The Savvy Aviator #53: The Dark Side of Maintenance
What Makes High-Time Engines Fail?I first started thinking seriously about MIFs about a year ago, when I was corresponding with Nathan Ulrich, Ph.D. -- a brilliant mechanical engineer, inventor, entrepreneur and Bonanza owner (but please don't hold that against him) -- about the causes of catastrophic piston-aircraft engine failures, with particular emphasis on high-time engines operated beyond TBO. Dr. Ulrich did some fascinating research on this subject by analyzing five years' worth of NTSB accident/incident data. I've reported on some of his findings in previous columns. Dr. Ulrich's analysis of NTSB data proves conclusively what I've long believed to be true: By far the highest risk of catastrophic engine failure occurs when the engine is young -- during the first two years and 200 hours after initial manufacture, rebuild or overhaul -- due to what we refer to as "infant-mortality failures" involving defects in materials and/or workmanship in assembling the engine. (Since replacing, rebuilding or overhauling the engine is a maintenance task, such infant-mortality failures are MIFs.) Unfortunately, the NTSB data was of little statistical value in analyzing the failure risk of high-time engines that are beyond TBO, simply because so few engines are permitted to operate beyond TBO; most are arbitrarily euthanized when they reach TBO. We don't even have good statistics about how many engines are flying beyond TBO, but we're pretty sure that it's a relatively small number. Consequently, it should come as no surprise that the NTSB data contains very few accidents attributed to failure of an over-TBO engine. Because there are so few NTSB reports concerning accidents attributed to over-TBO engine failure, Dr. Ulrich and I decided to examine all of them during the five-year period -- 2001 through 2005 -- to see if we could detect some pattern of what made these high-time engines fail catastrophically in flight. Sure enough, we did detect a pattern. About half of the accidents that the NTSB attributed to engine failure did not report engine time. Of the ones that did report engine time, only a relative handful reported times over TBO. And of those, about half reported that the reason for the engine failure could not be determined by investigators. But here's the fascinating part: Of the ones where the cause could be determined, about 80% were maintenance-induced failures! In other words, the engine failed not because it was beyond TBO, but because a mechanic worked on the engine and screwed something up!
How Often Do MIFs Occur?MIFs happen with astonishing frequency. In fact, hardly a day goes by that I don't receive an email or read a forum post in which a frustrated aircraft owner is complaining about some aircraft problem that is obviously a MIF. Recently, for example, I was contacted by the owner of a 1974 Cessna 182P. He explained that several months ago he'd put the plane in the shop for a routine oil change and installation of an STC'd exhaust fairing. A couple of months later, he decided to have a JPI EDM-700 digital engine monitor installed. The new engine monitor revealed that the right bank of cylinders (#1, #3 and #5) all had very high CHTs ... well above 400 degrees F. This had not shown up on the standard factory CHT gauge because its probe was installed on cylinder #2. (One good reason that every piston-powered aircraft should have a digital engine monitor.) At the next annual inspection (done by a different A&P), the inspector discovered some induction-airbox seals missing, which the owner is convinced were left off when the exhaust fairing was installed. The missing seals were installed during the annual, and CHTs returned to normal. Sure sounds MIFfy, doesn't it? Unfortunately, the problem was not caught and corrected early enough to prevent serious, heat-related damage to the right-bank cylinders. All three jugs had compressions down in the 30s with leakage past the rings and a borescope inspection revealed visible damage to the cylinder bores. Oil consumption increased from one quart in 12 hours to one quart in 2 hours, and the oil in the sump started turning jet black within 10 hours after an oil change. The owner is now faced with replacing three cylinders, and since he has no way of proving that the first A&P left out the airbox seals, he's on the hook for the cost of the three jugs -- probably around $5,000 including labor. Immediately after I replied to this Skylane owner, I spotted a post on the COPA forums by the owner of an older, pre-glass-cockpit Cirrus SR22 who was complaining about intermittent heading errors on his Sandel SN3308 electronic HSI. The owner indicated that these problems started occurring intermittently about three years ago when he had his Service Center pull the instrument for a scheduled 200-hour projection lamp replacement. Coincidence? You be the judge. This is a problem I know a little bit about, because I've seen it occur in my Sandel-equipped Cessna 310. (I was a very early adopter of the SN3308 EHSI, and have been flying behind one for more than 10 years now.) This problem is invariably due to inadequate engagement between the electrical connectors on the back of the SN3308 instrument and the mating connectors at the back of the instrument's mounting tray. Unless you are extremely careful to slide the instrument into the tray just as deeply as humanly possible before tightening the clamp screws, you've set the stage for flakey electrical problems that can cause a whole host of problems with the EHSI display, including heading errors. I can almost guarantee that the mechanic or technician who pulled the SN3308 out of the panel of that Cirrus to change the lamp was not familiar with this problem and failed to get the connectors fully engaged when he reinstalled the instrument. Apparently, the poor Cirrus owner has been suffering the consequences for three years. Chalk up another MIF! Not long after reading that post, I was back to the Cessna Pilots Association Web site and saw a post by the owner of a Cessna 340 who departed into actual IMC on the first flight after maintenance (not a very bright thing to do, IMHO), and discovered that all three of his static instruments -- airspeed, altimeter, VSI -- stopped working as the aircraft climbed through 3,000 feet. Switching to the alternate static source did not cure the problem. Fortunately for all on board, this particular Cessna 340 was equipped with duplicate co-pilot instruments (which have their own separate pitot and static sources), and those continued to work so the pilot was able to keep the dirty side down. Turned out that a mechanic who last worked on the airplane had disconnected a static line in the cabin and forgotten to reconnect it. So the static instruments were referenced to cabin pressure. As the aircraft climbed through 3,000 feet, the pressurization system started holding the cabin altitude constant, and you know the rest. MIF!
Why Do MIFs Happen?This was hardly an isolated case. I've read about at least three other similar incidents in pressurized singles and twins, all caused by failure of a mechanic to reconnect a static line. Interestingly enough, the FARs require a static system leak test any time the static system is opened up in any fashion. Clearly, many mechanics aren't taking this rule seriously. Problems like this can be absolutely deadly. Remember Aeroperú Flight 603, the Boeing 757 that crashed into the Pacific Ocean near Lima, Peru, on Oct. 2, 1996, killing all 61 passengers and nine crewmembers on board? Investigators found that the cause of the crash was static instrument failure caused by maintenance personnel who taped over the static ports in preparation for cleaning the airplane, and then neglected to remove the tape afterwards.
Now, it's true that most aircraft accidents are pilot-caused rather than machine-caused. Numerous studies indicate that about 75 percent of accidents are the fault of the flight crew. The 25 percent of accidents that are machine-caused are just about evenly divided between those caused by aircraft design flaws (13 percent) and those caused by MIFs (12 percent). Still, 12 percent of accidents is a pretty significant number. More than half of all MIFs -- 56 percent, according to one survey -- are errors of omission rather than commission. The majority of these omissions involved fasteners left uninstalled or not torqued properly. The rest involved things left disconnected (e.g., static lines) or other reassembly tasks left undone. Distractions play a big part in many of these errors of omission. A common scenario is that a mechanic installs some fasteners finger tight, then gets a phone call or goes on lunch break and forgets to finish the job by torquing the fasteners. I have personally seen some of the best, most experienced mechanics I know fall victim to such seemingly rookie mistakes, and I know of several fatal accidents caused by such omissions.