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11.2.5.3 Remedies for Damage to Accessory Equipment and Peripheral Devices

One advantage of the externally accessible engine equipment and peripheral devices is the possibility of inspecting and testing them with relative ease. Inspections can serve as a temporary measure in acute cases, when remedies can not be implemented sufficiently quickly for temporal and logistic reasons. Examinations can be conducted as separate procedures, but should preferably be done during maintenance and inspection. They can be specifically designed for the problem at hand in various ways. The following are several examples:

  • Visual inspection (e.g. leaks, overheating, signs of corrosion, fouling, wear, installation anomalies)
  • Testing functions (e.g. in electronic devices and pipes)
  • Crack detection, if accessible (e.g. penetrative testing)
  • Testing play (jamming, looseness)
  • Testing starting moments and operating forces

Knowledge of the frequency of the damage types is helpful for designing preventive measures (Ill. 11.2.5-1). Typical measures to minimize risks are:

Oil system:

  • Inspecting, analyzing, and assessing filters and magnetic chips at prescribed intervals (bearing damage, rubbing, foreign objects)
  • Inspecting oil conditions after typical run times and operation (aging, fouling)
  • Noticing unusual oil losses (signs of internal or external leaks, oil fires)
  • Looking for coking in the pipes and bearing chambers as early as during the development phase (danger of blockages, indication of oil fires or wear). Return oil lines and ventilation lines that pass through braces in the hot part area are especially susceptible to coking.
  • Being alert to increased oil temperatures during operation
  • Avoiding increased water content in the oil (condensation water in open oil tanks!). Water vapors and poor lubricating properties can lead to bearing damage and regulator instability (heavy vibrations in actuator systems, pipe fractures).
  • Noticing lines with unusual discolorations. This indicates hot gas encroachment and oil fires.
  • Preventing damage during installation (deformation, scratches)

Instrument and sensor errors:

  • Preventing corrosion and fouling on plugs and contacts
  • Only using approved auxiliary materials such as cleaning materials, corrosion protection, and lubricants
  • Conducting prescribed inspections after long standing times
  • Determining and eliminating the causes of false signals from instruments and warning systems as quickly as possible
  • Taking “isolated” incidents seriously during engine development
  • During development, always assume that operating fluids and auxiliary media will coat the engine parts, and ensure that this contact will not result in undesirable reactions. This is especially important with synthetics and elastomers.
  • No possibility of mistaking parts (plugs, lines, etc.)

Fuel system:

  • No areas may become worn
  • Titanium alloys should not be used as fuel lines
  • If stress corrosion cracking occurs in connecting parts (Volume 1, Ill. 5.4.2.2-1), the possibility of a sensitive production lot must be examined. The affected parts may have to be replaced immediately.
  • Preventing overheating of the fuel (e.g. formation of abrasive particles that can destroy fuel nozzles). This can be caused by remarkably low overtemperatures.
  • Only using seals from approved manufacturers
  • If wear damage and jamming occurs on axial piston pumps and regulators with silver plating, the fuel must be inspected for increased sulfur content.
  • Not deforming pipes during installation (increases risk of dynamic fatigue fractures)
  • Preventing damage during assembly (deformation, scratches)

Air system:

  • Wire mesh that protects motor bellows must be inspected for wear
  • Valve malfunctions must be corrected immediately, since they can cause compressor surges and/or excite dangerous blade vibrations.
  • Pipes must not be deformed during installation (increased risk of dynamic fatigue fractures)
  • Preventing damage during assembly (deformation, scratches)

Gearboxes:

  • Taking apparent “isolated” incidents seriously during the development phase, and implementingtargeted solutions.
  • Experienced, trained personnel for gearbox assembly
  • Storing gearboxes requires suitable conservation to prevent corrosion
  • If engines have stood for long periods in corrosive environments, the roller bearings must beinspected for corrosion
  • Paying attention to unusual noise development
  • If damage occurs in a gear train, the tooth contact patterns must be evaluated
  • Even if the assembly position is difficult, gears must never be snapped into place by tightening thecover over the bearings. This damages the bearings and/or bends the gear shaft, resulting in fatigue damage and shaft failures during later operation. It must always be possible to press gearbox covers shut by hand with no resulting gap.
  • Looking for oil leaks (also “sweating out” of oil)
  • Seals that leak a small amount of oil at splined connections should only be “improved” to totalseal effectiveness if it is clear that the splining does not require the oil to prevent wear.

Illustration 11.2.5-6: Accessory equipment can be subjected to high dynamic (LCF, HCF) and static loads (middle diagram). Special types of vibration are excited, depending on the excitement mechanism and mass distribution. These vibrations can cause both the aggregates and the fastening systems (bottom diagram) to fail. Because fasteners and fastened masses reciprocally influence one another, even “minor changes” (e.g. during modernization) require realistic verification in tests close to operating conditions (Ill. 12.5.3.4-5).

References

11.2.5-1 NTSB Identification: FTW94IA198, incident from June 17, 1994.

11.2.5-2 NTSB Identification: LAX94FA323, incident from August 13, 1994.

11.2.5-3 NTSB Identification: LAX87IA029, microfiche number 34708A, incident from October 31, 1986.

11.2.5-4 S.W. Kandebo, “USAF Targets Engine Mishaps”, periodical “Aviation Week & Space Technology”, March 29, 1999, pages 84 and 85.

11.2.5-5 M. Miller, J. Colehour, K.Dunkelberg, “Engine Case Externals Challenges and Opportunities”, Proceedings Paper of the “Isromac-7, the 7th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery”, February 22-26, 1998, pages 1604-1611.

11.2.5-6 G. Norris, “Boeing tackles 777 power problems”, periodical “Flight International”, 18-24 August,1999, page 6.

11.2.5-7FAA takes action on R-R AE3007 FADEC transistor failures”, periodical “Flight International”,12-18 September, 2000, page 14.

11.2.5-8 R.L.Johnson, R.C. Bill, “Fretting in Aircraft Turbine Engines”, Proceedings AGARD-CP-161 of the “Specialist Meeting on Fretting in Aircraft Systems, pages 5-1 bis 5-13.

11.2.5-9 P. Phelan, ”'Green' fuel sparks off fears of engine failures“, periodical “Flight International”, 17-23 August, 1994, page 6.

© 2020 ITTM & Axel Rossmann
11/112/1125/11253/11253.txt · Last modified: 2020/06/25 22:43 (external edit)

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