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Instrument Rating — Airplane
Instrument Oral Exam Questions: Alternate Airport Requirements (1-2-3 Rule)
Alternate-airport planning is a guaranteed instrument oral topic because it stacks three layers that applicants routinely blur together: when you must file an alternate (the 1-2-3 rule), whether an airport qualifies as your alternate (forecast weather at ETA versus alternate minima), and what you actually fly if you divert there (the published approach minimums — not the alternate minima). Keep the layers separate and the whole topic gets easy.
References: 91.169, the A/FD-style notes and the 'triangle A' symbology on approach charts, and the AIM/AC guidance on RNAV (GPS) alternate planning. Expect the examiner to hand you a real TAF and make you decide.
Layer 1 and 2: filing and qualifying
Must you file an alternate? The 1-2-3 rule: if, from 1 hour before to 1 hour after your ETA at the destination, the forecast ceiling is at least 2,000 ft AND visibility at least 3 SM, no alternate is required. Anything worse — or no weather reporting/forecasting available — and you file one. Special case: if the destination has no instrument approach at all, an alternate is always required regardless of forecast.
Does your chosen airport qualify? At your ETA at the alternate, the forecast must meet the alternate minima: standard values are 600-ft ceiling and 2 SM visibility for a precision approach, 800 and 2 for a non-precision approach — unless the airport has non-standard alternate minima (the 'triangle A' symbol on the approach chart; look up the published values) or is marked 'NA' for alternate use (often due to lack of weather reporting or unmonitored facilities). If the alternate has no instrument approach, it can still qualify if the forecast allows descent from the MEA, approach, and landing under basic VFR.
Layer 3, fuel, and the GPS wrinkle
What do you fly if you actually divert? The published minimums for the approach you use at the alternate — the 600-2/800-2 figures are planning filters only and cease to matter once the alternate becomes your new destination. This single distinction is the most commonly fumbled line in the topic. Fuel follows the same logic: 91.167 requires fuel to fly to the destination, then to the alternate (when required), then 45 minutes at normal cruise — and a smart answer adds personal reserves on top.
The GPS/RNAV wrinkle: for aircraft with non-WAAS (TSO-C129/196) GPS, standard planning guidance prohibits relying on GPS-based approaches at both the destination and the alternate — one of them needs a usable ground-based approach (and you must check RAIM). WAAS (TSO-C145/146) equipped aircraft may plan RNAV (GPS) approaches at both, but flight planning is generally based on the LNAV (or circling) minima line unless the equipment and procedure authorize better — with the expectation that vertical guidance may be available on arrival. Quote it carefully and offer to verify in the AIM/AFM: examiners respect precision about a rule that has changed over the years.
Practice questions with answers & rationales
Q1. Destination TAF for your ETA window: BKN025, 5SM. Do you need an alternate?
Answer: No — from 1 hour before to 1 hour after ETA, the forecast shows at least a 2,000-ft ceiling (2,500 broken is a ceiling above 2,000) and 3 SM (5 SM forecast). The 1-2-3 test passes, so no alternate is required — though the judgment add-on is that filing one anyway costs little and buys options, especially with marginal trends or TEMPO groups near the limits.
Q2. Same destination, but the TAF shows TEMPO 1,500 BKN during your window. Now what?
Answer: An alternate is required. The TEMPO condition inside the ±1 hour window breaks the 2,000-ft ceiling requirement — conditional and intermittent forecast lines count against the 1-2-3 test for planning. This is a favorite examiner move: the main forecast line passes, the TEMPO line fails, and they watch whether you read the whole TAF.
Q3. Your proposed alternate has an ILS and a forecast of OVC008, 2SM at your alternate ETA. Does it qualify?
Answer: Yes, if standard alternate minima apply: precision approach available → 600-2 standard, and 800 ft/2 SM forecast meets it... careful — OVC008 is an 800-ft ceiling, which exceeds the 600-ft requirement, and 2 SM meets the visibility requirement, so it qualifies. But check the chart for the 'triangle A': if that airport publishes non-standard alternate minima (say 700-2 or higher for that runway), the published values govern. Always state the lookup step.
Q4. You divert to your filed alternate. What minimums do you use for the approach?
Answer: The published minimums on the approach chart for the procedure flown — e.g., a 200-ft DH and ½-mile (or RVR) for a standard ILS — not 600-2. Alternate minima exist only to filter which airports you may file; once diverting, the alternate is simply your new destination. Stating this unprompted usually ends the line of questioning.
Q5. Can you file an airport with no instrument approach as your alternate?
Answer: Yes — if the forecast at your alternate ETA allows descent from the MEA, approach, and landing under basic VFR (91.169(c)(2)). Practically that means good VMC forecast at the alternate. It's also the reminder that the alternate's job is certainty: you're proving to the system you have somewhere you can definitely get into.
Q6. You're flying a non-WAAS GPS aircraft. Destination has only RNAV (GPS) approaches. What does that mean for your alternate?
Answer: Plan an alternate where you can fly a ground-based (non-GPS) approach — with non-WAAS equipment you may not plan GPS approaches at both destination and alternate, and you must verify RAIM availability for the ETA. With WAAS, planning RNAV approaches at both is permitted under current guidance, generally planned at the LNAV minima line. Offering to confirm against the current AIM paragraph is the right instinct — this guidance has evolved.
Q7. How much fuel must you carry on this IFR flight with an alternate?
Answer: 91.167: enough to complete the flight to the destination airport, fly from there to the alternate, then fly 45 minutes after that at normal cruising speed (helicopter IFR uses 30 minutes). Then the airmanship layer: that's a legal floor, not a plan — add personal reserve for vectors, holding, weather deviations, and a missed approach, and state a 'minimum fuel'/'emergency fuel' communication plan.
Common mistakes to avoid
- Applying 600-2/800-2 as landing minimums at the alternate after diverting — the most classic error in the entire instrument oral.
- Reading only the main TAF line and missing TEMPO/PROB groups inside the ±1 hour window.
- Forgetting that a destination with no IAP always requires an alternate, even in severe-clear forecasts.
- Never checking the 'triangle A' non-standard alternate minima or the 'alternate NA' notes on the chart.
- Misquoting the GPS rule (or asserting the non-WAAS rule for a WAAS aircraft). Know which box is in your panel.
- Quoting 45 minutes of reserve fuel as the plan rather than the legal minimum.