Forum: F-35 Lightning II

quicksilver wrote:

...forced the Navy variant to be single engine. Propulsion was/is part of the USAF URF constraint, and they will also tell you (with statistical justification) that modern single engine jets are safer than those with two.

I don't know where this mathematical fallacy got started around here but I've seen it repeated a couple of times now. It's not true. Let me try to drive a stake through the heart of this misconception once and for all.

Suppose you have a jet engine. This engine fails, on average, once per year. Suppose you fly a jet with this engine, on average, once per day. On any given day, the probability of that engine failing is 1 in 365 (1/365). Suppose further that if the engine fails my jet crashes and sometimes the pilot is killed as well.

What if use two engines on that jet, even though I can fly it with only one, then what? Am I better or worse off? Let's look at two numbers, number of engine failures and likelihood of jet crashes.

I add a second engine, which also has a failure rate of 1/365, then fly that same jet every day, I will on any given day have a probability of one of those engines failing being 1/365 + 1/365 = 2/365 or 1/182. Another way to look at this is that I will have twice as many single engine failures over the course of a year (one every six months).

But here's the key point. What is the probability that both those engines will fail at the same time? That is a different calculation: 1/365 x 1/365 = 1/133,225!

More importantly, over the course of a year, the probability of of my single engine jets experiencing a catastrophic (one engine out) failure is 365 x 1/365 = 1 or 100%. The probability of my twin engine jets experiencing a catastrophic (two engines out) failure is 365 x 1/133,225 = 0.0027 or 0.27%.

Granted, you are now paying for two engines and fixing engines twice as often. However, you are no longer losing jets or pilots - or only extremely rarely. Which aircraft would you rather fly? And no matter what failure rate you quote, the idea that two engines with the same mean time between failures (MTBF) are LESS safe than one is absurd and mathematically indefensible.

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This team oughtta be the 'B/S team' rather than the 'B&SS guys'. Very Happy

F-35C Carrier Variant Joint Strike Fighter (JSF)

http://www.globalsecurity.org/military/ ... /f-35c.htm

The JSF Basing and Shipboard Suitability (B&SS) team ensures that the F-35 Air System is compatible with and operationally supportable from all JSF basing options, and provides a single point-of-contact within the JSF Program for all matters relating to the JSF aircraft's suitability for operations at each of its basing options, which includes all CONUS/OCONUS shore bases, U.S. Navy aircraft carriers and L class ships, and UK Royal Navy carriers. Additionally, the team serves as the JSF program's primary interface with ship development programs such as the USN's future carrier CVNX, the USN's replacement for its LHA amphibious assault ship currently referred to as LHA(R), and the UK RN's future carrier CV(F).

The primary tool for assessment, verification, and design influence for basing and ship suitability is the BASS checklist which is comprised of over 300 line items consisting of relevant basing explicit requirements, prior military specifications and standards, and recognized good design practices developed in conjunction with LM. The JPO BASS team coordinates with LM and reviews BASS Assessment Reports (BARs) for each line checklist line item.

JPO BASS team led processes include maintenance of the ship integration watch list, which consists of potential ship alteration issues, and disposition of these through the Facilities Available Conference (FAC) which is a forum that has provided F-35 designers an understanding of existing shipboard facilities and interfaces. The intent is to influence designers to develop a JSF air system that fits within and is compatible with existing facilities and interfaces in wake of recognition that there are JSF contractual capability requirements that conflict with contractual ship compatibility requirements. In the event of ship alterations, this forum led by the BASS IPT, consisting of representatives from the JPO, LM, NAVAIR, NAVSEA, OPTEVFOR, USN and USMC requirements offices, and Naval Aviation type commanders, determines the best systems of systems approach to integration in time to properly budget and plan necessary ship changes.

Other BASS IPT responsibilities include helping to initiate and foster relationships for technical exchange with all critical govt BASS stakeholders external to the program, provide feedback on IPT trade studies, participate in BASS advisory panel reviews, coordinate with IPTs on status, risk, issues and requirements; and support mitigation of program-level risks associated with BASS.

The B&SS IPT draws focus to basing compatibility considerations and issues by serving as a reference source to the JSF team for the design and development of the JSF family of variants. The B&SS team coordinates with other teams within the JSF program to maintain a "big picture" design perspective and to ensure all appropriate issues are addressed.

Overall, the JSF Basing & Shipboard Suitability IPT is responsible for ensuring that the requirements are met. The JSF must be able to operate from US and allied shore-based facilities. Considerations for basing suitability, and shipboard compatibility in particular, include of a wide range of complex integration issues such as:
Approach flying qualities
Catapult hookup
Deck handling
Deck spotting
Elevator compatibility
Flight deck servicing
Hangar deck maintenance
Jet blast effects
Landing systems
Ship motion
Shipboard environment
Steam ingestion..."

Quote:

This team oughtta be the 'B/S team' rather than the 'B&SS guys'.

Sounds like one of those teams that could be either a really useful source of information or a bureaucratic nightmare depending how well organized and how well run it is.

stereospace wrote:

quicksilver wrote:

...forced the Navy variant to be single engine. Propulsion was/is part of the USAF URF constraint, and they will also tell you (with statistical justification) that modern single engine jets are safer than those with two.

I don't know where this mathematical fallacy got started around here but I've seen it repeated a couple of times now. It's not true. Let me try to drive a stake through the heart of this misconception once and for all.

Suppose you have a jet engine. This engine fails, on average, once per year. Suppose you fly a jet with this engine, on average, once per day. On any given day, the probability of that engine failing is 1 in 365 (1/365). Suppose further that if the engine fails my jet crashes and sometimes the pilot is killed as well.

