Aircraft Systems Related to Instrument Flight Rules (IFR) Operations

Anti-ice prevents ice from accumulating in the first place — you turn it on before entering icing conditions. Pitot heat, heated props, and windshield heat are anti-ice.

Deice removes ice after it has formed — pneumatic boots are the classic example, cycled once ice has accreted (IFH glossary).

• Clear (glaze) — glossy, transparent, formed by the relatively slow freezing of supercooled water. Favored by temperatures near freezing, large droplets, high liquid water content, and higher speeds. Dense and hard; large accretions can form horns.
• Rime — rough, milky, opaque; instantaneous freezing traps air pockets, so it's porous and brittle. Favored by low temperatures, small droplets, low speeds.
• Mixed — clear and rime together on the same surface. Shape and roughness matter more aerodynamically than type (IFH ch. 4).

When flying in visible moisture — rain or cloud droplets — with the outside air temperature between about +2 °C and –10 °C (IFH ch. 11). Monitor the OAT anytime you're in visible moisture. Small or narrow objects collect ice first, which is why a wing strut or other small protuberance works as an "ice evidence probe" — and why the tailplane, thinner than the wing, is a better ice collector than the surface you can see (IFH ch. 4).

It reshapes the airfoil:

• Max lift coefficient down — a 30 percent reduction is not unusual; large horn accretions can cost 40–50 percent (IFH ch. 4)
• Drag up 100 percent or more — 200 percent or higher with horns
• The wing stalls at a lower angle of attack and higher speed — ice that was benign in cruise can bite when you slow and pitch up on approach
• As little as one-half inch of leading-edge ice can cut lifting power by 50 percent on some aircraft (IFH ch. 11)
• Severe icing can trigger roll upset — airflow separation self-deflects the ailerons (IFH ch. 4)

• Trace — ice perceptible; accumulation slightly exceeds sublimation; equipment not needed unless the exposure exceeds about an hour
• Light — a problem only if prolonged (over 1 hour); occasional use of equipment removes or prevents it
• Moderate — even short encounters are potentially hazardous; use equipment or divert
• Severe — equipment fails to reduce or control the hazard; immediate diversion is necessary (IFH ch. 11)

Induction icing chokes off air to the engine rather than lift from the wing. The classic form is carburetor ice: moist air cooling through the venturi can form ice on the venturi walls and throttle plate at ambient temperatures between 20 °F and 70 °F (–7 °C to 21 °C) — no visible moisture required. The remedy is carburetor heat, which routes exhaust-heated air to melt the ice (IFH ch. 4). Fuel-injected engines use an alternate air source for a blocked intake.

Two immediate options: leave the area of precipitation or change altitude to above-freezing air — and remember the warmer air may be above you, which is why you noted the freezing level in preflight. If neither works, plan an immediate landing at the nearest suitable airport. Report the icing to ATC — aircraft type plus intensity — and request new routing or altitude (IFH ch. 11).

No. A FIKI airplane is safe only in the conditions evaluated during certification — and no aircraft is evaluated for every condition. Severe icing is outside the certification envelope by definition, and supercooled large drops (SLD) — droplets larger than 50 microns, found in and below clouds — are conditions no aircraft is certificated for. Ice protection equipment "gives a pilot more time to get out of the icing conditions" — it is not designed for indefinite exposure (IFH ch. 4, ch. 11).

• Pitot tube and static ports — the pressure instruments start lying, a serious hazard in IMC (failure modes covered under Task II.B)
• Stall warning systems — the vane or switch can freeze, and even a working warning may be useless because the iced wing stalls at a lower AOA than the system expects
• Windshield — you may break out at minimums unable to see forward
• Antennas — small and unprotected, they ice quickly, vibrate, distort signals, and can break off
• Propeller — ice on the blades costs thrust like ice on a wing costs lift (IFH ch. 4)

• Airframe (wings, empennage leading edges) — rubber pneumatic boots, inflated by vacuum/pressure pump discharge air; an oil separator keeps pump oil from deteriorating the rubber (IFH ch. 5). Boots are deice: cycle them after accretion.
• Propeller — normally anti-iced rather than deiced, so ice is never shed into the engine or airframe (IFH ch. 4)
• Windshield — an electrically heated plate giving a band of clear vision, or a spray bar feeding deicing fluid (IFH ch. 4)
• Pitot tube — electric heat; verify preflight with the ammeter or the POH method (IFH ch. 5)
• Intake/fuel — carburetor heat or alternate air for induction ice (IFH ch. 4); follow the POH for fuel-system provisions

Extending flaps (or increasing speed) increases the tailplane's negative angle of attack — the tail's job is to push down, balancing the nose-down moment of a CG ahead of the center of pressure. With ice on the tail, that added push can exceed the contaminated tail's reduced stall margin: the tail stalls, the downforce vanishes, and the nose pitches down, classically just after selecting full or partial flaps on approach (IFH ch. 4).

Symptoms:

• Elevator control pulsing, oscillations, or vibration
• Abnormal nose-down trim change
• Reduced or lost elevator effectiveness
• Sudden uncommanded nose-down pitch (IFH ch. 4)

Recovery — nearly the opposite of a wing stall:

• Immediately retract flaps to the previous setting and apply nose-up elevator
• Increase airspeed appropriately for the reduced flap setting
• Apply power for configuration and conditions — but note high power can aggravate the condition in some designs; follow the manufacturer
• Make nose-down pitch changes slowly
• If boots are installed, cycle them several times

A tailplane stall can occur below VFE, and the condition worsens with speed at the same flap setting (IFH ch. 4).

Ice can partially block or limit control surfaces, making control movements ineffective, and severe icing can cause roll upset — airflow separation ahead of the ailerons induces self-deflection and degraded roll handling, possibly without the usual accumulation cues or a perceived stall (IFH ch. 4). That's why FIKI aircraft POHs publish minimum speeds for icing conditions, and why your preflight controls-free-and-correct check and a look at hinge lines and gaps for contamination matter more before an IFR departure.

Most GA autopilots control two axes — roll and pitch. They come in two sensing designs: position-based, where an attitude gyro senses displacement from a reference (wings level, a pitch attitude, a heading), and rate-based, where the turn-and-bank sensor feeds rate information (position change over time) — the S-TEC line is the classic rate-based example (IFH ch. 5). A flight director computes and displays the same steering commands on the attitude indicator for you (or the coupled autopilot) to follow. Know which type you have, every mode you'll use, and every way to disconnect it — from the POH/AFM, not folklore.

• Severe turbulence or an inadvertent thunderstorm encounter — disengage altitude-hold and speed-hold modes; chasing altitude and speed increases maneuvering and structural stress. Fly attitude and accept altitude/airspeed excursions (IFH ch. 11)
• When you've stopped monitoring it — over-reliance turns the automation from a supplementary aid into your only source of control, and a mode confusion or servo failure then goes unnoticed (IFH ch. 11)
• When hand-flying proficiency is the thing eroding — currency on the gauges is part of your risk management

Use it deliberately: brief the modes, verify each annunciation, and stay ready to hand-fly.