Steep Descent + Intercepting the ILS from above

[Huntergreed]

On our return flight from Bordeaux to Manchester this afternoon, myself and @yorkie noticed a particularly low vertical speed at several points of the descent (featuring frequent and often quite harsh use of the speedbrake to maintain a constant airspeed)

I’m well aware it goes against normal practice, however I’m fairly convinced that given our steep rate of descent on the approach phase, we may have intercepted the ILS from higher than the glideslope (indeed the angle of approach seemed significantly higher than the normal 3 degrees used for ILS approaches until we were on final, particularly when we were clearly established on the localiser but not yet established on the glideslope).

A few questions I have related to this:

1) What circumstances would lead to an aircraft having to intercept the ILS from above the assigned glideslope? I’m particularly interested in this as the risk of intercepting an incorrect glideslope of 6 or 9 degrees, and what measures are used to mitigate against this?

2) Is this a frequent occurrence? Almost all approaches I’ve been on (certainly on a commercial aircraft) have either intercepted the ILS glideslope from below or, on a nice day, simply flown a visual approach.

3) Are there any airports (in or outwith the UK) where this is normal practice?

 

[gabrielhj07]

As you say it is not good practice to go about intercepting glideslopes from above, doing so often leads to a too ‘high energy’ approach, either in terms of kinetic or potential.

There are, however, a couple of reasons that spring to mind as to why this might have happened.

If the tailwind on the downwind leg was particularly strong, then maintaining proper airspeed would have required considerably more ground speed. Therefore a greater distance would have been required to descend, which may have been at odds with the hills to the North East of the airport. Without overshooting the localiser this may be likely.

Another possibility could be that the downwind leg was cut short for whatever reason, or was shorter than expected, leading to a sharper descent rate being required. Without examining the flight path/comparing it to other flights this is speculation.


RE some other points:

Flying around at the (relatively) low altitude of 3000ft in (I assume) a 737, a negative pitch of 6 degrees is inadvisable, and 9 would be quite dangerous. Of course there is a difference between pitch and FPA, an FPA of 6 might be fine, it depends on the Angle of Attack.
With respect to mitigation, the 737 (or A320 for that matter) will not intercept the glideslope from more than about half a diamond above. (As an aside, the A320 might accept glide star but wouldn’t capture).

It’s not exactly frequent, although of course it does happen.

I don’t believe that there are any airports where intercepting the glide from above is normal practice, indeed few specify. Not to say that there aren’t steep approaches, however, EGLC has 5.5 (used to be 7.5!).

 

[Huntergreed]

As you say it is not good practice to go about intercepting glideslopes from above, doing so often leads to a too ‘high energy’ approach, either in terms of kinetic or potential.

There are, however, a couple of reasons that spring to mind as to why this might have happened.

If the tailwind on the downwind leg was particularly strong, then maintaining proper airspeed would have required considerably more ground speed. Therefore a greater distance would have been required to descend, which may have been at odds with the hills to the North East of the airport. Without overshooting the localiser this may be likely.

I see, that’s very interesting thanks! I presume this would have been following a vectored approach as opposed to one of the published approaches? Or would this happen if the tailwind increased the airspeed to the point where the rate of descent was not able to reach the stated altitudes in the published approaches?

RE some other points:

Flying around at the (relatively) low altitude of 3000ft in (I assume) a 737, a negative pitch of 6 degrees is inadvisable, and 9 would be quite dangerous. Of course there is a difference between pitch and FPA, an FPA of 6 might be fine, it depends on the Angle of Attack.
With respect to mitigation, the 737 (or A320 for that matter) will not intercept the glideslope from more than about half a diamond above. (As an aside, the A320 might accept glide star but wouldn’t capture).

That’s correct, a 737-8200, with the speedbrakes very much in good working order

That’s also very interesting! I presume when you say it won’t intercept you’re meaning it won’t be able to use the approach lock function on the A/P? The pilot presumably could fly manually from any degree of interception with the G/S (1 and a half or 1 diamond above)?

Just curious if it’s possible that’s all - obviously it would never happen in practice! Not familiar with ILS procedures being mainly trained on VFR!

It’s not exactly frequent, although of course it does happen.

I don’t believe that there are any airports where intercepting the glide from above is normal practice, indeed few specify. Not to say that there aren’t steep approaches, however, EGLC has 5.5 (used to be 7.5!).

7.5 is absolutely crazy - I wish I could have tried that

5.5 isn’t far off 6 - would the aircraft capable of landing at EGLC be capable of safely and consistently landing at any airport using the 6 degree ILS G/S as opposed to the intended 3 degree one? (again obviously theoretical, but I am very curious!)

