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Unexpected cracking found on critical Boeing 737 Next Generation part

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Re: Unexpected cracking found on critical Boeing 737 Next Generation part

#71

Post by dredmorbius »

acjohnson55 wrote: post #13
phire wrote: post #7

Earlier quoted context omitted.

Sometimes they fly routes that are only 30min long. With a 30min turnaround, that could mean a flight every hour. If they kept one plane flying round trips all day, they might fit 16 flights into a day and theoretically burn through the 90,000 cycles in under 16 years.

Are there examples of routes like this you know of where a 737 is used?

Not positive of aircraft used, but flights may be as short as 2 minutes:

https://www.tripsavvy.com/the-worlds-shortest-scheduled-flig...

Re: Unexpected cracking found on critical Boeing 737 Next Generation part

#72

Post by dredmorbius »

newnewpdro wrote: post #24

If a part's made of aluminum and undergoes cyclic stress, even staying within its instantaneous modulus of elasticity, it will eventually crack. Note the aluminum curve in the graph here [0], it never levels off horizontally. My understanding is this effectively means the modulus of elasticity for aluminum parts is forever diminishing. Aluminum is a metal that's "work hardened", it's basically becoming increasingly b…

AFAIU, ultrasound is far more typically used than x-rays, as the acoustic properties of fractured castings are more distinctive than their xradiographs.

Apparently induction testing is also used (likely on a similar basis to ultrasound: properties deviate sharply from norm/spec).

Re: Unexpected cracking found on critical Boeing 737 Next Generation part

#73

Post by Obi_Juan_Kenobi »

axaxs wrote: post #69
alan5 wrote: post #2

> Engineers design pickle forks to last the lifetime of the plane, more than 90,000 landings and takeoffs, a term known as "flight cycles" in the aviation industry, without developing cracks. That number made me do a double take. Seems planes never stay on the ground for very long.

Not an aerospace engineer, but this seems like a weird way to measure airplane life. A transpacific plane could have 1/10 the cycles but equal airtime, and one would think that the stresses of flight are worth considering. Wonder why it's not measured in hours, like say, tractors, or miles flown. Do we measure any other engined vehicle this way?

A cycle involves pressurizing the cabin relative to the atmosphere approaching flight altitude, and then depressurizing during the descent.

The pressure differential may not seem like much, 6-10 psi in most cases, but the cabins are quite large, so these forces become substantial. Advanced airframes like the composite 787 use that extra strength, in large part, to lower cabin altitude (i.e. keep higher pressure) because it has such a substantial effect on passenger comfort.

Cycles is absolutely the best way to consider airliner lifespan because that is the most significant stress on the airframe.

Re: Unexpected cracking found on critical Boeing 737 Next Generation part

#74

Post by corbani »

newnewpdro wrote: post #24

If a part's made of aluminum and undergoes cyclic stress, even staying within its instantaneous modulus of elasticity, it will eventually crack. Note the aluminum curve in the graph here [0], it never levels off horizontally. My understanding is this effectively means the modulus of elasticity for aluminum parts is forever diminishing. Aluminum is a metal that's "work hardened", it's basically becoming increasingly b…

The modulus of elasticity of aluminum does not change with cycling (or basically with anything else but big temperature differences).

Fatigue is a damage accumulation process, that does not show in the most obvious properties, like modulus or strength.

The aviation regulations changed a long time ago from requiring to avoid cracks to requiring to have structures that can operate with cracks long enough to for them to be detected, as is exactly the case here. Of course nobody design planes to crack, but the regulations reflect the realization that completely avoiding cracks is impossible ...

Re: Unexpected cracking found on critical Boeing 737 Next Generation part

#75

Post by agurk »

axaxs wrote: post #69
alan5 wrote: post #2

> Engineers design pickle forks to last the lifetime of the plane, more than 90,000 landings and takeoffs, a term known as "flight cycles" in the aviation industry, without developing cracks. That number made me do a double take. Seems planes never stay on the ground for very long.

Not an aerospace engineer, but this seems like a weird way to measure airplane life. A transpacific plane could have 1/10 the cycles but equal airtime, and one would think that the stresses of flight are worth considering. Wonder why it's not measured in hours, like say, tractors, or miles flown. Do we measure any other engined vehicle this way?

There are a variety of ways that age of something can be measured with two common examples in everyday life being people and cars commonly measured in years and kms/miles respectively. The metric chosen is one that adds value for understanding the impact.

Aeroplanes and their components do have a lot of different ways of having their ages measured, depending upon what one cares about. Engines are a good example of something where the hours spent running is usually the most salient.

For fusalages, as we're talking about here, pressurisation cycles is actually very important in pressurised airframes. This is because it is the main source of material fatigue which is a major cause of issues - usually in the form of cracks. This was discovered the hard way with the de Havilland Comet[0].

Smaller non-pressurised planes are normally measured in total flight hours, but the effect of repeatedly pressuring and depressuring is so great that it's the biggest factor that will affect the life of the bigger airframes.

[0] https://en.wikipedia.org/wiki/De_Havilland_Comet#Accidents_a...

Re: Unexpected cracking found on critical Boeing 737 Next Generation part

#76

Post by morsch »

Transfinity wrote: post #68
elif wrote: post #46

Earlier quoted context omitted.

