We all know the speed limit is 250 knots below 10,000 feet. But why?

The Regulation

According to FAR 91.117(a), "unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 mph)".

If you're flying a piston aircraft, this might not mean a lot to you. However, it's an important factor for turbine and some turboprop aircraft. Jets don't slow down quickly, and turbine pilots often need to level off at 10,000 feet to bleed off airspeed before continuing their descent.

Multiple Deadly Mid-Air Collisions Resulted In This Speed Restriction

On December 16th, 1960, a United Airlines Douglas DC-8 collided with a TWA Lockheed L-1049 Super Constellation over the skies of Brooklyn, New York. United Flight 826 had missed its holding waypoint and proceeded over 12 miles off course in bad weather before colliding with TWA Flight 266 at over 300 knots. The crew of the DC-8 had just recently slowed from 400 to 300 knots prior to impact.

One of the contributing causes to the accident was listed as "the high rate of speed of the United DC-8 as it approached the Preston intersection."

In years prior, the FAA had been debating how exactly to limit speeds for inbound aircraft around the country as the jet age (and high-speed air travel) began.

This accident was just one example that further proved the necessity for better speed control at lower, more congested altitudes.

Mode C Transponders Aren't Always Required Below 10,000 Feet

According to FAR 91.215, nearly all aircraft are required to have an operating Mode C transponder at and above 10,000 feet MSL. These transponders automatically transmit pressure altitude in 100-foot increments to ATC. In part, this helps ATC separate VFR traffic from high-speed IFR traffic above 10,000 feet.

Below 10,000 feet (or within 2,500 feet AGL of the surface), these transponders are not required.

If you're not flying with a transponder, ATC can't easily determine your speed or track, and they have no way of knowing your altitude. This is another reason why speeds are restricted below 250 knots at altitudes below 10,000 feet.

As Height Increases, So Do Cloud Clearance Requirements

So why is there a difference in weather minimums at different altitudes? Starting at 10,000' MSL, you can fly faster than 250 knots. Accordingly, you'll need more visibility and distance from the clouds to see and avoid other aircraft. High speeds increase closure rates, so you'll have less time to react to oncoming traffic.

If you're flying in Class E or G airspace, your visibility requirement above 10,000' MSL is 5SM, day or night. You'll also need to stay 1SM horizontally from, 1,000' above, and 1,000' below clouds.

The opposite is true below 10,000 feet MSL. Cloud clearance requirements are loosened, and you can fly VFR with as little as 1 mile of visibility in Class G airspace during the day.

Class B Airspace

There isn't a specific speed restriction for operating in Class B airspace. If you're below 10,000 feet, you need to meet the standard speed restriction of 250 knots. However, if you're in Class B at 10,000' MSL or higher, you can fly faster than 250 knots (though ATC usually restricts aircraft speed for traffic flow and separation).

Most Class B airspace ends at 10,000' MSL, so this isn't much of a factor. However, some Class B airspace extends higher, like Denver's Class B, which extends up to 12,000' MSL.

According to 91.117(c), no person may operate an aircraft beneath Class B airspace, or in a VFR corridor through Class B, at an indicated airspeed of more than 200 knots (230 mph). This is done to help separate aircraft operating within Class B from those operating outside of Class B. Some aircraft flying below Class B may not be in contact with ATC, and the speed restriction of 200 knots provides ATC an added buffer to get traffic out of the way, should an airspace deviation occur.

What Do You Think?

Does the 250 knot speed restriction below 10,000 feet make sense? Do you think it should be increased or decreased, and under what circumstances? Tell us in the comments below.

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