Why it matters
An IFR flight plan rejected by FSS or the ATC system burns 30 minutes of pre-flight time per occurrence. The two most common rejections are alternate-weather mismatch (§91.169) and ICAO equipment-code suffix errors. Marathon Rank 10 checks both at filing time, so the two mistakes that cause them are caught before the plan leaves your screen.
Under the hood
- §91.169(a) destination weather check
If forecast at destination ETA is below 2,000ft / 3sm, alternate is required.
- §91.169(c) alternate forecast minima check
Precision approach 600ft/2sm; non-precision 800ft/2sm; no approach 1,000ft VFR.
- ICAO equipment-code suffix builder
Field 10a (equipment) + 10b (surveillance) generated from aircraft profile.
- PBN code derivation
A1, B2, C2, D2, O2 codes inferred from GPS + RNP capabilities.
Regulatory and safety context
14 CFR §91.169 is the single most-cited reg for IFR pre-flight planning rejections. The rule is mechanical but the destination-vs-alternate weather comparison gets misread under time pressure. ICAO equipment codes (the "/G" suffix is dead — ICAO codes have replaced it) trip up pilots who learned the FAA-domestic format. Marathon Rank 10 checks both against the rule text before you file.