EMI

Abstract

Lightning can produce many potentially-serious effects upon gas turbines, particularly those equipped with digital electronic control systems. Much of this can be avoiding or ameliorated by the judicious application of proper bonding between electrical components, engine frames and, eventually the airframe.

Resistance between electrical components and each interface should be less than 2.5 milliohms. This can be achieved by either the use of clean interfaces or flat cross-section bond straps as defined in the 3000070-family of parts.

With the introduction of the low-current full-authority digital engine control (FADEC) style of engine controls, it was discovered that spurious electrical ground paths and mini-ampere signals could raise havoc with the system logic. In a word, miniaturized sold state electronics are sensitive to transient voltages. In order to avoid, or at least minimize, the effect of accidentally or unintentionally induced currents, it was quickly understood that all electrical controls, components (switches, sensors, etc.) and accessories (PMAs, generators, etc.) should have a dedicated bond through the engine structure to the airframe.

The now-passé less-sophisticated analog controls operate on a zero-to-ten volt (dc) power range. Any EMI-induced currents and impressed voltages were relatively insignificant compared to the system’s operating levels. On the other hand, the digital controls operate on low-level “Yes-or-No” logic with power signals in the range of five vdc or less.

The exposure of aircraft to lightning strikes may seem relatively infrequent and therefore unimportant. However the magnitude and effect of any lightning-induced spurious signals now must be determined by engine tests so that the response of the engine to lightning strikes can be predicted and also quantified.

Lightning strikes on aircraft had generally been thought to be a case of the airplane just being in the wrong spot at the right (... or wrong) time. Experts supposed that the air vehicle intercepted an independent lightning “leader” -- the initial conductive spark traveling through the ionized air -- by chance and thus became part of the bolt’s path. However, extensive NASA flight-testing with a lightning-hardened F-106B -- which intentionally took 714 direct strikes between 1978 and 1986 -- demonstrably proved otherwise: the plane generates its own lightning. [Newton (14)]

Fisher and Plumer (18), referring to commercial aircraft data collected between 1950 to 1974, suggest that one strike can be expected for every 2930 flight hours. This is for any type of commercial aircraft.

The passage of the aircraft through the air -- under specific conditions, of course -- develops a static charge over the fuselage and triggers intracloud lightning. (This may be construed as a somewhat simplistic view -- but that is sufficient for this discussion.)

The avoidance of lightning is both common-sense thinking and an esoteric exercise.

First, one should avoid cumulonimbus thunderstorms altogether. Of course that would be construed as common knowledge; something found within pilot’s lore, as it were. But avoiding them at high altitudes and where the temperature is below -40° C is of particular interest.

At lower altitudes -- in the “teens” -- the industry definitely recommends avoiding the ±5° C regime, as cold temperatures appears to enhance the formation of lightning. (ibid.) Evidence of the aircraft “charging up” is the most effective warning of imminent strikes (Fisher, 18) The appearance of a corona -- also known as “St. Elmo’s fire” -- around the engine inlet would be very noticeable at night or under other conditions of low light.

A change in altitude may help to avoid a strike. The preferred direction is to go up -- a change as small as 1,000 feet may be significant. Then, if an aircraft is to get zapped anyway, it will more likely to be a cloud-to-cloud flash than a cloud-to-ground flash. This is less undesirable for the power of an in-cloud lightning stroke can be as little as a tenth of a ground stroke.

Another questionable suggestion is to stay with Jet A as a fuel (as if there is a choice, which is most unlikely). Accident data indicates that the greater volatility of JP-4 was probably a major factor in the loss of at least five aircraft from 1963 to 1988 (ibid.).

The direct effects of a lightning strike are the familiar electrical over-load scenarios due to the high currents and voltages.:

Engine flameouts, surges, and rollbacks may occur due to severe pressure and temperature disturbances introduced into the inlet that may adversely affect the performance of engine’s compressor section. A shock at the engine exhaust exit due to a strike at that location may back-pressure the turbine and cause similar disturbances.

Keeping the ignition system “On” is a prudent idea when evidence exists to the pilot and crew that the aircraft is developing a charge. This is particularly important when flying in freezing environments or where conditions favorable to lightning are known to exist. If a strike does occur, the aforementioned temperature and pressure disturbances may result in engine instability or even a shut-down. The auto-relight system, in concert with the continuously sparking ignitors, can help bring the affected engine back on-line with a minimum of engine down-time and certainly undesirable loss of thrust.

