Aircraft Icing

What structural icing is

Structural icing is the accumulation of ice on aircraft surfaces during flight. It forms when an aircraft flies through supercooled water droplets — liquid water that remains liquid below 0°C because it has no nucleus to freeze around — and those droplets freeze on contact with the airframe.

Ice can accumulate on wings, horizontal stabilizer, propeller, pitot tube, carburetor, and any other exposed surface. Even a small accumulation disrupts airflow, increases drag, adds weight, and reduces lift. The affected surfaces are no longer the shape the manufacturer designed — they're a different, less efficient shape that stalls at a higher speed and produces less lift at every angle of attack. (FAA-H-8083-28 Ch. 10)

Why pilots care

Ice is insidious because it degrades performance before the pilot notices anything wrong. A 0.5-inch layer of clear ice on the leading edge can increase stall speed by 30% or more, reduce lift coefficient by up to 50%, and increase drag by 100%. An aircraft in those conditions may not be able to maintain altitude even at full power, and the stall warning may activate at the same time the aircraft actually stalls.

For VFR pilots, icing is a particularly dangerous trap because it forces flight into IMC (clouds are where the supercooled droplets are) to escape — and most light GA aircraft cannot legally or safely operate in icing conditions. The correct decision is made on the ground, before departure.

Three conditions for structural icing

Structural ice forms when all three conditions exist simultaneously. Remove any one and no icing occurs. These are your pre-flight risk checklist before any flight in IMC or near-freezing temperatures.

Supercooled liquid water (SLW): Water normally freezes at 0°C, but in clouds, pure water droplets can remain liquid well below freezing — down to approximately −40°C — because they lack the ice nuclei needed to initiate freezing. These supercooled droplets are in a metastable state: they are liquid, but freeze instantly on contact with any solid surface. Your airframe is that surface. The smaller and more numerous the droplets, the more rapidly ice accumulates, and the type of ice that forms depends heavily on droplet size and temperature together. (AC 91-74B §3; FAA-H-8083-28 Ch. 10)

Ice types & temperature ranges

The FAA identifies three types of structural ice: rime, clear (glaze), and mixed. Each has a characteristic temperature range, droplet size association, physical appearance, and aerodynamic effect. Examiners regularly ask about these temperature ranges — know them cold.

Clear ice — why it's the most dangerous

Clear ice (also called glaze ice) earns the "most dangerous" designation for several compounding reasons that go beyond aerodynamic shape alone. AC 91-74B and AC 91-51A are explicit on this point.

The "clean aircraft concept" (AC 91-74B §5.3): Any contamination on the lifting surfaces before takeoff — frost, snow, ice, or slush — must be removed before flight. Even thin frost that you can "see through" disrupts the laminar flow pattern. This is a legal requirement under 14 CFR 91.527(b) for IFR operations, not just an advisory.

Runback icing

Runback icing is a specific and particularly dangerous form of clear ice that forms aft of the leading-edge de-ice protected zone. It is one of the primary mechanisms by which de-iced aircraft still crash due to ice — the crew saw the boots cycling, assumed they were ice-free, and did not know that ice was building aft of where they were looking.

How runback ice forms

When a pneumatic de-ice boot inflates and breaks the ice cap, or when thermal anti-icing heats the leading edge, the ice turns to water. That water flows aft along the wing surface under aerodynamic forces. If the OAT is below freezing and the water travels far enough aft, it reaches unheated, unprotected airfoil surface and refreezes. This deposit is called runback ice or ridge ice.

