Weeping Wings for Singles

Everyone has heard the rules for flying when icing conditions threaten. Here they are:

1. Always avoid icing conditions.
2. Should you begin taking on ice, take immediate steps to leave icing conditions. Take evasive action the moment you notice the first signs of ice.
3. If you absolutely, positively must fly in conditions conducive to icing, be sure your airplane has ice protection equipment.
4. Even if you have ice protection equipment aboard, and your airplane is approved for flight in known icing conditions, rule Number 2 still applies.

While every pilot can agree on the soundness of these rules, those of us who fly general aviation airplanes — particularly light singles — still face big dilemmas in the winter months. Why? Because most general aviation aircraft are not fitted out with ice protection gear. A heated pitot tube is usually the extent of our protection against the dangers of airframe ice.

Yes, there are a few single-engine airplanes with FAA approval to fly in known icing conditions. These include the Piper Malibu and Mirage, the Cessna P210, the Cessna Caravan, the TBM 700, and the Pilatus PC XII. Some piston twins have also earned known-icing approval: the Piper Aerostar; the Cessna 310; and the Beech B-58 Baron, to name a few. But for the vast majority of piston singles — the type of airplane best represented in general aviation — no method exists for preventing or removing airframe, windshield, or propeller ice.

Because of this limitation, strict adherence to rule Number 1 becomes a near impossibility. If we were to cancel our flights every time a briefer mentioned the chance of icing, most pilots in the United States would remain ground-bound for most of the winter and early spring. If we were to avoid every cloud that carried the potential for ice, we'd be endlessly deviating on most days, and we'd be breaking the regulations every time we climbed or descended through a cloud layer.

This state of affairs is changing. Pilots of more and more piston singles now have a fighting chance to safely deal with ice, thanks to Aerospace Systems and Technology's TKS liquid ice protection system. Better known as the "weeping wing," the TKS system works by forcing a glycol-based solution through tiny openings in specially designed leading edge panels. These panels are installed on the wings, horizontal stabilizers, and vertical stabilizers. Some of these panels have laser- drilled holes; others consist of a fine, stainless steel mesh. A windshield spraybar and propeller slinger ring are also included in the package.

The TKS concept is nothing new. Various versions of weeping-wing ice protection systems have been used since World War II on many types of large airplanes. These include the Aerospatiale Corvette, the Shorts Skyvan, the Vickers Viking, the Hawker-Siddeley HS-125, and the Cessna Citation S/II. However, AST is the first to offer TKS' weeping wing technology to the light single market. AST holds Supplemental Type Certificates that allow its system to be installed on the following airplanes:

• Beech Bonanza models 33, 35, 36, A36 (with vortex generators), A36TC, and B36TC.
• Cessna 206 and Turbo 206.
• Cessna 210L (1972-1976 model years), M (1977 and 1978 model years), and N (1979-1984 model years), both normally aspirated and turbocharged.
• Mooney M20J (models 201 and 205), M20K (models 231 and 252), and M20M (TLS).
• Aerospatiale General Aviation TB-20 and TB-21 Trinidads.

The TKS system's efficiency in single-engine applications has been heavily tested and proven effective. In 1981, the previous STC-holder (Kohlman Aviation of Lawrence, Kansas) conducted numerous tests in the NASA Lewis Center's icing wind tunnel. There, using Cessna 206 leading edges, it was learned that the TKS system functioned extremely well in both the anti-ice and de-ice modes of operation. With the glycol mixture flowing through leading edges under conditions simulating both clear and rime icing, no ice accumulations formed. When heavy accretions of ice were allowed to build, they were eliminated within four minutes of turning the equipment on.

Inflight tests in actual icing conditions — and behind a Cessna icing tanker — confirmed the wind tunnel results. A TKS-equipped 206 was even flown at low altitude over swamps to see if insects could plug up any of the leading edges' thousands of .0025-inch diameter holes. The wings stayed bug-free.

Operationally, the TKS is a no-brainer. To use the system as an anti-ice measure, turn the system on prior to entering icing conditions. For de-icing, turn it on the moment you notice ice accretion.

There's a two-position toggle switch (panel switch for the Aerospatiale Trinidads) that controls the glycol flow rate. In the low- flow position, the system's single main pump metes out glycol at 2.5 gallons per hour. This is a flow rate designed to make the system operate in an anti-ice mode. With the switch set to high-flow, fluid is pumped at 5 gph, a rate that will de-ice any buildups from protected areas. A smaller pump works downstream from the main metering pump, but it works as an adjunct. Its job is to maintain adequate fluid flow through the windshield spraybar, and to prime the main metering pump. The system operates at pressures ranging from 30 to 120 psi.

