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FAA Releases Updated Guidance on Instrument Landing System Intercepts

April 8, 2011

The FAA has released InFO 11009, "Failure to Comply with Minimum Crossing Altitudes at Stepdown Fixes Located on Instrument Landing System (ILS) Inbound Courses," and updated the guidance on this topic in the Aeronautical Information Manual (AIM). These changes are as a result of the NBAA Access Committee’s involvement in the Aeronautical Charting Forum.

On ILS approaches, step-down fixes are established for obstacle avoidance or for the purpose of traffic separation between aircraft underflying or overflying the final approach course or being vectored on the final approach course of the adjacent runway. For all practical purposes, the ILS glide slope remains stationary regardless of atmospheric temperature. Conversely, step-down fixes are published for a pilot to fly using indicated altitude, which varies with temperature changes. Therefore, the indicated altitude at each step-down fix in reference to the glide slope, changes with the temperature.

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 remain above all the published minimum step-down fix altitudes. However, on hotter than ISA standard days, an aircraft tracking a glide slope will fly below the minimum altitude for the published step-down fixes. This could result in loss of separation between parallel or crossing traffic maintaining assigned altitude by reference to an altimeter. To avoid a loss of separation, and a possible pilot deviation filing by ATC, pilots flying an ILS with step-down fixes prior to the final approach fix must comply with the minimum step-down altitude, even if it means remaining above the ILS glide slope until reaching the final approach fix.

On some approaches, a maximum or mandatory altitude may be published at a step-down fix for traffic separation purposes, e.g., the mandatory 1,500 ft. MSL altitude at DANDY on the TEB ILS Runway 06. The position of the glide slope at these fixes may be well above the published maximum or mandatory altitude at the fix. To avoid a loss of separation, pilots must comply with the published maximum or mandatory altitude at each step-down fix.

The ILS glide slope is intended to be intercepted at the published glide slope intercept altitude. This point marks the precision approach final approach fix (PFAF) and is depicted by the “lightning bolt” symbol on U.S. government charts or the beginning of the feather in the profile view on Jeppesen charts. Intercepting the glide slope at this altitude marks the beginning of the final approach segment and ensures required obstacle clearance during descent from the glide slope intercept altitude to the lowest published decision altitude for the approach. Interception and tracking of the glide slope prior to the published glide slope interception altitude does not necessarily ensure that minimum, maximum, and/or mandatory altitudes published for any preceding fixes will be complied with during the descent. If the pilot chooses to track the glide slope prior to the glide slope interception altitude, they remain responsible for complying with published altitudes for any preceding step-down fixes encountered during the subsequent descent.

There have been numerous pilot deviations filed at various airports around the country, including Los Angeles International Airport (LAX), Hartsfield - Jackson Atlanta International Airport (ATL), Salt Lake City International Airport (SLC) and Teterboro Airport (TEB).

The following additional advisory language will be added to the AIM 5-4-5 b:

2. The ILS glide slope is intended to be intercepted at the published glide slope intercept altitude. This point marks the PFAF and is depicted by the “lightning bolt” symbol on U.S. Government charts. Intercepting the glide slope at this altitude marks the beginning of the final approach segment and ensures required obstacle clearance during descent from the glide slope intercept altitude to the lowest published decision altitude for the approach. Interception and tracking of the glide slope prior to the published glide slope interception altitude does not necessarily ensure that minimum, maximum, and/or mandatory altitudes published for any preceding fixes will be complied with during the descent. If the pilot chooses to track the glide slope prior to the glide slope interception altitude, they remain responsible for complying with published altitudes for any preceding step-down fixes encountered during the subsequent descent.

View FAA InFO 11009, "Failure to Comply with Minimum Crossing Altitudes at Stepdown Fixes Located on Instrument Landing System (ILS) Inbound Courses"