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Is a 4 degree glide slope practical ?

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Is a 4 degree glide slope practical ?

Old 23rd Apr 2008, 07:29
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Is a 4 degree glide slope practical ?

In order to mitigate the noise of arriving aircraft it has been suggested that a steeper glide slope could be used. What would be the advantages and disadvantages of increasing the slope to 4 degrees ? I should welcome professional pilots views on this.
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Old 23rd Apr 2008, 07:35
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I think you'll find a number of modern jets will struggle to achieve stability on a 4 degree glideslope.
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Old 23rd Apr 2008, 07:40
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Doesn't London City have a 6 degree slope? Presumably the types of aircraft flying there are designed to accommodate that?
Old 23rd Apr 2008, 07:43
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Yes it would be nice & quiet under the approach chart.

Not so good for those living near the over run area though with all the noise of aircraft running off the end of the runway.

On a serious note 4 degrees is a bit too steep for most medium to large jets. I'm sure that lots of people can point you in the direction of statistics regarding the merits of stable approaches. One of the requirements being no more than 1000 fpm rate of descent. Sometimes this is not possible with slopes of up to 3.5 degrees so 4 degrees would be a non starter.

However there are a number of turbo props & a very small number of jets that are designed to operate into city centre airports where noise is super critical that can do 4 degrees & more.

Perhaps with time the normal (3 degree) glide slope will become steeper, as aircraft design evolves, but at the moment the safety issue out weighs the noise problems.
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Old 23rd Apr 2008, 07:51
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Marseille runway 31R has a 4 degree glide slope and the 737 doesn't have a problem with that as long as you configure nice and early. The majority of aircraft are not certified though to do autolands from approaches steeper than approximately 3,25 degrees. So a 4 degree slope might not be very practical for larger airports like LHR, LGW en STN since they operate quite regularly to CAT 3.
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Old 23rd Apr 2008, 07:52
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Not only possible but not unknown: 4.46 deg at CMF for example. Cannot remember MRS 32 but it is steep. Certainly 3.5 deg would be fine. Just requires a modified technique.
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Old 23rd Apr 2008, 07:54
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Yup, a 4 degree slope would not be workable on a 777. Maximum certified autoland is 3.25 degrees.
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Old 23rd Apr 2008, 07:56
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I regularly fly a 737-800 into Marseilles on a 4 degree glide. The ROD can sit near or even over 1000ft/min but so long as you brief that fact and both pilots are aware, it presents no problems other than a slightly more pronounced flare. Glasgow Prestwick in Scotland has a 3.5 degree glide which we use happily as do the large number of 747-400 freighters that operate there.
Basically where surrounding terrain makes it necessary it can absolutely be done and in a safe stable manner.
Edited to add that as people have said above since i started writing, Auto the pilot is not approved to land after one of these approaches.
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Old 23rd Apr 2008, 07:59
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Yes, but only a limited number of aircraft can maintain a steady descent-rate on this glideslope. (6 I mean, not 4) And even then, most companies (if not all) will only allow the Captain to execute the approach.

The advantage/disadvantage of a 4-glideslope lies in the powerrequirement. As was already mentioned, we would need less enginethrust to maintain a stable speed during final descent. Less enginethrust equals less engine-noise. However, that is also the risk (the thrust, not the noise).

As the descentangle/glideslope increases, the component of airplane weight along the direction of travel increases also. This causes the need for less enginethrust. (E.g. a rock. If you drop a rock of a bridge, the component of its weight is completely along its direction of travel - vertical. It does not need thrust to accelerate.)

Since there is reduced "excess"-thrust (thrust above idle, in this case) it will be more difficult to control/eliminate any excess speed. Larger, heavier airplanes might not be able to reduce speed with normal means at all.

