Intercepting the ILS

[Dont Hang Up]

PPRUNE is complaining I have not contributed for a while so here goes...

When I was taught (PPL IMC), the ILS intercept was always localiser first then glideslope. And the glideslope was always approached from beneath (typically straight and level). Trying to get onto the glideslope from above was a strict no no.

However, I am curious as to what is common practice with a modern airliner. In particular, we hear a lot about continuous descent. Is it actually possible to intercept the glideslope from a completely power idle descent (ATC permitting)?

[John Citizen]

Is it actually possible to intercept the glideslope from a completely power idle descent (ATC permitting)?

Yes, it sure is.

However the Airbus (A320) autopilot/Flight director wont capture/engage the Glideslope until the localiser has been captured/engaged first.

You do not have to be in level flight to achieve this.

You can be on (idle) descent or even rolling out of a (descending) turn !!

Sometimes ATC :mad: you around and so you got no choice to intercept it from an idle descent (sometimes from above the GS too)

[NigelOnDraft]

Joining the GS from above is not ideal... apart from any other reason, a modern clean airliner will not descend >3degreees without gear / speedbrake.

Ideally you do approach the GS from below, but descending iaw a CDA approach. The G/S requires say 700'/m descent, so if it works out well, you can be at idle, declerating from 200K to 180K at say 3-400'/m and intercept the G/S from below.

Of course, it doesn't always work out, and most will be well practiced in the "technique" to join the G/S from (slightly) above...

NoD

[Denti]

However the autopilot/Flight director wont capture/engage the Glideslope until the localiser has been captured/engaged first.


Depends on type, earlier 737 classics can and do capture the GS on its own and before they have done so on the localizer. For that reason our SOP was for a very long time to use only VORLOC until the localizer is captured and then switch to approach mode.

[Dont Hang Up]

Thanks for the input people.

And for exploding one of my misaprehensions...

apart from any other reason, a modern clean airliner will not descend >3degreees without gear / speedbrake


So an airliner will descend at shallower than 3 degrees unless some positive drag is introduced :ooh:. This rather negates my question because the aircraft in idle descent will tend to be coming at the glideslope from beneath anyway.

They really don't want to come down do they?

Reminds me of the old joke "If we lose any more of these engines we'll be up here all night!". :O

[NigelOnDraft]

So an airliner will descend at shallower than 3 degrees unless some positive drag is introducedRoughly... if you are trying to join from above, you need considerably more than 3 degrees...

An A320 series, 180K, 1+F, idle, still wind will tend to follow ~3 degree slope and hold 180K...

NoD

[RAT 5]

Most airliner ground speeds on 3 degrees at F5 = 800 - 900 fpm with a 10kts head wind. Thus if you can achieve >1000fpm you can capture from above. If started early enough it is very possible before 1000' agl. F15 & gear
(B737) or F5 & speed brake will give 1200-1500fpm. No drama and the G/S can be captured before 4nm if you begin the process soon enough. The secret is to realise what is happening, either caused by misjudgement or ATC vectors, and act promptly before the hairs on back of neck get prickly.

[Dont Hang Up]

F15 & gear (B737) or F5 & speed brake will give 1200-1500fpm. No drama and the G/S can be captured before 4nm if you begin the process soon enough. The secret is to realise what is happening, either caused by misjudgement or ATC vectors, and act promptly before the hairs on back of neck get prickly.


This was originally an innocent technical query on my part, but your words now seem to take on more significance when I read the thread on the Turkish Airlines accident.

It would appear the a/c was intercepting the G/S from above at less than 5NM much as you describe. The rest remains conjecture.

[4greens!]

if you do this and intercept the 6 or 9 degree sidelobes is there any cockpit warning (other than high v/s)?

[Atreyu]

There isn´t a warning on capture of a false G/S, however, rate of descent, and a check of Altitude vs DME distance on the approach plate should tell you something isn´t correct!

Atreyu:ok:

[4greens!]

all the ILS approaches I have flown were steam-guage variety. I was just wondering whether more modern equipment might detect possible differences in phase relationships between localiser signal and 3.6&9 degree g/s signals or some other subtle anomaly

[HiFli]

I find that ATCs around the world are inclined to try and get us to intercept the LOC and G/S at the same time. My conclusion is that they are using us to sharpen their FlteSim skills.

[pottwiddler]

Being the guy on the ground who twiddles the pots on these things (now I hammer the keys on them!), Localisers have more output power and are lower frequency, therefore have more usable range through signal strength. I can only presume that glides are intercepted from below as you flyboys establish on the Loc then turn in/line up which is best practise really.

One thing for me though is why headset-chair interfaces (ATCO's) say "descend with the ILS" now and not as before "descend with the Glideslope"? Strange!

Oh and sidelobes are really well supressed nowadays so 6 or 9 degree approach should be impossible now. Anyway I hope the driver notices that something was awry if he did descend at that rate!

