Preflight Preparation: Weather, Airworthiness, Cross-Country Planning
Weather
You must read METAR/TAF, prog charts, winds and temperatures aloft, freezing level, AIRMETs (Tango for turbulence, Zulu for icing, Sierra for IFR/mountain obscuration), SIGMETs, and PIREPs, and translate them into go/no-go decisions. Know the alternate rule: under 91.169 an alternate is required unless, for the period from one hour before to one hour after the ETA, the destination forecast allows at least a 2,000-foot ceiling and 3 statute miles visibility - the practical '1-2-3' rule. If an alternate is required, its weather minimums come from the alternate minimums section of the approach chart, not from standard minimums.
Airworthiness
Know 91.205 (instrument equipment list), 91.213 (inoperative equipment, MEL, placarding), 91.411 (altimeter and static system, 24 calendar months), 91.413 (transponder, 24 calendar months), and 91.171 (VOR check within 30 days, logged). Approach databases must be current for the procedure being flown.
Planning and performance
Account for fuel to destination plus alternate plus reserve (91.167), cold-temperature altimeter error, and terrain. 'From high to low, look out below' - extreme cold makes true altitude lower than indicated and inflates obstacle clearance requirements.
Worked example
Destination TAF at your ETA window reads BKN025, 5SM. Ceiling 2,500 ≥ 2,000 and visibility 5 ≥ 3, so no alternate is required. Change it to OVC015 and an alternate is mandatory.
Trap: treating the 1-2-3 rule as a regulation rather than the practical expression of 91.169(b), and using your own personal minimums in place of the charted alternate minimums.