Aeroengine Safety

Figure "Power gearbox with accessory components": Power takeoff gearboxes, such as this one from a modern civilian fan engine, are powered via a radial shaft by a rotor, usually the high-pressure rotor. The power takeoff gearbox in turn powers a variety of hydraulic and electrical components. The number of potential problems is equally great. This makes the engine one of the most compact collections of a wide variety of engineering fields. The variety of potential problems is correspondingly broad (Fig. "Statistic of accessory equipment damages and problems".2). The accessory equipment is usually supplied by sub-contractors according to the specifications of the OEM.

Figure "Statistic of accessory equipment damages and problems" (Ref. 11.2.5-5): This diagram is based on information from a large aircraft manufacturer. It applies to two-engine commercial aircraft and also takes the peripheral devices into sufficient consideration. Because it mentions different aircraft types, it probably also covers different engine types. Therefore, engine type-specific damages are probably largely eliminated from these data.
Over 50% of all inflight shutdowns can be traced back to damage and problems involving accessory equipment and peripheral devices. This demonstrates the importance of this technological field for operating costs and safety. Roughly 70% of unexpected costs during engine maintenance and overhaul can be attributed to the accessory equipment (detailed diagram)
The oil system is marked by especially high damage rates.
False fire warnings and instrument errors are next with a total of about 30%. There is a constant debate regarding the point at which monitoring devices are practical, since they are prone to malfunctioning and can unsettle the pilot.
The fuel supply system and compressor regulators also contribute heavily to shutdowns.
Air systems, generators, and auxiliary transmissions cause relatively few shutdowns, with a combined rate of under 10%.

Figure "Problems of accessory equipments": This compilation is intended to give an impression of the specific loads on selected accessory equipment. Keywords relating to the affected elements of the equipment and specific damage-relevant influences are provided. However, there is no claim to completeness with regard to either the equipment or the influences.

Gearboxes: Engines usually have an auxiliary gearbox, which is powered by an engine rotor via the radial drive. The gearbox in turn powers accessory equipment such as pumps and generators. During startup, the compressor is accelerated to the starting RPM. This task is sometimes accomplished by an intermediate gearbox. In shaft-power engines, the power-takeoff gearboxes are considered part of the engine, and not part of the accessory equipment. Typical auxiliary gearboxes run at high speeds, at least on the drive side. They have split housings, which are made from cast light metal alloys (Mg or Al alloys) and hold the shaft and axle bearings on both sides. Fig. "Problems of accessory equipments" shows the weak points of these gearboxes. Damage that disrupts the gear line causes important aggregates such as regulators (Volume 2, Ill. 8.1-16), generators, pumps, and starter generators to fail. This makes the possible damage consequences very extensive. If vibrations occur, the relatively large masses of generators, regulators, and pumps, which are one-sidedly affixed to the gearbox, can highly stress the flange and overload the typical low dynamic strength of the light metal materials (Fig. "Fatigue at light metal gearbox housings").

Control systems: Experience has shown that some electrical components of older engine types, such as boosters for the nozzle control system, were very sensitive to environmental factors, such as moisture and lightning strikes, due to insufficient shielding and protection (Volume 1, Ill. 5.1.3-6). Modern engines usually have electronic control systems that work together with mechanical-hydraulic systems. Although electronic regulators (FADEC, Example "Failing transistor leading to large scale inspections") evidently fail very infrequently (Example "Possible digital electronic engine control failure"), some incidents have been reported. Evidently there have been cases that are almost impossible to recreate since the device resumed flawless operation afterward.
Mechanical control systems have gliding parts with extremely tight play (e.g. relay valves) that are lubricated by the medium (fuel) being regulated. This means that the damage mechanisms of those tribo-systems will be active. The functioning of gliding parts with their typical small play is very sensitive to dimensional changes during operation. For example, in one case a minor increase in the volume of a steel piston caused jamming at low operating temperatures. If these parts are not deep-frozen after hardening during the manufacturing process, a remaining unstable structure can convert into one with larger volume (residual austenite). Of course, drive shafts can also fail. This is illustrated by an incident that was caused by a reduced lubricating effect (increased fretting) of fuel in fuel-lubricated control system spline shafts (Ref. 11.2.5-9).

