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Safety margins on CFM56-5B (A320) vs CFM56-7B (737NG)
I wanted to know about the safety margins for CFM56-7B on 737NG aircraft vs the CFM56-5B on A320 aircraft. I know that the current generation (and near-current generation) of engines operate precariously close to the operating margins of safety because efficiencies have been eked out again and again. I wanted to know if the 737 engines operate closer to the margins of safety, due to the smaller fan diameter and things generally being squished in as much as possible to have adequate ground clearance. I ask because uncontained engine failure on narrowbodies seem more prevalent on 737 types than A320 types.
Re: Safety margins on CFM56-5B (A320) vs CFM56-7B (737NG)
As a follow-up, do you know of any major incidents with the A320's sibling engine, the CFM56-5B?
https://en.wikipedia.org/wiki/CFM_International_CFM56#CFM56-5_series (A320)
https://en.wikipedia.org/wiki/CFM_International_CFM56#CFM56-7_series (737NG)
https://en.wikipedia.org/wiki/CFM_International_CFM56#CFM56-5_series (A320)
https://en.wikipedia.org/wiki/CFM_International_CFM56#CFM56-7_series (737NG)
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- Newbiepilot
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Re: Safety margins on CFM56-5B (A320) vs CFM56-7B (737NG)
1 in a billion per flight hour is the requirement for probability of a catastrophic failure. Propulsion rules aren’t quite the same as systems, but the logic from FAR 25.1309 still applies.
AC 25.1309–1 provides background for important concepts and issues within airplane system design and analysis.
Catastrophic failure condition rate
The circular provides a rationale for the upper limit for the Average Probability per Flight Hour for Catastrophic Failure Conditions of 1 x 10−9 or "Extremely Improbable".[5] Failure Conditions having less severe effects could be relatively more likely to occur; that is, an inverse relationship between severity and likelihood.
Fail-Safe Design Concept
This AC presents the FAA Fail-Safe Design Concept, which applies basic objectives pertaining to failures:
Failures of any system should be assumed for any given flight regardless of probability and such failures "should not prevent continued safe flight and landing" or otherwise significantly reduce safety
Subsequent failure during the same flight should also be assumed.
The AC lists design principles or techniques used to ensure a safe design. Usually, a combination of at least two safe design techniques are needed to provide a fail-safe design; i.e. to ensure that Major Failure Conditions are Remote, Hazardous Failure Conditions are Extremely Remote, and Catastrophic Failure Conditions are Extremely Improbable.
https://en.m.wikipedia.org/wiki/AC_25.1309-1
In general you can’t assume one is safer than the other. The required probability of failure is the same for all potential hazardous and catastrophic conditions. Most fault trees in the design process show probability of failure much less than 1x10^-9. -11 to -14 is more common, but you will have to go to the FAA or delegated representatives to get the exact numbers used in the certification plans and fault trees.
I would not say that. It is not a reasonable assumption. Higher core temperatures and higher compression ratios do lead to higher efficiency but I would not say they are precariously close to operating margins of safety. Those higher temperatures and pressures result in more wear and costlier overhauls to restore the engine, but that doesn’t mean safety is compromised. Engines are far more reliable than they were decades ago with lower pressures and temperatures. CFM56s have stayed on wing for 50,000 hours between engine overhauls (record set by a Tuifly 737-800). The CFM56 is the record holder for time on wing as far as I know due to reliability and safety.
I honestly believe the answer to your question is that neither is closer to the margin of safety. Airplanes simply aren’t designed that way. If anyone wants to defintively answer that question I hope they have an advanced degree in mechanical engineering and not just an aviation fan or pilot.
AC 25.1309–1 provides background for important concepts and issues within airplane system design and analysis.
Catastrophic failure condition rate
The circular provides a rationale for the upper limit for the Average Probability per Flight Hour for Catastrophic Failure Conditions of 1 x 10−9 or "Extremely Improbable".[5] Failure Conditions having less severe effects could be relatively more likely to occur; that is, an inverse relationship between severity and likelihood.