I do agree, approaches in Marseille are usually safe and stable. However, the margin of error (consider tailwind on final!) is smaller.
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Old 23rd Apr 2008, 08:07
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What's the point? Isn't the climbout path where most of the noise is? Quite apart from the fact that people that choose to live near airports should have to accept the noise...........
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Old 23rd Apr 2008, 08:13
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Yes, as others have posted. It can and does work in some instances, but is/would only be cat1. It would also need more seperation between arriving aircraft to allow them to slow up and configure for it. I expect there would also be an increase in missed approaches.
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Old 23rd Apr 2008, 08:16
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I regularly fly a 737-800 into Marseilles on a 4 degree glide.
Just out of interest are RyanAir now cleared to do the CDA approach to 31R?
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Old 23rd Apr 2008, 08:38
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so what really happens on a 4 slope?

You have to start configuring much earlier, like it has been said, in Marseille you should start the ILS with flaps 20, or you won't stand a chance (that is in the CRJ -700-900). On a 3 glide I can delay flaps 20 to around 2000' AGL if not speedrestricted by ATC. I also can delay the gear.

On modern aircraft the majotity of the noise is created by interference of slipstream around gear/flaps. The engines are at idle anyways and only give off a slight hiss, until - yes, just until - I have to start stabilising my approach by getting flaps and gear down and increasing engine power. Then the noise really starts...

So what do you gain by shifting to a 4 glide? - In theory, aircraft would fly about 30% higher off the ground, so at 10 miles 4000 iso 3000 feet - that is a gain, no doubt, and a big noise reduction.
Now at 6 miles out 2600 iso 2000 feet, still some advantage...
At 3 nm 1300 iso 1000 feet - not much difference.

Unfortunatly it won't really work that way - You have to get the flaps out at 10 nm, increasing the noise (remember, you don't fly slower, you just need more drag to maintain speed), somewhere between 10 and 7 nm you would need the gear to go down (on a CRJ - on an A330 starting down with anything less then gear down and flaps 2 would lead to a goaround). Final configuration must be achieved earlier to get down to Vapp, thereby increasing noise levels...

I think if a 4 slope wouldn't increase noise levels, they ought to be about the same, with a huge downside on safety - less stable approaches, higher rates of descent,...

In my opinion. not an option at all...


PS: The most silent approaches can be achieved at airports with little or no traffic, where ATC doesn't get in your way.
You just glide it down, and set it up to have the right speed and little flap hitting the glide at the descent point, take it down the ILS and drop everything at 1800 to 1500 feet AGL, to be established at 1000' exactly, not moving the throttles out of idle all the way...

Most satisfying, when done right...


Last edited by Admiral346; 23rd Apr 2008 at 08:45. Reason: more thoughts...
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Old 23rd Apr 2008, 08:39
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Cda Mrs

I'm off work today but if I remember I'll check our brief tomorrow and let you know.
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Old 23rd Apr 2008, 08:53
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Auto the pilot is not approved to land after one of these approaches.
Classic! I remember that one
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Old 23rd Apr 2008, 09:12
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Anyone else been to Chambery lately? That's always fun!!
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Old 23rd Apr 2008, 09:19
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Not so good for those living near the over run area though with all the noise of aircraft running off the end of the runway.

Funny, the last time I looked at anything relating to performance, you definitly got a landing dist/mass credit from having a steeper approach angle... but maybe they have found a way to eliminate that as well?

Re. 777 - so you cannot do an autoland, fair'nuf - how about max certified approach angle? Might be a different story.
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Old 23rd Apr 2008, 09:26
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London City is 5.5 degree (or was the last time I went in there a few years ago!)
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Old 23rd Apr 2008, 09:35
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yes the B 146 does the job

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Old 23rd Apr 2008, 09:35
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Point is you will have to add some safety margin making the approach even steeper: Like in LCY it's 5.5 degrees plus 2 = 7.5 degrees that you need to be certified for. If you aim at 4 for noise abatement you'll end up close to six in real life certification. That is way steeper than many a/c can handle. There where quite a few mods done to the A318s computers, avionics, callouts, trim, flaps and speedbrakes in order to get it certified for LCY ops.
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