Flying in Icing Conditions with TurboProps

In-flight icing is a serious hazard. By disturbing the smooth flow of air on the airplane, icing will increase drag, decrease the ability of an airfoil to produce lift and degrade control authority.

One of my first bad weather encountering with a Pilatus, which no has a real De-Icing system

Ice protection systems on transport category airplane generally operate on one of two principles:

1. De-icing 
2. Anti-icing

Anti-icing systems are designed to keep the protected areas of a surface entirely free of ice during an icing encounter. Anti-icing systems can be evaporative or "running wet". On a running wet anti-icing system, liquid water may run back to colder areas behind the protected areas and re-freeze - commonly referred to as "run back" ice. 

Note that runback ice can serve as accretion sites for additional accumulations in an extended icing encounter, and these accumulations may exceed the amount and position of ice demonstrated during certification.

Melting ice particles can also be a source for runback ice on surfaces protected by thermal anti-ice systems or can result from aerodynamic heating on the leading edge for an airplane operating at high airspeeds. If the temperature at the stagnation point on the leading edge is high enough, impinging ice particles may melt upon impact and then run back to colder areas behind the protected areas and refreeze.

De-icing systems are designed to remove ice after it has begun to accumulate on the airplane. Because some residual ice continues to adhere between system cycles, the surface is never entirely aerodynamically "clean". 

The Turbo-prop transport airplanes generally employ pneumatic boots to de-ice the airplane wings and tail surfaces which allow for the collection of ice between inflation cycles of the boots to remove the accumulation.

This accumulation of ice consists of "inter-cycle" ice which is the ice that collects on the surface between boot inflation cycles and "residual" ice which is the ice that is not completely removed after the boot inflates.

It was believed to be necessary to delay operation of pneumatic boots until a significant accumulation of ice was noted on the airframe - in some cases it was common practice to wait until the airplane had accumulated 1/4 to 1/2 inch of ice. This was intended to prevent the phenomenon of "ice bridging" where ice would continue to collect while the boot was inflated. It has been demonstrated in flight test trials that Transport Category airplane pneumatic boot designs certified to current requirements, are not susceptible to ice bridging and may be turned on and allowed to cycle even before the first signs of ice accumulation are evident.

Environmental conditions may exist in which the ice accretion rate or extent of the ice formation exceeds the performance capabilities of the airplane.  In such cases, the performance and flying characteristics of the airplane may not be maintained e.g the airspeed in cruise or the vertical speed in climb cannot be controlled and keeps decreasing. The flight crew must follow the AFM specific guidance and leave those conditions immediately.

As a general rule and based on experience, such environmental conditions have limited vertical extension. By descending several hundred or a few thousand feet, it is generally possible to exit such conditions.

Performing a descent combines three positive effects:

1. It allows the flight crew to trade altitude for airspeed increase while reducing the angle of attack
2. The outside temperature will generally increase and
3. The engines will provide more power.

The first effect of ice accretion on the airplane consists of an increase in drag. In turn, this will result in either a loss of climb rate in a constant airspeed climb or a loss of airspeed in level flight with a given power (or during any attempt to climb at a constant vertical speed or fixed pitch attitude). Aircraft as ATR has an APM (Aircraft Perfomance Monitor) to detect anomalies indirectly due to ice.

The lower the airspeed, such as in a climb or in a hold, the faster and more detrimental the performance decrease can be, which highlights the importance of early identification of airplane performance loss.

In addition to the effect of ice accumulation on the airframe, ice on the propeller blades will interfere with the aerodynamics of the blades reducing their efficiency and thereby reducing available thrust leading to a further reduction in climb rate or airspeed.

A degree of propeller performance can be regained by increasing the propeller speed setting.

1. First, this causes the propeller blades to operate at a lower angle of attack relative to the on-coming airflow which can increase thrust. 
2. Second, higher propeller speed may improve shedding of the ice from the propeller due to higher centrifugal forces on the blades.

Even with ice protection systems operating properly, inter-cycle and residual ice accretion on the airplane may also significantly reduce the maximum lift available. The airplane may stall at higher speeds and lower angles of attack than normal. For istance, aircraft as ATR with antice activated. the avionic system sets the stall threshold higher to provide modified minimum airspeeds in icing conditions to ensure that the same flying qualities and margin above stall as with a clean airplane are maintained when flying the airplane with the ice protection systems operating in the certified icing envelope.

