The 4 horsemen of landing gear apocalypse!

Flying Safety, August, 2004 by Doug Wiser

The Ogden ALC landing gear product engineering team has an unofficial motto: "We hold up the Air Force." Our small team of system engineers is responsible for the design and performance of most USAF landing gear systems. If you have a landing gear problem, odds are several engineers at Hill AFB have been burning the midnight oil working on it.

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Landing gear systems can be a very challenging business. A quick review of the second to last page of any recent Flying Safety magazine will reveal how challenging. Historically, roughly 20 percent of all non-pilot error mishaps are landing gear-related. In our team of 16 engineers, five lead engineers and one chief engineer, we have advised or served over 100 Class A and/or Class B safety mishap investigations.

Accordingly, we have an unofficial warning that we pound into the skulls of every new landing gear engineer: "Beware the Four Horsemen of Landing Gear Apocalypse." These unpredictable killers are: Hydrogen Embrittlement, Burns, Stress Corrosion Cracking (SCC), and Fatigue. Common and routine maintenance work can invite one of these horsemen into your landing gear!

Most modern landing gear structures are designed from very high-strength, low-alloy steels. The most common alloy is 300M, which can be heat treated to 280,000 PSI tensile strength. At this level, 300M has a 40 percent strength to weight advantage over 7000 series aluminum. This strength level is higher than virtually any other structural metallic alloy on earth (Titanium alloys wimp out at around 180,000 PSI, but are much lighter). Steel is also very stiff and cheap, with fairly common manufacturing processes. Accordingly, landing gear designers looking for a light, cheap, manufactureable material that will make the smallest possible structure love 300M ... We hate it! Along with this great strength come subtle, but serious environmental shortcomings. The ultra-high strength steels are quite vulnerable to three of the four horsemen.

Looking at the landing gear components depicted in photos 1 and 2, which do you think concerns the typical landing gear engineer the most? If you chose photo 1, you are mistaken! A typical OO-ALC-trained engineer will look at the part and think: "Which item is most likely to have been cleaned excessively or with unauthorized fluids?" Why? The most dangerous horseman is Hydrogen Embrittlement (HE). HE is a phenomena where atomic hydrogen atoms permeate the metallic structure, and disrupt the molecular bonds that give the alloy its strength and toughness. If sufficient hydrogen infuses, the material loses virtually all ability to withstand damage: the part may fracture into pieces just sitting on your workbench. The infused hydrogen is not detectable with any non-destructive instrument. If the hydrogen is not forced out of the steel (by heat) within a very short number of hours, it will form metal-hydride bonds that cannot be dissolved without melting the part. Once embrittled, no test/inspection will detect its presence, and you are in a very short race against time to prevent irreversible damage. In short, HE leads to sudden, undetectable, unpredictable failure of your landing gear. Photo 3 illustrates a typical HE failure. This F-16 main landing gear (MLG) failed in flight, with no external loads applied. Lucky circumstances upon the resulting emergency landing prevented a Class A mishap.

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What causes HE in high-strength steel landing gear parts? How about, every @ & # thing! Here's a short list: Water (tap and di-ionized), most cleaners on earth (except a tiny handful), rust strippers, every electroplating solution, hoof and mouth disease disinfectants, soft drinks, etc., etc. When steel landing gear parts are exposed to any fluid except authorized cleaning fluids (such as P-D-680), our engineers are programmed to suspect HE and react accordingly.

During routine depot repair, many operations are embrittling. Great care is taken to ensure any such process is followed by a 23-hour, 375[degrees] F. bake within the prescribed time window. Our landing gear depot facility is designed with process controls aimed precisely at this requirement. Several are computer-controlled and monitored, so no human error can accidentally bypass the required processes. In addition, we run test coupons continually to test our solutions and processes for HE (photo 4). It's a daily, hourly, constant battle; one that has been lost occasionally (power failures cause a missed bake that may lead to condemnation of vital and expensive components). In the early 1990's, every F-XYZ aircraft at base ABC was grounded for about a week due to HE of wheel tie bolts. An overzealous wheel shop troop found that dipping high strength steel wheel tie bolts in rust stripper sure cleaned them up nice and shiny! Shortly thereafter, several dozen bolts were failing on aircraft and in the inflation cage. The USAF has suffered fatalities due to tie bolt failures; but we were lucky that time. A quick grounding of several dozen aircraft and replacement of all suspect bolts solved the crisis. The suspect bolts were easy to identify; they were all brilliantly shiny!