Aircraft engines have become sophisticated systems within systems. From a maintainer’s point of view, an engine is one of the most important pieces of equipment on an airframe and the goal is to keep it on wing as long as safely possible. However, there are times when you have to make the tough call and either pull it or call in reinforcements. Knowing when to make this call can be a difficult decision.
While there are multiple types of engines — gas powered, diesel, turbocharged, turboprop, turbojet, turbofan and rocket, among them — delivering a vast range of power, the fact is that for technicians, they all fall into one of two categories: good or bad. A good engine is one that makes power, doesn’t leak and delivers problem-free service until scheduled removal time. Bad ones are absent any one of these traits.
Thanks to the steady evolution of engine technology, today’s powerplants are for the most part extremely safe and reliable — that is, if you operate and maintain them properly and follow the prescribed recommended inspections and service intervals. Rarely will one fail without warning, unless it ingests a foreign object of some kind. Peripheral components like sensors, pumps and filters can fail, but these are easy to address. Typical bad engine behavior begins with degradation of available power, a leak or some type of intermittent fault. These are the warnings signs of the trouble ahead.
Fortunately, most engines can be replaced in about a day with a good team. However, the logistics of obtaining a replacement, and the uncomfortable discussion of who pays for the work and repair, are the stuff of legend. For many maintenance managers there will come a time when a bad engine needs serious attention. Do you remove it early and face potential costs? Or do you tough it out, contrary to both your pilots’ wishes and your own instincts? Some real-world experience can help clarify the decision process.
The turbine engine has become the staple of business aviation in part because of its reliability and its superior thrust-to-weight ratio when compared to its piston counterparts. The core of every turbine engine involves multiple rotating blades attached to a hub shaft and spool assembly and stators attached to the case. At each stage, air and fuel are compressed, and then ignited. All of this takes place at extremely high temperatures and pressures, necessitating both precision and strength of all components. High-tech castings and forgings are made from exotic alloys to allow for expansion and contraction. Over time, the individual blades erode, causing a decrease in performance. You can see this degradation by either an increase in temperature or turbine speed and sometimes both.
Today, most digitally controlled turbines have an automatic feature that performs the necessary calculations and captures performance data. Plotting the results can show your trends and usually over time you will see a gradual decrease that remains well within acceptable limits. If you see a sharp decrease, you should be on the lookout for issues with the fan or sensors. Performing a borescope inspection is tedious and requires the appropriate equipment and training, so it is best to check sensors first, if possible.
The scope needs to have sufficient light and flexibility to see both rotor and stator blades, through each stage. Some engines require a comparator feature on the scope to actually measure the blade. Vigilance is essential while performing the inspection to ensure that no blades are missed. Considering that many engines have hundreds, even thousands, of blades, patience is equally essential to the process. Many engine manufacturers require that technicians be certified to perform borescope inspections per model family. Unless the engine exhibits gross damage or erosion, you may want to hire a qualified inspector from an approved service provider to conduct the work.
Engine speed sensors and wiring harnesses can degrade over time. Loss of insulation and loose connector pins are difficult to detect but can cause erratic readings. Faulty sensors can also confuse the autostart sequences, resulting in slow starts or no start at all. Newer engines can be hooked up to a laptop computer or display parameters on the aircraft’s multifunction display (MFD) and there provide error codes and other engine data. While this information can help narrow the problem source, you should use caution before swapping sensors and harnesses. Be sure to write down each step, so that you can back out any move that does not solve the problem. Exhaust gas temperature (EGT, also known as T5) is a vital component of a power check and over time the sensors degrade and can cause a faulty readout. Often overlooked during the troubleshooting process is the outside air temperature gauge or sensor, which can fool the computer into delivering an incorrect fuel flow.
Inside and Out — Leaks and Drips
Turbine engines are exposed to many types of extreme operating environments, from cold ambient temperatures to high exhaust gas heat, and thus the powerplant’s metallurgical properties need to accommodate a wide range. In addition, friction, expansion and contraction can cause high stress on internal oil seals. You will begin to see oil levels diminish without seeing drips or seeps. Periodic checks of the oil level are required to see if the oil’s loss is within limits. It is extremely rare for a leak to fix itself, and usually the problem only gets worse.
