Alternates and fuel

The rule reads like an addition problem. Minimum legal fuel covers the flight to the first airport of intended landing, then, only if an alternate is required, the flight from that airport to the alternate, then 45 minutes of additional flight at normal cruising speed.

When no alternate is required, the middle term simply disappears: destination, then 45 minutes. The reserve itself never disappears, and it is calculated at normal cruising speed -- not the most fuel-efficient power setting the pilot could find in the manual.

91.169 is written as an exception, not a blanket requirement: an alternate is required unless the forecast at the destination is good enough to skip it. The rule's negative phrasing is exactly why pilots use the mnemonic "1, 2, 3" -- it describes the one case where an alternate is not required.

An alternate is not required if the destination has a published instrument approach procedure, and if the forecast for the period from one hour before to one hour after the estimated time of arrival calls for a ceiling of at least 2,000 feet above the airport elevation and visibility of at least 3 statute miles. If either element is missing, an alternate becomes mandatory.

Not every nearby airport qualifies. An airport cannot be used as an alternate if its NAVAID is unmonitored, or if it lacks weather reporting capability -- both plain disqualifiers regardless of how good the runway or the approach looks on paper. For an airport that does qualify, the forecast at the estimated time of arrival there must meet the standard alternate minimums, which are stricter than the minimums published for actually flying the approach, because they have to absorb the uncertainty of a forecast made hours in advance.

Standard alternate minimums are a 600-foot ceiling and 2 SM visibility where a precision approach is published, and an 800-foot ceiling and 2 SM visibility for a non-precision approach. These standard values apply unless the approach chart booklet lists something higher for that specific airport.

Standard alternate minimums (600-2 and 800-2) are never printed on the chart itself -- if a triangle-A appears, it is a signpost pointing to a separate page, and the airports listed on that page all have minimums higher than standard. Boston Logan (KBOS) is a good real example: it appears on this excerpt with two different approach lines, each carrying its own footnote.

GPS adds one more wrinkle. Because GPS is not authorized as a substitute means of navigation guidance for flying a conventional approach at an alternate, an alternate whose only approach requires DME or ADF still needs the airplane to carry that DME or ADF equipment -- GPS cannot stand in for it there. For select RNAV (GPS) and GPS approach procedures, an alternate whose only instrument approach is GPS-based can be used only by aircraft with an approach-approved WAAS receiver meeting the applicable technical standard, or by TSO-C129/C196 receivers meeting the conditions published for that equipment.

The regulatory minimum assumes an optimistic flight: no holding, one approach, a direct track. Real flights add a hold on arrival, a vector that adds distance, sometimes a full missed approach before the actual diversion even begins. Building in an hour of reserve beyond the destination, or setting a personal fuel floor that triggers a diversion decided on the ground rather than improvised in the air, turns a regulatory minimum into an actual safety margin.