# 3 degree glideslope

## Recommended Posts

What is the 3 degrees referenced from? Is it 3 degrees from the ground (horizontal)? I was doing some calcs before and this would mean that at a distance of 4nm from the threshold you'd be expected to be at 810ft which is roughly half that of what most ILS charts i have seen require for that distance (4nm is 24,300ft, 3 degrees in a ratio of 30 horizontal units to 1 vertical unit, 24,300/30 = 810ft).

Anyone shed any light?

##### Share on other sites
 Help AVSIM continue to serve you!Please donate today!

Try law of sines, or work the tangent.  I get 1273 feet at 4 nm

##### Share on other sites

The glide slope signal is arranged to define a glide path of approximately 3°.  Some are noted as slightly more or less than 3.

An easy ref to staying on the GS is to half your ground-speed (approx IAS).

IF 120 kts then approx rate of descent should be 600fpm.

KingG is correct.

The average dist for the OM form the ILS antenna is 5nm, and the antenna is usually about 1000' down the landing rwy.

jerry

##### Share on other sites

Hi James,

your calculation is for percentage not degrees. With this calculation you have to use 5% slope : 5 ft in altitude for 100 ft in distance. That the same as 3 degres angle.

See here for so details.

Regards,

Thomas

## Create an account

Register a new account

• Tom Allensworth,
Founder of AVSIM Online

• ### Hot Spots

• Flight Simulation's Premier Resource!

AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!