What if use two engines on that jet, even though I can fly it with only one, then what? Am I better or worse off? Let's look at two numbers, number of engine failures and likelihood of jet crashes.

I add a second engine, which also has a failure rate of 1/365, then fly that same jet every day, I will on any given day have a probability of one of those engines failing being 1/365 + 1/365 = 2/365 or 1/182. Another way to look at this is that I will have twice as many single engine failures over the course of a year (one every six months).

But here's the key point. What is the probability that both those engines will fail at the same time? That is a different calculation: 1/365 x 1/365 = 1/133,225!

More importantly, over the course of a year, the probability of of my single engine jets experiencing a catastrophic (one engine out) failure is 365 x 1/365 = 1 or 100%. The probability of my twin engine jets experiencing a catastrophic (two engines out) failure is 365 x 1/133,225 = 0.0027 or 0.27%.

Granted, you are now paying for two engines and fixing engines twice as often. However, you are no longer losing jets or pilots - or only extremely rarely. Which aircraft would you rather fly? And no matter what failure rate you quote, the idea that two engines with the same mean time between failures (MTBF) are LESS safe than one is absurd and mathematically indefensible.

If you want to see the actual USAF engine-related Class A mishap rates for single vs twin, take a look at http://www.afsc.af.mil/shared/media/document/AFD-080819-035.pdf and http://www.afsc.af.mil/shared/media/document/AFD-100201-058.pdf.

Last edited by stereospace on Dec 19, 2011 - 12:37 AM; edited 2 times in total

Interesting charts. Notice that the vertical scales are different by more than an order of magnitude. The single engine scale goes from 0-10 mishaps per 100k EFH, while the twin engine scale goes from 0-0.6 mishaps per 100k EFH.

The worst F-15 mishap rate for the F100-229, is at 0.43 less than half the mishap rate of the best performing F-16 rate for the F100-129, at 1.01.
Twin = 2x safer (Worst case F-15/F-16 scenario)

The best F-15 mishap rate for the F100-100, is at 0.24 less than one quarter the mishap rate of the best performing F-16 rate for the F100-129, at 1.01.
Twin = 4x safer

The best F-15 mishap rate for the F100-100, is at 0.24 less than one seventh the mishap rate of the worst performing F-16 rate for the F100-200, at 1.84.
Twin = Almost 8x safer (Best case F-15/F-16 scenario)

This is a good example of why almost all commercial airliners fly with AT LEAST two engines.

Did you miss the F-16/F100-229 rate at 0? I would say it's the best performing; but, it also has only about one-third the flight hours of the F-16/F110-129.

I read that as a insufficient data. Impressive record so far, I agree. Mathematically, you're still better off with two of them. Beer

Mathematics is one thing.... How an engine failure in a twin affects the good engine - or not - is a consideration. How is this factored in? For example an USMC Hornet recently during CarQual was bingoed to NAS North Island as I recall by Navy (best practice to land ASAP at nearest field) after an engine failure. That engine failure affected systems and the other engine. USMC directed the aircraft to nearby Miramar but the only engine failed on finals - pilot ejected - 3 people killed on ground. Many other stories of an engine failure affecting the other eventually. Also many stories of single engine landings in same type aircraft. I'm just wondering how those events are factored into the mathematics?

I ain't sayin' life aint complicated or that math is the solution to all your problems. All I was sayin is that if you have a component that has a failure rate X, that if you have two of them in parallel your failure rate is better than X, not worse. This is the whole idea behind redundancy. I have read of some critical systems (fiber optic data busses) being made triple redundant so that an aircraft can absorb battle damage and still function.

Then do you account for the 'health monitoring' of the F-35 engine along with other ocmponents. I guess we have to wait for results from real world.

I was just illustrating a concept, not making any predictions. AFAIK, engines are so reliable now that a single engine is considered 'safe enough' for use over long stretches of water. I sincerely hope that turns out to be true. And I agree that health monitoring can only help.

stereospace wrote:

I ain't sayin' life aint complicated or that math is the solution to all your problems. All I was sayin is that if you have a component that has a failure rate X, that if you have two of them in parallel your failure rate is better than X, not worse. This is the whole idea behind redundancy. I have read of some critical systems (fiber optic data busses) being made triple redundant so that an aircraft can absorb battle damage and still function.

Your reasoning seems to make sense on the surface.
I have considered a different perspective though.
However, with two engines you have doubled the chance of at least one of them failing and I think the chances of it affecting the second engine is much greater than that of the 2nd engine failing by itself. The case cited by Spaz of the F18 crashing after the 2nd engine eventually failed points to this, you can't really take 2.engines, put them side by side ln a plane and consider them in isolation from each other. Consider them part of a complex system with a failure in one increasing the chances of affecting the other. My take on the issue...

The thing that apparently got this sub-thread started was the following statement:

"Propulsion was/is part of the USAF URF constraint, and they will also tell you (with statistical justification) that modern single engine jets are safer than those with two."

I don't know who in the USAF did that statistical analysis; but, it doesn't match the actual USAF historical data in the charts I referenced earlier. I agree with stereospace that it ain't true, even when you consider the probability of the first engine failure taking out the second engine.

Even if STOVL by lift fan requires a single engine (which I'm not convinced of), that wouldn't be the only reason why F-35 has a single engine. The Air Force wanted that, too, for its own reasons. And if the STOVL part of this program hadn't existed, the Navy would still be getting a version of the same plane as the Air Force.

stereospace wrote:

AFAIK, engines are so reliable now that a single engine is considered 'safe enough' for use over long stretches of water.

The A-7 was considered safe enough for Navy use in its day, and looking at the trends on the single engine chart, it appears the F-35C will likely be in much better shape.

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Forum: F-35 Lightning II

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