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Comment by scott00 | Hacker News Reader

Would using the ILS without the local altimeter actually be a safe way to land the plane? (Ignoring whether or not ATC is allowed to give you clearance for that.) Is the requirement of a local altimeter just overly cautious?

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It would not be safe at this airport, and the ILS procedure there explicitly forbids it. In the remarks at the top of the approach plate you'll see "When local altimeter setting not received, procedure NA." (NA = Not Authorized.) Here's the plate: https://aeronav.faa.gov/d-tpp/2607/06922IL24.PDF

The reason you need an accurate altimeter for this ILS is that the obstacle clearance surface for the intermediate segment - the part where you're flying straight and level without a glideslope - lies atop the very mountains these pilots crashed into. Vertical guidance on the missed approach is also barometric, and there are mountains in that direction too.

It would be safe. The ILS projects a localizer signal (left/right deviation compared to runway alignment) and glideslope signal (up/down deviation versus the intended flight path, usually about 3°).

The only thing you need a precision altimeter setting for is to determine the safe decision altitude (point at which you need to see the runway environment to continue, else initiate a missed approach) and a fairly theoretical chance to intercept a false glideslope lobe.

With the clear weather, knowing the decision altitude with precision was not important and the false glideslope is easy to identify by comparing outside view and descent rate needed to stay on the glideslope.

Not true. You also need an accurate altimeter to fly the intermediate segment, as well as the missed approach segment. There are mountains under the obstacle clearance surfaces on both segments. That is why this ILS procedure is explicitly not authorized when the local altimeter setting isn't available.

Any altimeter setting in the vicinity is going to be close enough in this scenario. It's still a visual approach, you can remain well above terrain until you're inside the GS service volume. I think it's an OK way to get yourself out of a crappy situation.

ILS, yes (the G/S vector and angle is calibrated to account for terrain), LOC on the other hand (i.e. non-precision with no G/S information) would have probably ended up in the same situation as it requires pilots to manually manage rate of descent. Not all runways have ILS equipment on both sides of the runway, let alone on all runways available or one that is active. At some airports it's possible you only run a 30% chance or lower that you can actually use the ILS to land, because the one with the equipment may not be the active one (and which runway is active changes depending upon wind and weather)

Yes, safe, and also way safer than doing a visual approach without any backup at night. Like the other commenter said, the local altimeter setting is only required for knowing when you arrive at Decision Altitude, which is not applicable for this visual approach.

I'm not sure of ATC's rules, but my assumption is that they would NOT clear me for an ILS approach in this circumstance. But that's moot, I would not ask for it because it's not authorized anyway. It would be a visual approach, we'd just happen to fly the ILS course and use the GS for vertical guidance.

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