Resurrecting and improving existing deicing systems
Charles Lindbergh's monoplane, Spirit of St. Louis, didn't need any deicing equipment when he launched on his epic 33-hour, 30-minute cross-ocean flight in May 1927 — of course, Lindbergh had the freedom to fly at any altitude and divert to any heading in seeking ice-free flying conditions. Since ocean-size block altitudes quickly became unavailable, the industry developed deicing equipment. In this era of New Age composite airplanes and integrated see-all, do-all avionics systems, ice-control hardware is also undergoing a revolution.
New ice-fighting technology includes systems that use the power of electromagnetism to jolt ice buildups from airframes and a modern adaptation of the old squirt and sling systems that spread a coating of low-freezing-point liquid that shrugs off performance-robbing ice on the windshield, prop, and airframe. There are even quick-heating tapes that instantly melt ice that has glommed onto airframe parts. The prosaic deicer boot systems, which led the way with their introduction more than 70 years ago, have been upgraded with modern materials and ice-sensing components.
Since 1994, when heavy icing and ineffective deicing systems caused the loss-of-control crash of an ATR-72 commuter airplane near Roselawn, Indiana, industry and government experts have had renewed interest in studying icing. The knowledge gained has resulted in the resurrection and improvement of existing deicing systems, and a sea change in how manufacturers design modern airplanes.
Boots
Deicing boots are glued onto the leading edges of ice-gathering surfaces — wings and horizontal stabilizers always, and vertical stabilizers, wing struts, and cargo pods sometimes. As pressurized air is pumped into chambers in the boots, the outer layer of the boot rapidly expands and changes shape. This cracks the surface of any ice that has formed over the boot. Boots have been in service for decades and they're still being installed because they work.
Installing deicing boots on Douglas DC-3 vintage airplanes was a time-consuming chore — early generation boots were anchored in place with hundreds of small machine screws that penetrated down through the upper and lower trailing edges of the boot material — one mechanic would hook the trailing edge with a J-shape hook and pull aft, and his partner would try to locate the hole and start the screw into the aluminum threads of a Rivnut.
Before long, Rivnuts and screws were replaced with high-powered contact cement — this bonded the boot directly onto the airframe. Physically removing a worn out or damaged boot was a matter of grab and tug — removing the old cement was not so easy. Solvents such as methyl ethyl ketone (MEK) and acetone were used to soften the old cement, but it was always a race as to what would soften first — the mechanic's brain or the glue. Boots are still glued into place, although some of Goodrich's boots are affixed using its patented FASTboot installation system that substitutes a factory-applied pressure sensitive adhesive — this cuts installation time and eliminates downtime related to adhesive cure.
A simple boot system
A typical deicer boot can be visualized by looking at a typewriter or PC keyboard. Each row of keys represents an air chamber that is installed span wise (from wing root to wing tip) with the spaces between the keys representing the attached edges of the chambers. When the boot system control switch is in the Off position, the boots are sucked flat against the wing — this maintains the wing leading-edge profile. When the pilot turns on the boot system an air pressure of approximately 15 psi is routed to the boots, inflating the air chambers.
Goodrich and other companies have developed ice-detection sensors — one model alerts the pilot when ice forms by sensing a change in the vibration frequency of a diaphragm when ice stiffens the diaphragm. Goodrich's SMARTboot has a detector located in the boot itself. These systems promise immediate ice detection and are much more dependable than the old "turn on the light and peek" method.
Icing studies
During studies that were part of the FAA's five-year In-Flight Icing Plan a number of hoary rules of thumb regarding deicer boot operation were discredited.
The first rule to fall by the wayside was that proper boot-operating technique is to wait for a "sufficient ice build-up for efficient ice removal," to quote the 1980s-vintage pilot's operating handbook (POH) of a popular high-performance single-engine airplane. Boot operating cycles were toggled one at a time by the pilot when he or she decided that there was "sufficient" ice buildup.
Boot-type deicing systems should now be activated as soon as ice is detected. The thought behind "wait and activate" was that continuous boot operation would allow an ice bridge to form over the expanded boot profile, leaving the airplane defenseless against ice. The icing studies proved that ice bridging was a myth.
Another discovery from the icing studies was a phenomenon called ice contaminated tailplane stall, or ICTS. A stalled tailplane can prevent recovery from an upset. (See " Wx Watch: Iced-Up Tails," November Pilot.)
