November 2016 Issue

Braking Action

Since your airplane’s brakes likely aren’t nearly as robust as those on the SUV you drove to the airport, maybe you shouldn’t depend on them so much.

Ever looked closely at the brakes on a personal airplane? The basic system isn’t much different than what’s on your car, motorcycle or SUV. Typically, you’re looking at a six- or eight-inch wheel assembly, with a brake disc of about the same diameter. The tires, of course, usually are much smaller than your car’s. Stopping power comes when hydraulic pressure is applied to the attached caliper, which then squeezes its brake pads against the disc, generating friction and slowing its rotation. That hydraulic pressure, of course, is applied and moderated by the pilot, usually by pressing on the top of the rudder pedals.

Cirrus SR22

According to the NTSB, the pilot of the Cirrus SR22 pictured here “taxied about 3.7 miles with the engine operating between 1200 and 1600 rpm.” The airplane's POH advises that if a 1000-rpm taxi power limit and proper braking techniques are not employed, the brake system may overheat, which could result in a brake fire.

While both your airplane and your car’s brakes likely employ a disc-based design, that’s about all they have in common. The brakes on your car, for example, easily can be twice the diameter, adding additional stopping power. It’s likely the calipers and pads are larger, too, containing more fluid to exert pressure over a wider portion of the disc. It’s also likely your car’s brake system is power-assisted and incorporates an anti-lock feature. Meanwhile, there’s a brake on each of the four wheels, not just two, and those wheel/tire combinations may be significantly wider than on your airplane. The bottom line is your airplane may weigh about the same as your car but has far less braking capability. Oh, and the airplane and its tire/brake assemblies often see pretty much the same speeds as the car when it’s on the ground, at least temporarily. So why are you driving around the airport like your Cherokee is a Chevy?

Taxiing 101

In fact, it’s not at all uncommon to see someone taxiing around an airport at an unnecessarily high speed. The larger the airport—and the longer the taxi route—the more likely pilots are to let their airplane build up speed until it’s time to slow down and turn into a ramp or taxiway. And even on a crowded ramp, you’ll occasionally see someone steaming around at the speed of heat. It’s all so wasteful. And risky.

While it’s uncommon to find much data on taxi-related accidents in the NTSB’s data—because such events usually don’t result in substantial damage or serious injuries—they do happen. The image below is what remained of a Cirrus SR22 after its pilot failed “to use the proper taxi procedure,” according to the NTSB, “which resulted in a brake system fire.” That event occurred on July 10, 2013. The two images above are from events similar in nature, at least to the extent the pilot lost control of the airplane while taxiing.

We’ll go out on a stout limb here and make this blanket statement: Short of a mechanical failure or an encounter with a larger aircraft’s wake, there’s really no excuse to run into something while taxiing. And when we do, it’s likely we either weren’t paying attention or were rolling too fast to stop when something got in our way. And taxiing too fast is relatively easy to do.

aircraft brake fire

aircraft brake fire

We don’t know all there is to know about the events depicted in these two images—the one at right occurred in 2006 at KHEF while the one above was in 2007 at KLAL—but it’s likely a moment’s inattentiveness while taxiing is at least partly responsible for both of them. We’d also guess more power was used than proved necessary, especially given the outcomes. Taxiing more slowly, while using the brakes sparingly, will go a long way toward preventing these kinds of events.

Say we’re taxiing to the opposite end of a 6000-foot-long runway. That’s a nautical mile, and if we taxied at 10 knots as measured by the panel-mounted GPS navigator, iPad EFB app or some other gadget, it would take us six minutes to accomplish the feat. For comparison, 10 knots is a fast jog for most humans, who generally walk at no more than three knots. But you’re in a hurry, so double that taxi speed to 20 knots. It’ll take you three minutes to taxi to the other end of the runway. You’ll save three whole minutes by taxiing at 20 knots instead of the less-risky, more-sedate 10 knots; less if the runway is shorter, which they often are. Are you really in that big a hurry?

Brake Systems

The schematic on page 10 details the major brake system components of a Cirrus piston single. As the accompanying description details, they’re fairly simple, lightweight and reliable systems. But there are some considerations we should keep in mind.

First of all is ensuring the system has adequate hydraulic fluid in it. The fluid typically used in general aviation airplanes complies with military specification MIL-H-5606, most emphatically not DOT 3, 4 or 5 fluid, as you might use in a car. Often referred to simply as “5606,” this fluid specification has been with us for some time. According to Royal Dutch Shell, “Since the 1940s, MIL-H-5606 hydraulic fluid, a mineral oil-based fluid, has been one of the most widely used types of fluid. This hydraulic fluid has provided excellent operational properties over the temperature range of –54 deg. C to 135 deg. C (–65 deg. F to 275 deg. F). A major deficiency of MIL-H-5606 fluids, which was recognized early in its use, was its high degree of flammability.” We’ll come back to brake fluid’s flammability in a moment.

