glideslope deviation

[cjam]

I have done a search but no joy.

How many feet low, is one dot low on he ILS when you are at 300ft, 200ft and 100ft?

I imagine different ILS's are different but for a standard 5% or 3 degree slope.

I ask because in visual conditions at my home port I like to take a visual aiming point in order to touch down about 1200ft into the runway (b 737) . The glideslope will always start to drift up showing me low on the ils. It doesn't bother anyone day VMC but I'm keen to know how low I am. The ILS is set up for a 78ft TCH and I want a 50ft TCH.

Thanks for your time, CJAM

[inbalance]

Talking about Heights above 3° GS Antenna:

At 300 ft you are, 300/TAN 3°= 5727 ft away from the antenna
At 200 ft you are, 200/TAN 3°= 3816 ft away from the antenna
At 100 ft you are, 100/TAN 3°= 1908 ft away from the antenna

1 Dot GS is 0,2° deviation.

For the 300 feet Point the deviation is 5724*TAN 0,2° = 19,9 Feet
For the 200 feet Point the deviation is 3816*TAN 0,2° = 13,3 Feet
For the 100 feet Point the deviation is 1908*TAN 0,2° = 6,6 Feet



TCH 78 ft is 78/TAN 3° = 1488 feet away from the antenna.

At TCH 50 ft the angle of the glidepath is 50/1488 = TAN 1,924°

So the difference between a 3° GS and the 1,9° GS for the assumed TCH of 50 feet is 3°-1,9°= 1,1 Degree.

A full deflection of the GS needle is 1 Degree. So crossing the threshold at 50 feet will give you more than a full deflection.

This stands for standard temperatures only.
Because with different temperatures the 50/78/100/200/300 feet points are moving.


Inbalance


edited for my bad english. I am ICAO level 4 only. :-(

[cjam]

Inbalance,
I could not have hoped for a better responce and answer to my question.
Thankyou very much. I really appreciate the time you took to answer.
Regards, CJAM

[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?

[ft]

Half sector (half deflection) is at 0.12 times the GP angle. For a three degree GP, this means 0.36 degrees. This gives 0.36/2.5 degrees/dot, or 0.144 degrees/dot.

One dot below gives a 2.88 degree glide path.

The difference x between on-GP height and one dot below height, h_1b, is thus, with d being the distance from the touch down point,

x = d*tan(3 deg) - h_1b (1)

The distance is given by h_1b as

d = h_1b/tan(2.88 deg) (2)

Insert (2) into (1) and you get

x = h_1b/tan(2.88 deg)*tan(3 deg) - h_1b/tan(2.88 deg)*tan(2.88 deg) =
= h_1b*(tan(3 deg)/tan(2.88 deg) - 1)

Or: The height below nominal GP when at one dot below is given by the actual height times 0.0418.

x = h_1b * 0.418.

100 feet -> 100*0.418 = 4.17 feet difference.
200 feet -> 200*0.418 = 8.34 feet difference.
300 feet -> 300*0.418 = 12.5 feet difference.

Using d*tan(0.12 degrees) instead of d*(tan(3 degrees) - tan(2.88 deg)) introduces an error which has an impact if you are going for three significant digits.

[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!

[ft]

cjam,
no, in actual practise it's "10 to 20 feet below when seeing 300", and that's good enough for real life. Three significant digits are not required. You can safely ignore the academics.

I didn't quite understand what you asked about in post #4 in the thread? You'll be 28' below GS at the threshold if you keep your GP antenna at 50' - is that the answer?

Regards,
/Fred

[inbalance]

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.

[cjam]

Got it!
Thankyou both for the time taken.

I will enjoy knowing approx how far below electronic glidepath I am after taking a visual aim point from 500ft AAL.