FAR Ninety-One Point One Seventeen, the Three Speed Limits Students Learn as One Number, and the Class Bravo Underside Where the Ceiling Is Lower Than the Airspace Above It

FAR 91.117 contains three distinct speed restrictions, and most pilots know only one of them. The regulation is titled “Speed Limits,” shows up on every written test and oral exam, and gets reduced by most students to a single number: 250 knots below 10,000 feet. That answer is incomplete - and the gaps are exactly where checkride examiners dig in.

What Does FAR 91.117 Actually Say?

The regulation has three sections. Section (b) is the one most pilots can recite. Sections (a) and (c) each set a limit of 200 knots, and both apply in situations that catch pilots who assumed they had the whole rule figured out.

Understanding all three requires knowing not just the numbers, but the specific geometry of where each section applies.

What Is the 250-Knot Rule (Section b)?

Section (b) is the baseline: no person may operate an aircraft at or below 10,000 feet MSL at an indicated airspeed of more than 250 knots - unless otherwise authorized or required by ATC.

That ATC override clause matters. If approach control needs you to maintain a higher speed for sequencing and issues that instruction, you can comply. The instruction is your authorization. But absent an ATC directive, 250 knots is the ceiling below 10,000 feet in any airspace section (a) or (c) doesn’t specifically reach.

Section (b) does not exempt pilots inside Class Bravo. If you are cleared into the Bravo and operating below 10,000 feet, section (b) applies unless ATC has authorized otherwise. The 250-knot default is the same whether you are in the Bravo or in uncontrolled airspace over a cornfield.

How Does the 200-Knot Rule Near Class C and D Airports Work (Section a)?

Section (a) sets a 200-knot limit whenever all of the following are true simultaneously:

1. You are at or below 2,500 feet above the surface (not MSL - AGL at that location)
2. You are within 4 nautical miles of the primary airport of a Class Charlie or Class Delta airspace area

The geometry here is frequently misunderstood. The restriction is drawn from the airport, not from the airspace boundary. You can be outside every magenta ring and blue dashed line on your sectional and still be inside that 4-nautical-mile bubble if the primary field is close enough. Knowing where the rings are drawn is not sufficient - you need to know where the airport is.

The 2,500-foot AGL ceiling also moves with terrain. Over a surface elevation of 1,500 feet, the restriction’s upper boundary sits at 4,000 feet MSL at that location. At 5,000 feet MSL over that same terrain - 3,500 feet AGL - section (a) does not apply and section (b) governs instead. Drop down to 2,000 feet AGL over the same terrain, still within 3 nautical miles of the field, and section (a) kicks back in. Your speed limit at any given airport depends on both your lateral distance from it and your altitude above the surface beneath you.

The reason for this restriction is the traffic environment around these airports. Regional jets on long finals, Cessna 172s in the pattern at 65 knots, helicopters operating low and slow - a fast aircraft moving through that mix at 230 knots is a hazard before it crosses into the airspace. The 4-nautical-mile buffer reflects that.

Section (a) includes the same ATC override clause as section (b). A controller can authorize higher speeds for traffic management purposes.

What Is the Bravo Underside Rule, and Why Does It Exist (Section c)?

Section (c) is the one most pilots miss entirely. It states: no person may operate an aircraft in the airspace underlying a Class Bravo airspace area, or in a VFR corridor through such an area, at an indicated airspeed of more than 200 knots.

The operative word is underlying. Not inside. Not within. The space below the floor of the Bravo shelf.

Consider this scenario: cruising at 9,500 feet, with the Bravo floor at your position sitting at 10,000 feet. You are not in the Bravo. You have not requested a clearance. You are squawking 1200 with no flight following. Section (c) applies. 200 knots maximum.

This is the regulation that catches pilots who reason: if I’m not in the Bravo, Bravo rules don’t apply to me. The FAA explicitly designated the airspace below the Bravo floor as a zone requiring speed discipline, separate from and independent of the Bravo itself.

The logic becomes clear when you compare the two environments. Inside the Bravo, you are in direct contact with ATC. A controller knows exactly where you are, is sequencing you, and is managing separation actively. That controlled environment justifies the higher 250-knot limit of section (b). Under the Bravo, you are frequently uncontrolled traffic, potentially without flight following, mixing with Bravo arrivals and departures climbing and descending through the same airspace. The FAA set a lower cap - 200 knots - and made it explicit in section (c).

The same applies inside VFR corridors through Class Bravo airspace. A designated VFR flyway is an accommodation, not a speed exception. Flying a published corridor does not exempt you from section (c). 200 knots still applies.

Does ATC Have the Authority to Override the Bravo Underside Speed Limit?

No - and this is one of the most important details in the regulation.

Sections (a) and (b) both contain the phrase “unless otherwise authorized or required by ATC.” Section (c) does not. There is no ATC override provision for the airspace underlying Class Bravo. A controller cannot authorize you to exceed 200 knots in the Bravo underside. The restriction is absolute.

This is a common oral exam question. When an examiner asks whether ATC can relieve you of the section (c) restriction, the answer is no. Sections (a) and (b): yes. Section (c): no.

The restriction also applies regardless of whether you are in contact with ATC. Flying VFR under the Bravo shelf with no flight following, no transponder response, no radio calls - you are still bound by section (c). The regulation does not require a radio call to be in effect.

Why Does Indicated Airspeed Matter?

All three sections of 91.117 use the phrase “indicated airspeed” - not groundspeed, not true airspeed. The number on your airspeed indicator is the legal measure.

This distinction matters at altitude. A pilot cruising at 9,000 feet on a warm summer day indicating 245 knots might have a true airspeed near 280 knots. That indicated reading is legal under section (b). The regulation does not care what the GPS groundspeed shows or what the true airspeed calculates to. At high density altitudes, where true airspeed diverges significantly from indicated, pilots have more room on the indicated scale than they might expect. The regulation was written to what the instrument reads.

How Should You Think Through This on a Checkride?

The Airman Certification Standards require knowledge of speed limits within the airspace topic area. Most students can recite the 250-knot rule. Fewer can accurately explain sections (a) and (c) or distinguish when each applies.

The most effective way to demonstrate this knowledge is to draw it out. Sketch the primary Bravo airport at center, the Bravo layers radiating outward, and shade the underlying airspace below the Bravo floor. Add a Class Charlie a few miles away, draw its 4-nautical-mile circle, and mark the 2,500-foot AGL ceiling. Label the remaining open airspace below 10,000 feet where only section (b) governs.

Write the speed limit in each zone. Then walk through a scenario using that diagram.

That approach shows an examiner you do not just know the rule - you know how to apply it to a real situation. That is what the ACS is testing.

Key Takeaways

• FAR 91.117 has three sections, not one. Two of them set a limit of 200 knots.
• Section (b): 250 knots at or below 10,000 feet MSL, everywhere sections (a) and (c) don’t reach. ATC can override.
• Section (a): 200 knots within 4 nautical miles of a Class C or D primary airport when at or below 2,500 feet AGL. The geometry is drawn from the airport, not the airspace rings. ATC can override.
• Section (c): 200 knots in the airspace underlying Class Bravo. No ATC override. Being inside the Bravo gives you a higher speed limit (250 knots under section b) than being under it (200 knots under section c).
• All three sections measure indicated airspeed - not groundspeed, not true airspeed.