Fail-Safe Design Concept
This AC presents the FAA Fail-Safe Design Concept, which applies basic objectives pertaining to failures:
Failures of any system should be assumed for any given flight regardless of probability and such failures "should not prevent continued safe flight and landing" or otherwise significantly reduce safety
Subsequent failure during the same flight should also be assumed.
The AC lists design principles or techniques used to ensure a safe design. Usually, a combination of at least two safe design techniques are needed to provide a fail-safe design; i.e. to ensure that Major Failure Conditions are Remote, Hazardous Failure Conditions are Extremely Remote, and Catastrophic Failure Conditions are Extremely Improbable.
https://en.m.wikipedia.org/wiki/AC_25.1309-1
In general you can’t assume one is safer than the other. The required probability of failure is the same for all potential hazardous and catastrophic conditions. Most fault trees in the design process show probability of failure much less than 1x10^-9. -11 to -14 is more common, but you will have to go to the FAA or delegated representatives to get the exact numbers used in the certification plans and fault trees.
brian415 wrote:i know that the current generation (and near-current generation) of engines operate precariously close to the operating margins of safety because efficiencies have been eked out again and again.
I would not say that. It is not a reasonable assumption. Higher core temperatures and higher compression ratios do lead to higher efficiency but I would not say they are precariously close to operating margins of safety. Those higher temperatures and pressures result in more wear and costlier overhauls to restore the engine, but that doesn’t mean safety is compromised. Engines are far more reliable than they were decades ago with lower pressures and temperatures. CFM56s have stayed on wing for 50,000 hours between engine overhauls (record set by a Tuifly 737-800). The CFM56 is the record holder for time on wing as far as I know due to reliability and safety.
brian415 wrote:. I wanted to know if the 737 engines operate closer to the margins of safety, due to the smaller fan diameter and things generally being squished in as much as possible to have adequate ground clearance. I ask because uncontained engine failure on narrowbodies seem more prevalent on 737 types than A320 types.
I honestly believe the answer to your question is that neither is closer to the margin of safety. Airplanes simply aren’t designed that way. If anyone wants to defintively answer that question I hope they have an advanced degree in mechanical engineering and not just an aviation fan or pilot.
Last edited by Newbiepilot on Sat Apr 21, 2018 2:34 am, edited 2 times in total.
Re: Safety margins on CFM56-5B (A320) vs CFM56-7B (737NG)
The CFM56 is widely regarded in the industry as one of, if not the most reliable powerplant available. Pretty sure it holds the record for time on wing before an overhaul. I’d suspect that any possible issues is because of freak coincidences or isolated manufacturing flaws.
Re: Safety margins on CFM56-5B (A320) vs CFM56-7B (737NG)
Other than Southwest, what operators have had uncontained failures on the CFM56-7s?
Re: Safety margins on CFM56-5B (A320) vs CFM56-7B (737NG)
The CFM is a pretty good engine...as mentioned above the engine is certified and has to be shown compliance with all applicable rules during that process. I'll take a CFM over the IAE on the A320 any day...uncontained engine failures have occurred and the past and will unfortunately continue to occur in the future, no matter the engine type. There are just too many engines in service and eventually a few will fail, but this is an extremely low percentage.
Re: Safety margins on CFM56-5B (A320) vs CFM56-7B (737NG)
The recent incident on Southwest was a contained engine failure. Disintegration of the inlet cowl will end up being the real issue I think.
Re: Safety margins on CFM56-5B (A320) vs CFM56-7B (737NG)
zanl188 wrote:The recent incident on Southwest was a contained engine failure. Disintegration of the inlet cowl will end up being the real issue I think.
Last I heard, a fan blade broke off, breaking into two pieces, hitting the fuselage. I believe that qualifies as an uncontained failure, does it not?
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