ILS and PAR Rates of Descent

I mentioned, briefly, in one of my posts breaking down instrument approaches to the water that there is a rule of thumb that rate of descent to maintain a three-degree glide slope equates roughly to groundspeed times 5, which has been useful for me when flying precise ILS and PAR descents. Let’s look at the rule compared to the actual rates of descent for a three-degree glideslope at 60, 90, and 120 kt groundspeeds: 

60 kts GS: actual = 320 fpm, rule of thumb = 300 fpm 

90 kts GS: actual = 480 fpm, rule of thumb = 450 fpm  

120 kts GS: actual = 630 fpm, rule of thumb = 600 fpm  

When pilots are flying at PAR at 90 kts groundspeed and hear “on glide path,” nail rate of descent at 450 fpm and they will likely hear, “on glide path” throughout the approach. 

When pilots are flying an ILS at 80 kts groundspeed and are on glide path, pin the VSI at 400 fpm and they will only have to make minor corrections to keep the approach on rails.