When icing conditions are forecasted, the effect on airplane performance must be anticipated both in flight planning and during the flight. Flight crews must anticipate the impact on airplane performance and target flight levels for the cruise which will ensure adequate margins above minimum icing speeds. The activation angle for stall warning is then decreased by a proportionate amount to provide the necessary margin above the predicted stall through the activation of an "icing mode".

With ice accumulated on airplane lifting surfaces, an airplane may exhibit stall onset characteristics before stall warning activation. Low speed cues such as buffet or instability in roll could likely precede an impending stall and must be interpreted as approaching the stalling angle of attack even if it occurs before stall warning devices have activated.

A consequence of this function, however, is that the airplane must be flown at higher minimum airspeeds regardless of whether the airplane has contamination on the leading edges or not in order to avoid inadvertent activation of stall warning.

MONITORING OF AIRCRAFT PERFORMANCE IN ICING CONDITIONS

In-service events have evidenced that many upset situations where associated with a lack of flight crew active monitoring linked to aircraft performance degradation.

These situations mainly occurred during operations in icing conditions where the aircraft had been flown at or below minimum icing speeds without recovery actions until the aircraft stalled or became unstable in roll.

In aircraft like ATR (see this post in italian), with no su much overpower/perfomance, the philosophy  is that active monitoring and aircraft performance expectations, in terms of climb performance and cruise parameters, are key to prevent any undersirde airplane state and to detect and to recover early enough any aircraft performance degradation.

The monitoring of the rate of climb (in climb) or of the airspeed (in cruise) should be tight to allow detection of icing conditions that may not be obvious from a visual standpoint (such as clear ice accretion for instance).

In climb, the AP/FD must be used in IAS mode that maintains the aircraft speed by adjusting the pitch , which impacts the rate of climb. Any other vertical mode (pitch hold, V/S) is prohibited.

The ATR recommends anticipating the entry into icing conditions: if the airplane is not in icing conditions yet but approaches icing conditions (for instance, a cloud layer above and/or TAT progressively decreasing), the target climb speed should be increased and the anti-icing systems engaged before actually entering icing conditions.

The flight crew should monitor the rate of climb to identify any possible loss of performance. At any time above Minimum Safe Altitude (MSA), a decrease in performance can lead the flight crew to choose a cruise level below the initial target.

Given the factors that influence the rate of climb (weight, temperature, turbulence, etc.), it may not be easy for the flight crew to detect a climb performance lower than normal. However the operational ceilings are defined when the rate of climb reaches a threshold of:

- 300 Ft/min in normal condition

- 100 Ft/min in icing condition

Since they are computed with a lower residual rate of climb in icing conditions, the operational ceiling values may be higher than in normal conditions. In such case, the operational ceiling is the lowest of the two.

Therefore if the climb rate decreases under 300 ft/min, it means that the aircraft is reaching its operational ceiling and a level off should be considered. At the latest when climb rate reached 100 ft/min or less, a level off will certainly not be enough to regain airspeed and the severe icing procedure has to be applied.

Any ice accretion will generate an increase in drag and a decrease in airspeed. Whatever the severity of ice accretion, there will still be a loss due to ice on unprotected areas (e.g. radome, wipers, spinners, ...). In most situations, the use of anti and de-icing systems will be enough to limit the loss of performance and it may even be almost transparent.

As soon as a loss of airspeed is identified, the flight crew should monitor that it stabilizes. If the airspeed keeps decreasing, the flight crew should take all necessary actions to maintain airspeed increased (normally at least + 10 kts).

It is recommended, if icing conditions are likely on the planned route, to choose a flight level that provides a cruise speed at least 30 kt above the minimum icing speed (for aircraft like ATR).

To regain or maintain airspeed, a first action can be to increase the rotation speed of the propellers (set to 100%) that helps de-icing the blades. If this is not enough and the IAS continues to decrease, the flight crew should prepare a descent strategy (MSA, escape route,...).

Therefore the only course of action would be to descend and wait for the speed to stabilize. If icing environment does not improve, further descent would be initiated.

In any case if the airspeed cannot be maintained, the flight crew shall immediately apply the severe icing procedure and initiate a descent.

My personal experience with a PC6 in Italy

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