Boot maintenance
Goodrich is the pioneer in deicing boot development, and has improved its boot products so much that the company now advertises free labor during routine boot inspection and boot replacement for the life of the aircraft with the purchase of a set of boots. Visit the Web site ( www.deicerboots.goodrich.com) for more information.
A second manufacturer of deicing boots is also beginning to penetrate the market. Ice Shield boots are manufactured by B/E Aerospace of Fenwick, West Virginia ( www.iceshield.com). The Ice Shield online installation and maintenance manual is a good reference source.
Boots take a lot of punishment from bugs, rain, ice, ultraviolet light, sand, and rocks. Since boots are seldom needed during the long hot days of summer, maintenance can be neglected for months. Boots can look good on the outside because of regular washing with mild soap and water followed by applications of Goodrich's boot maintenance products — AgeMaster, ICEX (Ice Shield's equivalent product is Ice Shield Plus), and ShineMaster — and still be in poor operating condition.
Looks can deceive
Owners of booted airplanes must take their airplanes to a shop that has the equipment to functionally test the boot system. Neglect and aging result in system leaks — leakage at the boots because of erosion, deterioration, or pinholes; leakage in hoses because of age and loss of elasticity; and leakage resulting from poorly sealing valves because of carbon contamination are common faults. A set of boots that does not inflate with vigor, or worse, does not inflate with enough power to crack off a lusty coat of ice can turn a mild icing encounter into a finger-crossing, prayerful gamble.
Poorly maintained boot-type deicing systems also contribute to premature vacuum pump failures because of the excess heat that is generated by a slow buildup to, or failure to develop, high enough pressures to actuate cycling switches.
Test kits are essential for detailed boot system troubleshooting. Until recently the most popular test kit available for troubleshooting and maintenance was the Airborne 343 pneumatic system test kit. Airborne Parker Hannifin stopped making dry air pumps in 2002, but it still loans and sells these test kits. In addition, Aerotech Components, makers of the "Clear View" inline pneumatic system filter, has recently introduced its CV-700 test kit, which is reasonably priced and comes with a comprehensive service and troubleshooting manual. For further information visit the Web site ( www.aerotechcomponents.com).
Deicing boot systems can be an expensive (and potentially dangerous) headache unless they're cared for with a regular preventive maintenance program.
Flight into known icing equipment
Many booted aircraft are not approved for flight into known icing (FIKI) conditions. FIKI certification requires additional equipment to ensure redundancy. A general guide for the equipment needed for FIKI certification is as follows. Ice-removal equipment must be installed on the wings and horizontal and vertical stabilizers (there may be further requirements — the Cessna Caravan requires ice-removal gear on the wing lift struts), the propeller(s) must be equipped with a deicing method, there must be a permanently installed method of deicing the pilot's windshield, and there must be a high-heat-capacity pitot tube, a high-heat-capacity stall warning transducer, an ice-detector light (a light dedicated to illuminating the left wing leading edge), control surface static dischargers (often called static wicks), and redundant alternators and pneumatic system sources.
The labor costs required to retrofit the extra plumbing and wiring to convert a non-FIKI airplane into FIKI status are quite high, so few airplanes are ever retrofitted. But it can be done — Flight Ice did it for its fleet of airplanes.
Flight Ice and the TKS ice-protection system
Flight Ice ( www.flightice.com) is a division of Flight Express, a large (92 airplane) courier and air freight company based in Orlando, Florida.
Flight Ice was able to get FIKI certification through the supplemental type certificate (STC) process for three aircraft types — Cessna T-210-series airplanes, Beechcraft Baron-series airplanes, and Cessna Caravan-series airplanes. The deicing method used was the existing Aerospace Systems and Technologies TKS "weeping wing" ice-protection system. The TKS weeping wing system is the same system that AOPA chose to install in the 2001 sweepstakes Beechcraft Bonanza. The exterior components of the TKS system — the leading edges bonded to the stabilizers and wings, the windshield and propeller fluid distribution tubing, and the ice-detection light — are identical for non-FIKI and FIKI systems approved for installation under AS&T's supplemental type certificate (STC). The difference is that the FIKI system has redundancy in the form of backup pumps, filters, and an additional engine-driven alternator.