While few aircraft accidents can be definitely attributed to the indirect effects of lightning there are two trends in aircraft design that aggravate the problem (Fisher and Plumer, 18).

The indirect effects of a lightning incident may be less apparent and certainly less imposing to the naked eye but can be equally disruptive. These repercussions are caused by the electromagnetic fields generated by the lightning’s currents. Field coupling, via cables and circuits, and voltage surges can be induced by the lightning flowing upon the surface of the airframe. The use of solid state components sensitizes the situation.

In addition, the increased use of non-metallics -- “composites” -- both in the airframe and the engines themselves result in greater indirect effects, along with the disturbances within the previously-mentioned solid-state electronics. The reason for this is that the gross electrical conductivity associated with metallic ducts and frames is missing. The high-resistance fabrics that make up the matrix of the composites disrupts the continuity and may force the current to jump and to arc to complete its path.

The strength and duration of a lightning strike is not the primary question when dealing with gas turbines, as engine problems are very rare. According to Fisher and Plumer (ibid.) “lightning effects on turbojet engines ... are limited to temporary interference with engine operation. Flameouts, compressor stalls, and roll-backs (i.e., reduction in spool speed) ... have been reported with fuselage-mounted engines. It is generally believed that these events result from disruption of the inlet air by the shock wave associated with the lightning ... sweeping aft along a fuselage. The steep temperature gradient may also be important.”

Lightning prefers to attach at extremities: the aircraft nose, the tail cone of the fuselage, wing tips and the ends of the empennage. A strike sweeping down the fuselage may disrupt the inlet air flow into an aft-fuselage-mounted engine and would probably not be noticed by wing-mounted engines. A wing tip strike would travel perpendicular to wing- or fuselage-mounted engines and little inlet distortion would be noticed. Therefore the typical business aircraft layout -- aft fuselage-mounted engines -- would appear to be the most susceptible configuration to a lightning strike from the standpoint of inlet distortion-induced surges and flame outs.

Newton (17) similarly notes that while “... single (engine) flameouts are fairly common on airplanes with aft-mounted small[1] engines ... (there have been) no (reported) accidents resulting from engine flameouts caused by lightning strikes, (and subsequent) in-flight re-lights (were) normal and no damage (occurred) in the engine.”

Plumer continues in the same positive vein: “There is no case on record, however, in which a successful restart or recovery of the engine to full power was not made while in flight (Emphasis added). There are no reports of lightning effects on wing-mounted turbojet engines, since lightning strikes do not often occur near the inlets of these engines ...” as the lightning attachment and exit points are at the extremities of the vehicle. (It is well to remember that these statements were written when engine controls were of the analog kind, not digital.) It is interesting to note that wing-mounted engines tend to be large and the shock wave from a lightning strike would probably be inadequate to noticeably disrupt inlet air flow.”

Plumer (ibid.) lists the indirect effects of 214 strikes on a variety of aircraft areas with only the following engine system-related items:

The concern, however, is not with the power of the stroke directly but rather with the currents and voltages induced in the various legs of the engine control electrical harnesses.

According to Naumann (16) the TFE731 control system has accrued twenty million (2*107) hours of operation with no reports of system problems due to HIRF or lightning. There were also twenty-six reported lightning strikes and “all aircraft returned safely” with “no reported control system damage.”

There are three methodologies for providing lightning protection and reducing RF susceptibility

• Shielding the sensitive hardware within metallic enclosures has been the accepted method in the past -- when the airframe and engine nacelles were made entirely of aluminum. Current airframers and nacelle designers are now turning toward the increasing use of composites in order to take advantage of the high strength-to-weight ratios, ease of manufacturability and structural toughness of non-metallics. These do not provide a modicum of the protection provided by metals.

• Another obvious ploy is designing into the devices the capability to withstand the onslaught of lightning strikes. This approach would, however, add even more weight; this approach would add volume; and this approach would add significant expense to the engine design. This approach would obviously not be a Politically Correct solution.

• Providing a complete conductive path from the each of the engine’s electrical accessories back to the aircraft structure metal-to-metal is representative of AE’s -- and the industry’s -- current thinking. This technique includes the use of bonding straps between electrical components and the main engine structures and also requires paying very close attention to the surface coatings and finishes between components.

Component bonding is not an difficult technical consideration -- at least from the mechanical standpoint. It is, simply, the consideration of keeping the resistance low across each faying surface. Resistances under 2.5 milliohms is the industry standard.