[Love_joy]

In practice, intercepting from above is not a problem, but it is taught as a no no because of the possibility of picking up a false glideslope.

I'm no expert on this, but seem to remember something about reflections of the glideslope beam forming repetitions of itself above the one that your looking for. Thus, if you manage to pick up a false glide, you will prob end up at the right place, but having flown a steeper approach. It is for this reason that you validate the glideslope on the way down, check altitude at 4nm etc...

It wasn't mentioned above, but you can fly a CDA with a verical speed of as little as just 100ft/min (as long as your below the transition level, otherwise the 500ft/min expectation from ATC applies). Rocking along at 200kts, 100ft/min almost approximates level flight, not quite, but almost.

[Midland63]

Sorry, just an SLF with no flying experience so naughty of me to post but can I just clarify NigelonDraft's statement at #3 "a modern clean airliner will not descend >3degreees without gear / speedbrake." You meant a modern clean airliner will not descend >3degreees without accelerating without gear / speedbrake." Right? If that is right, do flaps make any difference or does it take something really dirty like gear or s/b to inhibit the tendency to speed up?

Thanks.

[Blenk]

I was just wondering whether more modern equipment might detect possible differences in phase relationships between localiser signal and 3.6&9 degree g/s signals or some other subtle anomaly

There will be one, not so subtle difference and that is that the 6 degree sideband will reversed sense. Ie, the 90Hz sector is below the 6 degree line and the 150Hz sector above it. This still gives you 0DDM in the middle (a valid centreline) but if you deviate from that (say below) you will get fly down indications instead of fly up indications. All due to the GP beam is formed with antennas and reflections and the like.

As Pottwiddler said though these sideband are failry well supressed these days and the routine flight inspections if ILSs would pick up an anomolies.

The distance/altitude table would let you know though and I guess that you wouldn't "feel" quite right about the approach either. Come to think of it ATC might have also queried what you are doing if they cleared you for the ILS and you are twice as high as you should be!

[A Comfy Chair]

Midland you are correct, however it can become "impossible" due to flap limits.

The 767 when "clean" (no flap or gear) will accelerate significantly down a 3 degree approach. At flap 5, and about 180kts or less, it will 'just' hang on to 3 degrees without accelerating.

At flap 1, the aircraft will continue to accelerate, and quite quickly break the Flap 1 Limit speed... and so, in a way, it is "impossible" to do it, if that makes sense.

To intercept the G/S from above, with flap 5 out you need some additional form of drag. The most effective way to do this is Gear, although speedbrake will work if you are a reasonable way out and just need to have a slight increase in rate of descent.

Pottwiddler - "Descend with the ILS" means that you must be established within tolerance on the ILS (both LOC and G/S) before you descend. Descend on the Glideslope is a clearance to do just that... descend on the glideslope, possibly without being established on the LOC. It is a minor thing, but could create problems if the pilot interprets the clearance to descend on the G/S from wherever they are, and that is not what ATC are wanting. If the instruction is given when already established, there is not really any difference, but if the instruction is given prior to LOC intercept, then it may cause confusion, and apparently ATC are using new phrases to avoid that.

[Midland63]

@ A Comfy Chair - what you said makes perfects sense to me. Thanks for taking the trouble to answer my question so helpfully.

[Permafrost_ATPL]

Just to add to Comfy Chair's post...

On some types, like the NG, early stages of flaps can in some cases carry enough drag to slow down on a 3 degree glide slope. On a 737 NG, flap 1 will usually result in speed increasing but if flaps 5 are deployed within placard limits you will be able to stablise the speed and call for more flaps. With a tailwind, flaps 10 might be needed (for some reason most operators see the optimal flaps sequence on that type as 1,5,15 and 30/40, bypassing the very handy flaps 10).

HOWEVER, companies like my employer do not want you to "use flaps to slow down" because it increases flaps maintenance costs. So the want you to select the appropriate flap (typically 5) when intercepting the glideslope, to prevent the speed increasing in the first place.

Even though the speedbrakes will help reduce speed in the early stages of flaps, the NG FCTM says the use of speedbrakes should be avoided when flaps are deployed (but it is not prohibited). So when I see the speed increasing on the glide, I just drop the Dunlops.

Cheers

P

[Midland63]

Thanks Permafrost. In essence what you're saying is use flap to prevent yourself speeding up but not to slow you down. For slowing down, use speedbrakes (if clean) or gear (if flap already out), right?

Thanks.

[Permafrost_ATPL]

Correct. You basically want to try and deploy the next set of flaps as close as possible to the Vref for your current flap setting. My company's SOPs, for example, calls for speed being within 10 kts of Vref before deploying next flaps. If you just used the max placard speeds, you could be well outside those 10 kts. Flaps 5 placard speed on the NG is 250 kts, with a Vref of around 150 kts (changes with weight). So you could be 100 kts away from Vref without exceeding design limits! But like I said, it wouldn't do much good to your flap maintenance costs :-)

P