Well, yes and no - it's not quite as simple as "it won't move until at high power", its more a case of "It is not returned to closed until dropping from high power".
The actual EAD is very easy to implement, on the ground being a visual inspection of the valve seats and "replace as necessary", and only affects aircraft that have been stored for the last few months. And why the inspection isn't it part of the release from storage procedure - that's a whole different question.
Blancolirio has put out a short video outlining things and why only the b737 is affected (just now?)
https://www.youtube.com/watch?v=pHHMPXYEs9Y

Thanks for that link! Warren Brown gives a good explanation of the failure mode.

So... My pickup from that and from the comments, for the Boeing 737 + CFM56-3 engine, is that:

• The bleed air used to pressurize the passenger cabin is taken from the 5th and 9th stages of the engine compressor;
  
• The 5th stage valve for the bleed air is a passive non-return "check valve";
  
• The cabin pressure is controlled via a servo controlled valve on the higher pressure bleed air from the 9th stage;
  
• Normal takeoff and cruise has the high pressure 9th stage valve closed, or partially closed to regulate the cabin pressure, and the cabin is pressurized by the air pressure from the 5th stage (check valve is open);
  
• For low thrust such as during descent, the 5th stage pressure drops and is insufficient to pressurize the cabin, hence the 9th stage is used and that higher pressure forces the 5th stage check valve closed;
  
• If the check valve is jammed/corroded in its normally open position, the 9th stage high pressure bleeds back into the 5th stage to cause a compressor stall and engine shutdown.
  

Two main comments explaining that are:

https://www.youtube.com/watch?v=pHHMPXYEs9Y&lc=UgwMINUQxWPuJC9q2ZJ4AaABAg.9BY7Mn_G7Pi9BYIBJum_sR wrote:

... engines that have been stored for a long time have experienced corrosion because the valve is normally open, and the check/non return function might not work...

https://www.youtube.com/watch?v=pHHMPXYEs9Y&lc=Ugxlv3nKUtCAV6PRo414AaABAg wrote:

... the check valves are not a part of the CFM56 engine itself, but are a part of the nacelle and engine supporting equipment. They were developed by Boeing specifically for this installation, which is why Airbus wouldn't have them even though they use the same engines. Airbus will have a similar design for their installation.

Ouch! Multiple times over!!

So...

This may well have nothing to do with the engine manufacturer and may well be Boeing specific.

The fault of a normally open check valve stuck open is something that will not be noticed until the aircraft makes its first descent!

The stuck valve is common to all (both) the engines!

All the engines will have seen the same recent conditions that causes the stuck failure!

And that valve failure is very much NOT a fail to a safe state!!

Worse still, I can't see how that passive check valve can be physically tested whilst on the ground unpressurized.


OK, so a passive check valve is simpler and very much cheaper than a servo controlled valve. However, that also looks to be a potentially deadly too cheap a design that can't be tested and introduces a common mode failure with the use of that type of valve...

A work-around fix could be if it was possible (I doubt it!) to depressurized the cabin so as to then recover the engine operation, but I bet that isn't in the flight manuals!

Or the bleed air controller could be programmed to close the 9th stage bleed air valve and the cabin depressurization be damned! I bet that isn't in the flight manuals either!

Using an identical servo controlled valve for both the 9th and 5th stages is more complicated but at least that can reassuringly be fully tested by a control system before every takeoff.

Safer yet would be to take a flow restricted singular takeoff from only a single stage so that circular flow between compressor stages is simply not possible by the physical design.

Or as is done on some other more recent aircraft designs, better yet, and much more healthy to avoid breathing in fuel and oil, is to not use engine bleed air at all and instead use a dedicated electrically driven cabin aircon.


Scary stuff!

So what else lurks?...


All in our deadly greedy world,
Martin

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Quality control & full inspections cost & reduces profit.
Sadly, that is not Boeing specific!

A couple of things:
It is a controlled valve, not a passive one. Under some circumstances the valve is closed during take-off to allow all the engine power to propel the aircraft (that suggests to me that a fair bit of power is "lost" to cabin pressurisation and air conditioning.)

Since the failure to close is caused by corrosion either on the seat or the stem - in either case it is possible to inspect the appropriate part and spot the corrosion (either by partially dismantling it, or "poking a borescope in").

One question - and Sirrius hints at it - why does this part corrode in such a short period of time.......

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Bob Smith
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A couple of things - it is a controlled valve, not a passive one. ...

The 9th-stage (high pressure bleed air) is a controlled valve to modulate/control the cabin air pressure. The 5th-stage bleed air valve is a non-controlled passive 'check valve' to stop reverse airflow from the 9th stage blowing back through the bleed air duct/manifold back into the lower pressure 5th stage.

Note the AD:

Emergency Airworthiness Directive (AD) 2020-16-51 is sent to owners and operators of The Boeing Company Model 737-300, -400, -500, -600, -700, -700C, -800, -900, and -900ER series airplanes. (pdf)

The 5th-stage valve is a very simple passive flapper valve.

