Are the VFR weather minimums in 91.155(d) redundant because of 91.155(a)?

Question

14 CFR 91.155(d) specifies that:

... no person may take off or land an aircraft, or enter the traffic pattern of an airport, under VFR, within the lateral boundaries of the surface areas of Class B, Class C, Class D, or Class E airspace designated for an airport ... unless ground visibility at that airport is at least 3 statute miles ...

But we already know from the table in 91.155 (a) that for Class B, C, D, and E, the minimum flight visibility is 3 sm. So it's not possible for this rule (d) to apply without already triggering the rule (a).

My question is then what's the point of this additional rule? Am I misinterpreting the text?

Answer 1

Short answer: pilots need flight visibility, ATC needs ground visibility.

---

There are two different definitions of visibility in the regulations you mentioned: flight visibility and ground visibility. They're defined in 14 CFR 1.1:

Flight visibility means the average forward horizontal distance, from the cockpit of an aircraft in flight, at which prominent unlighted objects may be seen and identified by day and prominent lighted objects may be seen and identified by night.

Ground visibility means prevailing horizontal visibility near the earth's surface as reported by the United States National Weather Service or an accredited observer.

The practical difference is that only pilots can determine flight visibility; ATC and/or ground stations (AWOS/ASOS) determine ground visibility. The regulations aren't redundant because they're talking about different things and also about different needs:

• 91.155(a) applies to all airspace. At 5000' (or whatever), only flight visibility makes sense to determine if conditions are VFR.
• 91.155(d) applies to terminal (airport) airspace. That means ATC is also very interested in whether conditions are VFR or not, and they can only work with ground visibility.

Note that the full text of 91.155(d) also covers the case of uncontrolled airports with no weather reporting (emphasis mine):

(d) Except as provided in §91.157 of this part, no person may take off or land an aircraft, or enter the traffic pattern of an airport, under VFR, within the lateral boundaries of the surface areas of Class B, Class C, Class D, or Class E airspace designated for an airport—

(1) Unless ground visibility at that airport is at least 3 statute miles; or

(2) If ground visibility is not reported at that airport, unless flight visibility during landing or takeoff, or while operating in the traffic pattern is at least 3 statute miles.

Answer 2

A more complete quoting of 14 CFR 91.155 as it pertains to this question would be:

(d) Except as provided in § 91.157 of this part, no person may take off or land an aircraft, or enter the traffic pattern of an airport, under VFR, within the lateral boundaries of the surface areas of Class B, Class C, Class D, or Class E airspace designated for an airport -

(1) Unless ground visibility at that airport is at least 3 statute miles; or

(2) If ground visibility is not reported at that airport, unless flight visibility during landing or takeoff, or while operating in the traffic pattern is at least 3 statute miles.

There are not two rules in effect, there is only one rule concerning the visibility requirements in class B, C, D & E airspace - Three statute miles. This rule is mentioned in the text, and shown in the table. It can be either reported ground visibility, or flight visibility.

This rule's mention in one place does not cancel out or "trigger" another rule, you simply use the information available to comply. There is no conflict if you read the entire section in context.