1700Zulu
Sign in

Exam map/IREX

IREX · Lesson 01 · 5 min · Free preview

Do You Need a Weather Alternate?

Objective
Decide from a TAF and destination alternate minima whether weather alone forces a destination alternate (or equivalent holding fuel).

Why it matters. This sits on CASA’s common-errors list. The wrong call is not a paperwork nit — it is illegal fuel planning, or a diversion you never resourced.

Video briefing later — Mux or Cloudflare Stream, not files on this app server.

The only question this lesson answers

You have a destination, a TAF, and a set of alternate minima. Weather is the only issue on the table — navaids, night and TAF3 wait their turn.

Either the forecast forces a destination alternate (or holding fuel that is legally equivalent), or it does not. Walk the three-step drill every time so the exam — and the flight — get the same answer.

Three-step drill

  1. 1. Alternate minima yardstick

    Write the number first. You cannot judge the TAF until you know the ceiling and visibility you are measuring it against.

    • Usable instrument approach: use the ALTERNATE minima published on the DAP / approach chart. That is Part 91 MOS 8.08; see the current AIP ENR for the same rule in AIP language.
    • Day, no usable IAP: LSALT for the final route segment + 500 ft, and 8 km visibility.

  2. 2. Weather trap at ETA

    Read the TAF at ETA, and the window up to 30 minutes before. If any of the traps below is on, weather alone requires a destination alternate — or the holding-fuel out in step 3.

    • Cloud greater than SCT below the alternate minima (BKN or OVC under the figure counts; SCT on the figure does not).
    • Visibility below the alternate minima.
    • ≥ 30% PROB of visibility below the minima.
    • Crosswind or tailwind, including gusts, above the aircraft or runway limit you are using.
    • Thunderstorms, or ≥ 30% PROB TS.
    • No usable aerodrome forecast at all → you need an alternate that has a forecast.
    • The named alternate must not itself need an alternate (Part 91 MOS 8.07).

  3. 3. Legal outs — hold instead of naming an alternate

    You may carry holding fuel instead of naming an alternate when the rules allow it — for example, hold until 30 minutes after the condition ends.

    INTER is typically treated as 30 minutes of hold; TEMPO as 60. Confirm the current MOS / AIP ENR wording before you hang a flight on it.

    TAF3 — the 3-hour TAF and how it changes this picture — is a later lesson. Do not invent a TAF3 shortcut here.

Cockpit habit

  1. 1. DAP ALTERNATE block — write the yardstick.
  2. 2. ETA ± 30 on the TAF — hunt the traps.
  3. 3. Alternate versus hold — pick one and fuel it.

Takeaways

  • Minima first: usable IAP → chart ALTERNATE (MOS 8.08; current AIP ENR). Day, no IAP → LSALT + 500 ft and 8 km.
  • At ETA (and 30 minutes prior): > SCT below; vis below; ≥ 30% PROB vis below; cross/tail including gusts; TS / ≥ 30% PROB TS.
  • No forecast → alternate that has a forecast. That alternate must not itself need an alternate (MOS 8.07).
  • Holding fuel can replace a named alternate when the time-out works (INTER ~30, TEMPO ~60). TAF3 later.

Marked quiz

Exam-style MCQ — score it, then read the why.

3 items. Pick A–D, submit once. Wrongs land on Missed & Review — sign in to keep them on this account.

  1. Q1. ILS available. Alternate minima 600 ft / 2.0 km. TAF at ETA: BKN008. Weather alternate required?

  2. Q2. Day IFR. No usable IAP at the destination. LSALT for the final segment is 4500 ft. What alternate minima do you use?

  3. Q3. There is no usable aerodrome forecast for the destination. What does weather-alternate planning require?

Answer every item to submit.

Next. Next: navaid and night IFR alternates — when weather is fine but the aid or the dark still forces a plan B.