"hitting the glide"

[dogbros]

Is it true that you are not allowed to intercept the glideslope from above? (me thinks this is correct!!) If so, why is it illegal to intercept the glide from below at some airports because it involves a level segment. I know some airports require this as part of their noise abatement. So it is physically possible to do so but why can't aircraft capture the glide from above at other aerodromes. Comments please! <img src="redface.gif" border="0">

[gyrohead]

..Anytime soon this question is gonna get busted to "Questions" or "Tech Log"..just so ya know! <img src="tongue.gif" border="0">

[Bluejet]

Methinks it's not illegal to intercept from above, rather it's considered bad practice. In reality nothing wrong with it provided you know exactly where you are and are sure you've got the right one.

[Julian]

I agree with Bluejet, its purely good practice as far as I can see. I was taught to intercept from below.

If you intercept from above you end up chasing the GS, if you intercept form below you just fly into it and follow it down.

Julian.

[Matthewjharvey]

dogbros,

The reason why it is good practise to intercept from below (although at some airports around the world you will intercept from above) is that there are usually a number of 'false' glideslopes above the real one, caused by reflection (or something - those JAR questions soon disappear from memory) so if you were to intercept from above you may actually start following an incorrect glidepath. Intercepting from beneath you are bound to be on the correct one as there are no false glideslopes there.

[ 26 February 2002: Message edited by: The Greaser ]

[ 26 February 2002: Message edited by: The Greaser ]</p>

[HEATHROW DIRECTOR]

At very busy places it's virtually impossible to guarantee that a/c will be below the glideslope when they hit it. There are so many traffic problems that it is not always possible to descend a/c early enough to ensure that they are below the glide. In an ideal world, yes, but in a busy TMA.. not too often.

[dogbros]

Thanks for the posts. The reason this question was posed was because people tell me it is illegal to intercept from above. I realise that the "false glide" could be intercepted but isn't that one of the reasons we do an OM/4dme check? It is certainly more efficient to hit the glide from above because you are more likely to be at flight idle and also on a constant descent meaning no level segments which is kinder to the paying punters and crew due fewer thrust changes. The people who tell me it is illegal say it is in the rules/regs of which I have looked but cannot find. If this rule does exist could someone please let me know its whereabouts.

Thanks and happy flying

[Pegasus77]

More efficient is not always true... The steep descent path you need to intercept from above often requires the use of speed brakes or early geardown, to get rid of the excess of speed. And when you don't get it done in time... flying a go around is offcourse the least efficient :)

But to the question of it being illegal: Not at all, it might be considered bad airmanship, but even that is not always true. There is no rule as to intercept the g/s from above or from below.

There is, as already stated, near larger airports so much going on, that it is not always possible to have your ideal descent path. Also it seems to happen, so now and then, that ATC gives you a short-cut which you didn't expect before, and what would be better then... intercepting from above or flying a 360? <img src="smile.gif" border="0">

About intercepting a false glideslope: It is more likely to intercept a false localizer-signal, because measured in degrees you are way more off the localizer before intercept, than you are off the glideslope and therefore fly through more localizer sidelobes than through false glideslopes. The first available false glideslope is about 9 degrees steeper than the normal 3 degree glide, so there must be something really wrong there if you've intercepted that one. There is another false lobe at about 7,5 degrees, but the indications are reversed there, and must therefore seem very unlogical on the cockpit.

Still I consider it good airmanship myself to check a DME-reading with g/s-interception and then compare the altitude on the chart with actual. As well at the OM, offcourse, but first at g/s-interception.

Besides, if you have intercepted a false glide, it will be way steeper than a normal ILS, and it would be impossible to reduce your speed there, combined with a ridiculously high sinkrate, somewhere a bell should ring.

And for The Greaser: there are false localizer signals as well below the glide, but they are so close to the ground, I suppose anywhere near an airfield your GPWS has already rung 1000 times before you intercept such one.

[Capt Pit Bull]

Its a question of balancing the risks.

Having spent several years flying relatively small aeroplanes behind larger one, it is often a good idea (when familiar with airfield, no tricky terrain, certain of position, or visual) to sneak onto the glide from above.

Since wake gradually falls as it dissipates, when flying in calm conditions intercepting from beneath (in the time honoured fashion) increases the likelihood of meeting it.

When behind a heavy, I always try to stay above its flight path until intercepting the glide. Then, by the time you catch up to its wake horizontally, it will have fallen beneath you.

CPB

Who was once subject to a nasty wake encounter.

[Easy226]

Hi bluejet, ive always wondered why planes sometimes come over my house really low for no reason!! I guess youre based at Birmingham- if so , i live right under the ILS for runway 33 - 10 miles out!! Occasionally planes come over really low (compared to others).They usually just become established over my house as i have an airband radio.

[olivasnooze]

Some of the older AFDS do not capture the glide from above. ie the older 2A/P jumbo.