View Full Version : Why 3 degrees for ILS ?

1st Jul 2001, 04:18
How was 3 degrees chosen for the glideslope angle for most ILS's ?

As this is around 500-700 fpm for most planes, if this was a factor ... then why 500-700 fpm ?


1st Jul 2001, 05:47
I'm sure the 3 degree decision was pre-ILS era by quite some time. Pre-ILS is (roughly) pre-pressurization, 500-700 fpm is an ear friendly rate that gets the job of descending done in a reasonable period of time.

More likely the pioneers devoted some old-fashioned trial and error to this problem. At 5, 10 or 15 degrees they probably had problems engineering a respectable flare, especially with the marginal control authority provided by Bleriots, and whatnot. At 1 or 2 degress they probably noted that trees, telegraph poles, and smokestacks became troublesome obstacles.

The above is considerable conjecture, but I think 3 just worked.

1st Jul 2001, 07:11
The original slope was 2.5 degrees,with the advent of Cat2,3degrees adopted as standard..
US anti- noise advocates,at one time suggested 6degree slopes intercepting 3 degrees at 1000'--thrown out by the industry as too cumbersome(power on at intercept/dirty)
Remember a 90 degree 'slope gives you the least rollout!!!

1st Jul 2001, 07:20
Yeah, that may be, but after touchdown all the ladies would be wearing fur collars.

Nick Figaretto
4th Jul 2001, 04:33
I can see from your profile that you are not a pilot. It might therefore be interesting for you to know that, although it dos not sound like much, the difference between a 3 degree glideslope and a 4 degree glideslope is quite large.

On a 4 degree glide path, you might have severe problems getting established on the glidepath if you have a heavy aircraft, a little bit of tailwind, a little high speed, and are just slightly above the glideslope when intercepting the localizer.

4 degrees is the steepest "non-standard" ILS allowed. Though I am not sure wether there are some ILS'es that are steeper somewhere in the world. (Oh yes, I guess Cape Canaveral gas a 20 degrees glideslope for the space shuttle, but that's a different story)

4 degrees gideslopes are used to obtain obstacle clearance when there is high terrain (or obstacles) in the approach sector.

Glideslopes that are less than 3 degrees requires large amounts of thrust (or power), especially when the aircraft is in landing configuration. (Gear down and full flap = lots of drag!)

This can be very critical if a twin engine aircraft flies on one engine due to an engine failure. If you (inadvertantly) get below the glide slope, combined with low speed in this situation, it might be difficult to get back on the glide slope. Some (smaller) twin engine aircraft are not able to climb or even maintain straight and level flight with one engine out and gear down. Let alone with full flaps.

Hence, a three degree glideslope gives comfortable flying both in normal and abnormal situations, for all (normal) aircraft.


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