Ask the Expert: The Break System Deice Option

A rather popular optional break 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” – 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. Thus, brake deice uses “raw,” unregulated P3 air. The temperature of this air depends on both the OAT and, more importantly, the speed of the compressor: N1 or Ng speed.
To use brake deice properly, a few facts must be recognized. First, there is a limitation that tells us not to use it if OAT is above 15° C. Of course, in that warm of a situation brake freezing is of no concern. However, if used, wheel well temperatures can exceed a comfortable, safe value.
Second, one is almost assured of creating undesirably high wheel well temperatures if the system remains on too long with the gear retracted. That is why there is a timer in the control circuit that shuts off the system 10 minutes after the gear is up. The POH directions tell you to turn off the brake deice switch yourself if the system has not automatically terminated operation – evidenced by the proper annunciator light(s) extinguishing –
10 minutes after “Gear Up.”
Third, the bleed air that is tapped off of the Instrument Air line and directed to the brake assembly robs some of the normal “Little P3” flow into the other systems. This results in both interesting and critically important considerations.
At the “interest” level is the fact that at last 85% N1 is required to have enough air to operate both the brake deice option and the wing boots. Furthermore, on the ground, High Idle must be used if brake deice is to be effective. The temperature of the bleed air at Low Idle may not be sufficient to guarantee good brake deicing.
Also, don’t be surprised to see a momentary illumination of left, right or both Bleed Air Fail annunciators when using boots and brake deice together since pneumatic pressure can drop so low as to activate the low pressure sensing switches attached to the failure warning tubes.
At the “critical” level is the effect of brake deice usage on Rudder Boost. The Rudder Boost system on 200s and F90s operates considerably different than that installed on the 300-series, so I need to discuss this in two separate presentations. First, for the 300 and 350 …
In these airplanes, Rudder Boost is a mandatory, no-go system since without it the worst-case, engine-out situation requires the pilot to use more rudder force than the certification rules allow. The force applied by the Rudder Boost system comes from the same servo motor that the autopilot system uses for yaw damping. The force varies, depending upon the magnitude of the power differential between the two engines. This power difference is sensed by propeller torque transducers on the 350s but by raw, Little P3 pressure in the 300s. That leads to a problem.
With brake deice activated, the P3 pressure sensor for the good engine feels less pressure than it should – since air is escaping out of the brake deice manifold – and hence the system applies less rudder force than it should. Therefore, no takeoff or go-around should be initiated with brake deice in use since the pilot would not have the proper help in applying rudder force.
“So, we’ll tell the pilot to turn off brake deice on runway lineup and on final approach, right?” says Mr. Beechcraft engineer. “No!” responds Mr. FAA certifier. “Someday a pilot will forget that important step. You need to make it automatic and fool-proof.”
In response to that directive, Beech made a system change in the model 300 that deactivates brake deice whenever the Autofeather system completes its arming process … as both power levers are well-advanced. So if the crew does not turn off brake deice themselves when taking the runway, it will deactivate automatically when the Autofeather annunciators illuminate. Once the landing gear is retracted and a safe altitude is reached after departure, the Autofeather switch must be turned off before brake deice may be used to warm the main wheel wells to melt any ice that has accumulated there. Likewise, when we need to accomplish our once-a-day exercise of the brake deice valves, even in the summer, we must assure that the Autofeather annunciators are not on … by turning the Autofeather switch off or by retarding power levers. Just remember to reposition the switch back to Arm when the deice valve exercise is finished.
It is a very nice, simple, improvement that the 350 measures differential power via torque, not via P3 pressure. Although with brake deice on, slightly more N1 and ITT will be necessary to create the same torque, the yawing tendency relates to torque differential directly, so brake deice has no effect on the necessary rudder assist that is delivered. Therefore, there is no autofeather-related shutoff of the brake deice system in 350s.
In the case of the 200- and F90-series, Rudder Boost is not a no-go system. (None of the 100-series airplanes have a Rudder Boost system at all.) Although it is standard equipment, it is provided not by FAA directive, but simply to make the airplane somewhat easier to handle in one-engine-inoperative operations. It therefore follows that the brake deice system’s effect on Rudder Boost should be understood, but even if it impacts Rudder Boost negatively, it is not a critical concern.
First, with brake deice robbing air from the engines’ compressors, ITT may be about 20° C higher at a given power setting. If the torque value found on the “Minimum Takeoff Power” graph in the POH cannot be achieved with this additional ITT, then brake deice must be turned off so that Minimum Takeoff Power can be reached.
