Saturday, December 22, 2007


Aircraft wings and tails have forever been the collectors of enough ice to make them quit working like wings and tails. All it really takes is visible moisture, a cloud for example, and temperatures that approach freezing. The movement of the metal surfaces through the air is often enough to lower the temperature below freezing so even outside air temperatures above freezing will allow enough ice to form to give plenty of aerodynamic trouble.

Aircraft manufacturers have historically used de-ice devices to deal with ice, at least enough of it to satisfy the FAA’s paltry requirements to certify an aircraft for flight into “known icing conditions”. De-ice devices concede that the manufacturer will allow the aircraft to accumulate ice before activation is supposed to shed the ice, hence the term de-ice.

De-icing devices are typically rubber boots that inflate to break the ice off. The inflation pressures are supplied by an engine driven vacuum pump, bleed air taken from the compressor section of turbine engines or separate pumps electrically driven. Engine driven vacuum pumps have proven to be prone to failure just when they are needed most because of heat, sudden demands that fail their internal carbon blades, general deterioration or contamination just to name a few of the many reasons for un-annunciated sudden failure. Bleed air drawn from the compressors of little turbine engines is often not enough to completely inflate the boots, the air can often becomes contaminated with moisture and causes ice to form in the inflation tubes, and bleed air drawn from the compressor means less air for the engine to develop the necessary power to climb out of icing conditions or even to provide cabin heat.

Electrically driven air pumps is clearly the better idea because the pumps are single purpose and can supply full inflation pressures regardless of aircraft altitude, cabin heat requirements or moisture.

All of these systems share the same problem. They are designed to remove ice allowed to accumulate rather than preventing its accumulation. It is the accumulation of ice that has proven to be the undoing of many pilots and their aircraft because the manufacturers have hidden a very important fact. They don’t work effectively in the icing conditions that these airplanes regularly fly in and that most pilots think are safe to fly in. None of the federal agencies have taken strong action but there has been talk for decades about the problem. Nonetheless people continue to die each icing season.

Most deicing boots cover less of the airfoil than is required to remove dangerous accumulations of ice. Most deicing boots are impacted by the lack of adequate inflation pressure especially at altitude or with cabin heat on, and many deicing boots will accumulate moisture that will affect the amount of inflation. Couple this with some very unforgiving airfoils that have sharp stall characteristics and the use of deicing boots for known icing certification can be disastrous.

Federal authorities have been uniform in blaming pilots for icing accidents. Clearly these blame merchants are either not pilots, or have never flown in the clouds in the winter. Icing is unpredictable and even when forecast occurs only forty percent of the time. To blame a pilot for a crash because he didn’t predict weather even the National Weather Service can’t get right seems unfair since for the most part pilots are unaware of the limitations of their deicing equipment as manufacturers have not been honest about the limitations.


Neither the testing nor the equipment was ever designed to permit continuous flight in moderate icing conditions. The regulations require it but the manufacturers do not test for it and the aircraft will not handle it. Not a believer, look at the accident reports. They are replete with pilots who cannot believe their multimillion dollar aircraft can’t handle continuous moderate icing, a question they no doubt carry to their deaths.

Even the manufacturers can’t agree with either each other or the Government about when to de-ice. Some say wait until three quarters to one inch of rime ice accumulates before activating the boots. Others say, no, activate boots at the first sign of ice. Some say wait for less clear ice to accumulate before activation and others say activate the boots when such ice is anticipated. Some say don’t activate the boots on approach because it will slow the aircraft by ten knots or so and other say use the boots continuously. Some say ice bridging occurs if you use the boots too soon and too frequently and others say that’s an old pilot’s tale, dead old pilot no doubt.

Some NACA five digit airfoils, widely used in general aviation and some smaller turboprop commuter aircraft grow ice aft of the boots just because of their angle of attack in flight. Some will accumulate ice aft of the boots at the highest point of lift at twelve percent cord and others will react violently especially when the ice accumulates on the horizontal tail. Some airplanes will suffer an ice-induced tail stall for which recovery virtually no pilots have been trained. It is opposite normal stall recovery and may not be recoverable at all.
The manufacturers of big airplanes, transport category airplanes, have long recognized that the use of deicing boots is not a safe solution for the demands of air carriers who may have to fly in ice for a long time. Years ago they abandoned de-icing boots in favor of anti-ice systems. As one famous aeronautical engineer from a well known manufacturer has said: “You could not expect de-icing boots to effectively remove ice from an aircraft that had to fly from Paris to New York much of it in icing conditions so we heated the leading edges of the surfaces instead so ice wouldn’t accumulate.”

Anti-ice as it is called is the only safe way to keep modern aircraft safe in icing conditions. Heating the leading edges of the aerodynamic surfaces is the best way. In turbine aircraft, bleed air from the compressor of the engine is routed though the leading edges. It heats stainless steel strips and they will not allow the accumulation of ice. This requires lots of bleed air and that robs the engines of power and increases fuel consumption. It requires much more power than is necessary for the flight itself and typically is found in larger more powerful aircraft although it is also used in regional jets and many executive jet aircraft but sadly not all. Requiring more power means more expense to buy, greater expense to operate but greater safety is the prize. Other anti-icing options include, weeping wings which bleed glycol or other anti-icing fluids through tiny holes in a mesh leading edge, and electrically heated leading edge devices. The electrically heated leading edge devices will become more and more prolific once low weight high power electrical generators are introduced currently being developed for the newest transport category airliners.

Fortunately for jet operators, much of their flying time is above the weather including icing conditions but as many of the very light jets compete for scarce airspace and air traffic delays due to bad weather become more common, these low powered mostly de-icing equipped jets will suffer from accidents due to the limitations of this equipment. The propeller driven piston powered airplanes are simply doomed to suffer accidents in icing conditions because “certified for flight into known icing conditions” is a cruel hoax for which they are clearly ill-equipped. Small turbine propeller driven aircraft are equally hexed because their tiny engines just don’t have sufficient bleed air to do the job to inflate the boots under the most demanding of flight conditions.

The answer is straightforward. First, the Federal authorities must get their acts together and make a sensible realistic definition of “known icing conditions”. Second, the Federal authorities must ensure that manufacturers comply not only with the letter of the law but also the spirit of the law. If a manufacturer anticipates as it should that a “known icing” certified aircraft will be flown in lots of different icing conditions then it must ensure the aircraft will do it safely. Today that is not so. Third, the Federal authorities must mandate that all aircraft with a “known icing” certification be equipped with anti-icing equipment sufficient to prevent the accumulation of ice and that all power plants have sufficient reserve power to effectively operate this equipment. Fourth, the Federal authorities must carefully review prior known icing certifications and monitor new ones to ensure not that the aircraft meets the letter of the law but that it will be safe to fly in all reasonably anticipatable icing conditions. Under no circumstances should Federal authorities be allowing manufacturers to rewrite their flight manuals after certification to accommodate the reality of accidents in airplanes that never met the requirements in the first place.

Safe flight in icing conditions can’t be the luck of the draw, it must be totally predictable, repeatable and without chance. The only thing manufacturers have control over is to design and build in the capability of an aircraft to safely fly in icing conditions. A higher authority has control over the existence of and the severity of icing conditions that are likely to be experienced. Given man’s control over the former and his lack of control over the latter it is incumbent upon him, and well within the technology, to ensure that emergence safely from the latter is guaranteed.

Icing season 2007
Arthur Alan Wolk