This one's for instrument students — and for private pilots who are starting to daydream about the rating.
Picture a Thursday evening in late October. You're filing IFR from your home field to an airport ninety minutes away for the proverbial $100 hamburger (more like $200 these days). The forecast is that classic fall mush: a layer that might be 1,800 overcast at your ETA, might be 3,000, depending on which model you believe. Your finger's hovering over the "alternate" box on the flight plan. Do you fill it in or not?
That decision comes down to one regulation, 14 CFR 91.169, and it's the source of more oral-exam stumbles than almost anything else in the instrument world. Let's clear it up.
The 1-2-3 rule, in plain English
Every instrument pilot learns 1-2-3 on day one, and most of us spend the next hundred hours saying it slightly wrong. Here's what it actually means.
You must file an alternate unless both of these are true:
- Your destination has a published instrument approach (a Part 97 procedure), and
- The forecast at your destination, from 1 hour before to 1 hour after your ETA, shows a ceiling of at least 2,000 feet above the airport and visibility of at least 3 statute miles.
That's the 1-2-3: 1 hour on either side of your arrival, 2,000-foot ceiling, 3 miles visibility. Miss any piece of it — no approach, a ceiling forecast at 1,900, viz at 2½ — and you're filing an alternate. And notice it's an and, not an or. Both the ceiling and the visibility have to clear the bar.
One thing that trips people up: this is about the forecast, not what you actually find when you get there. If the TAF says 2,500 and 5, you don't need an alternate, even if the weather turns out lousy in real life. The rule lives and dies on what the forecast said when you planned.
The part almost everyone gets backwards
Say the forecast is ugly and you do need an alternate. Now the question becomes: which airports are you even allowed to write down?
This is where 91.169(c) comes in, and it's the single most misunderstood piece of the whole topic. To list an airport as your alternate, the forecast for that airport at your ETA has to be at or above its alternate minimums:
- If the approach plate publishes alternate minimums for that airport, use those.
- If it doesn't, the standard numbers are 600-2 for a precision approach and 800-2 for a nonprecision approach — a 600- or 800-foot ceiling with 2 miles visibility.
Here's the trap. Those numbers, 600-2 and 800-2, are planning minimums. They're the weather the forecast has to show for you to legally file the airport as an alternate. They are not the minimums you fly the approach to once you get there.
Let that sink in, because it's a favorite examiner setup. Once you actually divert and you're shooting the approach at your alternate, you fly the published approach minimums on the plate, which are almost always lower than 600 or 800 feet. The higher alternate numbers exist to give you a forecast cushion back at planning time. They evaporate the moment you're inbound for real. Mixing up "minimums to list it" with "minimums to land there" is one of the most common conceptual errors instrument students carry into the checkride.
When your alternate doesn't have an approach
You can also plan an alternate that has no instrument approach at all. In that case, 91.169 says the forecast has to let you descend from the MEA, make your approach, and land under basic VFR. In plain terms: the weather has to be good enough that once you leave the airway, you can get down and in visually. It's a legitimate option. Just make sure the forecast genuinely supports a visual arrival before you count on it.
The triangle-A trap
Pull up almost any approach plate and look for a black triangle with a white A inside it. That symbol means the airport has non-standard alternate minimums — the plain 600-2 / 800-2 assumption doesn't apply, and you have to check the alternate minimums listing for that airport. And if you see "A NA," that approach cannot be used to qualify the airport as an alternate at all, often because a required piece of ground equipment isn't monitored or the approach isn't available around the clock.
So never assume 600-2 or 800-2. Check the plate.
Don't forget the gas
An alternate isn't just a line on the flight plan. It's fuel in the tanks. 14 CFR 91.167 ties right into this. For IFR, you need enough fuel to fly to your destination, then to your alternate (if one's required), and then for 45 minutes at normal cruise after that. Skip the alternate under the 1-2-3 rule and you still carry the 45. Need the alternate, and that leg gets added on top. Planning an alternate you can't actually reach with legal reserves isn't planning — it's wishful thinking.
Legal isn't always the same as smart
One last thing, instructor to student. The 1-2-3 rule is a legal minimum, not a recommendation. Plenty of days the forecast clears 2,000 and 3 by a hair and you're technically off the hook. File an alternate anyway. Fall and winter fronts move faster than the TAF admits. Ceilings drop. That "definitely 2,100 feet" forecast becomes 1,500 while you're still forty miles out. Having somewhere planned, briefed, and fueled-for turns a bad moment into a non-event.
File the alternate when the reg says you must. File it plenty of the times it says you don't have to, too.
The takeaway
91.169 answers one question: do I file an alternate? The 1-2-3 rule — published approach plus a 2,000-and-3 forecast — tells you when you're allowed to skip it. 91.169(c) tells you which airports qualify, using planning minimums, not approach minimums. And 91.167 makes sure you've got the fuel to back the whole plan up.
Get those three ideas straight and you've beaten the trap that catches most instrument applicants. This is exactly the kind of question that shows up on the instrument oral, so it's worth drilling until it's automatic. The ASA Instrument Oral Exam Guide walks through scenarios just like these, and your Pre-Tabbed Instrument FAR/AIM flagged at 91.167 and 91.169 means you can put a finger on the exact language in seconds instead of fumbling for it under pressure.
For more resources to support your instrument training, visit NorthstarVFR.com.