Your pre-flight inspection is when you’ll determine the airplane has the right brake fluid in it, and that it’s of sufficient quantity. You’ll start by examining the brakes themselves, ensuring their security, that the pads have adequate material, that the discs are not rusty, cracked or broken, and that there’s no fluid on the ground near them. The brake lines themselves, which often are a combination of hard, metal tubing and flexible hoses, should be in good shape, also, with no abrasions, sharp bends or kinks.

Checking the quantity of 5606 fluid aboard can be a simple task, depending on the airplane, or involve two people and cowling removal. The airplane’s POH should detail where the fluid reservoir is located, how to access it and how to determine if it needs replenishing. It also should specify what fluid is required (usually it’s 5606, but there can be exceptions, especially among LSAs and experimentals).

Checking the brakes for operation and effectiveness really can’t be done until the engine is running and you’re ready to taxi. You’ve likely been holding them as you complete cockpit checklists and other tasks, so when you’re ready to taxi from the airplane’s parking spot, release the brakes. As the airplane begins to roll, gently but firmly apply the brakes again. You obviously should come to a complete stop. If not, if one brake seems softer than the other, or if the airplane is pirouetting on the ramp, shut down and seek a qualified technician’s assistance.

airplane brake systems

The typical personal airplane’s brake system closely resembles this schematic, adopted from a Cirrus Pilot’s Operating Handbook:
• Four master cylinders connected to the rudder pedals modulate the hydraulic pressure applied to each wheel via fluid in the system, which is replenished at the reservoir.
• A parking brake also is employed, and usually works by retaining system pressure when the rudder pedals are released.
• Depending on the airplane and its brake system, each caliper assembly may include only one piston, which applies pressure to the brake disc via the pads.
• More pistons/pads are desirable for maximum brake effectiveness, but soon designers can get into weight and complexity issues.

Using Your Brakes

Mooney right main landing gear

A Mooney’s right main landing gear and brake are shown. Item A is the brake disc—this one’s a little rusty (it’s obvious the airplane hasn’t flown in a while). Item B is the brake caliper (this one’s a two-piston system). Item C is the brake line itself. Item D is where you look for leaking brake fluid.

Presuming your brakes pass the test, you’re ready to taxi. How you taxi depends on whether your airplane has a steerable nose or tailwheel. Without a steerable small wheel on the airplane, e.g., if you have a castering nosewheel, you’ll be depending on differential braking to stay on the taxiway centerline’s yellow brick road. While there’s nothing fundamentally wrong with the castering nosewheel design, it does require greater brake use than the steerable kind.

That’s because the rudder simply isn’t effective for directional control on the ground at normal taxi speeds. Especially on crowded ramps or when conditions do not include smooth pavement and calm winds, you may find yourself using a lot of brake to control where the airplane goes. And that’s where 5606 hydraulic fluid’s flammability enter the picture.

aircraft taxiing procedure

Brake Fires

aircraft brake fire

aircraft brake fire

In the first few years after customer deliveries began, the Cirrus SR20/22 community became aware of a series of brake fires. Some of them were minor events; some were not. An FAA Airworthiness Directive (AD 2006-21-03) was issued, “intended to detect, correct, and prevent overheating damage to the brake caliper piston O-ring seals, which could result in leakage of brake hydraulic fluid. Consequently, this could lead to the loss of braking with loss of airplane directional control or brake fire.”

One component of the AD was an update to the airplanes’ operating handbooks, an excerpt from which is above right. The thing is...all that update does is recommend good operating practice, some things we already should be doing during normal operations. But we gotta save that three minutes.

Cirrus models are not the only ones posing a risk of brake fires. The two images at right are of a Piper Cherokee’s right wing after a brake caught fire. According to the NTSB’s probable cause finding, the fire resulted from the “pilot’s application of excessive brake pressure to maintain a safe taxi speed during a long, slightly downhill taxi with a strong quartering tailwind, which resulted in a brake fire.”

Slow Down

In our experience, a personal airplane’s brake system works best when it’s not being used. Yes, you may need to tap the brakes to make your desired runway turnoff. Yes, if you fly something lacking a steerable small wheel, you may find yourself needing to use brakes all the time to maintain directional control.

But our brakes are relatively fragile when compared with a modern automobile, and brake-related accidents are much more likely when we put stress on them. The simplest, easiest and cheapest way to do avoid stressing out our brakes is to reduce our taxi speed. A good strategy might be 10 and 10: No more than 10 knots taxi speed, and no more than 10 times 100 rpm for a power setting. Respect your brakes, and they’ll respect you.