10. Engine Mounts

Figure "Requirements for engine mounts": The requirements for the engine mounts, which have to be considered by the designer of the engine and the nacelle, surpass by far the bearing and the transfer of the already complex forces from the normal operation (Fig. "External loads at the engine mounts"). So the configuration of the connection bolts (fuse pins) at the joints of the mounts is something to worry about (Fig. "Engine mounts specific problems of elements").
Of great importance for the safety requirements for the operation is an optimal maintainability and an easy control/inspection for damages respectively failures like wear/fretting, corrosion or in an extreme case cracks..

Figure "External loads at the engine mounts": This sketch shows the conditions at a larger fan engine with high bypass ratio.The mounts for the loads into the casings are a matter of strength and stiffness and must be optimised. For this purpose the flanges and connections of the flaps and maintenance doors from the nacelle/cowl as well as the installations (e.g. thrust reverser at the fan) are used. Big nacelles have a high aerodynamic lift (Fig. "Backbone bending' by aerodynamic forces"). Especially its moment on the engine must be adjusted without unacceptable deformations, e.g backbone bending, (Fig. "Housing deformations near engine suspension" and Fig. "Problems by deformation of compressor housing").

Figure "Aircraft specific mounts" (Ref. 10-20 and Ref. 10-22): This picture shows examples of engine mount attachments. They depend on design features of the engine like:

• Bypass ratio,-Number of shafts (two, three) and axial position of the containment relevant rotors.
  
• Bearing inner structure of the engine: Casing struts (Fig. "Engine mount of an airliner"), position of the main bearings.
  
• Fan engine, turboprop engine or turbo shaft engine (helicopter propulsion).
  

Figure "Turboprop engine mounts / support" (Ref. 10-20, Ref. 10-21 and Ref. 10-22): The mount and suspension of turboprop engines, appropriate of external propeller gears (sketch below) is demanding. In this cases have additional high torques to the propeller and relative low frequent vibrations (possibility resonances) to be considered . For those loads stiffness/rigidity of the thrust frame have to be adjusted (upper sketch).

Figure "Engine mount of an airliner": A suitable load bearing inner structure (sketch below) allows, like here at a three shaft fan engine, a front mount on the fan casing (sketch in the middle). This requires naturally suitable rigidness e.g. design features. Frequently in other engine types the front mount is adapted to the core engine (Fig. "Design of an engine mount").

Figure "Engine mount of a fighter": In case of fighter aircrafts the engine is positioned in the fuselage. This can reduce the risk that the engine separates in a catastrophic manner when a failure of the not thrust loaded mount occurs. This showed a case where the rear mount failed. The engine lowered only till the the distance of few centimeters to the surrounding wall of the engine bay. The slight deviation of the of the exhaust direction hardly could be observed. So the failure was only identified during inspection on the ground.
Normally the thrust supporting mount uses a thrust pin (sketch lower left). The other mount is circumferential orientated and connected with hinges, using a ball joint (sketch lower right). Naturally, because of the cramped circumstances the maintenance and exchange of the engine have to be considered especially by design.

Figure "Design of an engine mount": This picture shows an example with front and rear mount direct connected with the core engine (Fig. "Aircraft specific mounts"). They must transfer forces and moments from the engine to the nacelle (Fig. "External loads at the engine mounts"). There are additional demands for OEMs which use typical design elements/features.

• Minimal elastic deformation of the engine casings in the region of the load initiation.
  

Used are circumferential lands.

• control of vibrations.
• fail Safe-feature by redundancy.
• suitable behaviour during overload (Fig. "Engine mounts specific problems of elements").
• screw joints/bolts at the support area to the pylon.
  
• joining rods (Fig. "Load transfer from the engine into the wing") with
• ball joints (Fig. "Engine mount of a fighter") and
• connection pins/shear pins (Fig. "Engine mounts specific problems of elements") as well as protection against slipping out.

