While reading the FS2004 manual in the autopilot section I came across something that stated you should intercept a glide slope from below and not above. I was'nt very clear on this point. How do I know at which altitude to intercept? Can anyone elaborate further please? Thank you.

Hi. I'm no esspert but I love answering questions! Intercept the glide slope about 3000ft above airport height. The distance is up to you,but if you can safely fly at that height from 15 or so miles away then do it and you will eventually see the horizontal bar come alive and may well move upwards at first and then as you approach the point at which the glide slope crosses your path on it's way down to the runway your plane will follow it down, or you can hand fly it and try to follow the bars!Remember that you will still need to keep the aircraft airborne as it follows the glide slope down: you will need to keep your eyes on the airspeed and adjust flaps and throttles as necessary.Andy.

The best way to learn this is to pull the approach chart for that ILS (free ones should be available on the web) and follow it as to altitudes, intercept radials, etc. This assumes no ATC, and that is probably best when learning this.Forget using the AP until you can do it by hand first, as ILSs are not hard approaches to fly by hand, as long as you can manage speed / descent rate (basic flying skills).Some folks make the mistake of thinking that the AP can do anything, and thus make demands of it which are unrealistic, such as intercepting the localizer at too great an angle and too fast a speed, then wondering why the AP can't handle it. Best to learn doing it manually first, as real pilots do. Then you'll better understand the demands of this approach, and what to reasonably expect your AP to accomplish under similar circumstances.Also, using a moving map GPS, such as the default GPS500 will give you some visual cues that will help (such as knowing when you are coming up on the localizer, so you can be prepared to turn to intercept it).Once you are good doing this by hand, then use the AP if you wish, but once you get good manually, you may find using the AP a bit boring! ;-)Regards,http://www.dreamfleet2000.com/gfx/images/F...R_FORUM_LOU.jpg

I understand and agree with you guys said but you still haven't answered my initial question. Why does the manual say to intercept the ILS from below and not above? Thank you.

Two reasons:I've read that the glideslope radio signal is "cleaner" on the underside. It's very clear when you start to intercept the glideslope and when you are centered. However, due to electronic design limitations (which I know nothing about), the top of the glideslope signal can be susceptible to lobes and false/spurious signals, and as a result the needle may fluctuate up and down before it locks on. Chasing those false up-and-down indications with the plane will not add to the approach stability. (Edit: I don't know if FS9 simulates this effect, but it is a valid real-life reason to not intercept from above.)Probably the more important reason is that the approach is much simpler to fly when intercepting from below. You will have been flying along straight and level for a minute or two, which gives you time to stabilize at the recommended approach airspeed, hang out some flaps, etc. Then at GS intercept you just reduce power a little, drop the gear if you need to, and start downhill. On the other hand, if you intercept from above you'll have to be diving at a pretty good clip just to get down to the GS, then continue to dive just to stay on the glideslope. This is not too hard in a 172 or 182, but in a fast, retractable-gear aircraft, it can be very challenging to keep the speed under control.

You will find much useful information about instrument flying in the Instrument Flying Handbook, the official FAA handbook for instrument pilots. The answer to your question is on page 7-34, which describes the false courses generated by glide slopes at high angles.The Instrument Flying Handbook and many other references available for free download from the Internet are described in the article "The Best Free Add-On for Microsoft Flight Simulator," available on the Microsoft Flight Simulator Web site at: http://www.microsoft.com/games/flightsimul...ree_add-ons.aspThat article contains links those free sources of authoritative and informative information on a wide range of aviation-related topics.Another great source of information is the AOPA Air Safety Foundation Online Safety Center, which includes many publications, online courses, and other resources, all available free even if you're not a pilot or a member of AOPA:http://www.aopa.org/asf/

>I understand and agree with you guys said but you still>haven't answered my initial question. Why does the manual say>to intercept the ILS from below and not above? Thank you.It is mostly due to a limitation of the auto-pilot by design. Some aircraft can capture from above but they are exceptions rather than the rule. Capturing from below allows the AP to stabalize on the GS better than if you are descending into the GS. Some AP's will not even capture a GS from above and you will fly right through it.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.precisionmanuals.com/images/forum/ng_driver.jpg