This upper limit of two-and a-half milliohms (0.0025Ω) reflects the current thinking with regard to bonding (Ref. 5, ¶4.2.26; and Ref. 3 ¶3.2.2.2) for terminations or connections or wherever any component or structure forms a part of a ground plane, shielding, a fault path or power/return signal. Fortunately, the structures of most of the engine electrical components are metallic and are sufficiently massive to provide excellent shielding and conductive properties.

The CFE738 engine specifications for both Dassault and MIL-B-5087B spell this resistance out in no uncertain terms:

“LRUs containing electronic or electrical parts must meet the 2.5 milliohms per bonding surface requirement, from any point on the (LRU’s) enclosure to the local metallic mounting structure (Ref. 2, ¶3.3.5.1) and an overall 10 milliohms to the engine structure, whichever is less (Ref. 5, ¶3.3.5.1).”

Actually, the concept is a lot of common sense woven around uncommon conditions. But interfering with the apparently-straight forward question of electrical continuity is the scepter of corrosion.

Aluminums and particularly magnesium alloys are ripe for corrosion damage, especially when the equipment is operated near salt water or under high-humidity conditions. The older TFE731s, for example, boast a light-weight cast magnesium thickly-painted gearbox and a similarly configured transfer gearbox in tandem. Beneath the paint, zinc chromate primer is found and exposed on all interfaces. Corrosion is not a concern on these assemblies: no electrons could penetrate this armor.

However these gearboxes mount accessories which bristle with low-voltage, low-current electrical devices -- such as the chip detector and the filter bypass switches -- which beg for electrical continuity with the front frame mount and the airframe.

The original models of the 731 used analog engine controls and rugged, hefty Teleflex™ power lever cables. Whether the individual control signals experienced a common ground or not was not important. With the advent of the Digital Electronic Engine Controls (DEECs), however, AE’s engine control is now able to sense differences in “earth” and could possibly react in an inappropriate fashion to currents and voltages induced in the electrical harness.

The initial “knee-jerk” reaction would be to add a bond strap from each and every component and harness connector all the way to the front frame. But in this day and age of advanced and improved maintenance considerations, a snarl of ground straps would be frowned upon ... just on general and esthetic principles. A companion problem is the possibility of a line mechanic not replacing a strap or perhaps routing it incorrectly such that chafing or an interference exists.

As most exposed parts of our AS engines are either plated, treated, painted or inherently corrosion-resistant any bond strap sites have to have special attention to guarantee continuity. This may be as simple as adding drawing notes that require masking at specific dedicated bolt pads or stud locations before the painting or the anodization process.

In general, all contact sites that will be used as a bonding path must be clean and free of paints, anodic coatings; greases; or any other dielectric materials. In fact document AFSC DH 14 (Ref. 10) “Electromagnetic Compatibility,” suggests that when possible weld all mating surfaces (!), hardly a viable solution for engine components and LRUs.

According to Dick Lerner (Ref. 17) “The most obvious factor (is) the cleanliness of the assembled joint and the surface treatment. Cleanliness is a function of manufacturing practice during assembly and must be clearly defined in the drawings and MOTs.”

“Joint finish is critical: Alodine or Iridite joint resistances can be in the order of one milliohm, while anodized or oxidized finishes are essentially an open circuit,” Lerner continues. “While joining pressure (bolt flange loading at the interface) is a factor, is it usually so great -- for mechanical reasons --- that it can be neglected for this discussion.”

Detail drawings and prints should specify the masking of any planned contact areas prior to anodization and/or painting. In the case of anodized aluminum components, where protective films are deemed necessary, ensure that the film material is a decent conductor. At Garrett, the use of MIL-C-5541 Type III corrosion-resistant wipe-on coating at the bond points is the recommended method.

Alodine/Iridite “touch-ups” are an acceptable method of spot-protecting an anodized part. Mil-C-5541 Type III is the Specification and a drawing’s note for parts that are to be treated (per Alodine 600) should follow this format:

XX. MASK INDICATED SURFACES ON FIND YY PRIOR TO ANODIZE. CHEMICAL FILM PER MIL-C-5541, CLASS III.

Be advised that this “touch-up” coating is quite thin and will wear away with repeated manipulations. Therefore it is not recommended for applications requiring surface wear resistance. It is typically used between mechanical assemblies requiring infrequent disassembly.

Since these strap attachment areas are hidden -- unexposed by virtue of the parts fitting intimately against one another -- corrosion should not be a major concern. And, because the entire engine is at the same potential, corrosion concerns may be ameliorated.

