PPRuNe Forums - View Single Post - glideslope deviation
View Single Post
Old 15th Feb 2009, 12:19
  #8 (permalink)  
inbalance
 
Join Date: Sep 2002
Location: germany
Age: 58
Posts: 210
Received 4 Likes on 2 Posts
CJAM Using your equation I think that

5724 x Tan 0.3 = 29

Keeping in mind that I am not very good at maths, would that mean that I was 29feet below glideslope and should indicate 1.5 dots low on the indicator?
Hello CJAM, thats correct.

CJAM hmmmm, thankyou.
I have to admit I am a bit confused now.
I guess there's not much difference in the two opinions, eg at 300ft I'll be about 12 to 19 ft low if the indicator is showing one dot low......I think!
also correct. Thats for 300ft.

The GS Pointer shows a deviation in angle, not in ft.
1 Dot below GS shows a deviation of 0,2 degrees below the 3 ° glidepath.
Like I wrote in my first post, the deviation of 1 dot below the GS at 300ft is 19,9 feet, but at 100 feet it is 6,6 feet only.
That is because the Glidepath beam gets narrower the closer you get to the antenna.

The threshold in your given example is 1488 feet away from the antenna.
Overhead the threshold 1 dot below GS is: TAN0,2 degrees* 1488 ft= 5,19 feet.

The 3° GS crosses the threshold at 78 feet.


You want to fly at 50 feet, that’s 28 feet lower than the GS.

78 – 50 = 28 feet.

1 DOT below GS over the threshold is 5,19 feet.

28feet/5,19feet per dot = 5,39 DOTs

So you have to fly 5,39 dot below the Glidepath of 3°

A full deflection of the GS is 5 DOT. Flying at 5,39 DOTs below the GS is more than a full deflection,
or like I also posted before:

The 50 feet point overhead the threshold is on an GS angle of 1,924 Degrees.
The treshold is 1488 feet from the GS antenna.

1488*TAN 1,924 Degrees = 49,986 feet, so nearly 50 feet

Flying at a full deflection = 5* 0,2 degrees will bring you on a 2 degree Glidepath over the threshold. 1488*TAN 2 = 51,9 ft

Flying at 4 DOTs below will fly you on a 2,2 Degree GS.
That is 1488 ft*TAN2,2 = 57,16 feet overhead the threshold.

Inbalance

gone flying, I´ll be back in 12 Hours.

Last edited by inbalance; 15th Feb 2009 at 12:34.
inbalance is offline