Forum: General F-35 Forum

I removed the link, cause I don't want to send more people to a post by someone who only writes to get hits and uses a position to blast anyone who doesn't agree with them.

Sufa Viper

F-16.net Sponsor

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.

I knew that one would get a rise from the audience.

Read it carefully. All it says is the the Navy was the only service pining away for a twin engine design. And, as one other poster noted, the USAF jet was going to be single engine for a variety of reasons, but principally because of cost(s).

At the time, there were several briefs floating around -- just chocked full of statistics -- showing favorable comparisons for single engine vs twin engine designs. Whether one agreed with the numbers or how the argument was framed, the attempt was made to argue the case (usually out of operational context).

What remains unanswered is how the F-35C design will fare in routine operations aboard ship -- probably the most stressing of operational cases, absent a shooting war. The Navy has no stats comparable to the USAF wrt one v two engines because they haven't flown single engine aboard ship since the A-7E, and the TF-41 wasn't in the same generation as the GE F404 or subsequent Mks.

Forum: General F-35 Forum

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