're using the VNAV - during an enroute descent or an approach. But like I said in my first post, it all really depends on whose box it is, it's software version, and how good the interface is.
Jetguy
 
airplay
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RE: Question On Descent In Vnav & ILS

Sat Sep 14, 2002 10:21 am

There is a common misconception amongst pilots that "psuedo-glideslope" profiles take you right down to the ILS approach minimums. In fact a feature commonly known as "nav-to-nav" allows a smooth transition from the FMS approach to the ILS. GPS lacks the integrity, accuracy and continuity required to perform precision approaches.

The so-called "glideslope" indicator should really be referred to as the vertical cue or pointer. It can represent VNAV or Glideslope deviation depending on the selected mode.

 
Guest

RE: Question On Descent In Vnav & ILS

Sat Sep 14, 2002 10:46 am

Airplay,
That's correct, the appropriate nonprecision minimums still apply. However, for all intents and purposes they are flown exactly as if they were an ILS. The "Nav-to-Nav" transfer only works when an ILS is be flown and is not employed during other approaches.
Jetguy
 
tsentsan
Posts: 1921
Joined: Mon Jan 28, 2002 10:48 pm

RE: Question On Descent In Vnav & ILS

Sat Sep 14, 2002 3:24 pm

Jetguy and Airplay,

Thanks for that. That pic is on a B777 (as you guys would probably have known). I had the impression that it was on the G/S.. actually I never had a landing that was visual or anything other than ILS, so I have the experience of watching a "Pseudo-Glideslope" appear, but thanks  Smile/happy/getting dizzy

Regards,
Tsentsan
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RE: Question On Descent In Vnav & ILS

Sat Sep 14, 2002 9:14 pm

Tsentsan,
Oh by the way, 310 kts is a legal speed at 10,000' outside of Class B airspace here in the States. The regulation that you are probably referring to requires a maximum speed of 250 kts BELOW 10,000'. The pilot, prior to descending would pull power, decelerate to 250 kts, then start his descent.
Jetguy
 
AAR90
Posts: 3140
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RE: Question On Descent In Vnav & ILS

Sun Sep 15, 2002 5:50 am

>With all of the FMS boxes that I'm familiar with, a G/S is displayed anytime
>you're using the VNAV - during an enroute descent or an approach. But like I
>said in my first post, it all really depends on whose box it is, it's software
>version, and how good the interface is.

Last sentence is most important. It _totally_ depends upon what the operator has installed. AA's FMC airplanes will always display a G/S anytime an ILS is tuned on the corresponding receiver. VNAV deviation is always displayed (normally on the ND) anytime in climb/descent with a VNAV profile _available_ (does not need to be engaged). VNAV deviation is never displayed when ILS G/S has been captured/tracking.

Again, this is just AA's current methodoligy --subject to change by management, or change in managers.  Nuts
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