How do pilots manage the speed during the approach?

Question

Once the initial approach fix is reached, what are the different "speed stages" to the runway?

I often read about landing speed and approach speed. For example on this chart, Boeing indicates various approach speeds.

I don't understand from what point an aircraft is supposed to be at the approach speed. Is there a specific distance threshold that you cannot cross above the approach speed? What about for the landing speed? Are there other key speeds than those two ones between the IAF and the runway?

Answer 1

The Boeing chart you reference appears to use for the approach speed column the Vref speed of the aircraft at its max landing weight in the landing configuration. This is the speed you want to be at or close to when you start the landing flare (the two 747 carriers I flew for used Vref+5).

Absent charted speed limits, controller instructions, or company policy, there is no specific distance threshold along the approach path that you cannot cross above that speed. The requirement is to be at the proper speed when you enter the landing flare.

However, if you were, say, flying the classic stabilized approach, you would be in the landing configuration at this speed from the FAF (final approach fix) to the landing flare. The FAF is typically around 5 miles from the runway.

Now, insofar as speed stages, the requirement is not one of specific distance points along your approach path (again absent charted limits, etc.), but rather what you have to do speed wise while changing from a clean configuration to the landing configuration. You're approaching the airport with the gear and flaps up. The critical element speed wise on the approach is the wing configuration.

Given the 747-200 speed of 150 kts in that chart, if you slow to that speed with the flaps up, you will stall. The minimum safe speed would be around 80 kts greater than that. Go to flaps 1 (which really doesn't put the trailing edge flaps at 1 degree but rather puts out the leading edge flaps or slats as some call them). Once they're out you can safely slow to Vref+60. At flaps 5 you can slow to Vref+40. At flaps 25 you'll be able to safely slow to the Vref speed in normal operating conditions.

Other aircraft have different flap settings and different speeds. The important thing to remember is that you're changing the shape of the wing to be able to fly more slowly. Where you will be along your approach path when you do this is typically not determined by distance per se, but by time. Do it too soon and you burn more fuel. Do it too late, and the problem becomes one of not being at Vref when you need to be.

Answer 2

I'm going to answer your questions in reverse order.

Is there a specific distance threshold that you cannot cross above the approach speed?

The short and simple answer is yes, there is such a point. More precisely there is a point, such that if it is crossed above the approach speed (with some margins) a missed approach becomes mandatory. The missed approach is not necessarily performed exactly at that point, but a landing is no longer permitted.

In this day and age an airliner is nearly never flown in such a manner that there is a gradual deceleration all the way to landing. Where that point, known as the stabilization point, exactly is depends on the aircraft flown, the airline policies and what type of approach is flown.

What has forced the industry and regulators into defining such a point is the finding that many accidents have been caused by what is known as an "unstabilized approach": Wikipedia: Unstabilized Approach.

In really broad terms a few different ways to define this point:

• 1000 ft AGL in IMC (sometimes the same in VMC also)
• 500 ft AGL in VMC (sometimes 1000 ft in VMC also)
• The Final Approach Fix (in non-decelerated non-precision approaches)
• Some much lower altitude for circling approaches and visual circuits

These will vary depending on the airline and the regulatory authority, so there is no definite reference where you can find these from. There are also other criteria than the speed that must be fulfilled at the stabilization point. There are some sources you can refer to for examples:

• Airbus
• Stackexchange question

Once the initial approach fix is reached, what are the different "speed stages" to the runway?

Referring to the first section of my answer the initial approach fix is not critical in determining your speed profile. Rather you determine the point at which you must be stabilized at approach speed first and work back from there and in this way determine the point where you would first have to start slowing down and configuring the aircraft step by step (flaps, slats, landing gear).

In an ideal world you would want a continuous deceleration from the point you decide to start decelerating all the way until your stabilization point. Flaps would be extended as soon as the max allowed speed for the next configuration is reached and the landing gear in a sequence that usually comes with some recommendations from the aircraft manufacturer.

That means that the location of the point where you start your initial deceleration depends on many variables: aircraft type, aircraft weight, altitude, wind conditions etc. In addition to that there are factors that make the world less ideal, like air traffic control, procedure speed limits etc.