Ground Icing: Risks - Contamination Penalties

During takeoff and climb-out, your aircraft is at a low altitude, high angle of attack, near maximum thrust, and at high drag with gear and flaps/slats out. This challenges, but is well within, your clean aircraft’s flight envelope. Adding even a thin layer of frost, particularly at the wing leading edge, however, can push you closer to the edge of the envelope. For too many pilots, contamination has pushed them over the edge.

AAIB investigator describes Challenger 604 accident in Birmingham, England (Jan 2002)

Related Information

Video: Probable cause - Challenger 604

Phillip D. Gilmartin, AAIB Investigator - Challenger 604 Accident

Accident - Birmingham, England

BIRMINGHAM, ENGLAND
JANUARY 4, 2002
CHALLENGER 604

Immediately after takeoff from Runway 15 at Birmingham International Airport the aircraft began a rapid left roll, which continued despite the prompt application of full opposite aileron and rudder. The left winglet contacted the runway shoulder, the outboard part of the left wing detached and the aircraft struck the ground inverted, structurally separating the forward fuselage. Fuel released from ruptured tanks ignited and the wreckage slid to a halt on fire; the Airport Fire Service was in attendance less than 1 minute later. The accident was not survivable.

Numerous possible causes for the uncontrolled roll were identified but all except one were eliminated. It was concluded that the roll had resulted from the left wing stalling at an abnormally low angle of attack due to flow disturbance resulting from frost contamination of the wing. A relatively small degree of wing surface roughness had a major adverse effect on the wing stall characteristics and the stall protection system was ineffective in this situation. Possible asymmetric de-icing by the Auxiliary Power Unit (APU) exhaust gas during pre-flight preparations may have worsened the wingdrop tendency.

N90AG's pilots should have been aware of wing frost during pre-flight preparations but the aircraft was not de-iced and the ice detector system would not have alerted them. It was considered that the judgement and concentration of both pilots may have been impaired by the combined effects of a nonprescription drug, jet-lag and fatigue.

Possible contributory factors were; the inadequate warnings on the drug packaging, Federal Aviation Administration (FAA) guidance material suggesting that polished wing frost was acceptable and melting of the frost on the right wing by the APU exhaust gas.

The investigation identified the following causal factors:

The crew did not ensure that N90AG's wings were clear of frost prior to takeoff.
Reduction of the wing stall angle of attack, due to the surface roughness associated with frost contamination, to below that at which the stall protection system was effective.
Possible impairment of crew performance by the combined effects of a non-prescription drug, jetlag and fatigue.

Aerodynamic Effects – Wing

Even small amounts of frozen contaminants (frost, ice, snow and slush) can change the shape and therefore air flow over aircraft surfaces.

Contamination on the wing can reduce perfor-mance by increasing drag and reducing maximum lift. This means the stall angle will decrease, probably so that the aircraft experiences stall onset symptoms before the stall warning device (horn or stick shaker) activates.

The disruption in air flow will also alter the flight characteristics, and can lead to a roll upset or pitch upset.

Further Information

Contaminated Wing Performance Data

Even small amounts of frost, ice or snow contamination similar to medium or coarse sandpaper on the leading edge and forward upper surface of a wing, can reduce maximum wing lift by as much as 30% and increase lift-induced drag by 40%. The changes in lift and drag significantly increase stall speed, reduce controllability and alter aircraft flight characteristics.

If you fly a modern aircraft with a “supercritical” or “laminar flow” wing, you really need to make sure that any kind of contamination – be it frost, ice, bugs or dirt – is removed from your wings.

Small amounts of frozen contamination can reduce maximum wing lift (Note: Airfoil is vertically oriented)

Aircraft with frozen contamination on wing

Jet aircraft with frozen contamination (frost) on wing

Frozen contaminants on the wing can cause both performance and handling problems.

Aerodynamic Effects – Tail

The lower surface of the horizontal tailplane is as aerodynamically critical as the upper surface of the wing. Contamination near the leading edge of the tailplane’s lower surface can cause a pitch upset.

Contamination in tail-elevator gap could restrict your ability to maneuver the aircraft.

Further Information

Role of the Horizontal Stabilizer

In most conventional aircraft, the horizontal tailplane generates lift in the downward direction. This counteracts the nose-down pitch moment generated between the weight and the wing lift. For more on the aerodynamics of the horizontal stabilizer, refer to A Pilot’s Guide to In-flight Icing.

Vertical forces and pitching moments

Frozen contaminants on tail leading edge

Frozen contaminants on the tail can cause a pitch upset

Engines

Frozen contaminants blocking engine inlets can cause a power loss or engine failure during takeoff. Frozen contaminants blocking the intake manifolds or filters may affect engine performance as well.

Illustration of ice build-up within the carburetor

Frozen contaminants ingested into the engine can cause foreign object damage (FOD) to turboprop and jet engines.

Further Information

Blocked Turbine Engine Probe

Some turbine engines use sensing probes forward of the compressor section to measure various pressures and temperatures at the engine inlet. These pressures/temperatures are compared to similar measurements at other points both within and aft of the engine. The comparison of the inlet pressures/temperatures to those downstream provides engine indications and, in some cases, governs the control of the engine (e.g., setting engine power).

In extreme environmental conditions, it is possible for frozen contaminants to deposit on the inlet probes. This may happen if the anti-icing system is not activated or malfunctions in very heavy precipitation conditions. Blockage of these probes would affect engine indications and performance. Monitor RPM and N1 to crosscheck these parameters.

Engine with frozen contamination coating the exterior and inlet.

Carburetor Ice

Piston engine carburetor induction systems are prone to developing ice in the venturi if the ambient temperature is at freezing point or slightly higher and the air is humid. Carburetor heat can be selected to prevent or remove the ice while on the ground and in flight, but carburetor heat should never be used during take-off.

Illustration of ice build-up within the carburetor. (from "Pilot's Handbook of Aeronautical Knowledge" - FAA-H-8083-25)

Frozen contamination on wing, prop and inlet

Frozen contamination in engine inlets or on propellers can interfere with their performance.

Instrument Effects

Frozen contaminants can also interfere with your airspeed, altitude and AOA instruments, and cause erroneous displays. If these readings are in question, reject the takeoff. If you are already past the decision speed, fly pitch attitude and power.

Further Information

Blocked Pitot Tube

If the airspeed indicator reads zero during the takeoff roll, the pitot tube is blocked. If you do not reject the takeoff, but continue to climb-out, the airspeed indicator will appear to function shortly after liftoff, but will give you misleading information. If the static ports are not blocked, the indicated airspeed will increase with altitude, not airspeed. As the airplane climbs, the indicated airspeed eventually will exceed the actual airspeed.

Do not be tricked into increasing the pitch attitude and/or reducing thrust - these could cause a perfectly flying airplane to stall.

Frozen contamination can block the pitot probe and cause it to provide erroneous or misleading information.

Blocked Static Probe

If the static ports are sealed, the airspeed indicator will appear to operate correctly during the takeoff roll. After liftoff, however, the altimeter and VSI will not show a climb. The altimeter will continue to read at field elevation.

The effect on indicated airspeed is less obvious. Because of the pressure trapped in the static lines, the difference between the pressure sensed at the pitot line and the pressure sensed at the static line will be less than the actual difference between the dynamic and static pressures. If the airplane actually climbs at a constant speed, the indicated airspeed will decay.

If you rely on the airspeed indicator, you may reduce the pitch attitude to maintain the erroneous airspeed, possibly causing the airplane to exceed its airspeed limitations.