Hi again. Your original question is answered as follows I think! The glide slope is composed of two beams, one horizontal and the other vertical, which form a '+' like the writing in a stick of rock,but which expands as well, ascending in a 3 degree (roughly) slope away from the airfield and out into the atmosphere. The range for the vertical segment- the localiser- is about 20+ miles and the reciever will lock on that and fly you towards the airfield. You could imagine this vertical element as an invisible paper-thin wall extending from the ground upwards and it is this that the localiser senses and captures and which you can see as movement of the needle and it is this which guides the airplane towards the airfield.The glide slope element has a smaller range and is the horizontal part of the '+' and this also expands as it leaves the airfield and rises, as it were, into the sky! However, to allow the instruments to 'capture' this and fly the airplane down to the runway you also have to intercept it and if you imagine these as two wafer thin bands of radio waves, then to descend from a height greater than the glide slope element, when you are already quite close to the airfield is foolhardy, as instead of a 3 degree slope you may be on a 10 degree slope attempting to capture the slope from above and the airplane will probably fly right through the signal.Andy.

Lemonadedrinker's reply betrays some confusion about the components, specifications, and operation of an Instrument Landing System (ILS). For a more accurate and detailed description, see the references described in my previous post in this thread, particularly the Instrument Flying Handbook.An additional authoritative and reasonably easy-to-follow description is in the Aeronautical Information Manual (the FAA's Official Guide to Basic Flight Information and ATC Procedures), section 1-1-9, which you can read online at the following link:http://www.faa.gov/atpubs/AIM/Chap1/aim0101.html#1-1-9This description includes several diagrams that help illustrate the components and function of an ILS, including the glide slope.(The table of contents for the AIM is located at http://www.faa.gov/atpubs/AIM/index.htm -- if you prefer, you can download a PDF version of the AIM from the FAA's Air Traffic Publications Web site at http://www.faa.gov/atpubs/)

If you visualise it in your mind or on a piece of paper it becomes clear(er).Envision the glideslope as a line running up from the runway at a 3 degree (usually angle).If the aircraft is level above this line it will never intercept the line, if it is level below it it will at some point cross the line.To intercept the line from above it will have to dive at a steeper angle than the line itself but how will it know that angle?Of course this is oversimplified. In reality there's a vertical range to the signal and the system will know that it's above the line and to descend but if it's far above it still won't know how fast to descend.Typically you should plan to be 2000ft above runway elevation at 10nm out. This puts you in a good position to intercept the glideslope.Of course if there's high terrain on the approach all bets are off (but then the glideslope will often give a steeper approach path and/or may be offset).

Hi. Naturally I don't think the glide slope IS a stick of rock or wafer thin pieces of paper spread out in limitless space but as an analogy and an attempt at a simple explanation it seemed ok especially as the original poster was trying to grasp why he couldn't or shouldn't attempt an approach from above the glide slope. You might get a signal but your approach would be more tangentially into the glide slope than an approach from below the signal and therefore much more difficult for the airplane to lock on to in a way that would carry you down to the runway safely.It is a bit like thinking there are two ways to go to the next room : 1. through the door.2. through the wall beside the door.To approach an airport from above the gs is a bit like 2 as it may be possible, but not advisable or sensible.Andy.

Hi Andy,I think what "Extrapilot" is referring to is that you appear to have confused the orientation of the localiser and glideslope beams in your post. The localiser beam is the lateral(left to right) component, whereas the glideslope beam is the vertical(top to bottom) component. I'm sure you know this; it's the post that is confused.BR,Frank

Hi. That is more than likely in fact! Although logic might assume they were the way I tried to describe; my analogy was paper thin itself as the operation of the ILS is way, way beyond my understanding--how does the airplane follow the glideslope,for instance, once it's locked onto it? I'm going to post a few screenies of my latest attempt to land via the ILS, inspired by the original post, and it is a perfect example of only flying with people who know what they are doing!!Andy.

Thank you all for a very thorough explanation.

Again, this thread contains some entertaining and creative explanations of why one generally intercepts glide slopes from below (using the altitudes specified on the chart for the ILS that you're flying).The real answer is explained in the references that I cited. For example, AIM section 1-1-9 (d) notes:"4. Pilots must be alert when approaching the glidepath interception. False courses and reverse sensing will occur at angles considerably greater than the published path."(http://www.faa.gov/atpubs/AIM/Chap1/aim0101.html#1-1-9)The Instrument Flying Handbook, referenced in an earlier message in this thread, provides the following information on page 7-34:"2. False courses. In addition to the desired course, glideslope facilities inherently produce additional courses at higher vertical angles. The angle of the lowest of these false courses will occur at approximately 9