Page 16 - June 2015 Volume 9, Number 6

Ask the Expert
The Brake Deice Option
by Tom Clements
Arather popular optional system that you will find installed in lots of King Airs is Brake Deice. This system is only offered on King Airs with dual main landing gear wheels, so you will never find it on a member of the 90-series, with the exception of the F90 group, since they, like their bigger 100-, 200- and 300-series brothers, have dual main wheels.
Since it is optional (not standard) equipment, the brake deice description, along with its operational Limitations and Procedures, will usually be found in the Supplements section of the POH rather than in the Systems Description section.
The winter-time phenomenon of frozen, locked brakes was never an issue in the early days of King Airs since they had rather large, single, main gear wheels and tires. It appears that any slush that the tire throws upward while taxiing, taking off, or landing, misses the brake assembly. Soon after the first model 99s and 100s appeared in the late 1960s, Beech began receiving reports of locked brakes due to ice accumulation. The smaller dual wheels and tires appeared to throw slush up quite readily onto the brake assemblies. If the brakes were warm due to friction from usage, the slush would turn to water, run into the assembly, and then freeze solid if and when the assembly temperature dropped below freezing ... such as when stopped in the run-up area or on the ramp, with cold OATs and no frictional heating taking place any longer since the tire was not rolling.
Somehow, the test pilots in Wichita (a place that can have some very cold winter temperatures!) were lucky enough to have avoided this experience during the certification flight testing of the airplanes and initially the reports were attributed to poor piloting technique, not to a design deficiency. “Well, the pilot shouldn’t have used the brakes so much to melt the slush in the first place!” Or, “He was asking for trouble when he set the parking brake!” Or, “The pilot should have sprayed some deicer fluid – isopropyl alcohol – on the assemblies before he taxied out!”
Then one winter day on the ramp outside the factory’s delivery center, Mrs. Olive Ann Beech herself and her companions were inconvenienced by a lengthy time delay when it was found that her A100 had succumbed to the locked brake scenario. It was Standard Operating Procedure at the factory to always have a “safety pilot”
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– mechanic, actually – in the cockpit monitoring the brakes during the towing process. (Once or twice a tow bar had come loose and the affected airplane rolled along with no back-up means of control, so the cockpit rider became the order of the day.) Perhaps the safety rider in Mrs. Beech’s A100 was unconsciously riding the brakes? No one will ever know for sure, but the fact is that now the chairwoman of the board herself was inconvenienced by the frozen brakes phenomenon. Is it any surprise that the deice option got designed, certified, and available within less than a year?
As you well know, a small percentage of the air that leaves the PT6’s compressor flows into lines that direct this “bleed air” into the cabin. The larger line sends its bleed air to the Flow Control Package, and from there, to the cabin’s environmental systems for pressurization and heating purposes. This branch is called “Environmental” bleed air and is also referred to by the slangy term “Big P3.”
The smaller line – known as “Instrument Air” or “Pneumatic Air” or “Little P3” – sends its air into a regulator where its pressure gets reduced to about 18 psi above ambient pressure and this is what the Pneumatic Pressure gauge in the cockpit is reading.
Before the Little P3 line leaves the wheel well area, those airplanes with the brake deice option connect a tap off to it and this branch goes to a solenoid control valve and through a flexible hose on the main gear strut down to the distributor manifold around the brake assembly.
JUNE 2015