Why it is especially dangerous

• It forms where no protection exists. De-ice boots cover the leading edge — typically 6–10 inches aft of the stagnation point on each surface. Runback ice forms behind this coverage, where no boot inflation will ever reach it.
• Aft ice is aerodynamically worse than leading-edge ice. A ridge of ice even half an inch tall, located 20–30% chord back from the leading edge, creates a sharp step in the airflow that triggers early separation. FAA research shows that mid-chord ice ridges can cause catastrophic lift loss at angles of attack well below the clean stall AoA.
• It is invisible from the cockpit. Leading-edge ice can sometimes be seen with a flashlight on the wing at night. Runback ice aft of the boot zone cannot be seen in flight without a wing inspection camera.
• Activating boots too early can make it worse. If boots are activated before ice has built to a sufficient thickness (typically at least 0.25–0.5 inches), the thin ice cap may not fully break — and can shatter into a rough layer that adheres to the boot surface. AC 91-74B §5.1.1 recommends allowing ice to accumulate to the certified thickness before activating pneumatic boots.
• SLD dramatically worsens runback. Supercooled large droplets — from freezing drizzle or freezing rain aloft — impinge on the airfoil much further aft than the design droplet sizes for which de-ice equipment is certified. SLD can form runback deposits 18–24 inches aft of the leading edge, far beyond any protected zone. The 1994 ATR-72 accident at Roselawn, Indiana (NTSB/AAR-96/01) is the seminal SLD runback case: boots were cycling normally while SLD built runback ice aft of coverage, ultimately causing loss of control.

The key takeaway on runback icing: Seeing the de-ice system operating normally does not mean the aircraft is ice-free. Runback ice can be accumulating aft of the boots even while the boots are cycling on every interval. If the aircraft is not FIKI-certified and you're in conditions conducive to clear ice or SLD, exit the conditions — don't trust the boots to cover the whole story. (AC 91-74B §4.2)

Tailplane icing — the hidden hazard

The horizontal stabilizer (tailplane) accumulates ice faster than the wing and presents a hazard pattern that is opposite to — and more confusing than — a wing stall. Pilots who understand only wing stalls may respond incorrectly to a tailplane stall and make it fatal. AC 91-74B devotes an entire chapter to this scenario.

Why the tailplane ices faster than the wing

The horizontal stabilizer has a much smaller chord length than the wing — typically one-quarter to one-third the wing chord. The smaller the chord, the thinner the boundary layer, and the more efficiently the surface collects impinging water droplets. Ice accumulates on the tailplane at a higher rate per unit of surface area than on the wing, often at a 2:1 ratio or greater. (AC 91-74B §4.4)

The tailplane as a tail-down force surface

This is the key concept: the horizontal stabilizer does not produce upward lift — it produces a downward force (tail-down force) in most flight regimes. The wing's center of lift is typically aft of the aircraft's center of gravity, which creates a nose-down pitching moment. The tailplane counteracts this by pushing the tail up (i.e., producing a downward force on the tail surface), and to do so the stabilizer operates at a slightly negative angle of attack relative to the free airstream.

This means ice contamination affects the tailplane differently than it affects the wing:

• Wing ice → wing stalls at a lower positive AoA → nose pitches up, then drops as lift is lost
• Tailplane ice → tailplane stalls at a lower negative AoA → tail pitches up → nose pitches down uncontrollably

Symptoms of a tailplane stall

• Sudden, uncommanded pitch-down movement
• Elevator buffet or vibration (easily confused with wing buffet before a wing stall)
• Decreased or ineffective elevator authority
• Abnormal pitch oscillations that cannot be controlled with normal inputs
• Nose pitching down during flap extension on approach — the classic trigger scenario

The symptom trap (AC 91-74B §4.4.2): Elevator buffet before a tailplane stall feels similar to pre-stall buffet from the wing. If a pilot misidentifies it as an impending wing stall, the instinctive response is to push the yoke forward. Pushing forward in a tailplane stall makes it worse. The correct response to tailplane stall is the opposite of the correct response to a wing stall — and the difference depends entirely on understanding the mechanism.

The flaps-and-icing scenario

Flap extension is the most common trigger for a tailplane stall in icing conditions. The mechanism is precise and well-documented, and the rule it generates is one of the most important icing rules a pilot can know: do not extend flaps if you suspect ice contamination on the tailplane.

