Plastic Pilot

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 !

The speed that matters in turbulence is the maneuvering speed, a.k.a. Va. Most pilots feel good with Vx, Vr, Vy, Vno, Vne, Vfo, Vfe, Vlo, Vle, but Va sometimes sounds mysterious.

To make it simple, Va is the maximal airspeed at which the pilot can move the controls at full deflection without damaging the airframe.

Va is not marked on the airspeed indicator, because it varies with weight - and this is where the things get fuzzy if you don’t like theory. Let’s try to make it easy…

For a given airspeed, the forces generated by controls at full deflection will be the same. But if the plane is heavy the reaction will be slower because of inertia - you know, pushing a heavy car is harder to push a light car. Heavier plane, less effect, so higher Va, lighter plane, more effect, lower Va.

When the flight gets turbulent, it is important to fly at a speed below Va, if full deflection is required to compensate in case of turbulence. We all agree on that, but when it comes to defining the best speed in turbulence, there are two schools…

Generally, american instructors teach to speed up to Va in turbulence. The idea is to have a good speed reserve. Turbulence can modify airspeed, and by flying at Va the risk of turbulence induced stall is lower.

On the other hand, european instructors teach to maintain speed below Va, and even slower. When moving from updraft to downdraft, the higher the horizontal speed, the more sudden the change. So reducing airspeed makes the ride smoother.

What about stall then ? The difference between Va and stall speed is large, and even if turbulence generates a stall, it would not last long enough to bring the aircraft down to the ground. Obviously the speed reduction must remain within reasonable limits, particularly when flying at or below 1′000 feet above ground.

The risk of a temporary stall evolving into a spin is remote, because turbulence won’t cause a long stall and out of  balance condition lasting long enough.

I’d be happy to get some feed-back from you in comments:

• What did you learn ?
• What is your experience with  moderate to severe turbulence ?
• Do you know the values of Va for your aircraft ?
• Is all of this souding like a big enigma ?

I’m looking forward for your comments, we will see if there is such a big difference between US and Europe or not…