What is the 250 knot speed limit? ​

In the US, all aircraft must fly at or below 250 knots indicated airspeed (KIAS) when below 10,000 feet MSL. This is an FAA regulation (14 CFR 91.117) that applies to virtually all operations in the lower airspace.

Why 250 knots ​

Two main reasons.

Bird strikes. The majority of bird strikes happen below 10,000 feet, where birds actually fly. At 250 knots, a bird strike is already serious. At 350 knots, the impact energy is roughly twice as high. Slowing down gives pilots more reaction time and reduces damage if a strike occurs.

See and avoid. Below 10,000 feet, IFR and VFR traffic mix together. VFR pilots are looking out the window to spot other aircraft. The closure rate between two aircraft - one at 250 knots and one at 120 knots - is already 370 knots. If the faster aircraft were doing 400 knots, there would be almost no time to see and avoid.

The 250 knot limit keeps speeds manageable in the airspace where traffic density is highest and where visual separation is part of the safety system.

What counts as 250 KIAS ​

The limit is in indicated airspeed, not true airspeed or groundspeed. Indicated airspeed is what the pilot's airspeed indicator reads and is directly related to aerodynamic forces on the aircraft. At 10,000 feet, 250 KIAS is roughly 290 knots true airspeed. At sea level, they're nearly the same.

Wind doesn't matter. An aircraft with a 50 knot tailwind doing 250 KIAS has a groundspeed of 300 knots, and that's perfectly legal.

Exceptions ​

The rule isn't absolute:

• ATC can assign speeds above 250 knots below 10,000 feet if needed. This is rare but it happens.
• Aircraft that can't safely fly at 250 knots are exempt. Some military jets and a few clean-wing aircraft have minimum safe speeds above 250 knots. They fly at the minimum safe speed instead.
• Class B airspace has an additional 200 knot limit below 2,500 feet AGL within the surface area. This applies even to aircraft that would otherwise be allowed to fly faster.

Outside the US, rules vary. ICAO recommends 250 knots below FL100 (roughly 10,000 feet), and most countries follow this. Some countries apply it below FL100 regardless of the local transition altitude.

How controllers use it ​

Controllers know the 250 knot limit is there and plan around it. When sequencing arrivals, speed assignments below 10,000 feet are always at or below 250 knots - typically 210 knots for base turn, 180 knots on final approach, and 160 knots for close-in sequencing.

For departures, the pilot accelerates after takeoff but caps speed at 250 knots until passing through 10,000 feet. After that, they're free to accelerate to cruise speed (often 300+ knots). This creates a predictable speed profile that controllers can plan around.

The limit also compresses traffic on departure. Everyone is doing 250 knots below 10,000 feet, so spacing stays consistent. Above 10,000 feet, faster aircraft pull away from slower ones - a B737 doing 280 knots separates from a CRJ doing 250 knots.

In radarcontrol.io ​

Departure aircraft automatically follow the 250 knot restriction below 10,000 feet. They'll accelerate through the lower altitudes at 250 knots and then climb to their filed cruise speed above 10,000 feet.

You can assign speeds below 250 knots for approach sequencing - s210, s180, s160 are all common commands for slowing arrivals. The restriction is enforced on departures but not strictly on arrivals, giving you flexibility to assign speeds as needed for sequencing.

Controllers in the real world also have this flexibility. The 250 knot limit is a maximum, not a target. Slowing aircraft well below 250 knots is standard practice in terminal airspace.

Related: How arrivals are sequenced | What is a SID and STAR? | What is a flight level?

Guides: Command reference | Arrivals guide

Play SoCal TRACON - practice speed control and arrival sequencing at LAX.