Other suitable protective films are: silver or gold plating (or other plated metals of good conductivity); Oakite #36; Iridite #14, and Iridite #18P, and both Alodine #1000 and #600 (the latter is Garrett’s choice).

Do not use any conductive paints to establish an electrical or RF bond. Such methods are prone to disruption, disturbance and damage during routine or special maintenance.

Anti-friction bearings, wire mesh vibration cushion mounts, or lubricated bushings shall not be used as a bonding path. Piano hinges may not be used as a path if a lubricant or any nonconductive element is used in conjunction with the hinge.

The recommended minimum bonding contact area is -inch diameter (.028 in2) [(2), Figure 1.]

The individual threads of screws and bolts by themselves alone are not acceptable to fulfill the bonding requirements for a number of reasons, according to Parmer (9).

• Single high-strength steel bolts are relatively poor conductors due to their alloying materials. In contrast, ductile and malleable materials possess a plethora of extra poorly-bonded electrons which freely carry the current; strong metals do not. The use of multiple fasteners, however, avoids this concern.

• Commonly-used anti-seize compounds, such as Fel-Pro, Lubriplate, etc., while carrying a metallic fraction, have non-quantifiable resistances as they are applied by brush and should not be considered acceptable.

• Installation compounds for inserts (per AE specification S8562) have non-quantifiable resistances also. All threaded inserts and studs are installed per AS Specification FP5039 (“Installation of Dissimilar Metals into Magnesium and Aluminum for Maximum Corrosion Protection”) which, in turn, specifies the use of an epoxy-amine primer per PSC5401 (NPC61611). The ramification is that these threaded items are effectively isolated from their cases, electrically (and corrosion-wise) by the primer, which is basically only a paint.

In addition, individual bolts may not provide particularly good electrical contact between the sides of the mounting through-holes. (As opposed to relying upon the bolt head’s contact area) The exception to this is the case of large mating flanges, with many fasteners, where the sum of many small contact values is sufficient.

Mating case flanges are acceptable paths due to the large surface areas involved. According to Fisher and Plumer, p203 (6), “Basically, the riveted skins and substructures found in ... aircraft are adequate to safely conduct lightning currents ... Even with the profusion of electrically insulating primers and wet sealers now used to coat joined surfaces, the great number of rivets or fasteners needed to meet mechanical strength requirements provides the necessary electrical path even when the fasteners and holes themselves have been coated with primers or wet sealants.“

Springs are not acceptable bonding paths due to the metallurgical concerns noted above. In addition, the contact loads will generally be significantly less than that found in a well-torqued fastener. The CFE738, for example, has experienced variations in measured bonding resistance with regard to the N1 (fan) monopole. This sensor uses an Inconel compression spring between the front frame and the interface plate and the resistance can be as high as 50 milliohms or lower than 2.5.

According to Perez (12), however, the manufacturer (Simmons) of that sensor has received no reports of bonding problems throughout their history. In light of that report, CFE has elected to continue with that design.

Rivets are acceptable if a minimum of three -inch rivets are used per junction and they are match-drilled (per Parmer (ibid.)). Rivets expand when set (as opposed to bolts, which neck-down when torqued: they fill their holes tightly -- guaranteeing good surface contact. Fisher and Plumer (ibid.) not withstanding, the components should not be painted or coated prior to riveting.

Conductive pastes, caulkings, and other sealing compounds may be appropriate if flange pressures of at least 10 lbf/in2 are achieved. Unless a special material is used, restrict the environment to 500 °F.

Bonding straps -- ground straps, if you wish -- are a common enough method of guaranteeing an electrical path between components and cases. The usual strap is MS25083 “Jumper Assembly, Electrical, Bonding and Current Return” although M83413/8-G (“Connectors and Assemblies, Electrical Aircraft Grounding: Type IV Jumper Cable Assembly, Lead, Electrical”) is similar. When referring to the MS part, be advised that pages 1 and 2 of the specification document apply to aluminum jumpers only and, as such, this material is not appropriate for power plant use. The resistance of these aluminum jumpers, “-1” and “-2”, exceeds the 2.5 milliohm limit at overall strap lengths of almost six inches.

The MS25083-3 strap is a quick-disconnect configuration and the line resistance is excessive. Once again, do not use.

Only the MS25083-5 strap has provisions for the common No. 10 fastener; the MS25083-4 and MS25083-6 are for -inch and -inch diameter fasteners, respectively.

