Why, and how, are barometric altimeters accurate enough for approach and landing?

Question

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.
• 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.

Answer 1

A difference of 1 hPa results in an error of 27ft in the altimeter. To have the altimeter showing 3000ft when the aircraft is actually at sea level, the altimeter must be off by 111hPa. Standard air-pressure at sea level is 1013 hPa (that's converted to 29.92 inHg for you Americans).

For comparison, the pressure in Hurricane Katrina reached as low as 902 hPa. So if you were flying into a hurricane but left the altimeter setting at 1013 (which is used as the standard setting at cruising altitude), you could theoretically pop out of clouds at what you thought was 3000ft but instead hit the ground.

Ignoring the fact that nobody tries to land in a hurricane, this mistake is prevented by the airfield pressure being reported by ATC, in the ATIS, and even an alert from the aircraft if it is advanced enough.

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.

Pockets of low pressure as you describe just don't exist to that degree. Hurricanes take hundreds of kilometres to reach their low point. And anyway, this is why airliners have a requirement for the approach to be stabilized at a specific point - typically 1000ft above the aerodrome level. Any significant deviation from the glideslope below this point mandates a go-around.

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.

Altimeters are calibrated with multiple static ports and designed so AoA changes do not noticeably affect the altimeter. For the rest of this example, I mean I suppose a pilot could just forget the basics of flying but it's unlikely. Pilots know how to select the correct pitch and power to guarantee maximum climbing performance.

You didn't mention that altimeters are also affected by temperature, and in cold air they will read higher than reality. There is no correction for this, and so some approaches will have limitations in very cold weather. EDIT: See John K's comment below.

Answer 2

Pressure rise at the static ports, caused by crosswinds or sideslip, is compensated by having a second static port on the opposite side of the aircraft.

The traditional static system simply connects both in a T junction, modern types use two separate static pressure sensors and combine the data in the ADC to avoid leaky plumbing.

This design also adds redundancy in the event of one side being plugged.

Answer 3

Baro altimeters are generally required to be accurate to +- 50ft when set to local altimeter setting, and the lowest you can descend on baro altitude (a Cat 1 ILS with a decision altitude) is 200 ft, so there is ample margin in the most critical case (to descend any lower on an approach, that is, Cat 2 or 3, which has a decision height, not altitude, you need a radar altimeter).

The actual altitude sensitivity of a typical baro altimeter is less than 10ft. That is, the needle will move to show that much change, especially when there is some vibration source present to overcome bearing "stiction" in the mechanism (On gliders without any vibration present, they are a bit less sensitive and tend to stick a bit until the pressure change is enough to overcome bearing friction, and they may not indicate a change until 20 or 30 feet unless you tap on the panel if the unit has aged a bit).