Decision Altitude distance

5 Answers

1. 
   

   
   

   Chris on Mar 12, 2019

   It all depends on the glideslope angle. For a “standard” 3 degree glidepath, 1 nautical mile of ground distance will be ~318 feet of altitude. Also remember that slant range is going to be slightly more than ground distance.

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2. 
   

   
   

   Russ Roslewski on Mar 12, 2019

   The DA-to-threshold distance is purely a function of trigonometry.

   You’re basically creating a triangle. You need to know:
   The DA.
   The runway threshold elevation (NOT the Touchdown Zone Elevation)
   The design Threshold Crossing Height
   The glideslope angle

   So, let’s say (from a random, but real, example)
   DA = 1476 (200 HAT)
   Threshold elevation = 1263
   TCH = 49
   GS = 3.00

   1476-1263-49 = 164 ft
   164 / tan 3.00 = 3129 ft

   So, DA is 3129 ft from threshold.

   Note that in this example, the ILS DA is at 200 HAT, but since the TDZE is higher than the Threshold, the DA is actually 213 ft above the Threshold, which is used to calculate the DA distance.

   You can see that although many, many ILS procedures have a 200 ft HAT and a 3.00 deg glideslope, the exact distance to that DA point will vary depending on the threshold elevation and the threshold crossing height. But the variability is pretty small. 3.00 deg/200 HAT procedures will have a Threshold-to-DA distance of right around 0.5 nm (0.515 nm for this example).

   Note also that I used the readily-accessible values from the approach chart (and the airport diagram for the threshold elevation) and these are rounded to the nearest foot, so for the ultimate in accuracy, you would need the source documentation.

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3. 
   

   
   

   Chris on Mar 12, 2019

   A 200 DH on a 3.0 degree glide-slope, will put you .62 nautical miles or 3816 feet from touchdown, and the the touchdown is typically 1000 ft down the runway. So 2816 feet from the start of the landing surface ignoring any displaced threshold or underrun.

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4. Best Answer
   
   

   
   

   Russ Roslewski on Mar 12, 2019

   Chris’s comment (made while I was writing my novel) prompts me to add to my post:

   The distance I calculated above is the distance along the ground to the DA point. Slant range will be slightly longer, of course, but pretty minuscule at typical glideslope angles.

   I calculate for my example that the slant distance is 3133 ft vs 3129.

   I should also add that, the easy-estimate way is to take the DA HAT, minus the TCH from the chart and divide by 318 (or 300 if you want to do it in your head).

   200-49 = 151, divided by 318 or 300 is 0.47 or 0.50. Close enough for me!

   Also remember that visibility is in statute miles. 0.50 nm = 0.58 sm.

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5. 
   

   
   

   John D Collins on Mar 13, 2019

   To add to the above, with a DH of 200 feet and 0.5 SM visibility, when the weather is at minimums, you need a working approach light system to continue the approach as the distance to the threshold is greater than a half a statute mile, so at the DA it would not be possible to see the threshold, but you should see the approach lights all the way to a two or three lights past the 1000 foot marker. A half mile visibility is 5280/2 = 2640 feet. At a DA of 200 and a TCH of 50 feet, and the TDZE at the same elevation as the threshold, the threshold is (200-50)/tan(GS angle of 3 degrees) = 2862 feet. At smaller airports and runways, the TCH is usually lower than 50 feet, more typically 40 feet, and the resulting distance from the DA point is further away. At my airport, the minimums are 200-1/2 and the threshold is the same as the TDZE with a TCH of 41 feet, so the distance to the DA point is 3034 feet.

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