Platform altitude is not a term used in US IFR, what is its precise meaning? In the US, cleared for the approach permits descent to the charted altitudes on the approach procedure. If there is a crossing restriction, the approach clearance comes in sequence afterwards in the clearance, So maintain 3000 until crossing ABCDE, cleared approach. When cleared on a STAR with a maintain an altitude instruction overrides any altitude restrictions on the STAR. The terminology used to follow the vertical crossing restrictions would be “Descend Via”. “Descend via except” can be used as well but it defeats the purpose of the STAR.
A typical ILS approach I fly has an IF about 5 NM from the FAF at 3000, and the IF segment has a minimum altitude after crossing the IF of 2500 that intercepts the GS. I normally intercept the GS from the 3000 ft altitude rather than doing a step descent to 2500 as it takes less work and will nominally intercept the GS from below about 1.6 NM earlier. Controllers are required to issue an altitude that is below the GS with the approach clearance. The standard service volumes for the localizer is 18 NM and 10 NM for the GS. At 10 NM from the threshold, an ILS GS is 3323 ft above the threshold, so even with a sea level runway, an aircraft maintaining 3000 MSL from 10 NM or further out would intercept the GS from below and at roughly 9 NM from the threshold.
Airborne_Again wrote:
You’re supposed to follow the published profile down to the cleared altitude.
That’s pretty obvious but there’s difference between published minimum altitude and cleared altitude. If STAR has e.g. 3 segments with minimum altitudes 6000, 5000, 3000 and you’re cleared to 4000, obviously you have to stay above 6000 in the 1st segment, above 5000 in the 2nd and above 4000 in 3rd before cleared for lower, regardless published 3000 for 3rd segment.
Emir wrote:
If STAR has e.g. 3 segments with minimum altitudes 6000, 5000, 3000 and you’re cleared to 4000, obviously you have to stay above 6000 in the 1st segment, above 5000 in the 2nd and above 4000 in 3rd before cleared for lower, regardless published 3000 for 3rd segment.
Indeed.
NCYankee wrote:
Platform altitude is not a term used in US IFR, what is its precise meaning?
It’s a colloquial term and thus it doesn’t have a precise definition. It usually means the altitude you fly before commencing final descent.
NCYankee wrote:
Platform altitude is not a term used in US IFR, what is its precise meaning?
The intermediate segment essentially.