Figure "Engine mounts specific problems of elements" (Ref. 10-6):
Overload during operation: This develops by unbalances or the failure of a mount element (Example "Engine mount maintenance deficit"). Not to endanger the structure of the wing or the nacelle (Fig. "Engine mounts specific problems of elements"), certain connection bolts (fuse pins) may only fail in a defined sequence (Fig. "Separating an aeroengine for safety"). This is also required to avoid the fracturing of the wing tanks (Fig. "Fracture of the engine connection at the pylon"). With this feature the design will be a difficult task. In one respect, the connection bolt must meet the high safety requirements against a failure under the normal long time operation loads. To those belong especially dynamic loads.These are apparently not always sufficient known (Fig. "Design of an engine mount"). Thereby the integrity of those highly loaded components must not be affected by wear and corrosion. On the other hand the bolt must breake as planed early enough when extremely overloaded. This performance will be achieved by mainly shearing the bolt at overload, in contrast to bending by the normal operation loads.
To meet the demands, it is possible to use hollow connection bolts with special cross section features (Fig. "Fracture of the engine connection at the pylon").

Wear: Especially two kinds of relative movements can occur at the connection bolts and hinges.

• Siding wear by the planned macro movements.
• Fretting as result of micro movements by elastic strain differences. Wear can be reinforced by corrosion.
  

As a consequence of an unsuitable lubricant e.g. containing MoS₂ (see German edition Volume 5 Ill. 22.4.1-1) in combination with humidity and/or accumulated wear/abrasion paricles this can lead to jam of hinges and so to overload.

Corrosion represents especially for connection bolts a great danger (examples 10-10.1/.2/.3). The bolts consist of high strength steels. Those are corrosion resistant against water (e.g. condensation /atmospheric moisture) and especially sea atmosphere not. Therefore a suitable coating as corrosion protection is used Is this damaged, consumed or worn by fretting over long operation times typical corrosion pits can develop. Those notches lower the fatigue strength and promote fatigue cracks/ fractures (Example "Cracks and corrosion in engine mounts").

Damages during maintenance, respectively assembly form notches that trigger fatigue failures. Plastic deformations (burrs, bulgings) can be unfavourably influence contact conditions as cause for an overload (Example "Fatigue cracks at the connection bolt of an engine mount"). Also with galling/seizing of the bolts must be reckoned (volume 4 Ill. 16.2.2.5-10).

Assembly problems always must be seen in connection with deficiencies of „human factors”. First there is the accessibility by hand and visual (Fig. "Demands on the engine mounts of an agricultural aircraft" and German edition Volume 5 Ill. 19.1-8).
A typical problem are unsufficient fastening torques of the nuts (Fig. "Demands on the engine mounts of an agricultural aircraft"). They increase the vibration stresses of the bolt. From this problem suffer also other bolts like those so fix lockings that serve against slipping out. (Example "Fatigue fracture of fastening bolt securing connection"). Also possible is the loosening of an unsufficient tightened nut (Example "Engine separates by turbine dusk failure").
Forgotten securing elements also have been the cause of the failing of a connection because the bolt slipped out (German edition Volume 5 Ill. 19.1-8).

Lay out design of engine mounts is very demanding. It is possible that not all operation loads are known, e.g. in a special application (cargo use, agricultural use) respectively operator specific (Fig. "Demands on the engine mounts of an agricultural aircraft"). Thereby the utilizable material strength, depending from long time influences like corrosion or wear, have to be regarded.
A further problem is the controlled failing at extreme overloads (Example "Engine separates by turbulence" and Example "Separating an engine purposeful at dangerous unbalances?"). To this belong external loads like from accelerations (inertia forces) by turbulences(Example "Engine separates by turbulence") and the impact of an emergency landing (e.g. on water, Fig. "Separating an aeroengine for safety"). Also other forces must be considered like extreme unbalances (Example "Separating an engine purposeful at dangerous unbalances?") as a result of failing components (fracture) of a suspension/mount (Example "Engine mount maintenance deficit" and Example "Separated engine after fatigue fracture of an 'anchor rod'") must be considered. .
Dangerous influences like fire (titanium fire, hotgas) or fragments (in case of containment) must also be considered (Fig. "Stresses during blade fragment containment"). If there are new findings, retrofits can be necessary (Example "Destroyed engine mount by uncontained fragments").

