We’ve extensively used this rule throughout the article. It’s not written in any official publications, so it’s just a rule-of-thumb. It’s a fairly easy way to estimate if the aircraft will be at an appropriate altitude for any instrument approach, without turning it into a trigonometry equation. The 300’ per mile rule is conservative, however. Most ILS approaches have a glideslope angle of 3°. The 300’ per mile rule works out to an angle of a little over 2.8°. So for the standard 3° ILS approach, 300’ per mile works just about perfect because it will result in the aircraft being slightly below the glideslope. In ZLA, there are no ILS approaches with a glideslope angle less than 3°; but there are approaches with much steeper glideslopes; the POC ILS 26L has a glideslope angle of 3.76° (about 400’ per mile), and the VNY ILS 16R has a glideslope angle of 3.5°. Some non-precision approaches have angles provided for pilot information between the FAF and the runway. If you look at the L35 RNAV 26 approach chart earlier in the lesson, you’ll see an angle of 3.69°, which is about 390’ per mile.
The point of this section is to reinforce that the 300’ per mile rule is not a strict rule that is never to be broken. It is just a guideline that will work for any instrument approach.