Your instructor is waiting when you arrive at the airport for an instrument proficiency check. You wave a friendly hello and offer a cheery comment on the perfectly imperfect weather for the flight—but there’s to be no chitchat today, as he opens with an unusual question.
"Quick, what’s 886 plus 54?"
Nine hundred forty.
He tosses an approach plate onto the table. "Tell me a communications frequency for Ontario. Suppose the minimum descent altitude for the LOC RWY 26R approach is 1,120 feet. What happens to Category D minimum visibility?"
You study the plate and wonder whether he has lost his mind, or is playing some cagey situational-awareness game. Doubtless the latter, although the former possibility has crossed your mind on occasion.
This time, in his eagerness to pounce, he may have overplayed his hand. The approach plate he threw down is an IAP for California’s Chino Airport, not for nearby Ontario International Airport. And the minimums he was grilling you on don’t match up with those published for the approach. In a droll voice, you volunteer to fetch the appropriate approach plate. In response, he requests that you refocus, please, and answer the question.
All right. On a second inspection, the numbers do make sense. The decision altitude for the straight-in ILS approach is 886 feet. That would increase 54 feet to 940 feet, according to a note, "when local altimeter setting not received." The note also requires using Ontario’s altimeter setting, and increasing the "S-ILS" visibility minimum in all categories and "S-LOC" visibility minimum in category D by a quarter-mile.
When might the local altimeter setting be unavailable? Looking up the airport, you learn that if you arrive between 21Z and 07Z, the tower is not operating. (When that’s the case, the airport also would be unavailable as an IFR alternate, says a note in the IFR alternate airport minimums in the IAP publication.)
Now it occurs to you that neither the government-issue approach plate for Chino nor the low-altitude en-route chart provides an ATIS or communications frequency for Ontario—not something to be looking up during a critical flight stage.
ATC could provide the altimeter setting—and it would be given in the airport’s most recent METAR—but making sure to obtain it "to comply with published procedures" is necessary, just as is applying appropriate ceiling and visibility minimums, to demonstrate your proficiency.