AE has developed a flat woven strap similar to the one used on the T800 control: LH10489. A family of sizes are listed on PN 3000070, which used lugs which accommodate either #10 or -inch bolts. The current approved source is United Avionics, Naugatuck, CT. Not only does United make the T800 part they are heavily involved with Lycoming as a harness supplier.

Note: The initial RFT inspection showed that the (6-inch long) dash three parts had an end-to-end resistance of about 0.25 milliohms.

One pithy question has to do with the “round” vs. “flat” strap cross section. The T800-LHT-800 helicopter engine uses a flat strap for the Engine Control Unit (ECU), which is supposed to have the width (W) greater than five times the thickness (t): W> 5t. This geometry reduces the ease with which an induced current can flow over the strap: The lines of flux prefer a circular path as produced by a round strap.

This configuration is, apparently, The Strap of the Future as dictated by the ramifications of digital fly-by-wire, FADECs, and current knowledge of microwaves, EMI, RF, and lightning.

It would be well to note that bonding straps, or “jumpers,” are not always the best solution as they may represent a longer path between surfaces -- arcing may still result. Keeping a jumper’s length less than the air gap distance is a good rule of thumb to follow. (Gibbons (15))

For lightning bonding protection straps, Fisher and Plumer (ibid.) recommends the following guidelines (where the concern is with respect to resisting the effect of massive currents of a lightning strike -- not controlling electomagnetic Interference levels):

• Avoid all sharp bends -- current reversals lead to field changes and induced shear stresses may be high

• Must withstand common jet fuels (Jet A and JP-4, etc.), turbine lubricants (Mil-L-7808, Mil-L-23699, etc.) and engine shop chemicals

• Must be coated except for terminal contact points for corrosion resistance, wicking prevention, and unravel or fraying resistance

GE’s Jim Durham (of the Lynnclay, Ohio Nacelles and Installations Group) (7) witnessed a test in which a round strap vaporized while the same power surge did not affect a “5:1” flat strap. GE (or at least Durham) is adamant about the use of flat, straight straps. The 738’s circular cross-section FADEC straps routed IN A “loop” (i.e., 180° bend) are “in direct conflict with GE’s policies. If the straps aren’t changed by FFRR, (and if I had my way) I would recommend keeping the ‘AOG’.”

But then he offered that “We’ve got FADECs flying now without any field problems that have not been designed to this latest thinking.”

It is good to remember that the purpose of the bond straps is to insure that all parts and components of the engine and associated systems are existing at the same potential: so that they have the same relative ground. A lightning strike that passes through any bond strap such, as used on a jet engine, would vaporize same. They are too small to carry, by themselves, the enormous current density experienced in a lightning strike. The straps’ purpose is to prevent variations in potential -- the engine structure itself is “massive enough” -- and to keep harness shield currents as low as possible (to reduce the chance of induced currents in the control circuits).

GEAE Electrical Engineer Tom Carter (8) agrees with the 5:1 strap configuration and notes that the AC impedance is what is important. Lightning power peaks immediately then decays - which makes it a pseudo-AC experience. He also reiterates the stance that bends should be avoided and the straps kept as short as possible (to circumvent the jumping over air gaps). The choice between solid or woven construction appears to make little difference to his way of thinking. GEAE’s company line is basically “Don’t use straps if some other path is available.”

An interesting dichotomy exists between RF protection and lightning defense requirements, according to LTI’s Mike Dargi (11). On one hand, RF requires closely-bonded electrical components, so that the ground plane -- the electrical “zero” -- is common to all components and the airframe. Here, bonding is of utmost importance.

Lightning protection, on the other hand, would like to have all the electrical components totally isolated from the engine -- such that no current path exists at all!

But then you get an opposing opinion from GE’s Durham (ibid.), who remarked that “Lightning and EMI-RF -- meet one requirement, (you will) meet the other ‘reasonably well’.” These two schools of thought cannot both be correct.

According to the 1985 document, Working Draft of the (Proposed) Criteria for Aircraft Lightning Protection (4):

“The effects of lightning need not be considered if the total authority of the electrical/electronic power plant (i.e., engine) control system results in a variation in engine thrust or power of no more than 10 percent, and (in addition) after system failure:

Lightning electromagnetic energy can affect the engine systems by several ways by virtue of induced coupling, where voltage and/or current transients can result at interconnecting wiring interfaces of each individual LRU. System performance degradation can be either permanent or temporary. Component damage is permanent and are usually manifested as dielectric breakdowns and thermal effects. The temporary effect is a system functional upset, where (for example) signal processing systems; power generation; and control systems may be susceptible to electrical transients.” (3)

In order to mimic the installation completely, it may be necessary to provide a reasonable simulation of the aircraft-to-engine connections that are potential current paths. These may include the following potential current and/or voltage paths but will be found to be peculiar to each installation and engine configuration:

The lightning testing of an engine occurs in two phases: static and operating. The static testing determines the magnitude of lightning-strike-induced currents in each leg of the engine harness. When this is known the engine is subjected to those currents -- in each leg -- while the engine is running. The pertinent engine parameters ... spool speeds; ITT, thrust, etc. ... are monitored to ascertain the level of response to this so-called bulk-injected “lightning.”