Second, in this series of King Airs, unlike in the 300 and 350, Rudder Boost is an all-or-nothing proposition, not varying the force based on the difference in engine power. The sensor that initiates the rudder boosting force is a “Delta P” switch tied in to the left and right Little P3 lines, looking at raw, unregulated P3 pressure. With Brake Deice on, since some of that air is being diverted overboard, the Delta P switch will see less of a difference so rudder force will not be applied until more power is added on the “good” side.
Realizing that brake deice usage during takeoff has these negatives associated with it, and knowing that a brake assembly is not going to freeze up while the wheel is turning, I strongly suggest that you always turn off Brake Deice as part of your Runway Lineup procedure on any model King Air you fly.
The actual valve that opens to direct P3 to the brake assembly is a bit of an oddity. Instead of being a simple solenoid valve similar to the Instrument Air Shutoff valve aft of the firewall, this valve is electrically activated but pneumatically operated. The solenoid opens a port that permits P3 to provide the force that actually moves the valve. This means that the valve does not move unless the engine is operating, supplying raw Little P3 pressure.
Long ago, it was found that these valves were prone to not operating when they should: Either not opening when turned on or not closing when turned off. Analysis of the situation convinced Beech that regular exercise was necessary for the valve to remain relatively trouble-free. Now, all of the POH supplements direct us to cycle the valves once each day, regardless of OAT. I encourage you to incorporate that into your checklist procedures as you begin the descent into your home base airport on the last leg of the day. In that manner, if a valve fails to close, (1) it won’t be subject to too high temperatures for too long since you are using descent and approach power settings and (2) the engine will soon be shut down at the hangar, eliminating the concern about hot bleed air continuing to flow.
The F90-, 100- and 200-series contain a single, green advisory annunciator that illuminates to indicate that power is being sent to the brake deice valves. In no way, however, does that annunciator actually confirm proper valve operation. One or both valves could fail to function and yet the light is still there. Hence, when doing your once-a-day, in-flight test, do not merely observe the annunciator. Make certain that both sides show a minor drop in torque and a minor increase in ITT when brake deice is selected on and, perhaps even more important, make sure they return to normal when the system is turned off. You will also observe a small fluctuation in cabin altitude as the bleed air supply is slightly affected during the test.
The 300s and 350s contain separate, left and right, brake deice advisory annunciators and these are triggered by actual valve movement to the open position. Nice! However, I still encourage you to monitor torque and ITT when you do your in-flight test.
I want to make a comment or two about that 10-minute timer that shuts off the system after landing gear retraction. First, the timer does not start unless brake deice is on. In other words, you could be well into the climb or cruise and think, “Dang! I forgot to turn brake deice back on after that winter takeoff to thaw out the wheel wells!” No worries, it will come on now and operate for up to 10 minutes.
On the other hand, suppose you turned on brake deice shortly after takeoff and ran it for, say, six minutes. Now, in the descent for landing, you’ll have four minutes available for preheating the brake assemblies before extending the gear, right? Wrong! Once the timer starts with the gear up, it runs its course completely even if the switch is turned off prior to the end of the 10 minutes. Again, no worries: Once the gear leaves the wheel wells, brake deice will function with no time constraint at all. For what it’s worth, you can also pull and reset the Brake Deice circuit breaker to reset the timer and allow up to another 10 minutes of gear-up brake deice operation.
To conclude, I’ll run through a typical winter usage scenario where there is snow or slush on the ramp and/or taxiways. On starting, I’ll not only take the first engine’s condition lever to High Idle, but the second one as well. Now, after starting, the brake deice switch gets turned on and I check for the annunciator(s). As I begin to taxi, I will pull the condition levers back to Low Idle but if I need to stop in the run-up area or hold short of the runway, I will again select High Idle. Back to Low Idle as I roll onto the runway and I will turn the brake deice switch off now.
Unless an engine fails, I will leave the gear extended for the first 400 or 500 feet after liftoff, remembering to be below the gear retraction speed limit as I finally bring them up. Once I am high enough that an engine power loss would be relatively uneventful – maybe pattern altitude or above – I will now turn Brake Deice back on, check the time, and run it for five minutes or more. If I get distracted, I know it will shut itself off but I’ll eventually back it up by turning the switch off, too.
If the temperature at the destination airport is also close to or below freezing, then I will turn the brake deice switch on either before or right after the gear is extended. I’ll leave it on until performing the shutdown procedure and, if I get any lengthy stops while taxiing, I will remember to select High Idle while stationary. Got it? Good!

If you have a question you’d like Tom to answer, please send it to Editor Kim Blonigen at kblonigen@cox.net.