Pumps: Various pumps are used in engines to transport and pressurize fuel and oil:

• Turbopumps (fuel)
• Axial piston pumps (fuel)
• Gear pumps (oil)

Each type has specific weaknesses. For example, axial piston pumps with the usual silver-plated guides are very sensitive to sulfuric contaminants in the fuel, which can cause catastrophic damage in a very short period (Volume 1, Ill. 4.5-14). Gear pumps can jam due to differences in thermal strain between the gears (steel) and housings (light metals) at low temperatures, which can cause play to be bridged (Volume 1, Ill. 5.1.5-2).

Turbopumps are relatively failure resistant. However, their pressurization is limited and their dimensions are relatively large. Due to the upstream filters, blocking of the rotor can only be caused as a consequence of OOD. In the case of bearing damage, pumps can run hot and fail
(Volume 2, Example 9.3-6.3). Of course, pump failures can also be caused by failure of their drive systems (Volume 2, Ill. 6.2-19). A typical cause for drive shaft failure is fretting in the splining (Ref. 11.2.5-8). The gears and/or housings of oil and fuel pumps can be damaged by cavitation (Volume 1, Ills. 5.3.1-1 and 5.3.1-11.2). One result of this may be an inability of gear pumps to provide the necessary exit pressure.

Generators and starters: The frequently reported damage cases indicate that a special weak point in these devices is the splining, which is usually used in the drive system of the gearboxes. This is usually due to fretting, which progresses until the splining loses contact and the shaft spins freely (Example "Failure mechanism of splined socket connections" and Volume 2, Ill. 6.2-18).
Blocking damage in generators (Example "Secondary damage complications") has also been reported. Especially serious consequential damages can occur in the gearbox if the rated break point (Volume 1, Ill. 4.5-12) on the drive shaft fails to break. This can also cause bearing damage. It seems to be difficult to ensure sufficient lubrication of these devices over long operating times.

Pipes: Damages and problems relating to pipes have been extensively covered in Volume 2 (Ill. 9.3-2). Leaks in fuel- or oil-carrying lines always result in fire danger (Volume 2, Example 9.3-6.1), making them very relevant to safety.

Leaks or fractures of pipes carrying hot air can also lead to fires and/or the failure of neighboring aggregates. For example, air extracted from the rear area of the high-pressure compressor to cool the turbine inlet stator has temperatures above 500°C. Pipe connections such as V clamps or swivel nuts must be given special attention (Fig. "Problems of auxilary gearboxes" and Volume 1, Ill. 5.4.2.2-1). Another problem is O-ring seals (Examples 11.2.5-1 and 11.2.5-2) that fail due to overstress or damage (e.g. aging). A problem that especially affects titanium pipes is damage and fatigue cracking due to fretting at wear points.

Motor bellows braced with wire netting, which are used especially on hot air lines, are very problematic. Typical damage includes collapsing of the bellows after the wire bandage is worn through (by expansion movement due to internal pressure fluctuations).

Oil tanks: These are usually voluminous thin-walled structures with a filigreed internal structure. The inner structure normally consists of metal sheet walls that are known to be sensitive to high-frequency vibrations. Leaks and fractures of the innner structures (fuel baffles) are the result. An important factor is the oil intake. It must be directed to the areas where the oil collects due to G-forces. This moveable device operates as a pivoting mechanism with bearings or a flexible element (snorkle, Ill. 11.2.5-5). The bearings of the pivot mechanism are threatened by brinelling (local fatigue and wear due to vibrations) and mechanical friction in the bearings can prevent the oil intake from functioning. Flexible snorkles are made from synthetic materials. These can corrode and/or swell if they are fouled or come into contact with non-specified fuels, which can cause them to stop functioning (Ill. 11.2.5-5). Problems with filler necks must not be underestimated. Leaks resulting from seal failure and/or oil losses have been reported. Vibrations caused the lid, which had not been properly secured, to open (maintenance error).
Fastening devices: In the following, this term refers to devices that fasten the accessory equipment and peripheral devices to the engine. This includes clamps, brackets (usually made from sheet metal), couple levers, and dampers (Fig. "Fastening of accessory equipment"). These components must withstand dynamic and static loads. If possible, matched elasticity and damping properties should minimize the loads. Especially brackets, which are used to fasten gearboxes and regulators, must be given sufficient consideration during engine development or when changing over to different standards. The interplay of the various stiffnesses (housing, pipes), the mass distribution, and the damping (friction) with the many possibilities of vibration excitement (engine, gears, pumps, etc.) is extremely complex. Safe predictions are not possible through mere analytical means. Experience has shown that only realistic tests on complete engines are sufficiently informative (Ill. 13.6.3.4).

