AIM Text
- 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.
- 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.
Source: FAA Aeronautical Information Manual · current edition · paragraph 5-1-10.
The short answer
This AIM section is FAA advisory guidance cautioning pilots flying IFR to airports in mountainous terrain to consider filing an alternate even when forecasts would technically relieve the requirement. It identifies three situations where failing to plan an alternate could create a critical fuel situation if weather is worse than forecast. It supports the alternate-airport rules in 14 CFR 91.167.
Research Notes
Common Questions
Why should I plan an alternate even when the forecast says I don't need one?
Per the AIM, at high-altitude mountainous airports a slight worsening from forecast can place the airport below published IFR landing minimums, and a pilot who elected not to carry alternate fuel based on forecasts could face a critical situation if weather is less than forecast.
Which airports have all instrument approach minimums above the standard alternate thresholds?
The AIM names three U.S. high-altitude airports where all MDAs are greater than 2,000 feet and/or landing visibility minimums exceed 3 miles: Bishop, California; South Lake Tahoe, California; and Aspen-Pitkin Co./Sardy Field, Colorado.
How does special required equipment affect alternate planning to these airports?
Per the AIM, an approach requiring special equipment such as DME or glide slope to reach the lowest minimums means other minimums may demand better weather than 14 CFR 91.167(b) specifies, and an inflight equipment malfunction could leave the airport below landing minimums for all remaining alternatives.