https://youtu.be/rr-UpZI2cbE

At around2:50 “somebody on foot...”, is it an idiom? Does that mean somebody was walking on the runway? The Japanese subtitle says somebody at the foot of their airplane. Which is correct?

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"They spotted somebody on foot on the runway out there."

"On foot" means that someone was walking (or running) on their own feet, (which is a weird phrase that means they are walking instead of driving or biking or riding, etc). It was said this way, because normally no people would be out on the runway, except when they are inside of a plane. So people are often riding on a plane while on the runway, but almost no one walks around out there. But this time, someone was walking on foot, out on the runway, which is strange and dangerous. So the plane was being told to "hold position" or to stay in place and wait for a few minutes, until they could make sure it was safe (to make sure no person was standing in the way, so that no accidents can happen).