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Comment by jcutrell | Hacker News Reader

Things I use as a pilot of a single engine piston:

- Temp dew point spread - Barometric pressure - TAF / forecast - Wind shear - Freezing level - Cloud tops - Cloud level AGL - visibility measured in actual distance

All of these things can’t be measured easily by a human. Visibility is probably the easiest, but can you tell me what the dew point is easily?

If I use it, imagine the critical importance for airliners.

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Armchair sim-only pilot/aviation enthusiast, simply not having accurate wind speeds is enough to prevent a jet from landing because you don't have data to run performance calculations.

Can you land without running performance? Sure. But now a standard landing is an emergency since you can't verify you have enough runway to come to a complete stop. (See SWA 1248 for what happens when you don't run landing performance calculations correctly in iffy conditions).

This is, of course, ignoring all the other issues that simply not having accurate barometric pressure and cloud levels when flying IFR. If you're flying an approach that descends over mountains through a cloud layer and lack accurate weather data (specifically barometric pressure) then the chances of your EGPWS shouting "TERRAIN! TERRAIN! PULL UP!" because you're low due to an incorrect altimeter reading are non-zero. Do this in a mountain range and the EGPWS calls may end up being too little, too late even if you immediately perform terrain escape maneuvers given you still need accurate altimeter readings for EGPWS to function correctly.

Accurate METAR reports are safety critical for aviation. Full stop.

Jets plug these numbers into the flight computer to determine landing speed (Vref), needed runway length, and sometimes configuration (flaps etc.). So without weather I'd expect commercial flights to simply stay on the ground.

Isn't there a way to rough out the dew point from the cloud base altitude? I seem to remember that from ground school.

I remember being the other way around, you could estimate height of the cloud based on dew point and base altitude.

Yup! 400' AGL for every 1 deg C of temperature spread.

Yah! So you can find the dew point by flying up to just below the cloud and look at the altimeter and do some math ;)

Yes, but generally this would be used the opposite direction. If cloud bases are unavailable, or if weather is changing rapidly, you could use say barometric pressure + dew point to determine "is this about to go south towards low clouds and low vis?"

(Also, cloud bases are part of the METAR info in the first place, so it's a "neither" situation)

Wind shear is easy. feel the wind on your face, then jump as high as possible and feel the wind up there.

this is an advanced technique that they don't usually teach until the commercial certificate.

Oh, so that’s what turbulence is.

Wind shear is one of many various causes of turbulence in aviation. Certainly a common one, however.

I got curious and Google'd some of this:

> The dew point in relation to the temperature gives the pilots information about the humidity, and can affect visibility. If the dew point is close to the temperature, humidity is high, which can cause hazy conditions, or even fog.

It's also an easy way to get a rough estimate of the bottom of the clouds: 400' above the ground for every 1degC of difference between the temperature and the dew point.

This is useful for a couple of things, including whether or not you can legally take off or land when flying under visual flight rules.

I’m very much willing to believe you when you say you can do it practically, but there’s something logically or epistemically funny about measuring visibility using vision.

Haha - yes, this is really just a language problem.

Visibility for pilots is a precise measurement, not a classification. "Can I see" is a different question than "how many miles can I see", and even still "how many feet can I see down this runway."

These measurements dictate different flight rules, for example which approach I can take into a given airport.

Take this plate for example: https://flightaware.com/resources/airport/CHA/IAP/VOR+RWY+33... - There are two approaches you can take, categorized in the bottom, that use this plate. One is circling, the other is straight in. Your aircraft category (approach speed ranges) dictates the visibility requirements; in this case, a small Cessna needs a statute mile of visibility to execute either of these approaches.

However, a larger airplane - an airliner, for example, needs different visibility for the circling versus the straight in. How different? In this case, half a mile for category C (1 3/4 vs 2 1/4).

What if visibility is less than a mile?

Then you need to do something like this approach instead: https://flightaware.com/resources/airport/CHA/IAP/ILS+OR+LOC...

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