Cirrus Design offers an adaptation of a TKS system for the Cirrus SR22. This optional system reinforces the Cirrus safety philosophy and has become a must-have item for more than 90 percent of new Cirrus buyers. Termed "TKS Lite" simply because it was certified without some of the parts of the standard non-FIKI system — there's no leading-edge weep system on the vertical stabilizer, no dedicated spray bar or fluid pump for the windshield (fluid flowing aft from the propeller system has proven to be adequate), and the fluid reservoir is smaller than standard. Testing has proven that the Cirrus system conforms to regulatory icing system requirements and will provide up to an hour of protection.
Other manufacturers that have chosen the TKS-style system in whole or in part include Mooney and Adam Aircraft. Visit the Web site ( www.weepingwings.com) for more information.
Shocking the ice off
At least three companies are developing ice-shedding systems that jolt ice off wings and stabilizer surfaces. Goodrich's Electro-Impulse De-Icing System (EIDI), Ice Management Systems' Electro-Expulsive Separation System (EESS), and Cox and Company's Electro-Mechanical Expulsion Deicing System (EMEDS) all shed ice by pulsing high-current electrical flows through opposing conductor coils that are located in close proximity to the control surface leading edges. This creates an expulsive shock that causes the wing skin, or an erosion shield on the wing skin, to flex and vibrate at a high frequency. This has been proven to crack ice into salt-size particles that don't damage the airplane or engines.
Expulsive systems have one unusual characteristic — the expulsive shocks created by the system can be easily heard during a preflight walkaround inspection (see " Turbine Pilot: Little Big Jet," July 2002 Pilot).
The icing studies mentioned above added volumes of knowledge to how in-flight airframe ice accumulation changes flight characteristics — this data has changed the way airplanes are being designed. Prior to the icing study, airframe icing was an afterthought in the airplane design process. Icing certification was looked into after the design was finalized. This has changed. Raytheon's Premier I business jet is the first clean-sheet airplane design that factors icing characteristics into the initial stages of an airplane design. Raytheon chose Cox and Company's EMEDS to protect the Premier.
The Northcoast Technologies Thermawing system
Lancair, builders of the speedy composite Columbia 300, 350, and soon-to-be-certified turbocharged 400, has chosen the Thermawing system by Northcoast Technologies, of Chadron, Ohio, as its deicing tool.
This system uses a special alternator, a solid-state controller/processor, and flexible expanded graphite-foil heating elements that can be applied to any surface that's prone to icing.
During no-ice conditions the Thermawing alternator operates as a second alternator — splitting the aircraft load with the primary alternator.
When sensors detect icing temperatures, the impingement area, or leading edge, of the tape is heated to above freezing, causing a running-wet condition. Ice is melted and runs aft to build up on the "shedding zone."
At regular intervals the controller switches the Thermawing alternator offline, raises its voltage output to 70 volts, and delivers a surge of power to the shedding-zone heating elements. A complete airframe deicing cycle takes between seven and 10 seconds from start to finish.
The low-budget icing tool
Oregon Aircraft Design, of North Plains, Oregon, sells Ice-Away, a product it calls a propeller ice inhibitor. One application stays on the prop for several hours in icing conditions. Keeping the propeller ice-free prevents imbalance-caused vibration, and permits engine power to be fully utilized. Visit the Web site ( www.oregonaircraftdesign.com) for more information.
The Roselawn tragedy provided the push that increased icing knowledge. Only a very few owner-flown single-engine airplanes are equipped for icing encounters. Improved forecasting and increased icing knowledge are the only ice-fighting tools for many pilots — that's why it's critically important to always have a guaranteed escape route when icing conditions exist along your route of flight. It's OK to stay home or run away to warmer air at the first sign of ice. This will ensure you live to fly another day.
E-mail the author at [email protected].
Traveling Boot Cobblers
If winter arrives and you find yourself flying more than you planned — and your deicing boots are so patched and rough looking that deep down inside you know that they're really only along for the ride — the traveling team of deicing boot system experts from Leading Edge Aviation in Oklahoma City may be just what you need.
Under the BootsEdge Repair Corporation ( www.bootsedge.com) flag, a team from the company will travel to your airplane, evaluate the condition of the existing boots, and rejuvenate all repairable boots with its FAA-approved patchless repair scheme.
If the boots are deemed to be beyond repair, the team from Boots-Edge is able to install new boots (Goodrich or Ice Shield) quickly and at a competitive price.
In addition, the BootsEdge team will provide instructions for any additional repairs that are needed to return your deicing boot system to full health.
Contact BootsEdge Repair Corporation at 888/687-0945 or 405/720-6004. — SWE
