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Consider this ATC instruction from a ground controller:

N123, continue down Alpha, turn right at Kilo, hold short of RWY36.

N456, follow N123.

Does this instruction mean N456 can turn right at Kilo without further ATC instruction?

What about:

N123, continue down Alpha, turn right at Kilo, cross RWY36.

N456, follow N123.

Must N456 hold short of RWY36 and await clearance to cross the runway?

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The controller is required to give explicit clearance for crossing a runway. "Follow" is not clearance to cross a runway.

From the ATC phraseology:

7-3-2

d. When an aircraft/vehicle is instructed to “follow” traffic and requires a runway crossing, issue a runway crossing clearance in addition to the follow instructions and/or hold short instructions, as applicable.

EXAMPLE−

“Follow (traffic), cross Runway Two−Seven Right.”

or

“Follow (traffic), cross Runway Two Seven−Right, hold short Runway Two−Seven Left.”

The AIM doesn't spell this out as explicitly. AIM 4-3-18-5

  1. A clearance must be obtained prior to crossing any runway. ATC will issue an explicit clearance for all runway crossings.

The explicit requirement for runway crossings is somewhat new (2010 I think) and was the more interesting part. I neglected the question about crossing taxiways:

Does this [follow...] instruction mean N456 can turn right at Kilo without further ATC instruction?

Yes. Also from 7-3-2

a. Note 1. The absence of holding instructions authorizes an aircraft/vehicle to cross all taxiways that intersect the taxi route.

That said, I would not expect the only instruction from the controller to be "follow (traffic)". I would expect to be cleared to a specific destination as well.

"N456, taxi to 36. Follow N123"

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    $\begingroup$ In practice, the instruction is usually "N456, follow the Cessna ahead, cross rwy 36" or "N456, follow the Cessna ahead, hold short rwy 36 on A". The controllers tend to be very clear about "cross" & "hold short" instructions, not leaving you guessing. In rare cases when they don't specify, it's usually when the lead aircraft has been cleared all the way to where-ever, including clearance to cross the intervening runway, and that's what the controller intends for the trail aircraft to do to. N456 absolutely needs to confirm that, though. NEVER assume you're cleared across a runway! $\endgroup$
    – Ralph J ♦
    Commented Sep 7, 2018 at 22:51

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