The mechanism — step by step

1. Ice accumulates on the tailplane during the approach in icing conditions. The pilot may not be aware — the tailplane is not visible from the cockpit, and de-ice boots may be cycling normally without clearing all contamination.
2. Flaps are extended as part of the normal approach checklist. Nothing feels wrong yet.
3. Flap extension increases the wing's nose-down pitching moment. Wing flaps increase camber and shift the center of lift. The resulting nose-down couple requires the tailplane to generate more tail-down force to maintain level pitch.
4. More tail-down force = higher negative angle of attack on the tail. The horizontal stabilizer must deflect further into negative lift to balance the increased pitching moment.
5. If ice has already reduced the tailplane's stall margin, the additional AoA demand from flap extension can push the tail past its stall angle.
6. The tail stalls: the nose pitches violently downward. The horizontal stabilizer is no longer producing sufficient tail-down force, and the wing's pitching moment goes unchecked.

Recovery from tailplane stall

AC 91-74B §4.4.3 specifies the recovery procedure:

• Immediately retract the flaps to the previous (or a lesser) setting. This reduces the wing's pitching moment and lowers the AoA demand on the tail, allowing it to un-stall.
• Do not extend flaps further — this is the instinct that kills: the pilot senses a pitch-down and tries to add flaps to increase lift. This makes the tailplane stall worse.
• Apply nose-up trim and back pressure only after the flaps are retracted, as the aircraft allows.
• Increase airspeed — a higher airspeed reduces the angle of attack on all surfaces, which helps un-stall the tail.
• Do not re-extend flaps until the tailplane has been deiced or the icing environment has been exited.

The rule, stated plainly (AC 91-74B §5.2): Do not extend flaps to full deflection in icing conditions or when tailplane icing is suspected. If the nose pitches down after flap extension, retract the flaps immediately. Do not mistake tail-stall buffet for pre-wing-stall buffet — the recovery is opposite. A pilot who pushes forward or extends more flaps in response to a tail stall will not recover.

Pitot-static icing errors

Structural ice doesn't only degrade the wing — it blocks the small holes your pressure instruments breathe through. Three instruments live on the pitot-static system: the airspeed indicator (ASI), the altimeter, and the vertical speed indicator (VSI). The system has exactly two inputs ice can attack: the pitot tube (a ram opening facing the airflow, plus a small drain hole underneath) and the static port (a flush opening sensing undisturbed ambient pressure). Which hole the ice plugs determines exactly which instruments lie to you, and how.

The key to reasoning through any blockage question is knowing what each instrument compares. The ASI is the only instrument that uses the pitot tube: its diaphragm holds total pressure (dynamic + static) and its case holds static, so the needle shows the difference — dynamic pressure, which is airspeed. The altimeter and VSI use only the static line: the altimeter compares a sealed aneroid wafer against static pressure in the case; the VSI compares current static (inside its diaphragm) against slightly-old static that trickles through a calibrated leak into the case.

What ice does to each instrument

Work every blockage the same way: ask what pressure gets trapped, then ask what each instrument compares. The four cases:

The two ASI behaviors are the testable ones, and they're mirror images. Entire pitot blocked: total pressure is trapped in the diaphragm while case static keeps changing — climb and the case pressure falls, so the needle rises like an altimeter. This is the insidious one: in a climb the ASI reads higher and higher while the airplane may actually be slowing toward a stall. Static blocked: now the case is trapped instead, so the comparison runs backwards — the ASI under-reads in a climb and over-reads in a descent, the "inverse altimeter." Meanwhile the altimeter quietly freezes at the altitude where the ice sealed the port, and the VSI pins to zero no matter what the airplane is doing.

Why only the ram hole? When ice plugs the ram opening but the drain hole stays clear, the trapped pressure leaks out the drain and the line equalizes with ambient — the diaphragm and case match, and the ASI sinks to zero. It takes both holes iced to trap total pressure and turn the ASI into an altimeter. That little drain hole is the difference between the two failure modes.

The escape hatches — and their own errors: Pitot heat prevents most of this; turning it on before entering visible moisture near freezing is the habit. If the static port ices, the alternate static source (vented inside the cabin in most unpressurized aircraft) restores the instruments, but cabin pressure is slightly lower than ambient — so the ASI and altimeter both read slightly high and the VSI shows a momentary climb when you open it. No alternate static? Breaking the VSI glass exposes the static line to cabin air — same effect, but the VSI then reads backwards. (FAA-H-8083-25 Ch. 8)

Reading icing forecasts and products

Several weather products specifically address icing. Knowing what each one tells you — and where it comes from — is a checkride topic and an operational necessity for any instrument pilot.

