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Each aircraft type has specific performances, and performance can even vary for the same type, depending of the load on board. Air Traffic Controllers (ATCOs) sometimes use speed restrictions to maintain separation between aircrafts flying along the same route, but as they don’t know all the details of daily aircraft performance, some standards have been defined.

If you’re pilot, or a well-equipped aviation fan, you might have heard the terms “minimum clean” or “final approach speed”, and wondered what it was. This is usually used by ATCOs when issuing clearances like “Flight XXX, reduce to minimum clean / final approach speed”.

“Final approach speed” is rather easy to understand: the speed to which the pilot slows down immediately before landing. But what about minimum clean ? This as nothing to do with the cleaning done by cabin-crew before the approach starts, but with aerodynamic configuration of the airplane.

For take-off and landing, the flaps are extended, to create more lift at lower speed, and to help slowing down. Flaps are the mobile part of the wing on the aft edge, which seems to slide behind the wing. The landing gear is also extended for obvious reasons and it does contribute to reduce speed.

Without extending the flaps, the wing can generate less lift (the upwards force making the plane fly), so it’s not possible to slow down too much before the flaps are extended. In pilot’s jargon, the plane’s configuration is said “clean” when no “dirty” elements like the flaps or the landing gear slows it down.

After take-off, the process of retracting the gear and flaps is known as “cleaning up” the plane, or configuration. The minimal speed to which a plane can slow down without using flaps is thus known as “minimum clean”. Pilots normally wants to maintain speed higher than minimum clean as long as possible, to avoid the extra fuel consumption coming with flaps extension if flying level.

One of the funny thing when flying high-performance single engine planes like Saratoga or Bonanza in the IFR system is the speed range. Flying 160kts on final makes integration with airliners possible, but the final approach speed is much lower (80 or 90 knots, compared to 120 - 140 for a typical airliner).

A controller once asked me to reduce to final approach speed to permit a departure, when I was still more than 10 Nautical Miles away from the airport. Before slowing down, I asked to confirm, mentionning that my final approach speed was 85 knots. After that, he simply asked me to reduce speed to 120 knots.

Here again, knowing ATC business helped to keep good co-operation and an optimal traffic flow. For various safety and procedural reasons, pilots don’t like to retract flaps on final, so when speed has been reduced, it normally don’t increase again, except on go around. If on that day I slowed down as required, the airliner behind me (well behind, but 95 is really slow) would probably have received a “go around” clearance… or more likely my own approach would have been interrupted by ATC !