April, 2006

Holy Smokes! These brakes will get your attention!

Today let’s talk about what some of us usually consider to be a boring topic, our aircraft’s brakes. You know, those thingies on the wheels that we usually ignore until they screech at us, or we push the pedal to the metal and there is—nothing there! What got me to thinking about it was an account of an aircraft catching on fire due to the overuse or misuse of its differential braking system. To wit:

NTSB Identification: DFW06LA038 14 CFR Part 91: General Aviation Accident occurred Friday, December 09, 2005 in Houston, TX Aircraft: Cirrus Design Corp. SR22, registration: N302BY Injuries: 3 Uninjured. This is preliminary information, subject to change, and may contain errors. Any errors in this report will be corrected when the final report has been completed. On December 9, 2005, about 1500 central standard time, a single-engine Cirrus Design Corp. SR22 composite airplane, N302BY, was substantially damaged during a fire on the right main landing gear brake following an extended taxi at the David Wayne Hooks Memorial Airport (DWH) near Houston, Texas. The airline transport rated pilot and two passengers were not injured. The airplane was registered to and operated by a private individual. Visual meteorological conditions prevailed and an instrument flight rules (IFR) flight plan was filed for the planned 14 Code of Federal Regulations Part 91 personal flight. The cross-country flight was destined for the Mobile Regional Airport (MOB), near Mobile, Alabama. The pilot reported that the airplane was initially parked facing a westerly direction on Ramp B. After engine startup the pilot performed an approximate 270-degree right turn and taxied south on Taxiway K. About 2,000 feet along Taxiway K the pilot made a right turn onto taxiway E followed by a near immediate left turn onto Taxiway F. Approximately 3,000 feet along Taxiway F the pilot stopped and performed the "Before Takeoff Checklist." Shortly after completing the checklist, and before the pilot resumed taxiing, the tower radioed and reported that the airplane's right main landing gear was on fire. The Cirrus is a beautiful airplane from any angle. It is setting sales records and is very innovative. Note the fully turned castering nosewheel in the above picture, like the Mirage Marathon design. The pilot and his 2 passengers were able to egress the airplane unassisted. The airplane's onboard fire extinguisher was discharged into the fire to no avail. Two mechanics from a nearby hangar were able to extinguish the fire using large capacity fire extinguishers. The 4,100-hour Air Force pilot had previously completed five and a half hours of flight training at a Cirrus certified training center to obtain his SR22/20 Cirrus pilot certification. This training was required in order to qualify as a pilot on the operator's insurance policy. Since receiving this certification the pilot had accumulated about 150 flying hours between the SR20 and SR22 combined. According to photographs provided to a representative of the National Transportation Safety Board, the right main landing gear and lower right wing sustained substantial thermal damage. A review of the accident database revealed several similar events involving brake fires on Cirrus SR20 and SR22 airplanes. (End of NTSB report.)

Cirrus Aircraft has of course commented on this and they are busily trying to make any necessary changes that would help to prevent a recurrence. The following story appeared on the AvWeb site:

Don't Ride The Brakes Cirrus Design plans to issue a Service Bulletin on all its aircraft in response to a spate of brake fires (the most recent of which occurred Dec. 9) that have caused serious damage to at least five aircraft. The SB will call for the installation of color-changing temperature sensors on the brake components so that pilots can tell -- during the preflight -- if the brakes have been previously overheated. Cirrus spokesman Ian Bentley said overheating can cause failure of an O-ring. Failure of the O-ring allows flammable brake fluid to leak onto potentially hot parts. If they're hot enough, the brake fluid ignites and causes a stubborn fire that can really make a mess of a low-wing plastic airplane. The latest such incident occurred at David Wayne Hooks Memorial Airport in Houston and appears to be typical of the others. According to the NTSB preliminary report, the pilot had taxied almost a mile before stopping for his run-up and that's when the tower called him to tell him the right gear was on fire. He and his two passengers got out and tried to put the fire out with the on-board extinguisher, which apparently was not up to the task. A couple of mechanics from a nearby hangar managed to put the fire out with larger extinguishers but not before both the gear and wing were substantially damaged. Improper Brake Use To Blame, Says Cirrus Bentley said Cirrus has studied the incidents extensively and determined there are no design or equipment faults at work, and for Cirrus this leaves only "operator error" as the cause. Unlike many aircraft pilots may be transitioning from, Cirruses have a free castering nosewheel and are steered only with differential braking, plus some positive or negative contribution from the rudder (dependant on relative wind). Bentley said some pilots may have a tendency to overuse the brakes to compensate for excessive power settings or may simply be riding the brakes. Last June, the company e-mailed all its customers an owner service advisory that warned them not to overtax the brakes, which Cirrus also says are more than adequate for an airplane with the performance and weight of a Cirrus. Bentley said that after the first reported brake fire, Cirrus and the brake manufacturer, Parker Hannefin, intentionally overheated sets of brakes and cycled them 19,000 times without a failure. Bentley said the components must be subjected to "significant overheating" to cause a fire. Thanks to the multi-function display installed on most Cirruses, Bentley said they have hard data to show how the overheating occurs. The MFD records engine RPM and speed. If the speed drops and the engine RPM stays the same, the only place that energy can go, according to Cirrus, is into the brakes. (Emphasis added—Ed.) "This isn't speculation. There's a lot of real information around," he said. In at least one case, it appears the plane was taxied with the parking brake on, says Cirrus. (Ed. note—Hey, that sounds like the way I back my car down the driveway!) Company Looks At Prevention First Bentley said Cirrus is doing everything it can to prevent brake-related fires. "We do take these things very seriously," he said. In most cases, he said, the fires have occurred on club or rental aircraft that see a lot of different pilots (the most recent incident was an exception). Cirrus believes the heat sensors will be most beneficial in those cases because they'll give pilots an indication of whether the brakes have been abused by those before them. And, although Cirrus says the factory-installed brakes are a common installation on aircraft of similar size and exceed design standards, the company will be offering -- as retrofit kits only -- higher-capacity brakes. The larger brakes have bigger rotors capable of dissipating more heat, thus reducing the chance of overheating. Bentley said the kits will likely be most popular with fleet operators, who will see some of that investment returned in longer brake life. The beefier brakes will not be offered as an option on new aircraft, but if you tend to taxi in a fully loaded airplane one mile downhill with a blustery quartering tailwind after landing fast, perhaps the retrofit kit is an appropriate investment. (Emphasis added—Ed.) (End of Cirrus response)

