MIAMI – Before we dive into the topic, let’s explain briefly how a wing creates lift and how lift is augmented by Flaps and Slats.
An airliner’s wing is designed to generate the optimum amount of lift with the minimum drag for a specific speed (Mach) and at normal cruise altitudes (35-41,000ft). This is especially important for long haul airliners that spend 85-95% of the flight in the cruise.
Flaps and Slats allow the aircraft to generate lift at lower speeds, for take-off and landing, by increasing the size and curvature of the wing.
For take-off, we extend flaps and slats to give us the most efficient use of the runway length available. We use (Airbus) Flap settings 1 or 2 for take-off as we are generally heavy. This provides the optimum lift with a small drag compromise, with take-off speeds between 145-165 knots.
After take-off between 1000 ft or 3000ft, depending on local terrain or noise requirements, we reduce the take-off thrust, lower the nose to accelerate and retract the flaps/slats to the “clean” configuration as seen in these images.
Clean vs Dirty Landings
We remain in the “clean” config for the entire flight until we need to reduce our speed for the approach and landing. We then “dirty-up” by selecting sequential flap/slat selections, as ATC reduce our speeds for traffic spacing.
Before our approach, while manoeuvring towards the airport, or in a holding pattern, ATC may ask us to “reduce to Minimum Clean speed” On most of the Airbus aircraft I have flown, this is approximately 220 knots. ATC are able so sequence aircraft safely towards the approach runway/s at similar speeds.
To fly at slower speeds we need to extend flaps/slats to sufficient lift to keep the aircraft aloft. When fully configured with full flaps/slats extended. ATC may ask to reduce to “Minimum Approach speed” which for the A330/A350 is approximately 135 knots.
Featured image: Virgin Atlantic A330. Photo: Francesco Cecchetti/Airways