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Reports are there were fog and rain at La Guardia at the time of the incident. They were on a short final, and it’s entirely possible they were not visible to the fire truck’s crew.
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I have over 1,000 hours as PIC in the NY area alone, in all seasons and all weather, and I was in the region last night, awake when the accident happened. LGA was reporting 4SM -RA BR FEW045 BKN090 OVC110 at the time of the accident. The weather wasn't anywhere close to what I'd call "bad" here. Sprinkles and some high clouds.

I can almost guarantee you the airplane was visible from taxiway D.

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You can also see from the videos that they could have easily seen the plane.
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At night with multiple runways it can be very hard to see a plane on final.

Still, I'm always hesitant to cross an active runway.

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Yes ARFF should still look before crossing, but the weather wasn’t great with limited visibility and thus even if they looked it’s possible they didn’t see anything.
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I mean, isn't it obvious that they didn't?
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It’s obvious that either they didn’t, or they did but they didn’t see the plane. We don’t know which.
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