Originally Posted by
JammedStab
From the FAA letter....
"Discussion: What this means to pilots is that on some approaches, outside the Final Approach Segment, on a cool day, you might be able to follow the glide slope and all the published stepdown altitudes may pass below your aircraft. The next day, after a warm front passes, you could follow the same glide slope and (because the temperature is hotter this day) those same stepdown altitudes now protrude into the glide slope and require pilot action to ensure compliance with the published minimum altitudes (stepdown fixes). On both days your flight path on the glide slope was the same, but on the hotter day, the stepdown altitude, crept up into your glide path. High barometric pressure produces the same effect as high temperature.
Regardless of cause, pilots are cautioned to adhere to published step-down fixes located outside the Final Approach Segment on an ILS approach. If a pilot elects to follow the glide slope while outside the Final Approach Segment he should be fully aware that this technique needs to be closely monitored and, if necessary, action must be taken to meet all stepdown altitudes. Examples of airports where multiple altitude deviations have occurred include, but are not limited to; LAX, ORD, ATL, SLC."
While I have heard of this issue, this is the first time I have seen it worded this way. Bottom line is...on both the cold and warm days given in the underlined area above, you had the same terrain clearance as you were on the glideslope(which did not change its angle. But due to altimeter errors based on temperature, there was a technical deviation on the warm day. I understand the seriousness of nit descending below minimum altitudes but...If this temperature effect is the cause of all cases, it sounds more like a nitpicking exercise by the FAA than a safety issue.
This is an extraordinarily poorly written note by the FAA.
If the aircraft is on the glide slope (assuming that it is accurate at that range) then the aircraft will always have the same vertical distance from terrain as the glide slope is a 3 degree path from a fixed point on that terrain.
What they should be saying is that the glide slope obstacle clearance surface is only calculated to 20 miles outside that distance there is no guarantee that the glide slope does not penetrate terrain.. The minimum safe altitudes for the fixes on the STAR are calculated to ensure that an aircraft at that altitude will be clear of terrain regardless of the pressure setting or temperatures and therefore should be obeyed.