IM SAFE (sometimes written without the space, IMSAFE) is an acronym for a checklist used to determine if the pilot of an aircraft is in condition to fly. It's a list of things that you should check off to make sure you're OK to get in the cockpit. Note that 'ability to fly the airplane' includes your judgement. This is used in what the FAA calls 'proper' ADM, or Aeronautical Decision Making.
- I - Illness. Are you suffering from any sickness or symptom or injury that might affect your ability to fly the airplane? Use your judgement, obviously - a case of athlete's foot is probably okay unless it distracts you like crazy, but a sinus condition that might be affected by altitude (pressure) change might mean you should cancel your flight. Be smart and conservative.
- M - Medication. Are you taking any meds, either prescribed or over the counter, which might have an affect on your ability to fly the airplane? Anything that has warnings about operating machinery, or warnings about drowsiness, or maybe even warnings against high altitude?
- S - Stress. Are you under any emotional or psychological stress which might affect your ability to fly the airplane? This can be either personal or even related to the flight. 'Get home syndrome' is cited in a large number of general aviation accidents - the first factor of many that result in injury or death. Typically, pilots know that there are situations or conditions that make flying less than ideal - but they feel they need to get home. Maybe they're hundreds of miles from home and didn't expect to stay the night but the weather is coming in; perhaps they're desperate to make it back in time for an event, whatever. This is stress, because the pilot is feeling pressure to take an action that may not be safe. Or, of course, if the pilot is suffering depression, or anxiety, or any other form of stress that is affecting their thinking.
- A - Alcohol. "Eight hours, bottle to throttle." If the pilot has consumed any alcohol within the previous eight hours or their blood alcohol content may be higher than absolute minimums (as determined both by conservative judgement and the local legal minimums, which in the US are 8 hrs and .04 blood alcohol content for a private pilot) then they should not fly. This, obviously, is a good place to consider...ahem...less-than-legal intoxicants as well.
- F - Fatigue. Did you get enough sleep? Will you be able to make the entirety of your planned flight or leg without becoming tired? If not, then delaying long enough to take a nap is probably a really, really good idea.
- E - Eating. Has the pilot had enough to eat recently? Having your blood sugar plummet three hours into a four hour flight is definitely not a good thing. Make sure you're not hungry, and if the flight is long enough, bring a snack!