Durability testing for mechanical engine control units and accessories O Greenwood 18 October 2018 One principle: cost. Lifecycle cost should be the driver for all decisions related to product robustness. Three stages: discover the in-service reality of our products, including SEE and

Accessories Ainonline.com; Rolls-Royce plc, The Jet Engine, 2015

Fuel metering unit

Fault descriptions Event Date Event Type Primary Fault Fault Category Component Fault 16/12/2014 Unscheduled LVDT OPEN CIRCUIT Confirmed - LRU 10/03/2015 MVDS SEAL 14/08/2014 MMV DYNAMIC SEAL WEAR 03/12/2014 27/01/2015 CONNECTOR 02/03/2015 Dump valve 06/02/2015 Position indicator POSITION INDICATOR 18/02/2015 MV SEAL 17/02/2015 23/02/2015 LVDT 24/02/2015 25/02/2015 19/02/2015 LP PORT FILTER 27/02/2015 01/03/2015 MMV Servo Valve 12/03/2015 06/03/2015 DUMP VALVE 09/03/2015 Dump Valve 23/12/2014

Strategy Required information What are the functions of the accessory? The unit in service What are the functions of the accessory? What types of loads and stresses is the accessory subject to during its lifecycle? Unit failures In what ways can the accessory fail to perform its functions? Which failure modes are most disruptive in service? Does the life data for the accessory suggest trends in the rates of each dominant failure mode? Causes of failure What physical mechanisms cause the relevant failure modes?

Stresses -40° to +80°C SAT; -40° to + 180°C fuel temperature; humidity; vibration and shock; 30 000 cyclic high fuel pressure cycles; blowing sand and dust; fuel contamination; exterior contamination; fungus attack; exposure to water, ice and salt; lightning; electromagnetic interference; and, occasionally, fire.

Unit failures FMEA – 839 failure modes but just 8 cause >90% of unit failures. FMEA considered most of the failure modes but does not always provide the failure mechanism which is crucial for test. FMEAs cannot easily take account of interactions between stresses.

Mechanisms Wear; Corrosion or stress-corrosion; Build-up of fuel lacquer / debris; Cavitation erosion; Compression set; Shrinkage of polymer and subsequent high cycle fatigue through vibration; Fatigue and deformation of elastomeric material caused by swelling when exposed to fuel followed by vibration; Fretting (due to contamination); Low cycle fatigue due to thermal cycling (mismatch of adjoining materials’ coefficients of thermal expansion).

Empirical life Dump valve

Empirical life LVDT

Parametric life

Unit hazard plot

Test, fail, fix BS5760 BS5760

Screening BS5760 BS5760

HALT and HASS Thermotron.com

ADT Nelson, W., Accelerated Testing, 1990

The role of energy in degradation McPherson, J.W., Reliability Physics and Engineering, 2013

Reliability testing currently Service Test EID open circuits MVDS wear MVDS cavitation Dump valve leakage TOS filter erosion MVSV inlet blockage Electrical receptacl e failures Thermal cycling x 2 Some detected, declared an overtest   Contamin ation x 2 Not severe enough to cause leak Detected twice but not fixed until in service No fuel lacquer but some debris on filter SIMOP Not detected Vibration x 2 Detected This table relates to the FMU702/801.

Root cause analysis