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When To Slow For The Approach In Busy Airspace.

You know when someone is about to get scolded by Air Traffic Control (ATC) when the controller asked in a annoyed tone, “Flight 1234 say airspeed”. Flight 1234 responds “210 knots”. The controller then responds “For future reference please let us know before you start slowing because if it was a busy day we would probably put you in the back of the line”. Flight 1234 then apologized.

Shortly afterward my co-worker stated “Flight 1234 should read his AIM”. I then turned to him and asked him to explain why. He said because the AIM states that if our cruise speed varies by 10 knots or 5% of our planed airspeed we need to tell ATC. I then turned to him and said that would have happened a long time ago when he passed through ten thousand and slowed to 250 knots since he was probably cruising around 450 knots before descending.

This scenario brings up a question often asked of people transitioning to faster aircraft with a wide range of airspeeds. When can you begin slowing for your approach? When you as the pilot in command deem it necessary unless you were assigned an airspeed by air traffic control. In the case above the pilot was not assigned an airspeed therefore there was no violation for slowing early.

Be courteous.

Before you slow for your approach in very busy airspace it is common courtesy to keep your speed as fast as practical until it is necessary for you to slow for the approach. This might mean doing 250 knots to the marker in very busy airspace if you know the flight following you is in a hurry to get to the airport.

Of course you know your airplane and if you need to slow down earlier for a safer more stabilized approach then by all means slow down. If you also need to slow for operational purposes it is a good idea to let ATC know in advance especially in busy airspace.

The Regulations.

The AIM states that we must advise ATC any time cruising airspeed varies plus or minus 5 percent or 10 knots, whichever is greater, from that given in the flight plan. (AIM 5-5-9, AIM 5-3-3). While on an arrival we are no longer in cruise flight and do not need to advise ATC of changes in our airspeed unless you were assigned an airspeed or the arrival you are flying has an airspeed restriction.

If ATC has assigned you an airspeed you must comply with that airspeed and notify them if you cannot maintain the airspeed assigned. Before you key the mike and tell ATC you are unable to maintain the assigned airspeed also have in mind a good reason so that they are more willing to work with you.

For example, if you have a mechanical problem and need to slow to 180 knots but were assigned 240 then tell ATC you must slow because you are having a mechanical problem. The next thing you should hear from ATC is if you need any further assistance and you must be prepared to say yes or no depending on your mechanical problem.

Operating in Busy Airspace.

Now that we know we are legal we must also consider the operation. If you are arriving into busy airspace the best thing to do is keep your speed up as long as practical until you need to slow for the approach. If you are like the person above who decided to slow on his own and not notify ATC you may be legal but you are not being very courteous.

Furthermore, it you are equipped with TCAS or other collision avoidance system, you should be aware of who is behind you. This helps if you were planning on slowing early when maneuvering for the approach. Normally ATC will give you airspeed assignments but on a slow day they may not. On a slow day speed is at your discretion and slowing early may be a good idea for passenger comfort especially if it is busy.


If you are maneuvering for the approach or on an arrival you can begin slowing as early as you want but if you are in busy airspace you should let ATC know you are slowing and why. This information will help the air traffic controller plan on how to work you in with the other traffic.

Although it is legal to begin slowing whenever you want it is a bad idea to start slowing when you are far from the airport. Many times you may be 5 miles from the airport but because you might be following 20 other airplanes ATC might vector you for many more miles before placing you in position for the approach.

Flying is a series of decisions based on previous experience. Many times it is beneficial to slow early for the approach especially if it is going to be a visual approach and you are high over the airport and being vectored for a down wind. If you are at a major metropolitan airport they usually want you to go fast .

There are many different approach control facilities throughout the country and some consistently keep you high while being vectored even if there is no traffic in front of you. These facilities are few but if you know which ones they are from past experience you might start slowing early before your approach.

Generally speaking it is best to keep your speed up if you have not been assigned an airspeed until you are being vectored towards the final approach course. Remember there is nothing that says you can’t slow early, but be courteous and let ATC know you are slowing and they will work around your request.

Safe Flying!

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