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For the purposes of airspeed restrictions within airspace, the FAA regulations list the following area description (emphasis mine):

14 CFR 91.117 (C)

No person may operate an aircraft in the airspace underlying a Class B airspace area designated for an airport [...] at an indicated airspeed of more than 200 knots (230 mph).

Should we understand the described airspace to be any and all airspace underlying a Class B where that specific Class B is designated for an airport? Or, should we understand the described airspace to be any and all airspace designated for an airport (Class C, D, or E) which is underlying a Class B?

Are there letters of interpretation or other regulations which clarify the grammatical ambiguity of this regulation?

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Yes, there is a legal interpretation on this:

[...] under the express language of the regulation, aircraft operating in the airspace underlying Class B airspace - irrespective of whether the underlying airspace is controlled or uncontrolled - may not exceed the 200-knot speed limitation

So it is indeed any and all airspace underlying class B.

And note the exception to the rule:

We note § 91.117 (d) does provide that, if the minimum safe airspeed for any particular operation is greater than the maximum speed prescribed in § 91.117, then an aircraft may be operated at that minimum speed.

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    $\begingroup$ Therefore, the retention of the phrase, "designated for an airport" is superfluous and is to blame for the ambiguity of the regulation. $\endgroup$ – J Walters Jan 10 '16 at 18:11
  • $\begingroup$ Indeed. "Designated for an airport" does appear in some of the descriptive language in the paragraph introducing class B airspace in FAA Order 7400.11C, but there was no reason to put it into that FAR. Likewise with some other regs-- see for example aviation.stackexchange.com/questions/65697/… . However there are some other regs where the inclusion of the phrase "designated for an airport" arguably functions specifically to exclude E4 (Class-E-to-surface "extension") airspace and thus is not superfluous. $\endgroup$ – quiet flyer Jun 20 '19 at 12:22
  • $\begingroup$ If we were including parenthesis is in algebra equations, the phrase would like like this: (the airspace underlying (a Class B airspace area designated for an airport)) . "Designated for an airport" modifies "a Class B airspace area" rather than modifying "the airspace underlying". And yes it appears to be completely superfluous here. $\endgroup$ – quiet flyer Jun 25 '19 at 15:32
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To add to Pondlife's answer, the reason for this rule is that many VFR aircraft will try to fly under Class B airspace to avoid dealing with ATC so the airspace underneath the shelves of class B can be congested. Therefore the FAA put this lower speed limit in place to allow for better VFR separation of aircraft under Class B airspace.

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