Cables and cable connections: Two different types can be defined: electrical cables and mechanical cables (especially in older engine types).
Electrical cables connect sensors, actuators, electronic regulators, ignition systems, fire warning and extinguishing systems, etc. Dangers include consequential damages such as overheating or wearing through of the cable insulation. Plugs are a special weak point. Corrosion and fouling can increase the transition resistance until the cable fails (Volume 2, Chapter 9.5).
Mechanical cables in the form of flexible shafts or helix cables (teleflex cables) are used for actuation, feedback (e.g. to the regulator), and synchronization of movements (Volume 2, Ill. 6.2-14). They only fail in very rare cases due to wear, corrosion, and jamming (e.g. fouling, icing; Volume 1, Ill. 5.4.1.2-7).

Figure "Problems of auxilary gearboxes": Although auxiliary gearboxes account for a relatively small portion of inflight shutdowns (Fig. "Statistic of accessory equipment damages and problems"), they are susceptible to many different potential damage mechanisms due to their wide variety of elements (gears, housings, bearings, shaft connections, seals, oil supply).

These damages can be be causally influenced by different phases of the part life. This is shown in the following with the aid of selected examples:
Production: Light metal cast alloys, and especially Mg sandcasting (typically used in older engines), often have continuous porosity. This porosity can be sealed through infiltration and compacting (peening) of the surface during production. However, overhaul procedures (degreasing, cleaning, stripping) can reopen and increase the porosity. This occurs through initiation of synthetic material infiltration and/or chemical corrosion of the substrate. The result is leaks through which oil or fuel “sweats” out. Material flaws and weak points such as fields of porosity or oxide skins can crack under dynamic loads.
Improperly soldered or welded injection nozzles can fracture due to dynamic fatigue and fall into the gears.
If the sides of narrow teeth are case-hardened, internal flaws may develop. They are the result of hydrogen embrittlement and can cause the tooth to fracture during operation (Volume 1, Ill. 5.4.4.2-3).

Overhaul: Penetrative crack detection requires stripping and degreasing, as well as disassembly of all steel parts (bearing seatings, threaded inserts). This can prevent chemical attacks and corrosion due to local cell action. Because these fastening elements must be reinstalled later, larger inserts or coatings are used on the fitting surfaces of the housings to compensate for wear and material removal (operation, disassembly). This can limit the number of possible overhauls.
Gear shafts wear at the seal surfaces of the seal rings. Circumferential grooves are created on the shafts. In these zones, the parts must be reworked (ground) and coated to restore the original diameter (e.g. chroming). The usual bronzing is also removed in the course of this repair process. It is later reapplied in a hot leachate. This repair process can cause cracking and serious damages. These cracks can not always be safely detected afterward through magnetic crack detection (Volume 1, Ill. 5.4.2.2-2). During operation, these cracks can lead to unpreventable failure of the part.

Operation: There is a high probability that design-specific weak points in gearboxes will make themselves known during development and testing. This makes it possible to prevent them before serial production begins. This is the most likely reason for the many reports of gearbox problems during development of new engines, but relatively few damages are recorded during operation (Fig. "Statistic of accessory equipment damages and problems"). Gearboxes are subjected to considerable dynamic loads through the fastenings, gears, and power input and takeoff. For this reason, special attention must be paid to flaws and weak points that could promote dynamic cracks. This is especially true for gears.
A special problem is presented by the oil conditions in the gearbox. If gearboxes have “dead” zones in which larger amounts of oil can collect (even during certain flight maneuvers), it is called oil hiding. In extreme cases, this can cause oil shortages in the main bearings and even engine failure.
Nozzles, which frequently take the shape of pipes attached to the gearbox wall with clamps, can be excited to vibrations and suffer dynamic fatigue fractures. This can result in spontaneous damage if fragments become trapped between gears. If there is merely a localized lack of lubricant, then bearings and gears may be damaged over time. This type of damage usually makes itself known early with metal shavings on magnetic plugs. Experience has shown that dynamic fatigue fractures in oil nozzles occur in notch areas such as transitions to distributer parts, soldering, and weld seams. For this reason, these notch areas should not be located in regions that are potentially subjected to high dynamic loads, i.e. near the clamps.

Long standing times are a part of operation, especially in military engines (Example "O-ring failure resulting in crash"). Standing times can have damaging effects on gearbox components such as bearings (corrosion, brinelling) and seals. In O-rings, reported problems are aging with embrittlement and lasting deformations. It is also possible that shaft seal rings may leak and/or run hot if they encounter dry surfaces where the oil film has run off.