As for direct GHG emissions, the most efficient mode of travel depends on many factors involving the models, occupancy, and route, but generally speaking, planes can become more efficient than trains for distances >= 700 miles.

Source: https://www.ucsusa.org/clean-vehicles/getting-there-greener Turns out motor coaches (e.g. Greyhound busses) are the most efficient at pretty much any distance.

Their number for travel by rail: 0.37 - 0.43 pound of CO2 emissions per passenger-mile.

Germany's department of environment number for travel by rail: 38 gram/kilometer = 0.13 pound per passenger mile.

(Source (German): https://www.vcd.org/fileadmin/_processed_/8/9/csm_Emissionen...)

Re: Unexpected cracking found on critical Boeing 737 Next Generation part

#77

Post by peteretep »

alexis_fr wrote: post #49
hansthehorse wrote: post #12

There is a 9 year old documentary on two Boing whistleblowers describing cracking probability on the 737ng. It's an hour long: https://www.youtube.com/watch?v=IaWdEtANi-0

Extremely interesting. Especially, around 19-21 minutes, we find the same problem as on 737 Max: Boeing has the delegated authority from the FAA to certify their own processes and parts. I’m never flying Boeing again. They were hammering crucial parts to fit, provoking crackling; redrilling holes when the hole was 3 inches apart, endangering the whole structural integrity; painting it all green and sending that to pr…

> I’m never flying Boeing again

I suspect you were intending this to be obvious hyperbole, but also this will be very difficult if you’re flying at all regularly. While you can have a look at the equipment the airline thinks it’ll be flying at booking time, there’s no guarantees about which plane will actually show up, and the airline isn’t going to refund your ticket because they decided to swap out their A350 for their 787 for operational reasons.

Re: Unexpected cracking found on critical Boeing 737 Next Generation part

#78

Post by kube-system »

m0zg wrote: post #67
kube-system wrote: post #52

Earlier quoted context omitted.

> Another source tells us Boeing quickly reported the issue with the single plane to the FAA last week, and now more planes with similar cracking have been found.

How many? Could be as few as 2. Out of thousands.

Really, the only thing we know is that it's not a single plane.... which is the cause for concern.

2/7000 is not a very low incidence rate as far as safety concerns go. Many cars are recalled for far less severe issues with orders of magnitude lower incidence rates.

Re: Unexpected cracking found on critical Boeing 737 Next Generation part

#79

Post by greedo »

my_first_acct wrote: post #58
samstave wrote: post #48

Earlier quoted context omitted.

Thank you for this! 76 machines destroyed, carrying (obstensibly) nukes.. from 7.5mm flight miles == .00001% risk of failure. Did I do the math correctly? With ZERO nuke detonations... And a flight carcass of nearly 70 years old now???

A handy website [1] notes at least a couple of B-52 nuclear incidents. In 1961, a B-52 carrying nuclear weapons broke up in mid-air over North Carolina; the bombs did not detonate [2]. In 1966, a B-52 crashed due to a mid-air collision with a refueling tanker over Spain. In this case, the non-nuclear explosives in two of the nuclear weapons detonated, scattering radioactive material [3]. So I would not say "zero nuke…

Most people would not refer to the incident at Palomares as a "nuke detonation," since there was no nuclear detonation. No fissile reaction etc, just conventional explosives that are part of the weapons trigger. To call it a "nuke detonation" is disingenuous.

Re: Unexpected cracking found on critical Boeing 737 Next Generation part

#80

Post by azernik »

samstave wrote: post #60
hn_throwaway_99 wrote: post #57

Earlier quoted context omitted.

That's not the definition of subjective. When GP says "That’s not subjective; you just made it up." it means that talking about airplane safety (which can obviously be objectively measured with things like crashes per landing/takeoff, crashes per mile, etc.) with the moniker "it's subjectively the safest" doesn't make any sense. It would be like saying "That plane is subjectively the heaviest" or "Today is subjective…

I completely disagree with this. I subjectively make my statement based on that data I have available to ME * Objective would be I have failure data in front of me. Now, I personally have heard of far fewer B-52 accidents in my lifetime, and the plane is almost double mine, and as such, I deem it safe as heck. Also, guess what - B-52s carry nukes not passengers. How many nukes have gone off for a B-52 crash vs how ma…

> and the plane is almost double mine

I assume you're talking about lifespan? The B-52 may have been in the air for a long time (though only 15 years longer than the 737), but it has also been flying in much smaller quantities; there have only ever been ~750 B-52s built, while there have been over 10K 737s built. And in addition, a commercial airliner is usually in the air for a much higher percentage of its lifespan than any military airplane.

So for example, the 737-600/700/800/900 series has had 9 accidents (6 full-loss equivalents)... over 100 million flights.

http://www.airsafe.com/events/models/rate_mod.htm

Accident rate info for the B-52 is harder to come by, especially since a lot of the "accidents" are complications from battle damage in Vietnam, but I'm seeing 10 peacetime full-loss incidents just among those that have their own Wikipedia pages:

https://en.wikipedia.org/wiki/Category:Accidents_and_inciden...

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