The static testing allows the determination of the magnitude of induced currents in the various legs of the engine harness. This is achieved by introducing 1000 amperes into the front of the engine -- the return route is through a stand-off array of 14-gage wires -- and individually measuring the currents with simple ring current transducers. These values are proportional to the current strengths that could be expected in a real lightning strike and, thus, are extrapolated to the realistic lightning-strike values. This scaling usually by a factor of 1000x. These values are used in the Operational Testing.

For the “real world” testing, the engine re-attached to the thrust frame and run at about 80% T/O Fn. The values calculated during the individual harness testing are injected around the harness: “bulk injection.” (As opposed to “pin injection” wherein the current is introduced through the connector pins.)

The flight engine inlet or a reasonable facsimile should be used. No matter what it may look like, the materials and construction techniques should be representative of the real thing.

If at all possible, obtain scale drawings of the engine installation so that the relationship between the engine and the A/C pylon can be ascertained. The installation of a “ground plane” -- a vertically-mounted sheet (or plate, if you wish) of aluminum -- will be required and can be used to model the aircraft engine pylon. No matter what the configuration of the pylon, it can probably be safely assumed that the pylon will extend from the engine inlet back at least to the turbine case. That depends, of course, whether the test installation has a thrust reverser, an exposed mixer nozzle or is completely podded.

This pylon “ground plane” which will be solidly bolted to the engine support structure. It will be used to accept aircraft-engine interface mockups which will generally include the following

• Thrust mount: large-diameter welding cables can be routed from the front frames normal mount lugs to the ground plane.

• Fuel line: a representative length of the appropriate diameter of armored line should be fixed to the ground plane by a bulkhead fitting

(Note: Starter/generator cables are isolated by the devices’ windings and any Teleflex™-style mechanical throttle cables are not appropriate bonding paths in any event and therefore need not be included.)

A review of a proposed or an existing engine configuration should be performed to avoid having to reconfigure the components to conform to current thinking.

Following are two examples of bonding audits that were performed on the CFE738 and the TFE731-20. The bonding paths for each electrical LRU were traced through all interfaces ultimately to the airframe mounts. The surface coatings are noted and suggestions were provided to produce the best bonding path. It was interesting to find that many electrical components were designed to be installed onto painted or anodized surfaces without any apparent regard whatsoever for the electrical path.

Note: Attachment I is a chart showing the electrical components and the paths necessary to complete electrical bonding to the airframe. Materials and coatings are listed. Bonding methods, comments, and suggested changes are noted.

The accessory mounting points on the aluminum bypass duct have to be stripped of “Clear-Coat” protective paint at the contact points for the following components:

Note: The mounting brackets on the lower fan bypass duct are riveted to the duct before the entire assembly has been Clear-Coated. Since rivets expand when tightened (as opposed to a bolt, which necks down as it is torqued), they fill their holes tightly -- guaranteeing a good contact. Therefore no treatment of the duct-to-bracket interfaces is necessary.

The Eldec™ fuel flow meter sits in two silicon-rubber-cushioned saddle clamps and can only attain a ground through the fuel tubes -- which is not an acceptable path. Signal noise initially experienced at Dassault has reinforced the contention that the meter requires airframe bonding. GEAE and Eldec™ will arrange the use of one of the meter’s through-bolts as the anchor for a bond strap. AS will provide an Alodined area on the bypass duct flange for the strap. Be advised that the flow meter body must not be anodized at the strap’s contact point. A strap has been added to the parts list.

NOTE: The duct drawings have been revised to subsequently omit the Clear Coat, relying instead upon anodization alone for corrosion protection.

Hispano-Suiza, the manufacturer of the cast aluminum gearbox, has agreed to AE’s request that the following mounting and contact points be Alodined:

2.3 • PMA stator pad (the PMA’s contact surface is already bare per the print)

2.4 • Oil filter adapter pad (which mounts the oil filter bypass indicator) -- the indicator is made of CRES and the adapter’s contact surface is already bare per the print.

