The barometric altimeter is used as the primary altitude indicator during approach and landing right up until the aircraft crosses the threshold (after which the radar altimeter takes over and makes the “thirty...twenty...ten” calls), as it - unlike the radar altimeter - can’t be fooled by mountains or canyons lying across the descent path.
Yet barometric altimeters have problems of their own:
- They require an accurate knowledge of the local surface pressure at all times... and atmospheric pressure can change very rapidly, both from time to time and from place to place.
- They require that the aircraft be at a specific, constant angle of attack in order to give accurate readings; if the aircraft is flying at a higher angle than that for which the pitot-static system is calibrated, ram air will be forced into the static ports, resulting in a falsely low altimeter reading.1
- They rely on the aircraft’s static ports, which can be blocked by things such as rain, icing, or duct tape (fortunately, this is uncommon nowadays, as most static ports are heated, and most maintenance techs quadruple-check that they haven’t left anything covering the ports).
Given these vagaries of barometric altimeters, why don’t we see lots of crashes from situations like these:
- You break out of the clouds at what your altimeter says is 3000 feet; unfortunately for you, it turns out the pressure’s dropped since the airport’s altimeter setting was last broadcast, and you’re about to hit the ground (either this or the following could, in the wrong circumstances, be exacerbated by torrential rain, which commonly occurs in conjunction with low-pressure systems, and can potentially be ingested by insufficiently-heated static ports, resulting in an even-more-falsely-high altimeter reading).
- While on approach, you enter a localised low-pressure cell that the airport’s AWOS, being at the airport rather than back along the approach path, doesn’t know about. Seeing your altitude apparently ballooning up, you idle the throttles and push the nose over to correct the altitude excursion; unfortunately, since you were never actually too high in the first place, this action instead sends you below the glidepath, and straight into the ground.
- During a go-around, you set TOGA power on the throttles and pull back on the yoke; as the elevators are quicker to respond than the engines, the aircraft pitches up before it can accelerate much, resulting in the aircraft’s angle of attack temporarily increasing. Ram air enters the static ports, the altimeter says you’re lower than you actually are, and, in an attempt to avoid hitting the ground, you pull the yoke as far back as it’ll go, stalling the aircraft.