AIRMET Zulu (AC 00-45H §7.5)

AIRMET Zulu is issued when moderate icing is expected and a freezing-level forecast is always included regardless of icing activity. Key parameters:

• Issued every 6 hours, valid 6 hours, with updates as needed
• Moderate icing: rate of accumulation that would be hazardous if prolonged
• Always contains the freezing level: altitude(s) at which temperature is 0°C
• No AIRMET Zulu does not mean no icing — it means no moderate icing. Trace and light icing are always possible in clouds in freezing temperatures without an advisory

PIREPs (UA / UUA)

Pilot reports are the most operationally current icing information available. Look for the IC field in a PIREP. Intensity codes:

• TRACE: Ice barely perceptible. Accumulation slightly greater than the rate of sublimation. De-icing not required.
• LGT (Light): Rate of accumulation may create a problem if flight is prolonged more than one hour. Occasional de-icing required.
• MOD (Moderate): Rate of accumulation is a problem if prolonged. Short encounters (15–30 min) are manageable; longer encounters require immediate exit or use of de-icing equipment.
• SEV (Severe): Rate of accumulation such that de-icing or anti-icing equipment fails to reduce or control the hazard. Immediate diversion necessary.

A PIREP may also note the ice type (RIME, CLR, MIXED) and whether SLD (supercooled large droplets) were encountered. An SLD PIREP anywhere along your route is an immediate go/no-go flag regardless of the aircraft's FIKI certification, unless specifically certified under 14 CFR Part 25 Appendix O for SLD conditions.

Freezing levels and the Graphical Forecast for Aviation (GFA)

• AIRMET Zulu text (always includes the 0°C isotherm altitude)
• Constant Pressure Charts (winds/temps aloft) — find the 0°C temperature altitude
• FA (Area Forecast) or GFA — graphical freezing level and icing layer forecasts
• Icing Severity and SLD products on aviationweather.gov — overlay icing probability, severity, and SLD by altitude and time

Multiple freezing levels: In warm front environments, a layer of 0°C to +4°C warm air can exist above a colder surface layer. This creates freezing rain aloft — rain that forms above the warm layer, passes through it as liquid, and becomes supercooled below. Clear ice accumulates rapidly from large supercooled rain drops. In the clouds just below a warm front, expect this condition when the sounding shows a warm layer above the surface cold layer. (AC 00-45H §7.5.3; FAA-H-8083-28 §10.3)

Known Icing and legal implications

Under 14 CFR 91.527, a pilot in command operating under IFR may not fly a large or turbine-powered aircraft into known icing conditions unless the aircraft is equipped for flight in known icing. For certificated small aircraft, flying a non-FIKI-certificated aircraft into known icing conditions — meaning conditions where you have received a forecast or PIREP indicating icing — is not permitted. An AIRMET Zulu covering your route makes those conditions "known." The aircraft being certificated for "IFR" and the aircraft being certificated for "flight into known icing" are two entirely different things.

Red flags

Checkride questions you'll actually be asked

The examiner's go-to icing questions for PPL, IR, and commercial orals. The temperature range question is nearly universal.

Would-You-Fly scenario

Educational example only. This scenario is designed to teach the questions a pilot should ask — not to make a decision for any actual flight. Always consult official FAA/NWS sources.

You're an instrument-rated private pilot flying a non-FIKI Cessna 172 on an IFR flight plan. Your route takes you through a cloud layer between 4,000 and 9,000 ft MSL. Current OAT at 6,000 ft (from the FB) is −3°C. The GFA shows light to moderate icing in the layer. AIRMET Zulu is active for your route with the freezing level at 3,500 ft MSL. A PIREP filed 45 minutes ago by a Cessna at 7,000 ft reports light icing, rapidly becoming moderate.

You're 20 nm into the layer, on top is at 9,500 ft (above your aircraft's service ceiling for the weight), and you're in IMC with visible ice beginning to accumulate on the leading edge.

Pilot takeaway