6
$\begingroup$

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?

$\endgroup$
  • $\begingroup$ Are you flying a simulator? I ask noting Boeing and 'speed stages'. ? $\endgroup$ – user6035379 Nov 12 '16 at 14:24
  • $\begingroup$ In additional of the existing good answers: Boeing provides in their operations manual an overall sequence ("pattern") on how to do the approach, from a configuration perspective (including flaps, landing gear, etc). This look like this for a B747-400 in an ILS approach with autopilot (from FCOM page 131 -- Large document!). From the configuration, speed limits are imposed as explained in the answers. $\endgroup$ – mins Nov 12 '16 at 14:54
13
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ Does that mean, we need to fly greater than the Vref speed + wind components speed till the flare point? $\endgroup$ – user9935 Apr 2 at 3:34
  • $\begingroup$ @Vijay Assuming the wing is in the landing configuratuon, no, there's no requirement to fly faster than that unless you want to. Remember, though, that if the wing is not in the landing configuration for the specified Vref, you need to keep your speed above the minimum safe speed for the configuration the wing is in. $\endgroup$ – Terry Apr 2 at 18:35
  • $\begingroup$ Oh.. That's it. So we should keep the Vref speed with the wings at final landing configuration and maintain till the touchdown zone, Right? Sir. $\endgroup$ – user9935 Apr 2 at 20:18
  • 1
    $\begingroup$ @Vijay When you start the flare, ideally you will be at Vref (or Vref+5 or whatever your company desires for a little extra padding). From the start of the flare until touchdown (usually but not always in the touchdown zone) your speed and rate of descent will be decreasing, especially the rate of descent as the majority of the descent rate should be gone when the wheels touch. $\endgroup$ – Terry Apr 3 at 2:37
  • $\begingroup$ @Vijay I'm not qualified to answer aviation.stackexchange.com/questions/61898/… as the 727s and 747s I flew did not have FMCs. I retired in 1999. Back then we typically disengaged the autopilot before the application of flaps and hand-flew the airplane from that point. I typically hand-flew from the top of descent since I enjoyed that. Some if our aircraft had auto-land, but the op specs of both the 747 operators I flew for specifically forbad doing that because auto-land was not maintained. $\endgroup$ – Terry Apr 3 at 2:52
3
$\begingroup$

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:

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.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.