2.7 • Bonding strap-gearbox tether-lanyard bolt bosses are to be locally Alodined

The N1 Monopole is an enigma according to Engineering’s Perez (13.): “Sometimes it (the resistance to the engine’s front frame) reads 50 milliohms -- other times ... a lot less. I hope that (the CFE738 fan monopole) don’t need a bond strap.” The front frame drawing’s mount pad has been anodize-masked and the mounting plate (the “interface plate”) drawing will be changed to a full Alodine treatment (vs. anodization).

The N2 monopole is in the PMA stator which mounts to the AGBX (see item 3.3 above) and the contact is Alodined.

The aircraft main thrust mount is bond-strapped through its own threaded insert to the bypass duct which is, in turn, bolted to the front frame, which provides plenty of area and a number of fasteners.

The FADEC mounts are anodized; however the flange contact points to the front frame are locally Alodined. In addition, multiple bolts are used to attach the mounts to the engine structure. The spots on the mounts where the FADEC straps contact are Alodined per the drawing (or more generically, coated per MIL-C-5541, Type 3). In addition, the discrete FADEC mount contact points on the front frame are Alodined.

The FADEC straps currently used are round and “loop” in 180-degree arcs which is in conflict with industry guidelines. Pragmatic restrictions forced the retention of the existing attachment points so the straps do have to loop.

Note: Attachment II is a chart showing the electrical components and the paths necessary to complete electrical bonding to the airframe. Materials and coatings are listed. Bonding methods, comments, and suggested changes are noted.

The engine-to-aircraft electrical connector bracket bolts to the painted aluminum bypass duct flange.

The surge control solenoid pack bolts to the painted aluminum bypass duct pads. The contact areas must be locally Alodined and left un-painted.

The ignition exciter box bolts to the painted aluminum bypass duct pads. The contact sites must be locally Alodined and remain un-painted.

The N1 compensator bolts to the painted aluminum bypass duct flange. The contact point must be locally Alodined and left un-painted.

The DEEC is mounted on isolators and is therefore must be bonded with straps (2) to the titanium fan containment housing. It is suggested that the straps be of the flat configuration and as straight as possible. Note: As the two parts are isolated, concerns regarding the power of the dissimilar metal’s galvanic couple appear to be unwarranted.

The front frame is effectively isolated from the thrust mount and a flat straight bonding strap should be incorporated between the two.

The titanium LPC Case and the bypass duct (painted aluminum) bolt to the titanium front frame. The mating surfaces of the Duct must be stripped of paint and Alodined.

The LPC case is joined to the magnesium transfer gearbox by an anodized aluminum housing support tube. The mating flanges need to be Alodined.

The N1 monopole is mounted against an Inconel compression spring. Inco is not a good conductor as its alloying agents, nickel and chromium, are resistive (as far as metals are concerned). A captive, easy-to-install ground strap is suggested between the sensor’s connector and the rear bearing support.

This exercise includes Table I which is an outlined list of the pertinent components, listing the material, the surface coating(s), and any appropriate suggestions or remarks (from a bonding standpoint).

Chart I is an electrical flow path from the lowest-hierarchy part leading to the front frame, then to the engine mount, and eventually to the airframe. Suggested coating changes are noted on the inter-component leader lines.

The TFE731 accessory and transfer gearboxes both are cast magnesium and have been anodic coated -- effectively insulating them electrically. These are the tricky parts, as any MgO2 coating will be non-conductive. There are several painful options:

• Provide bonding pins -- either round or diamond in cross-section -- but these will, in time, may come to have a loose fit and the desired electrical path may be broken.

• Rely on stud or bolt contact. As high-strength fasteners are notoriously poor conductors this is a somewhat conflicting suggestion.

• Provide uncoated contact surfaces that are “buried”, i.e., hopefully unexposed to the environment. If these unprotected magnesium surfaces do get attacked however, the corrosion products and the resulting electrical path will, in all probability, become a dead-end.

• Bond straps are easy to implement and trouble-free -- but maintenance-rich. It may be necessary to “flat strap” from the oil pump and the fuel control directly to the front frame (plus several AC-installed accessories). While bypassing the gearbox entirely may ameliorate the situation, the straps could be as long as 12 inches.

