Aerodynamic Degradation: Ice and Ageing - ATPL 081 — Principles of Flight

Every lesson in this module has quietly assumed one thing: that the wing the air meets is the wing the designer drew: smooth, clean, and exactly the intended shape. This closing lesson is about the two ways reality breaks that assumption. The fast way is contamination: ice, frost and snow rebuilding the aerofoil's shape and surface in minutes. The slow way is age: dents, waviness and accumulated modifications degrading an airframe over years. Both tell the same story: the coefficients in the book belong to the drawn wing, and the air only ever sees the real one.

Where ice forms in flight

In-flight icing comes from supercooled water droplets: liquid below zero degrees, waiting for something to freeze onto. What they freeze onto is whatever meets the airflow head-on, so the build-up map is a map of the aircraft's forward-facing surfaces:

leading edges of the wing, the tailplane and the fin;
pitot probes (and, though flush-mounted rather than forward-facing, the static ports, which ice can crust over), whose blockage corrupts the airspeed and altitude indications: the instruments subject takes that thread;
windshield;
engine intakes, where shed ice can be swallowed by the engine;
propeller leading edges, costing thrust and throwing ice against the fuselage;
antennas and other protrusions.

One detail matters later in this lesson: thin surfaces collect ice more efficiently than blunt ones, so the tailplane, with its slender leading edge, typically carries proportionally more ice than the wing.

What contamination does to the wing

Ice hurts twice: as shape (horn- and ridge-shaped accretions rebuild the leading edge, exactly the region that sets how cleanly the flow can round the nose at high angle of attack) and as roughness, which drains energy from the boundary layer over whatever it covers. A boundary layer that arrives at the pressure rise already tired separates early, and everything in this lesson's list follows from that. Item by item, as the exam asks:

Lift — the maximum lift coefficient falls, and falls hard: leading-edge ice can cut $C_{L\,max}$ by a third or more of its clean value. Even a roughness layer with almost no thickness (sandpaper frost) costs a serious share of the clean maximum.
Drag — rises substantially: roughness raises friction drag over every contaminated surface, and the disturbed, early-separating flow adds form drag on top. The aircraft needs more thrust for the same speed while its wing lifts less.
Stalling speed — rises, straight from the stalling-speed formula: a smaller $C_{L\,max}$ under the square root. The margins built on the clean stalling speed quietly shrink.
Critical angle of attack — falls: the contaminated wing stalls at a smaller angle than the clean one. This is the treacherous item, because the stall-warning system is calibrated for the clean wing's stalling angle: an iced wing can stall before the warning that was supposed to precede it.
Stability and controllability — degraded on several fronts at once: ice rarely builds symmetrically, so the two wings stall at different angles and the stall breaks with a roll; ice ahead of ailerons and elevator blankets them in disturbed air, blunting their response; and the tailplane (carrying proportionally more ice) can stall itself, and since the tail normally pushes down to hold the nose up, its stall is an abrupt NOSE-DOWN pitch, most likely when flap extension increases the downwash angle arriving at it and drives the tail past its own reduced stalling angle.

Icing at take-off

Now put contamination on a wing that is about to rotate, and note first that ground contamination is different from in-flight ice: overnight frost and snow blanket the UPPER surfaces, exactly the suction side that generates the lift. The take-off speeds are scheduled for the clean wing (fixed factors above its clean stalling speed, with the rotation attitude well below its clean stalling angle) so that the clean $C_{L\,max}$ leaves a healthy margin between lift-off and the stall. A layer of frost or snow moves both ends of that margin at once (the stalling speed comes up, the stalling angle comes down) so an aircraft rotated at the computed speed, to the normal attitude, can find itself within a few degrees of a stall it was never supposed to meet, just after lift-off, possibly rolling off on whichever wing is dirtier. Add the extra drag degrading the acceleration and the climb gradient, and the conclusion is the rule the whole industry flies by: the clean-aircraft concept: take-off with frost, ice or snow adhering to the lifting surfaces is prohibited, and de-icing and anti-icing exist to make that rule achievable on a winter morning.

The ageing aeroplane

The slow degradation needs no weather. An airframe that has flown for decades differs from its type-certificate self in a hundred small ways:

dents and skin waviness from years of handling;
chipped and repainted surfaces;
panels and fairings that no longer sit quite flush;
seals that leak air where the structure should be closed;
modifications, accumulated over the years (antennas, sensors, reinforcements), each adding its own patch of parasite drag and interference.

None of these is dramatic alone; together they explain why an old airframe cruises a little slower, burns a little more, and misses its book performance figures: the book was written for the aeroplane as drawn.

The mechanism concentrates in the boundary layer. On a smooth young skin, the flow runs laminar over a useful forward stretch before turning turbulent; every scratch, rivet ridge and paint edge on an ageing skin trips the transition earlier, so a larger share of the surface wears the thick, high-friction turbulent layer: friction drag up. The roughened, thickened boundary layer also arrives at every pressure rise with less energy, separating a little earlier than the clean wing's would: a small loss of $C_{L\,max}$ to go with the drag. The physics is this module's oldest pair (friction in the boundary layer, separation under an adverse pressure gradient) applied to an aeroplane that has simply lived.

What the exam asks

This lesson is 'know where, know what it does, know why take-off is the worst place'. The overview, following the learning objectives:

Describe the locations on an aeroplane where ice builds up during flight

Explain the aerodynamic effects of ice and other contaminants on lift ( $C_{L\,max}$ ), drag, stalling speed, the critical angle of attack, and stability and controllability

Explain the aerodynamic effects of icing during take-off, and the clean-aircraft concept it leads to

Describe the effect of airframe deformation and modification of an ageing aeroplane on performance

Explain the effect on the boundary layer of an ageing aeroplane: earlier transition, more friction drag, earlier separation

Where ice forms in flight

Check you understood

What contamination does to the wing

Icing at take-off

The ageing aeroplane

What the exam asks