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Comment by tomohawk | Hacker News Reader

Before the days of GPS in the cockpit, a relative of mine was a 747 captain doing routes across the Pacific. On one flight, all of the gyros went offline.

He had been trained in the US Air Force in celestial navigation, so he was able to use that to get the aircraft within range of a beacon, and land safely.

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Boeing 707s were equipped with sextant periscopes. It's popular belief that 737s used to be as well, though I believe that was only the case for a handful of 737s purpose built for the USAF.

wouldn't a simple compass have sufficed if they weren't off route?

Inverting compass readings into position is highly problematic because of interference, and noise.

There'd probably be too much metal / electrical interference for a regular compass to work.

Planes don’t have built in compasses?

edit: yes they do https://en.wikipedia.org/wiki/Flight_instruments

Airplane compasses are indeed affected by the electrical system, but not so much as to make them unusable. A maintenance process known colloquially as "swinging the compass" involves turning on the electronics, positioning the aircraft over a compass rose painted on the ground, and noting the error at various bearings.

As for the metal, most metal airframes are aluminum.

A compass would probably be of marginal utility if you're flying a "great circle" route across an ocean. For instance, if you were to fly non-stop from Chicago to London, your initial route will have you heading north east, but at the end, you'll be heading south east.

That's why ships would follow a loxodrome instead of a great circle route. They'd still arrive in port, though.

While this is true, an aircraft has very critical fuel requirements - taking a longer and less efficient route combined with a faulty navigation system is a factor best avoided.

I was about to correct you sand say "no, they'd follow a rhumb line", but then I looked it up and realized they're two terms for the same thing. TIL!

All airplanes (or almost all? I'm sure someone will know of an exception) have compasses. But without data on wind direction and speed, a compass isn't much good over any appreciable distance. Once off the ground, an airplane is like a boat crossing a swift river. The boat won't land where it's pointed; it'll move with the current as it crosses and end up landing downstream of where it's pointed. So it is with airplanes. They don't fly where they're pointed; they move with the wind as they fly.

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