Too High on the Glideslope

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Hop Harrigan
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Too High on the Glideslope

Unread post by Hop Harrigan » Mon May 02, 2016 8:52 pm

Recently on a vectored approach to the George rwy 29 ILS, the localiser was captured at the moment I intercepted the GS. The autopilot (Stec55) will only intercept the GS from below with the result that I flew through the GS and wound up 200' high on the GS. ATC then advised (admonished) me that I was high on the GS.
I was able to manually capture the GS from above but it made for an uncomfortable approach.
What does one do? Try to capture the GS or ask for another approach?
Why does this happen? Are ATC conditioned to aircraft with high descent rates and not really geared for a -500'/min rate and are they aware that the GS needs to be captured from below?
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Re: Too High on the Glideslope

Unread post by Iceberg » Tue May 03, 2016 9:21 am

Hop, it seems you might have turned inbound too soon, or you did not do a good job of capturing the localizer timeously.
I usually fly the localizer with help from the heading bug. The glideslope must come in from way above. When it centers I drop the wheels, throttle back and ride it down manually using the yoke and trim only.

To capture the localizer I set the VOR on the same inbound course and use that to help to capture the localizer.
I use a 45 degree intercept angle until the VOR needle is halfway in. The localizer should then come alive and then you turn the remaining 45 degrees in neatly to capture it.
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Re: Too High on the Glideslope

Unread post by Hop Harrigan » Tue May 03, 2016 12:57 pm

Hi Karl,
Agreed...thats my technique as well but this was a vectored approach under control of the George ATC, so I was following his vector instructions. So either he turned me onto the intercept too late or descended me too late. Either way I wound up too high and too close to intercept from below the GS. Am just wondering how to better handle it next time around?
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Re: Too High on the Glideslope

Unread post by SandPiper » Tue May 03, 2016 5:32 pm

Not much you can do, crappy vectoring, unless:

query in good time, if you notice it
do a quick 30 or so degree turn towards the loc before gs capture
intercept gs from above, it is acceptable if done correctly

none of the above are perfect sollutions, or

Go around. :D
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Re: Too High on the Glideslope

Unread post by Denali » Tue May 03, 2016 5:58 pm

Hop
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Re: Too High on the Glideslope

Unread post by gear up » Tue May 03, 2016 7:48 pm

This was a similar thread, hope some info here helps...

viewtopic.php?f=9&t=166400&p=1668643#top
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Re: Too High on the Glideslope

Unread post by Flugmaour » Tue May 03, 2016 8:34 pm

How far out were you when you intercepted the LOC? Often happens that you will be vectored and intercept the LOC quite far out while you are still way above the GS. Obviously as you get closer to your initial approach altitude the GS will start to come back up so you can intercept from below.
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Re: Too High on the Glideslope

Unread post by foxbat » Wed May 04, 2016 9:41 am

SandPiper wrote:Not much you can do, crappy vectoring
I'm not sure what the ATC manual says regarding vectors but I would regard a vector that intercepts LOC and GS simultaneously as perfect and what we would strive for every time in a commercial environment. In many parts of the world (Heathrow as an example) the standard is to intercept from above for noise abatement.

It's up to you to manage the aircraft's systems to achieve the desired result, and if that's not working or not possible with your autopilot then disconnect and fly manually.
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Re: Too High on the Glideslope

Unread post by Hop Harrigan » Wed May 04, 2016 5:46 pm

Hi Foxbat,
That sounds fine for your heavy which has a descent rate in the thousands. My preferred descent rate is between -500'/min and -700'/min (flap speed and pax comfort considerations...I'm unpressurised).
Around -700'/min is what I require on approach just to track the GS. So intercepting the GS from above will always exceed my preferred descent rate.
Am wondering if the controller might be sympathetic to a request for an earlier localiser intercept to get in below the GS?
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Re: Too High on the Glideslope

Unread post by foxbat » Thu May 05, 2016 8:19 am

I'm sure the controller's will be sympathetic to your needs but will primarily be trying to fulfill their mandate of moving as many aircraft as efficiently as possible, which means vectors with simultaneous localiser and glideslope intercept are ideal.

Have a running calculation going in your head of the track miles you need from you current altitude. If its looking like it will be tighter than you'd like then ask the ATC for "estimated track miles to touchdown". If that confirms your calculation then ask for additional track miles.

Quiter airports will most often accommodate your request. The busier the airport the more likely you'll end up at the back of the que so best to make your request earlier in that situation, even before TOD.
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Re: Too High on the Glideslope

Unread post by Hop Harrigan » Thu May 05, 2016 1:41 pm

Hi Foxbat,
Ok that makes sense...thanks for the advice
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Re: Too High on the Glideslope

Unread post by SandPiper » Thu May 05, 2016 4:34 pm

foxbat wrote:
SandPiper wrote:Not much you can do, crappy vectoring
I'm not sure what the ATC manual says regarding vectors but I would regard a vector that intercepts LOC and GS simultaneously as perfect and what we would strive for every time in a commercial environment. In many parts of the world (Heathrow as an example) the standard is to intercept from above for noise abatement.

It's up to you to manage the aircraft's systems to achieve the desired result, and if that's not working or not possible with your autopilot then disconnect and fly manually.
ICAO PANS-ATM (Doc 4444) in respect of facilitating ILS GS capture from below and from level flight pre-established on the corresponding final:

The final vector shall enable the aircraft to be established in level flight on the final approach track prior to intercepting the specified or nominal glide path if an MLS, ILS or radar approach is to be made.”
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Re: Too High on the Glideslope

Unread post by SandPiper » Thu May 05, 2016 4:48 pm

Also:

The noise abatement procedures at Lhr are not meant to intercept the gs from above, not even doing the Cda approach.

Doing the Cda approach there is even a level flight segment of 3nm? allowed to intercept the gs from below.
"Tweaking" the rod to intercept the gs while descending is a fun thing to do. :D
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Re: Too High on the Glideslope

Unread post by foxbat » Thu May 05, 2016 5:14 pm

Thanks for the references Sandpiper.

Interesting to note that the manual requires the vectors to provide a level flight segment. 90% of the approaches I've done into LHR required intercept from above.
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Re: Too High on the Glideslope

Unread post by quintdxb » Sat Oct 22, 2016 7:02 pm

Interesting thread. Unfortunately being vectored in such a way that the glideslope is intercepted before the localizer happens all too often.

Many airliner and business jet autoflight systems will not capture the Glideslope unless the Localizer is captured. This can lead to some rapid re-programming on the autoflight panel as the altitude selector will probably need to be set to a lower altitude and a mode such as vertical speed engaged with a high descent rate as Hop notes. Alternately, it may be quicker to disconnect the autopilot and establish a conservative attitude that will result in capture.

In any event, it is a destabilising situation at a time where things should really be stable and proceeding as planned. The negative impact on flight safety is obvious.

It seems that Hop has a set of ballpark figures that he is comfortable with and that is a very good thing in my opinion.

Ultimately, however untidy the actual intercept is (you may elect to miss the approach right there at the poorly vectored intercept), the next "decision gate" would be the Outer Marker or 1000' point. If you are still IMC at this point and still above glideslope, it is probably better to give it away and go around.

As a matter of interest, what aircraft type are you flying Hop?

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