 

[niggill617@gma]

On our return flight from Bordeaux to Manchester this afternoon, myself and @yorkie noticed a particularly low vertical speed at several points of the descent (featuring frequent and often quite harsh use of the speedbrake to maintain a constant airspeed)

I’m well aware it goes against normal practice, however I’m fairly convinced that given our steep rate of descent on the approach phase, we may have intercepted the ILS from higher than the glideslope (indeed the angle of approach seemed significantly higher than the normal 3 degrees used for ILS approaches until we were on final, particularly when we were clearly established on the localiser but not yet established on the glideslope).

A few questions I have related to this:

1) What circumstances would lead to an aircraft having to intercept the ILS from above the assigned glideslope? I’m particularly interested in this as the risk of intercepting an incorrect glideslope of 6 or 9 degrees, and what measures are used to mitigate against this?

2) Is this a frequent occurrence? Almost all approaches I’ve been on (certainly on a commercial aircraft) have either intercepted the ILS glideslope from below or, on a nice day, simply flown a visual approach.

3) Are there any airports (in or outwith the UK) where this is normal practice?

Mentour Pilot video

/www.youtube.com/watch?v=ZiNHsukIkCg

 

[Ediswan]

I’m particularly interested in this as the risk of intercepting an incorrect glideslope of 6 or 9 degrees, and what measures are used to mitigate against this?

One mitigation not mentioned so far is to compare altitude/height against distance to go.
A standard approach chart will give this. Here is the profile for Manchester ILS/DME RWY 23R


If you appear to be on the glideslope, but are substantially higher (or lower) than that profile indicates, something is wrong, time to go around.
For example, height (AGL) on a 6 degree glideslope would be roughly double that of a 3 degree glideslope.

 

[gabrielhj07]

I presume this would have been following a vectored approach as opposed to one of the published approaches?

AFAIK all approaches from the South/South East to runways 23L/23R are vectored anyway, the published approach ends about at waypoint DAYNE at FL80.

if the tailwind increased the airspeed

Tailwinds increase the groundspeed

a 737-8200

Ah! A MAX!

That’s also very interesting! I presume when you say it won’t intercept you’re meaning it won’t be able to use the approach lock function on the A/P? The pilot presumably could fly manually from any degree of interception with the G/S (1 and a half or 1 diamond above)?

On the 737, assuming you had selected APP on the MCP, the aeroplane would self-arm VOR/LOC & G/S. G/S won't capture, so it would remain armed, even if the aeroplane captured the localiser.

5.5 isn’t far off 6 - would the aircraft capable of landing at EGLC be capable of safely and consistently landing at any airport using the 6 degree ILS G/S as opposed to the intended 3 degree one? (again obviously theoretical, but I am very curious!)

The aircraft that fly into London City are required to meet certain standards for the approach. Taking the BA's A318s as an example, they are both fitted with a 'STEEP APPROACH' mode, which lets them fly the ILS at full config with the speedbrakes extended for the whole glide. Other aircraft, such as the BAE146, will deploy their air brake as well.

 

[BananaRepublic]

…..Taking the BA's A318s as an example, they are were both fitted with….….

Past tense.
Both retired and scrapped I believe.


.

 

[gabrielhj07]

Past tense.
Both retired and scrapped I believe.


.

Yes, October last year at St Athan.

 

[D1024]

I’m well aware it goes against normal practice, however I’m fairly convinced that given our steep rate of descent on the approach phase, we may have intercepted the ILS from higher than the glideslope (indeed the angle of approach seemed significantly higher than the normal 3 degrees used for ILS approaches until we were on final, particularly when we were clearly established on the localiser but not yet established on the glideslope).

Been away for a few days...

Just looked at a replay and the download data for your flight. You definitely 'cut the corner' on approach on this one - straight over Marple and turned final over Bredbury and a little bit higher than would be normal for an ILS Approach. Having also looked at the METAR (Aerodrome Met reports) for the time of your approach I would strongly suspect the pilot was conducting a Visual Approach as weather conditions were favourable and there was no other traffic immediately ahead of you. The wind was consistant West-North West throughout so a tailwind not really relevant.

 

[YorkshireBear]

Pretty sure this happened to use recently on a trip into Manchester too. Flight too long ago for me to check though. Either that or I was just imagining things.

 

[TheEdge]

Past tense.
Both retired and scrapped I believe.

That's sad.

