AIM ¶ 5-1-10 — IFR High Altitude Destinations
AIM 5-1-10 explains why IFR pilots flying to mountainous, high-altitude airports should plan an alternate even when weather technically exempts them.
In Plain English
AIM 5-1-10 cautions pilots flying IFR into mountainous, high-altitude airports to plan for an alternate airport even when forecast weather would otherwise exempt them under 14 CFR 91.167. The FAA highlights three risk scenarios:
- High MDAs or visibility minimums. Some airports (e.g., Bishop, CA; South Lake Tahoe, CA; Aspen-Pitkin Co./Sardy Field, CO) have MDAs above 2,000 feet AGL or landing visibility above 3 miles. A pilot relying on the standard 1-2-3 rule could legally skip an alternate even when forecast weather is below the actual approach minimums.
- Marginal MDAs. Other mountain airports have MDAs only 100-300 feet below 2,000 AGL, so a slight worsening of weather can push conditions below landing minimums.
- Special equipment required. Approaches needing DME, glide slope, or similar equipment to reach the lowest minimums leave little margin if equipment fails inflight, potentially eliminating all usable approach options.
This is recommended FAA guidance, not a regulation — but treating it as best practice protects you from arriving with insufficient fuel to divert.
AIM Source Text
FAA AIM ¶ 5-1-105-1-10. 5-1-10. IFR Operations to High Altitude Destinations
Pilots planning IFR flights to airports located in mountainous terrain are cautioned to consider the necessity for an alternate airport even when the forecast weather conditions would technically relieve them from the requirement to file one. REFERENCE- 14 CFR section 91.167. AIM, Para 4-1-19 , Tower En Route Control (TEC). The FAA has identified three possible situations where the failure to plan for an alternate airport when flying IFR to such a destination airport could result in a critical situation if the weather is less than forecast and sufficient fuel is not available to proceed to a suitable airport. An IFR flight to an airport where the Minimum Descent Altitudes (MDAs) or landing visibility minimums for all instrument approaches are higher than the forecast weather minimums specified in 14 CFR section 91.167(b). For example, there are 3 high altitude airports in the U.S. with approved instrument approach procedures where all of the MDAs are greater than 2,000 feet and/or the landing visibility minimums are greater than 3 miles (Bishop, California; South Lake Tahoe, California; and Aspen-Pitkin Co./Sardy Field, Colorado). In the case of these airports, it is possible for a pilot to elect, on the basis of forecasts, not to carry sufficient fuel to get to an alternate when the ceiling and/or visibility is actually lower than that necessary to complete the approach. A small number of other airports in mountainous terrain have MDAs which are slightly (100 to 300 feet) below 2,000 feet AGL. In situations where there is an option as to whether to plan for an alternate, pilots should bear in mind that just a slight worsening of the weather conditions from those forecast could place the airport below the published IFR landing minimums. An IFR flight to an airport which requires special equipment; i.e., DME, glide slope, etc., in order to make the available approaches to the lowest minimums. Pilots should be aware that all other minimums on the approach charts may require weather conditions better than those specified in 14 CFR section 91.167(b). An inflight equipment malfunction could result in the inability to comply with the published approach procedures or, again, in the position of having the airport below the published IFR landing minimums for all remaining instrument approach alternatives.
Oral Exam Questions a DPE Might Ask
Q1Why does the FAA recommend planning an alternate for IFR flights to high-altitude mountain airports even when not legally required?
Per AIM 5-1-10, MDAs and visibility minimums at these airports are often higher than the forecast weather minimums in 14 CFR 91.167(b), so a slight worsening of weather could place the airport below landing minimums with insufficient fuel to divert.
Q2Name the three U.S. high-altitude airports cited in AIM 5-1-10 where all MDAs exceed 2,000 feet and/or landing visibility minimums exceed 3 miles.
Per AIM 5-1-10, these are Bishop, California; South Lake Tahoe, California; and Aspen-Pitkin Co./Sardy Field, Colorado.
Q3How can an inflight equipment malfunction create a critical situation at a mountainous IFR destination?
Per AIM 5-1-10, if special equipment such as DME or glide slope is required to achieve the lowest approach minimums, a malfunction may force the pilot to use higher minimums on remaining approaches, potentially leaving the airport below the published IFR landing minimums.
Related Paragraphs in AIM Chapter 5