Recently, I was controlling tower at a very busy airspace without the luxury of radar assistance. I was, for lack of better words, shocked at the number of people who were unable to follow a sequencing instruction.
What is a Sequence?
The sequence is the most basic of tower controlling instructions. It allows a controller to inform a pilot of “his place in line” and who he is meant to follow. This is especially important to follow in a busy airspace since sequencing in this scenario plays a great role in the flow of traffic. Not following the sequence could lead to getting ghosted, if you interfere with other traffic. When it is not busy, often times, the sequence will be the logical progression of traffic.
Why are sequences important?
They are the main tool used to order planes when radar control is unavailable. The only time when they are not required is when aircraft are inbound on the ILS/GPS after being handed off by the approach frequency. In that case, the aircraft will already be in a line so a sequence is not necessary.
How to correctly follow a sequence
Below, you will find a perfect example of a correctly followed sequence. The spacing does vary depending on the aircraft used, of course. Larger, commercial jets will require greater spacing than the TBMs used in the example, but the same principle can be applied to all aircraft. An additional point to note would be the area where the aircraft bearing the callsign “CHATTA” turned base. A common solecism in following sequencing (more to come later in the post) is turning base too early. This does not help anyone, since it confuses the controller, and will lead to the pilot needing to go around.
Common mistakes with following sequencing and how to avoid them
- Simply not following the sequence
- This is the most common error, and it is the easiest to explain – count the planes ahead of you and make sure you understand. In the image below, you can see how not following the sequence can be dangerous. The aircraft with the callsign “ROO” was not cleared to land and cut the sequence, leading to some dangerous separation with the other aircraft. (Clearance was prolonged for the sake of example, in most cases, both aircraft would already be cleared to land when this screenshot was taken)
Turning base too early
- As I previously mentioned, this one is also fairly common. Solving this simply requires attention to the speed of aircraft ahead of you. If you are in a 777 and the aircraft ahead of you is a prop, you will need to allow for more separation. The only way to perfect this is to get practice judging speeds and turn radii for aircraft.
Prolonging the sequence
- The final error I will discuss in this post is what I also see a lot of, but is less detrimental to the flow of traffic. Flying a very long downwind leg can lead to a longer wait time for aircraft inbound. In some cases, the ATC will squeeze one or two aircraft ahead of the aircraft on longer downwinds. It is certainly a minor mistake, since it is easily solved by the controller, and is certainly better than turning base too early. Prepare for resequencing if you do make this mistake.
Yes, I am aware that most pilots on expert are able to do this, however, there are definitely still some that may not understand some of the points brought up in this post. This post is a success as long as one user, somewhere, learns something and applies it the next time they fly.