Too young to have had the chance to fly on the proper Speedbird 1 and never had the cash to fly on the A318s, always wanted to.

 

[niggill617@gma]

Thunder & Lightnings - English Electric Lightning - Survivor XM135

Cold War British military aircraft, including history, surviving airframes, profile drawings, photos, walkarounds, links, references. Also viewing guides for UK airfields and the Test Flying Memorial

www.thunder-and-lightnings.co.uk

No ILS for this frightening flight.

 

[Trackman]

I know little about flying aircraft so please bear with me.
Trying stay OT from the OP.

Does approach not tell them the vector/course and tell them they can intercept the ILS signal?
Also when do pilots switch off the ILS? Over the runway?
If you are in the wrong glidepath in the ILS transmission area, would the ILS takeover or a big no from the flight computer?

 

[Huntergreed]

I know little about flying aircraft so please bear with me.
Trying stay OT from the OP.

Does approach not tell them the vector/course and tell them they can intercept the ILS signal?

Sometimes they are “vectored in” (told which heading/altitude/speed to fly in, and they’ll do it this way) and sometimes they’ll follow a STAR (Standard Terminal Arrival) which is a predetermined approach programmed into the flight plan which will get them automatically established with the ILS.

An approach which intercepts from above will almost always be vectored, I find it unlikely a published STAR would use this procedure!!

Also when do pilots switch off the ILS? Over the runway?

They don’t really switch it off as it’s just a radio signal. Pilots will switch off the autopilot at some point and manually land the aircraft (as is FAA /CAA law as far as I’m aware), but some aircraft, certainly the A320 family, are capable of fully automated landings in CAT III ILS equipped runways.

If you are in the wrong glidepath in the ILS transmission area, would the ILS takeover or a big no from the flight computer?

The ILS will pick up the glideslope in multiples of 3 degrees (it’s possible to intercept the 6 or 9 degree glideslope, hence why intercepting from above is generally a bad idea!)

 

[Trackman]

Sometimes they are “vectored in” (told which heading/altitude/speed to fly in, and they’ll do it this way) and sometimes they’ll follow a STAR (Standard Terminal Arrival) which is a predetermined approach programmed into the flight plan which will get them automatically established with the ILS.

An approach which intercepts from above will almost always be vectored, I find it unlikely a published STAR would use this procedure!!

They don’t really switch it off as it’s just a radio signal. Pilots will switch off the autopilot at some point and manually land the aircraft (as is FAA /CAA law as far as I’m aware), but some aircraft, certainly the A320 family, are capable of fully automated landings in CAT III ILS equipped runways.

I've heard of STAR, think they are charts the pilots have.

---
Yes I meant switching off the autopilot not the ILS!

 

[Huntergreed]

I've heard of STAR, think they are charts the pilots have.

That’s correct. They are charts that outline different approaches to runways at an airport. When programming the flight computer for the approach, you would put in a SID (standard instrument departure), then all waypoints, then a STAR for the approach. This will be followed unless ATC give vectors on approach (which is very common, more common than actually following the STAR in my limited experience of this!)

Yes I meant switching off the autopilot not the ILS!

Ah apologies!! Company policy differs I believe! With airlines you would switch off the A/P on final still well away from the airport (when the runway is in sight), and at some airlines you’ll keep the A/P engaged until short final. With others I imagine it’s pilot discretion!

 

[Trackman]

That’s correct. They are charts that outline different approaches to runways at an airport. When programming the flight computer for the approach, you would put in a SID (standard instrument departure), then all waypoints, then a STAR for the approach. This will be followed unless ATC give vectors on approach (which is very common, more common than actually following the STAR in my limited experience of this!)

Ah apologies!! Company policy differs I believe! With airlines you would switch off the A/P on final still well away from the airport (when the runway is in sight), and at some airlines you’ll keep the A/P engaged until short final. With others I imagine it’s pilot discretion!

I will look at some you-tube videos about this, many thanks again.

 

[BananaRepublic]

That’s correct. They are charts that outline different approaches to runways at an airport. When programming the flight computer for the approach, you would put in a SID (standard instrument departure), then all waypoints, then a STAR for the approach. This will be followed unless ATC give vectors on approach (which is very common, more common than actually following the STAR in my limited experience of this!)………..

A STAR defines the flight plan arrival route linking from the airway/ air route structure, to the terminal holding fix (or fixes) for the associated airport.
They don’t lead to final approach or to runways.
The terminal holding fix is the end limit of the STAR.
From the terminal fix onwards is covered by the airfield’s approach procedures, which cover both radar controlled or procedural approaches.


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