We should always see material strength in connection with the dimensioning/design. The strength must be with sufficient safety higher than all the design loads in the normal operation. This is also true over the whole scheduled operation time under estimated influences like corrosion and wear. Here problems with later material changes can emerge. An example is, when a part made of wrought/forged material is changed to a casting (Example "Cast materials requiring high caution").
Also repairs (e.g. weldings) in highly stressed zones of a mount must be considered very critic.
A defined failing under overload must consider effects like embrittlement during shock/strike (Fig. "Brittle material behaviour at impact").

Figure "Engine separation risks" (Ref. 10-6, Ref. 10-24): The separation of an engine under low thrust/ power or standing will be considered by the designer for the case of overload (Example "Separating an engine purposeful at dangerous unbalances?", Ref. 10-25). This situation can be expected if the engine failure is primary. Thereby a damage of the wing, the nacelle and/or a parallel engine must be avoided (upper sketch). For this an optimal failing sequence of the connection bolts shear from front and rear mount as scheduled (shear bolts, Fig. "Fracture of the engine connection at the pylon").
In case of high engine power/thrust (start, climb, upper sketch) the risk of a catastrophic secondary failure is far higher (Fig. "Fracture of the engine connection at the pylon" and Example "Separated engine after fatigue fracture of an 'anchor rod'"). When the rear mount fails (Example "Fatigue cracks at the connection bolt of an engine mount") the engine first tilts upwards (phase „1a“). Thereby the leading edge of the wing with flaps and installations can be dangerous damaged. 'Shoots' the separated engine forward in front of the wing, accelerated by the short time still acting thrust (phase „1b”), it will be caught up in the next moment by the airplane. Thereby the danger of a collision with the parallel engine exists. This can be torn out or catastrophic damaged.

Figure "Load transfer from the engine into the wing" (Ref. 10-24): Looking at engine mounts (sketch at the left side) we are bound to include also the structure of the pylon (lower sketch at the left side) to the wing, respectively when required to the nacelle. Of influence are stiffness/rigidness/elasticities and damping as well as failure limits and failure sequences under overload. Thereto belongs, that safety important structures and other engines will not be destroyed. Also the dimensioning of the engine mounts is affected. In the pylon we find also connections with typical elements, similar to the mounts at the engine. To these belong deflection rods (lugs), connection bolts (fusion pins) and ball joints. They all are exposed to similar failure mechanisms (Fig. "Fracture of the engine connection at the pylon").
The shown configuration suffered an catastrophic accident when during the start an engine with the pylon separated. Thereby the flap control of the wing was badly damaged.n and the crash unavoidable (German edition, Volume 5 Ill. 20.2-2 and Ill. 20.2-3). The cause were during installation of the engine overloaded structures at the mount to to the pylon of the wing.

Figure "Fracture of the engine connection at the pylon" (Ref. 10-6):Concerned is an airliner in cargo version. The engine 3 (at the airplane inner side right, upper sketch) broke and separated with the pylon during climb flight in about.2000 m above ground. Thereby a loud bang occurred. It swayed to the outer side and backward and damaged the wing leading edge with the flaps. Then it hit the engine 4. This separated also with it's pylon from the wing.
Because the engines of this type of aircraft are only difficult (outer engine) or nor at all (inner engine) to see from the cockpit, the crew believed that only the function of the engines is concerned. Because only the flaps at one wing were damaged the maneuverability of the airplane was limited. It crashed in a big dwelling house.
The connection of the pylon to the wing (upper sketch) is so designed, that in case of a separation due to extreme overloads, the damaging of a wing tank does not occur (Fig. "Engine mounts specific problems of elements"). A `clean' separation of the engine nacelle and/or of the pylon shall be guaranteed with the shear of the connection bolts (shear bolts/pins).
The pylon is designed as a torsion box with 4 compartments (bulkheads). Those in front provide the connection to the engine, the rear (upper sketch) to the wing. The associated connection bolts (fuse pins) are called primary. The forces from the engine are absorbed by the front bulkhead. The next bulkhead adopts vertical and lateral reaction forces, drag of the engine nacelle as well as torsion. The four attachments (mounts) to the wing are loaded over beams of the pylon. Primary, drag forces are guided by a compression strut into the diagonal connection (diagonal brace). An additional strut (side brace) takes the side thrust.
Hydraulic systems which can be damaged by the failing of the pylon can be designed multi redundant. In this case there are four independent systems.
After this flight accident the inspection intervals of the connection bolts (fuse pins) of the concerned aircraft type were reduced. The result was a multitude of failure notes from the operators. The majority dealt with cracked bolts in different mounts/connections from the pylon to the wing.