Besides high consumption, the most common sign of internal oil leaks is coking of the turbine blades, but sometimes leaks reveal themselves in more noticeable ways.
“The engine smoked a large amount on shutdown. There was so much smoke it would cover the vertical tail, horizontal tail and aft fuselage,” said John Bucher, an aviation maintenance consultant based in Castlewood, South Dakota, and former chief of maintenance for Honeywell’s corporate flight operations.
Sometimes an engine manufacturer can be slow to respond to a request for information or action involving an errant powerplant, and you have to persist to ensure the issue gets addressed and resolved.
Bucher recalls the manufacturer was reluctant to even acknowledge there was a problem with his smoking engine, that is, until airport ground control “sent the fire trucks one time upon shutdown” and he forwarded them the bill for the fire department. “We also notified the aircraft manufacturer we were going to ground the aircraft because of the smoking problem of this engine.” Discussions among the engine and airframe manufacturers and the operator ensued and resulted in the aircraft being ferried to a service center for repair.
After that, the persistent Bucher said, “The problem was corrected and the plane continued in service with this problem never to surface again.”
Most engine manufacturers have a good understanding of serviceability issues. Obvious discrepancies such as low power and leaks rarely cause controversy. However, the conflict comes when there’s disagreement between the operator and manufacturer about the severity of the problem.
“In the last 20 years, some OEMs have moved to what I term a “manageable risk philosophy” toward their products,” said John Gibson, a veteran business aviation maintenance manager who heads Latitude 45 Aviation Consulting Services in Brush Prairie, Washington. “This is often demonstrated by an inspection that may be due immediately, or in a very short time interval if the engine has less than so many cycles or hours on it. But if it has exceeded a certain time interval, it can continue to the next shop visit or some future extended time interval. The reasoning being, if the part hasn’t failed by a certain time, it probably isn’t going to.”
The problem with this practice is that the burden comes back to the maintenance manager, who must continue to either inspect something that will eventually lead to a removal, or pull the engine before it gets to that point. This difference in risk tolerance will cause friction between the operator and manufacturer and could lead to extensive cost penalties if the former elects to pull the engine early. This is also true for third-party cost control programs.
“I’ve found it a good practice under those circumstances to make a deal with the OEM,” said Gibson. “If the engine is pulled, inspected and there is no problem found, I’ll foot the bill. But if on disassembly there are issues, then I’ll look to the OEM to cover the costs of R&R, lease engine and repairs per the warranty process or engine maintenance agreement. This has been a successful process for me, and I think a fair and equitable deal for the OEM.”
A misbehaving engine is a pain multiplier. Each level of suffering magnifies — from the passengers, pilots, schedulers, leadership and, ultimately, the head of maintenance. Considering that most business aircraft have two engines, think of them as two opportunities to force your early retirement. Each engine OEM has very specific removal guidance and criteria. Most engines have condition monitoring features and the data produced can be reviewed by any number of analysis services. If upon review, the OEM says that the engine is safe to operate, but your gut tells you that it is time to replace it, you have arrived at a pivotal career moment, made even more momentous if there’s no replacement or loaner available. If you make the wrong call, pain and expense will follow.
Keeping your engines healthy and safe is a major responsibility. If you are not the engine expert for your operation, having a trained and experienced technician on staff is critical. In the last century of engine development and service, the only thing that has not changed is the importance of a knowledgeable maintainer.
FADEC Time-Limited Dispatch
Fault codes presented by a full authority digital engine control (FADEC) can be an indicator of serious issues. Always follow your maintenance manual when troubleshooting these codes. However, some codes may be allowed to remain in service under Time Limited Dispatch (TLD) authorization via the aircraft/engine Master Minimum Equipment List (MMEL). To learn more about TLD for FADECs see FAA MMEL Policy Letter 45 at: http://tinyurl.com/hbarkux