“Tired Metal” and its Effect on Components
“Tired metal” or using its more correct name “metal fatigue” is something I don’t think pilots have a good understanding of, in fact, I’m not sure the metallurgists really understand why it occurs. Here is the dictionary definition: “Metal fatigue is the progressive and localized structural damage that occurs when metal is subjected to loading and unloading, called cycles.” If the loads are above a certain threshold or the cycles continue to mount, a crack will begin to form at the molecular level of the metal and grow until eventually the metal fails from fatigue. When metal fails from fatigue it does not do so in a slow kind of ripping or tearing fashion. Instead, the metal just fractures as a piece of glass broken over one’s knee. The failure of this aluminum crank arm is a good example.
The first recorded account of metal fatigue was in Germany by William Albert in 1828 involving the failure of an iron mine hoist chain. Every time the hoist lifted a load of iron ore out of the mine it constituted a cycle, the cycles built up and the hoist failed from fatigue. In 1842, King Louis-Phillippe of France held a big celebration at the Palace of Versailles and provided a special train to bring Paris’s and probably France’s most important people out to the palace. On the return trip the locomotive’s axle failed from fatigue killing 55 of the country’s most important people. As one can imagine, this was a huge accident for the day. Aviation has certainly had its share of metal fatigue accidents. Some older pilots might remember two de Havilland Comet passenger jets that broke up in midair within months of each other in 1954. Interestingly, the fatigue was caused by the repeated pressurization and de-pressurization of the cabin, the effects of which were not readily understood by the engineering community at the time. Or, in 1968, the rotor blade failure on a Los Angeles Airways Skorsiky S-61 during a passenger flight from LAX to the Disneyland Heliport. 18 passengers and three crew were killed. The crash also killed the airline which never flew again. One of the more bizarre metal fatigue accidents occurred in 1988 to an Aloha Airlines B-737. The aircraft had 89,090 takeoff and landing cycles, second most in the world at the time. In cruise flight at 24,000 feet a large portion of the forward upper cabin failed from fatigue and was ripped off the fuselage. 90 people were onboard but, remarkably, there was only one fatally, a flight attendant who was not seat belted as the pilot made an emergency descent and landing.
Determining the fatigue life-limit on components
Depending on the model (R22, R44 or R66), there are 15 to 21 airframe “fatigue life- limited components” which are listed in the front of the aircraft logbook and in the appropriate maintenance manual. For example, the main rotor blade’s life limit, sometimes called the “fatigue life” or “service life”, is 2200 hours, which means when that hour limit is met the blade is no good anymore and cannot be rebuilt, overhauled or in any way reused. Chopped up into small sections to be used as training aids is the best use of run out rotor blades I’ve seen.
A stress/strain analysis is used to determine a component’s fatigue life. Stress is defined as an object’s internal resisting force and is determined by the load on an object or part divided by the part’s cross-section area. Whereas, strain is the amount of deformation or mechanical movement that occurs when a load is placed on a part. Two factors must be determined to define a part’s fatigue life. First, how much load the part is exposed to during flight and second, how much load can the part withstand until it fails from fatigue. The in-flight loads are determined by an in-flight survey. A strain gauge is a device used to measure the amount of deformation or strain at a specific place on a part. It is a small metallic foil pattern that converts the mechanical movement into an electrical signal. As the strain on an object changes the electrical resistance in the foil pattern also changes.
When these strain gauges are connected in an array called a Wheatstone bridge (not to be confused with the rock band of the same name, seriously!) the change in resistance and the amount of strain can be measured and recorded. As you can see the strain gauges are very small, approximately five square millimeters, and are placed at numerous locations on one of the helicopter’s components, such as a rotor blade. The aircraft is then flown in literally hundreds of different conditions (i.e. at different bank angles, airspeeds, weights, power settings, centers of gravity, in cruise, climbs, descents, power on and power off) recording the strain on, in this example, the rotor blade. Once this survey is completed on the blade, the engineers know the amount of strain or load the blade encounters during flight.
Next, the amount of load or the length of time at a given load the blade can endure until failure is determined. This is done during a ground test in a machine designed to duplicate the highest in-flight loads on the blade. The machine runs 24/7 until the metal fails from fatigue. Once the engineers know how much load the blade is exposed to and how much load the blade can withstand until failure, the fatigue life can be calculated, 2200 hours in the case of the main rotor blade.
Why is this important for pilots?
There certainly are margins built into the fatigue life calculation, so it does not mean that the blade will fail at, say 2205 hours. However, pilots need to understand that the way they fly the helicopter dramatically affects the amount of loads the aircraft has to endure. Three areas the pilot has full control of that can greatly increase the in-flight loads are weight, speed and power. Exceeding the maximum gross weight limit, flying above the VNE, pulling too much manifold pressure and especially combinations of the three substantially increase these loads and could cut into a part’s fatigue limit causing a premature failure. This is especially true of the combination of excessive speed and power which is why both the R44 and R66 have airspeed limits (R44 -100 kts/R66 - 65 kts) at power settings above maximum continuous power (MCP). Excessive power (manifold pressure) by itself will increase the loads but not significantly. I would not be concerned if, at a hover, I saw my manifold pressure was an inch or two above my limit (Who has not been in this situation before?). Now, that’s not a license to ignore the manifold pressure limits, but it’s fairly common, so just reduce the power and continue to march.
There was a fatal R22 accident in the Central American country of El Salvador a few years ago that was a direct result of the way the pilots flew the helicopter. Two pilots, who owned a construction company in El Salvador, bought a Beta II to fly to their various construction sites. Since avgas is not readily available throughout the country, they always filled both tanks. Each pilot weighted over 100 kgs (220 lbs) so a simple calculation shows they were way over the gross weight limit (100 lbs over). Time was important to these two successful business men so they would use all the horsepower the engine could produce (remember it’s a Beta II so 180 hp is available at sea level), to fly as fast as they could, to get around to their different construction sites. By consistently exceeding the weight, speed and power limits one of the rotor blades failed from fatigue at less than 1000 hours–that’s less than half of the fatigue life. Certainly, this is an extreme case but, nevertheless, an example of the pilot’s effect on metal fatigue. Limits are there for a reason–respect them!