Figure "Destroyed engine mount by uncontained fragments": Merely by chance a spectator made at the same day before the previous landing pictures of the airplane. On those photos can be seen, that the engin 3, compared with the other engines was slightly angular tilted upward (Example "Connecting bolt failing by corrosion"). With this the question arises after an already existed failure at the engine suspension which with enough attention during routine inspections could have been identified.

Figure "Demands on the engine mounts of an agricultural aircraft" (Ref.10-18): The engine mount, respectively the thrust frame (sketch in the middle) seems for agricultural aircrafts subject of especially high, unexpected loads. The diagram shows events/incidents related to the mounts of three different engine types (2 piston engines, 1 turboprop) over a period of about 15 years. It concerns fractures of the connection bolts/fuse pins and engine components like tubes and angle sections. The upswing of the curve is primarily dictated by turboprop drive refitted aircrafts. A faulty design as cause seems unlikely. More likely are regarded:

• The authorized operation data don't match sufficient the real ones.
• Deficits with the assembly of the engine. To this hint fatigue fractures of connection bolts which are connected with differences to the assembly instructions. Thereby an unsufficient accessibility seems to play a role (human factors). So it came to a too low torque at the bolting (German edition Volume 5 Ill. 23.3.1.1-2).As remedy the repeatedly inspection of the engine mount, respectively the trust frame for cracks was instructed by an airworthiness directive.
  

The picture shows the position of a typical failure: In a welding zone of the lower tube from the engine frame, near the engine mount (sketch in the middle), a crack was found. If it comes to a fracture the engine may separate from the aircraft.

Figure "Separating an aeroengine for safety":With the size if the engines, especially of the fan the dynamic loads in case of a rotor blade/fan blade fracture increase. Typical are unbalances and high torsion moments as result of a sudden retarding of the rotor by seizure. Thereby the structure of the airplane /wing, pylon will be endangered by a catastrophic failure/fracture. The question arises if this can be practicable avoided by a controlled failure of shareable connection bolts/ fusion pins which fail at extreme overloads. So the risk can be minimised, that components (e.g. tanks, actuation of flaps) get damaged with catastrophic results. But those systems have also problems. They must not fail by the normal high dynamic operation loads (see Example "Dealing with catastrophic engine seizure").
A further situation for a purposeful separation of the engines by an controlled explosion blow is an emergency ditching into water. The high water drag of the engines can force the airplane to tilt forward even at optimal horizontal flight attitude when both engines immerse at exactly the same moment.
However it must be reckoned due to a sloping position. In such a case the airplane will be tilted in a manner that even the fuselage will break. To avoid this situation both engines should be controlled separated before the water will be touched. This must occur not until the connection bolt at the engine mount fails uncontrolled during touching the water. Otherwise extreme loads act at the airplane.
Therefore obviously a technology from the aeronautics is discussed. There explosive predetermined studs are attached, which when a dangerous overload occurs, or possibly automatic by sensors, can be triggered by the pilot before touching the water. A benefit would be that the predetermined explosive stud could be sufficient over-dimensioned to rule out a failing by normal operation loads. A grave problem may exist in the possibility of an unintended ignition. Therefore the reliable triggering must be guaranteed under all situations.