Commercial and Business Aviation Advisory Circular (CBAAC) No. 0147


No. 0147


Airborne Icing Update


This Commercial and Business Aviation Advisory Circular (CBAAC) provides air operators with new information concerning operations in airborne icing conditions that has resulted from the investigation into the fatal accident of a Comair EMB-120 in January 1997.


U.S. National Transportation Safety Board (NTSB) Abstract of Final Report, Comair Flight 3272, EMB120RT N265CA, Monroe MI, January 9,1997.


In its report on the Comair accident, the NTSB has made many recommendations to the Federal Aviation Administration (FAA) based on information that was gathered during its investigation. The recommendations that concern ice bridging, the effect of residual ice, operational use of pneumatic de-icing boots, and monitoring of the auto-pilot in icing conditions contain information that will be of immediate benefit to the pilots of aeroplanes fitted with pneumatic de-icing boots and/or auto-pilots. The information will aid pilots to make better informed decisions when faced with operations in airborne icing conditions.


Several generations of pilots operating aeroplanes with pneumatic de-icing boots have been cautioned against the dangers of ice bridging. Pilots were -- and are -- advised against activation of the pneumatic de-icing boots before sufficient ice has built up on the leading edge -- generally between ¼ and 1 inch -- out of concern that the ice would form the shape of the inflated boot, resulting in the boot inflating and deflating under a shell of ice, making de-icing impossible. Despite the widespread belief in this phenomenon within the pilot community and its coverage in numerous technical publications, its existence cannot be substantiated, either technically or anecdotally. At a recent conference held in Cleveland to investigate ice bridging, the major manufacturers of pneumatic de-icing boots reported that they had been unable to reproduce ice bridging under any laboratory/wind tunnel conditions, and that any operational report of ice bridging investigated by them had been determined to be a report of residual ice.

Residual ice is the ice remaining on a pneumatic de-icing boot surface after an inflation cycle. Wind tunnel tests have shown that a higher percentage of the ice on a pneumatic de-icing boot breaks away if the ice is allowed to build up to ¼ to 1 inch prior to the pneumatic de-icing boot activation. Even in this case, some ice may adhere to the boot after inflation, and be removed after a subsequent boot cycle. If, however, the boots are inflated with a thin layer of ice on the boot surface, as little as 40% of the ice may be removed during the inflation cycle. This is not ice bridging, but residual ice. When pneumatic de-icing boots with an automatic cycle are selected on with a thin layer of ice on the boots, typically some residual ice will remain on the boots after the first and second inflation/deflation cycles, but be totally cleared following the third or fourth cycle. If the pneumatic de-icing boots are left on automatic the clearing pattern will repeat every third or fourth cycle. To repeat, the ice remaining on the pneumatic de-icing boots under such circumstances is not evidence of ice bridging; it is evidence of residual ice.


Any contamination on a wing leading edge will degrade performance. The degree of degradation depends on many factors, and can be quite dramatic on modern high performance airfoils at the low end of the speed range. While the effect on performance of what might appear to be insignificant amounts of residual ice may not be noticeable to the flight crew between boot cycles at cruising speed, it could seriously degrade performance as speed decreases, for example, while slowing the aircraft to configure for landing or in the landing flare, resulting in stalls at low altitude or unexpectedly hard landings. For this reason pilots should respect guidance in the aircraft flight manual (AFM) concerning the minimum airspeeds to be maintained in icing conditions, and ensure that there is no residual ice on the pneumatic de-icing boots prior to landing by cycling the boots passing the outer marker if Instrument Flight Rules (IFR), or at some convenient time on final if operating Visual Flight Rules (VFR).


Pilots of aeroplanes fitted with pneumatic de-icing boots will find direction on operational use of the boots in the AFM. In most cases the AFM will direct pilots to delay operation of the pneumatic de-icing boots, either in the manual mode or automatic mode (if fitted), until ¼ to 1 inch of ice has built up on the leading edge. As pointed out above, this guidance is almost universally included to prevent the occurrence of ice bridging. In its report on the fatal accident of a Comair EMB-120 in January 1997, the NTSB has concluded that a small amount of rough ice had built up on the wing as the aircraft slowed to configure for an approach, but this small amount was sufficient to cause the aircraft to stall without warning as speed decreased. As a result, the NTSB recommends that, for modern turboprop aeroplanes:

"…leading edge deicing boots should be activated as soon as the aeroplane enters icing conditions because ice bridging is not a concern in such aeroplanes and thin amounts of rough ice can be extremely hazardous."

Unless specifically prohibited by the AFM, it is recommended that pilots of turboprop aeroplanes equipped with pneumatic de-icing boots with an automatic cycle select the boots on automatic as soon as the aeroplane enters icing conditions. The pneumatic de-icing boots should be left on until the aeroplane has departed the icing conditions. If the automatic cycle has a FAST/SLOW option, the FAST option should be selected in moderate or severe icing conditions.


When the autopilot is utilized in icing conditions, it can mask changes in performance due to the aerodynamic effects of icing that would otherwise be detected by the pilot if the aeroplane were being hand flown. It is highly recommended that pilots disengage the autopilot and hand fly the aircraft when operating in icing conditions. If this is not desirable for safety reasons, such as high cockpit workload or single-pilot operations, pilots should monitor the autopilot closely. This can be accomplished by frequently disengaging the autopilot while holding the control wheel firmly. The pilot should then be able to feel any trim changes and be better able to assess the effect of any ice accumulation on the performance of the aeroplane.


The contents of this CBAAC will be included in the next revision of the guidance material for air operator airborne icing training programs.

M.R. Preuss
Commercial & Business Aviation

Commercial & Business Aviation Advisory Circulars (CBAAC) are intended to provide information and guidance regarding operational matters. A CBAAC may describe an acceptable, but not the only, means of demonstrating compliance with existing regulations. CBAACs in and of themselves do not change, create any additional, authorize changes in, or permit deviations from regulatory requirements.

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