1. 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

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

3. Fuel metering unit

4. 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

5. 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?

6. Stresses -40° to +80°C SAT; -40° to + 180°C fuel temperature;
humidity;
vibration and shock;
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.

7. 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.

8. 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).

9. Empirical life
Dump valve

10. Empirical life
LVDT

11. Parametric life

12. Unit hazard plot

13. Test, fail, fix
BS5760
BS5760

14. Screening
BS5760
BS5760

16. HALT and HASS
Thermotron.com

17. ADT
Nelson, W., Accelerated Testing, 1990

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

19. 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.

20. Root cause analysis