• Both the Transfer and Accessory Gearboxes require that all threaded inserts and studs be installed per FP5039 (“Installation of Dissimilar Metals into Magnesium and Aluminum for Maximum Corrosion Protection”) which, in turn, specifies the use of epoxy--amine primer per PSC5401 (NPC61611). The ramification is that these threaded items are effectively isolated from their cases, electrically and corrosion--wise. Magnesium corrodes very easily by virtue of its location at the highest position in the electromotive force series (or the galvanic series). Magnesium will become anodic and corrode when placed in contact with any metal and will be particularly active in concert with iron--based metals. In addition, magnesium corrosion is accelerated when subjected to electrical currents, e.g., providing bonding paths.

AFSC DH 14 (10) suggests using an aluminum washer under the fastener head with the screw and nut cadmium-plated. This electromotively-intermediate metal, aluminum, was selected as it lies between magnesium and the ferric alloys, providing more than a modicum of corrosion protection.

Protective finishes, such as paint and plating, should be used with car (10). When dissimilar metals are in contact, do not cover the surface of only the anodic material (i.e., the higher electromotive material): either cover the surface of both materials, or only the cathodic material.

The most effective means of minimizing corrosion (other than avoiding the use of dissimilar metals altogether) is to exclude moisture from the bonded areas with edge seals or conductive pastes, both of which are problems for maintenance personnel.

In order to guarantee the 2.5 milliohm component-to-component bonding requirement now in effect throughout the aircraft industry, these suggestions -- “strip”; “Alodine”; and “bond strap” -- should be accommodated, no matter how painful.

It would prudent to pay attention to comments offered by Forest Hover who would have “ . . . more confidence in these solutions if it were a military program -- something more regimented; something more structured. (In the private sector) nobody reads the manual -- and if (the procedure) is not obvious . . . it doesn’t get done (properly)..”

The accessory pads -- for the oil pump, FCU/HMU, starter/generators -- should have one circular area, concentric to an upper outside mounting stud, masked during anodization and NOT coated per Mil-C-5541 (i.e., “Alodined).

The pieces that mate to the gearbox should have a similarly-sized ring free of anodization so that they match. The HMU flange should have yet another similar contact patch on the outboard side for contact with a bond strap that will connect to the front frame.

The bond strap will have to have 3/8” terminal on one end and a ____ on the opposite.

Alodine 600 is a chemical conversion coating -- a local surface treatment -- that is “. . . applied non--electrically by spray, brush, or immersion after all heat treatments and mechanical operations have been completed.” It is time--dependent with respect to the thickness: “The primary difference between a Class 1A and Class 3 MIL-C-5541 Chemical Film coating is thickness . . .” (Note: there is no Class “1” nor “2.”) According to Jim Laird, “Two minutes is sufficient application time for Class 3.”

The Alodinic film coating is thin (t ≈ .0002”) and not as hard and nor as resistant to damage as an anodic coating. Therefore, subsequent handling, assembly, and disassembly will, in all probability, result in the deterioration of the Alodined surface and the part will end up with the base material exposed.

Note: also designed as a gearbox restraining strap in the case of a blade-out situation

Bonding and Current Return Electric Bonding Assembly: MS25083-5BB6; #10 holes; 6-inches long; pure tinned copper AWG 12 7 x 37, AWG size 36 strand

Engine Component or System	Interference	Outage
Engine rpm gauges	0	4	2%
Engine exhaust gas temperature	0	2	1%

	Suggestion	Pro	Con
1	Bond through strap onto bare flange -- then over-paint assy	• Typical feature on many engines	• Several straps req’d -- will look “busy” • Braided bond straps wick and retain fluids -- particularly at the lug • Field repair might result in paint removal
2	Bond via strap through stud or bolt into case	• Looks normal • Inexpensive • Easy	• Insert electrically isolated from case by epoxy
3	Bond with strap via though-bolt to case	• Looks normal • Inexpensive • Easy	• Case painted -- electrically isolated
4	Bond between stripped mating flanges	• No additional parts	• Exposed margins susceptible to corrosion
5	Bond between mating flanges masked at discrete internal locations	• No additional parts	• Difficult to guarantee sufficient electrical contact
6	Conductive gasket between stripped mating flanges	• Isolates bare magnesium from atmosphere	• Additional parts • Exposed margins susceptible to corrosion • Alter component stack-up
7	Conductive gasket between stripped mating flanges with external beading	• Isolates bare magnesium from atmosphere	• Additional parts • Complicated parts • Alter component stack-up
8	Conductive O-ring between bare groove and bare face	• Isolates bare magnesium from atmosphere	• Additional parts • O-ring groove required