The system's glycol tank varies in capacity according to the airplane. In a Mooney installation, the tank can hold up to six gallons. With the switch set in anti-ice mode, that works out to a 2.5-hour endurance. Trinidads can accept the largest tanks, which are able to hold 7.8 gallons and allow 3.3 hours of anti-ice operation. Bonanzas have 7- gallon tanks, Cessna 206s can carry 7.2 gallons, and Cessna 210s would have 6.3-gallon tanks.

Let's pause right here for some emphasis. It's very important to understand that the installations we've been discussing are *not* approved for flight in known icing conditions. The TKS system is approved for the singles listed earlier by virtue of STCs. This means that the equipment will not adversely affect the performance or flight characteristics of the airplanes in question. It does *not* mean that the systems have proven their ability to comply with the certification requirements regarding performance in icing conditions. The system may work — and does work — very well when it comes to protecting an airframe in icing conditions, but it has not yet earned FAA approval for flight in known icing conditions.

For now, the TKS system should be viewed as an escape tool only — good for buying a little time while you sort out an ice-free alternative plan of action. It may allow you to fly for hours in icing conditions, but not legally.

This may soon change. Officials at Mooney Aircraft Corporation recently indicated a strong interest in pursuing known-icing certification using the TKS system in its TLS and Ovation. With these aircraft, the TKS system will have dual pumps; known-icing certification requires a second pump for redundancy. Mooney already installs the TKS system as an STC'd option on its new aircraft, as well as on other Mooneys approved for the TKS kit.

Until its first known-icing approval in a piston single, the TKS system will continue to serve many of us admirably as a vital tool in dealing with winter's worst weather. It may not be a license to mill around indefinitely in icing conditions (but then again, even the most powerful, certified ice protection systems can't handle the heaviest ice accretions) but it's still a whale of a lot better than having nothing at all.

Compared to conventional pneumatic-boot leading edge protection, the TKS system has many advantages. Topping the list is its ease of operation: You simply turn it on. With inflatable boots, the pilot must be careful not to exercise the boots too often. One-quarter to three-eighths- inch of ice must be allowed to build before inflation. Cycling the boots continuously can allow ice to continue to build as it forms a cap over the air space produced by the expansion of the boots. With the weeping wing, this can never happen.

Another benefit is fluid runback. The glycol solution runs aft of the leading edges, preventing ice buildups on rivet heads, skin joints, or other surface irregularities. In effect, the entire wing is covered with glycol. The solution, according to AS&T, will not damage paint or other exposed surfaces. With inflatable boots, ice can easily form aft of the boots, and create very dangerous accretions.

There's no airspeed penalty, either. Conventional boots usually knock a few knots off cruise speeds.

Still another advantage is the installation itself. Unlike inflatable rubber boots, which deteriorate with age, the TKS system uses leading edge panels made of titanium and/or stainless steel. They'll never rot or need replacement. It's a one-time installation, and a one-time wallet hit.

That hit is a doozy, though, and this brings up some of the TKS system's drawbacks. A TKS kit will set you back anywhere from $16,750 (35- series Bonanzas) to $21,500 (Cessna 206s). That titanium isn't cheap, and neither is the 96 to 175 hours' worth of labor for installation, which adds another $4,000 to $7,000 to the price.

Weight is another problem. The weight of the components runs between 40 and 49 pounds. Add the glycol solution (it weighs 9.2 pounds per gallon) and you've got a 100- to 108-pound crimp in your airplane's useful load.

Glycol solution availability could also be a pain. First of all, it's not a pure ethylene glycol solution. It's a mixture consisting of 85 percent glycol, 5 percent isopropyl alcohol, and 10 percent deionized water. The solution is called AL-5, and it's sold by Canyon Industries of Phoenix. It's available in quantities ranging from 2.5 to 55 gallon, and sells for about $7.80 per gallon. Most large FBOs would have AL-5, but it would still be a good idea to have your own stash.

For some, the weight penalty alone is enough to veto any thoughts of a TKS system. For others, the five-digit price tag and its uncertified status are turnoffs. But for the 100 or so who have bought the system so far, peace of mind is the chief objective. As anyone who's been there knows, a curtailed useful load and big price tag vanish in importance when ice builds fast, airspeed drops off, airports are scarce, and surface weather is rotten. Those are the times when you'd exchange a big check for a clean wing in a heartbeat.

For further information, contact Aerospace Systems and Technologies, Inc., 110 Riverfront Road, Suite C, Lawrence, Kansas 66044; telephone 913/865-5511; fax 913/865-5544.