You’d expect modern-day commercial flying to feel as smooth as glass from takeoff to landing. In reality, however, flying usually feels more staccato, with frequent changes in speed and altitude beyond simply the climb and descent. You often feel these changes throughout the flight, but especially during the descent. Planes often break their descent and level off multiple times as they approach the airport. Airliners also slow down at various points throughout the descent, and they almost universally need to suddenly reduce speed below 10,000 feet.

Modern air traffic operations are a carefully coordinated performance that balances safety and efficiency to create the most effective system possible. Air traffic controllers are meticulous in ensuring that planes are properly separated while keeping a constant flow of arrivals that’s manageable while still maximizing the airport’s total capacity. Couple this with Federal Aviation Administration (FAA) regulations, and you get a system that is enormously complicated while being invisible in nearly every way apart from the noticeable slowdowns.

250 Knots, Plus Or Minus 10 Knots

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Pilots and air traffic controllers use SIDs (standard instrument departures) and STARs (standard terminal arrival routes) for any operation in or out of most major airports. These procedures include altitude and speed constraints, but what’s more or less non-negotiable is that all aircraft must fly slower than 250 knots below 10,000 feet. This is an FAA regulation, and the rules allow a margin of ten knots above or below this speed, meaning that aircraft cannot fly faster than 260 knots in most cases.

This means that, regardless of the arrival procedure that the aircraft is operating, it must slow down to roughly 250 knots by the time it approaches 10,000 feet. How suddenly this occurs depends on the flight’s approach to the destination as well as how the pilots plan to hit 250 knots by 10,000 feet. Sometimes the speed may fall off gradually, and you might not even notice the crew has crossed this target speed, while in other cases it may be more abrupt, depending on whether the crew uses spoilers, pitch, or reduced thrust to bleed off airspeed.

Aircraft are permitted to exceed this rule when their minimum clean wing speed is greater than 250 knots. This is not common during approach, but it does occur with departures, and specifically, with heavy jets loaded close to their maximum gross weight. Pilots need to advise ATC that they require a climb speed above 250 knots, and ATC then acknowledges this. Meanwhile, if pilots need to exceed a speed restriction listed out in a SID or STAR, they must request an exemption from ATC, and this is essentially always granted.

The Importance Of Monitoring Speed

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Modern commercial aircraft are typically fairly easy to hand-fly until you push them past typical airline flying. The biggest risk is losing airspeed and stalling, because swept-wing transport aircraft can be very difficult to recover from a stall. Air France 447 is an infamous example of a high-altitude stall that led to tragedy, and it demonstrates the difficulty of recovering an airliner in such a condition. Even if the crew had responded appropriately to the stall, it still would have taken thousands of feet to regain control of the Airbus A330.

Pilots are therefore very conscious of their speed and are cautious to stay within safe operating margins. By the time the aircraft approaches 10,000 feet, crews have usually decelerated enough so that the plane doesn’t need to bleed off too much airspeed, although this depends on the STAR being flown and the handling characteristics of the aircraft type. What passengers then feel as the pilots slow down to comply with the 250-knot restriction depends on a specific airline’s operating procedures.

Pilots usually descend at a rate that also effectively bleeds off airspeed. In addition, the use of flaps can help slow the aircraft down, although this is not their primary function. Speedbrakes and spoilers (essentially the same control with different names depending on the manufacturer) are commonly used by some airlines, while other carriers may discourage their use. Exact procedures also differ by flight, as different actions may be needed at different airports than at others, depending on the arrival route, weather, and congestion.

Speeds On The STAR

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STARs are intended to streamline IFR traffic into specific pathways en route to the runway. To keep flights organized and on time, the STARs usually set out altitude and speed constraints, which help maintain a steady flow of arrivals. Depending on the specific procedure being used, there may be multiple speed restrictions from top of descent to landing, and pilots are required to comply with the published speeds even if they are not directed to follow the STAR’s altitudes. So long as the aircraft is following the lateral path of the STAR, it must comply with the speed restrictions.

STARs mainly serve to keep planes safely organized and separated. By having aircraft fly slower, this makes it easier for controllers to monitor and handle aircraft in the vicinity of the airport as they become increasingly clustered. In addition, you often find that controllers at major airports like Heathrow issue holding patterns or speed restrictions to arriving aircraft due to congestion. This allows for a steady flow of arrivals into the airport and keeps planes spaced apart, although pilots do sometimes request exemptions from assigned speeds (but rarely in excess of 250 knots).

While departures often exceed the 250-knot restriction for performance reasons, this isn’t usually required for aircraft approaching on a STAR. Departing aircraft will usually climb out faster than 250 knots if their minimum clean wing speed is greater than the speed restriction (as in, with no flaps deployed) to ensure safe maneuvering, but arriving aircraft may have already started deploying flaps while passing through 10,000 feet or intend to do so soon. In addition, because the aircraft is lighter towards the end of the flight, the safe maneuvering speed decreases.

The Impact Of Speed On Performance

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At slower speeds, large transport aircraft can have sluggish handling and can be easily stalled, while recovering from a stall in a swept-wing airliner is hardly a quick process. Flaps are incredibly important to ensure that the aircraft has lift, and crews also need to keep the speed high enough for safe maneuvering with the flaps retracted. Each airline has different operating procedures, but the common requirement is the minimum clean speed plus a certain margin to allow for safe maneuvering, and this trumps the 250-knot restriction.

During descent, however, the aircraft’s reduced weight and flight profile generally make it more forgiving, and crews are focused on establishing a stable final approach. As such, the flaps come out relatively high, and the aircraft continues to slow down gradually. Meanwhile, once the aircraft is set up on its final approach to the runway, it’s generally expected to be flying close to its VREF (1.3 times the stall speed in landing configuration), aligned with the runway, and on the glideslope.

Maintaining a stable approach is crucial to maintaining safety, and if an aircraft hasn’t achieved a stable approach in full landing configuration by a certain altitude (typically 500 feet for most airlines), then standard operating procedures dictate that the crew must perform a go-around. Meanwhile, ATC controllers and STAR procedures help organize dozens of flights operating in different phases within close proximity of one another, enabling the system to work as a whole.

Additional Speed Restrictions At Low Altitudes

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In addition to the general 250-knot speed restriction, the FAA further prohibits flights from exceeding 200 knots if they are flying underneath a Class B airspace (the limits of which are shaped like an upside-down wedding cake). In addition, aircraft flying within four nautical miles of a Class C or Class D airspace must not exceed 200 knots, or if they are flying within 2,500 feet of the ground.

Commercial airliners, however, don’t often find themselves in these specific circumstances, and these regulations mainly impact general aviation traffic. Once again, however, if an aircraft requires a higher clean operating speed than what regulations permit, then it’s permitted to do so. This isn’t something that ATC necessarily approves, but the crew is responsible for the safe operation of the flight, and if this means flying faster than what regulations permit, then they have the authority to do so.

Other nations may structure similar regulations differently, but a general speed restriction below 10,000 feet is very common. Many regulators give controllers the discretion to cancel speed restrictions for aircraft below 10,000 feet to provide additional separation between arriving and departing flights, but the United States is slightly different. The FAA permits individual controllers to waive speed restrictions that are published in SIDs/STARs, but controllers do not have the authority to waive the 250-knot requirement (pilots may exceed the limit on their own authority if necessary).