Are you speeding?

The universal speed limit in U.S. airspace we all remember, and mostly obey, is the 250-knot indicated airspeed restriction below 10,000 feet msl. This rule, 91.117 (a), is very broad and applies to all aircraft. The only exception is if the minimum safe airspeed for any particular operation is greater than 250 knots. Few airplanes qualify for that exception. Some versions of the Boeing 747 need to maintain a faster indicated airspeed to climb efficiently so they got a break, but for those of us in bizjets, no dice.

Observing the 250-knot limit below 10,000 is pretty easy on climbout, but in some jets it may demand a significant power reduction. On descent, we all want to go as fast as we can until the very last minute, so we delay the power reduction as long as possible. Autothrottle systems are very good at making this calculation and they manipulate power, sometimes changing the rate of descent to nail the 250-knot limit. Even if you don’t have autothrottles, some flight management systems (FMS) issue warnings to slow down when they calculate your airspeed will bust the limit.

Can controllers’ radar spot a 250-knot scofflaw? Maybe. But it’s complicated. What controllers track is groundspeed, not airspeed. An indicated airspeed will yield a different true airspeed—actual speed through the air—depending on altitude and air temperature. Air density, in other words, is a big variable. So, a controller will see targets moving at different groundspeeds depending on atmospheric conditions even when all pilots are observing the indicated airspeed restrictions.

Then, of course, there is the wind factor. Even when airplanes are flying at the same true airspeed, the controller will typically see different groundspeeds based on whether the wind is pushing or retarding the groundspeed.

ADS-B delivers more precise information to controllers than radar can, but the system sends our velocity, not indicated airspeed. Velocity is the rate of change in position, which is just another form of groundspeed. So, it’s very difficult for a controller under most conditions to know for sure that a pilot has busted the 250-knot speed limit by 20 or 30 or maybe even more knots.

Two other broad indicated airspeed limits that are easier to forget than the 10,000 one are the 200-knot cap when flying within 4 nautical miles of an airport in Class C or D airspace below 2,500 feet agl. That’s what we oldsters called the airport traffic area at an airport with an operating control tower. Oddly, this limit does not apply to the airport traffic area around the primary airport in Class B.

The other easy to forget limit is 200 knots when flying below a layer of Class B airspace or in a VFR corridor through Class B. Even when we remember these limits, it can take some searching on the electronic chart display to know when you’re below a Class B layer. And if you’re flying in a Class B airspace, the only limit that generally applies is the 250-knot restriction at 10,000 feet.

The 200-knot limit in a control tower’s airspace also applies on departure and that limit is easy to bust in many bizjets. Typically, your initial cleared altitude is low, often 3,000 feet agl, so it usually takes a significant power reduction to restrain both the climb rate and airspeed not to bust either limit.

These general airspeed limits are intended to give pilots and controllers more time to separate traffic that would otherwise be flying at wildly different indicated airspeeds. Makes sense to me.

Operational speed limits

What has become more common every day are operational airspeed limits designed to maximize the efficiency of traffic flow and to procedurally keep traffic within certain airspace.

Most standard terminal arrival routes (STARs) into busy airports contain airspeed restrictions, along with altitude assignments along waypoints on the route. Controllers will typically say cleared to descend on the route but implied in that clearance is to also observe every speed restriction.

Unlike the general 10,000-foot limit, busts of speed restrictions on a STAR will be more apparent to controllers because we are in line with other traffic. The altitude and speed assignments on the route are designed to maintain the desired separation between arriving airplanes so traffic is delivered to the arrival runway spaced most efficiently. It is essential that pilots observe the arrival restrictions for the system to work.

Fewer, but not uncommon, are speed assignments on a standard instrument departure (SID). Usually, the SID airspeed restrictions are broad, such as maintain a certain airspeed to this fix, or not exceed a speed until a fix or altitude, or more broadly maintain a minimum airspeed until reaching a specified altitude. The intent is the same: to separate departing traffic in trail to make more efficient use of airspace.

Controllers often use nonpublished airspeed assignments to maintain separation when vectoring traffic, or when lining up in-trail traffic to join a STAR. Controllers will ask if you can maintain a desired airspeed or Mach number and we have to be honest about what our airplane can do. If we are too slow for the prevailing traffic, controllers need to find an alternate route or altitude for us to maintain order in the arrival or departure stream.

In general controllers won’t assign an airspeed faster than 170 knots to the final approach fix, which most turbine airplanes can handle. If you can’t, again, be honest about what your airplane can do, especially when the controller says, “Maintain your best forward speed to X, what will that be?”

Airspace restriction speed

An airspeed restriction that controllers take very seriously is one designed to keep you within certain airspace. That is particularly important when the procedure involves a large change in heading.

One of the more famous, or perhaps infamous, restrictions is at Westchester County Airport (HPN) in New York. Departing from the main Runway 16 points you at the crazy busy airspace of LaGuardia and JFK located just a few miles away. The departure procedure demands you start a right turn when reaching 800 feet (not quite 400 feet above the runway) all the way around to a heading of 320 degrees, leveling at 3,000 feet and not exceeding 190 knots indicated airspeed.

The speed limit is crucial because the faster the speed the greater the radius of a turn. So, flying too fast, or banking into the turn too late in the climb, gets one uncomfortably close to LaGuardia and JFK airspace and the controllers will be yelling.

Published procedural airspeed limits are very common in Europe where airspace is congested, and in many areas high terrain is nearby. The only way to remain in the protected airspace is to fly at or below the assigned airspeed limits, and controllers there take that very seriously.

The good news in the growing maze of airspeed limits is that autothrottles are becoming much more available for personal and business turbine airplanes. With the autothrottle speed set at the assigned value, we can devote much more of our attention to being certain we are flying to the lateral and vertical limits of our clearance and not being a speeding scofflaw.

J. Mac McClellan is a corporate pilot with more than 12,000 hours, and a retired aviation magazine editor living in Grand Haven, Michigan.