Is there a moral to the story for the rest of us?

The question for us then as consumers and as fellow airplane builders (Cirrus is an airplane builder too!) is this: “What similarities are there between the Cirrus and our Celerity and Marathon aircraft, and what can we do as builders and aircraft operators to prevent a similar problem?” The similarities are obvious—castering nosewheel on the tricycle gear Marathon, and also a castering tailwheel on the conventional geared Celerity. Both aircraft need differential braking to some extent in order to maintain directional control on the ground. Another similarity—Both aircraft also feature composite construction, and let’s be honest here, in case of fire, composites can burn more readily than aluminum! So common sense tells me that we should pay attention to the lessons of the Cirrus and learn what we can from their experience. I do not know the design details of the Cirrus, but I am quite sure that their conventional disk brakes are like about 90 percent of other airplane’s brakes out on the flight line. With all brakes that I am aware of, including disk brakes, kinetic energy is changed to heat energy during the act of brake application. Disk brakes appear to me to have the heat energy that they generate concentrated in a substantially smaller area (puck against rotor) than drum brakes (brake shoe against brake drum), and there may also be less steel surface area (rotor versus brake drum) available to dissipate the heat. Therefore we need to look at two things:

How can we minimize the heat generated by our brakes? And,
What can we do to enhance heat dissipation once it has been generated?

A couple of ideas for you to consider

Let’s first address the idea of pilot technique. Cirrus addressed it by saying, in part, that the pilot may be at least partially responsible. I highlighted one of their first conclusions above, which says that:

“If the speed drops and the engine RPM stays the same, the only place that energy can go, according to Cirrus, is into the brakes.”

Man, does that ever make sense! How many times have you seen the prop on somebody else’s airplane (not yours of course!) turning at a fast taxi rate, while the aircraft nose bobs up and down due to continued brake stabbing in order to keep the taxi speed in check? How much more sensible it would be to just pull back on the throttle and add speed as needed, rather than having to continuously reduce speed as necessary! Then there is the matter of the rudder. The rudder should be considered the primary method of directional control for most practical purposes while taxiing, particularly at speeds where the rudder does have some effect. Just don’t depend on the rudder for initiating directional control when you are starting out from a dead stop—that is what directional braking is for! Nevertheless the rudder should be fully deflected as you start the turn from a dead stop so the initial prop blast will assist with initiating the turn. Or better yet, gain a little speed as you taxi forward a bit before kicking in the rudder to make your turn. When you are not turning, drop your feet to the floorboard so they’re not resting on the toe brakes. Pilots will subconsciously apply toe brakes while taxiing even when they know that they should not be used, just because of their foot position telling them that this is okay. Remember, the throttle is primary for speed control, not the brakes, and act accordingly. The second Cirrus conclusion that I highlighted is a somewhat tongue-in-cheek comment on how some people treat brakes. It is intended to just get your attention, so you won’t do the same stupid thing that somebody did.

Prevention through proper design, configuration and installation

There are two things for you to consider here. Number one is that many wheel pant designs may prevent free airflow over the heated parts, especially the very aerodynamic, tight fitting wheel pants. Remember, the brake parts are air cooled just like your engine! Before fabricating your own wheel pants for your Marathon or fixed gear Celerity, look carefully at some production aircraft wheel pants first to see how much air flow their wheel pants permit, as opposed to installing “close cowling” around the tires. As homebuilders we sometimes have too much of a tendency to “improve on the design” of something without considering the possible negative consequences. Another suggestion is that you install only metal tubing to conduct the hydraulic fluid down near the wheels and brakes, not rubber or Tygon® type material.

Inspect, and then re-inspect!

Many pre-flight inspections are much too routine. You know, “Kick the tires, light the fires, and let’s go flying!” Spend a little extra time to look for any signs of overheating, such as bluing of metal parts. And after flying, carefully (cause they’re HOT) feel and smell the parts to see if they’re getting excessively hot. Hopefully this has given you some ideas that could help prevent a potentially catastrophic situation from occurring. Good luck!