Cylinder Safety Program

Jimmy Tubbs
Cylinder Safety Project• Statistics:

– 27,000 each 520/550 cylinders produced 

nov 2002 thru June 2008

– 24 confirmed head-to-barrel head thread 

failures

– 7 failures on cylinders produced after 20– Additional similar type failures (in FAA lis

resulted from two additional failure mod

• Head separation at top of barrel• Head & barrel disengaging
Cylinder Safety Program

Time Line

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Cylinder Safety Project-Design History• AEC Replacement Cylinders:

– Designed from continental cylinders– FAA knew of high continental cylinder he

barrel separation history

• Was discussed in AEC certification doc• The Service Difficulty Reports from th

show more than 1200 Continental heabarrel separations between 1974 and 
Cylinder Safety Program

AEC and OEM Head-Barrel Separations 1974 To Today
Cylinder Safety Project-Search For Cau• FAA/NTSB Meeting In San Antonio In 

– Wide Ranging Issues Discussed– Most Pressing Issue Was HB Separations– Suggestion To Develop Lab Testing Tech

Identify Root Cause For HB Separations
Cylinder Safety Project-Search For Cau• Lab testing in 2006-

2007 time frame

– Higher interference 

fits were the result

• Tests on Lycoming 

cylinders very conclusive

• Continental tests-not 

so much 

– Engine Testing 

Needed
Cylinder Safety Project-Search For Cau• AEC Proposed Root Cause Engine Tes

FAA In Late 2010 and Again in Early 2

• AEC Developed A Test Program Plan

– Briefed FAA on Plan June 2011

• Instrumentation, Engine Build and Te

Started December 2011

• Report 1202-2 Dated Feb. 20, 2012
Conducted instrumented engine tests in late 2011 and early 2012
Cylinder Safety Program• Cylinders instrumented at critical location with high temp strain 

thermocouples
Cylinder Safety Program• A range of interference fits were tested

Engine Cylinder Position Seal Band Interference Thread Interference

#1 0.008 #2 0.01 #3 0.012 #4 0.014 #5 0.016 #5 Glazed 0.017 

#6 0.018 
Cylinder Safety ProgramTest Results:The test data

plotted as afunction of CHT

Exhaust Side Strain Trend Plots vs CHT

Phase 2 Tests, TO Power

200220240260280300320340360380400420440460480500520540

300 320 340 360 380 400 420 440 460 480 500 520 

CHT, Deg. F

Strain, Microinches Cyl #5 Cyl #2

Cyl #4
Cylinder Safety ProgramEffect of Bad Break-In
Results of NTSB Meeting Nov. 2012• Suggested FEA Correlated to Engine T

Data 

• AEC Agreed On The Need 

– Delta G

• Used AEC 3-D Model• Set Range of Boundary Conditions To 

Assembly and Operation
Computer 

Model
Stress with no 

seal band friction

Shows seal band 

contribution essential
Stress with no seal 

band slip

Thread stress within 

Material limits with seal band contribution
Cylinder Safety Program• Analysis Supports AEC Position:

– AEC631397 safe when operated within li

• With a margin

– AEC Inspection and Removal at TBO 

• Pre-April 2006• All manufactured with less than .011 i

interference

• Give inspection credit for engine moni

system
Cylinder Safety Program• What does the testing and analysis te

– Stresses become critical if:

• CHT above published limits• Break-In attempt does not seal piston 

and cylinder bore

– New assembly practices will not prevent 

failures, but does increase margins
Cylinder Safety Program• Effects of Higher Interference Fits

– Higher interference fits will not prevent 

head-barrel separations

• However, other failure modes caused 

excessive heat may cause failure befobarrel separation

– Most of the failure modes more 

catastrophic than head-barrel sepa
Cylinder Safety ProgramIndications ofOver Heat
• GOOD
Cylinder Safety Program• Causes of cylinder overheating

– Induction leaks– Mixture– Malfunctioning or blocked injectors

• Air side of turbocharged engines

– Ignition timing– Spark Plugs

• Loose• Broken insulators• Bad resistors
Cylinder Safety Program• What other factors are relevant?

– CHT gauges not required for many airpla– On many with CHT, only one cylinder mo– CHT gauges not required to be calibrated– Mechanics not trained adequately about 

engine Break-In procedures
Cylinder Safety Program• Factors (Cont)

– In the old days-

• Chrome bores were very difficult to Br• Oil burners were considered a nuisanc

safety issue

– Hundreds of head-barrel separation

past years show that it was just acas a part of aviation

– We now know the root cause for head-ba

separation, but changing is difficult!
Cylinder Safety Program• Discussions have been held with the 

Airplane Directorate and GAMA

– They believe that mandating old airplane

retrofitted with CHT gauges on all cylindpractical

– Also, they believe it would be difficult to 

calibration requirements on older airplan

– May be a way to address for new airplan

• ASTM F44
Cylinder Safety Program

– Specifics

• AEC suggests the following (as incor

in MSB 13-3):

1. Recommend a repetitive inspection a

removal at overhaul of cylinders prosince end of MSB 06-2 to April 20061. AEC does not have assembly data 

these cylinders, so cannot segregsubsets
Cylinder Safety Program

Specifics (Cont)2. Recommend a repetitive inspection a

removal at overhaul of cylinders prosince 2006 with less than .011 seal binterference.1. The testing shows that this would 

adequate safety margin for tempeexcursions

2. Give Credit For Engine Monitors

1. 50 Hr vs. Annual Inspection
Cylinder Safety Program

• AEC Should:

– Provide owners/operators with d

information on the issue of high ctemperatures and glazed cylinder » SIB 12-2

– Encourage Engine Monitoring
Cylinder Safety Program

• FAA Advisory Data such as SAIB could 

useful

• NTSB Advisory?• Root cause information should be sha

– AEC working on publications– Mike Bush just published an article 

another to follow
Cylinder Safety Program• The NTSB:

– Can promote a technical root cause base

approach to this safety issue

– Can encourage root cause issues to be pr

to the aviation public by the FAA

Questions?