IIRC: Surprisingly, it is merely gravity operated to initiate closing! No springs!!! No positive return spring force/pressure to force closing. Hence very easy for the valve to jam open for even minimal resistance on the flapper hinge.

Also from my reading around, the air bleed system is Boeing's implementation and the Boeing 737 uses a unique variant of the CFM56. Airbus A32x series aircraft are different.


Ouch!

Stay safe!!
Martin

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A couple of things:
... Under some circumstances the valve is closed during take-off to allow all the engine power to propel the aircraft (that suggests to me that a fair bit of power is "lost" to cabin pressurisation and air conditioning.) ...

And yep, the rate of air changes in the passenger cabin is kept meager in order to conserve fuel.

Don't know what happens if there is a cabin depressurization... Presumably, engine performance is reduced as air bleed from both the 5th and 9th stages hit some maximum flow rate limited by the cabin ducting or, hopefully, a purpose designed flow rate limiter. At least the bleed air is leaving both the 5th and 9th stages so hopefully there won't be a compressor stall due to blowback into the 5th stage. However, dread to think what that does to the compressor pressures for keeping the engine going!


Also, never understood why on (all?) passenger airliners, the cabin air exhaust is merely a dumb spring loaded pressure relief valve. Is there no energy gain in using an air exhaust turbo-assist for pressurizing the incoming air?... (Or is fuel too cheap and passenger comfort is expendable so as to 'not bother'?...)


Stay safe!
Martin

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... The 5th-stage valve is a very simple passive flapper valve.

IIRC: Surprisingly, it is merely gravity operated to initiate closing! No springs!!! No positive return spring force/pressure to force closing. Hence very easy for the valve to jam open for even minimal resistance on the flapper hinge...

Further thought:

For such a critical part, there really should be a sensor on there to detect correct operation before engine start and to confirm full operation whilst the engine is still on the ground!


Ouch!

Stay safe!!
Martin

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... Under some circumstances the valve is closed during take-off to allow all the engine power to propel the aircraft (that suggests to me that a fair bit of power is "lost" to cabin pressurisation and air conditioning.) ...

And yep, the rate of air changes in the passenger cabin is kept meager in order to conserve fuel.

Not really, as the cabin is a constant loss system - and stopping the air coming in just means that the air inside will (fairly rapidly) escape, leading to cabin depressurisation and all that entails. This valve being closed is used to extract every bit of thrust from the engines during take-off under very extreme circumstances (e.g. short runway and max load).

---

Bob Smith
Member of Seti PIPPS (Pluto is a Planet Protest Society)
Somewhere in the (un)known Universe?

... Under some circumstances the valve is closed during take-off to allow all the engine power to propel the aircraft (that suggests to me that a fair bit of power is "lost" to cabin pressurisation and air conditioning.) ...

And yep, the rate of air changes in the passenger cabin is kept meager in order to conserve fuel.

Not really, as the cabin is a constant loss system - and stopping the air coming in just means that the air inside will (fairly rapidly) escape, leading to cabin depressurisation and all that entails. This valve being closed is used to extract every bit of thrust from the engines during take-off under very extreme circumstances (e.g. short runway and max load).

All the press Martin remembers about that "low circulation" was dumbed down. Truth is cabin altitude.

AgentJayZ (an aero-engine rebuilder) has posted this explanation:
https://www.youtube.com/watch?v=6rj3mrJalmw

---

Bob Smith
Member of Seti PIPPS (Pluto is a Planet Protest Society)
Somewhere in the (un)known Universe?

AgentJayZ (an aero-engine rebuilder) has posted this explanation:
https://www.youtube.com/watch?v=6rj3mrJalmw

Thanks Rob, and here's an interesting update from AgentJayZ:
https://www.youtube.com/watch?v=T57NcrvKmOg

Thanks for those.


Mentour Pilot gives a succinct explanation:

YouTube: Boeing 737 Emergency Airworthiness directive! - Explained

So, as assessed, it's all part of the Boeing add-ons for supplying pressurized air to the passenger cabin and other facilities.


Despite the warm fuzziness from that YouTube expose, note this apt comment:

https://www.youtube.com/watch?v=FV4FjT6-xk0&lc=Ugyhsm9m2JHbbBOF18R4AaABAg
wrote:

It's a check valve not the bleed air valve. The check valve is there to prevent 9th stage from back flowing to the 5th stage. Please read the AD again. The bleed air valve cannot be a check valve as it would flow (in the forward direction) uncontrollably. Your diagram shows the check valve. It's vulnerable to stiction/corrosion because it's a passive valve (i.e., you can't send a signal/command to close it). It is a big deal if the generic cause of stiction/corrosion causes both (passive) check valves to stick open and both engines require shut down. Generic cause issues like this are worthy of serious attention.

(My emphasis added.)


Note also that ALREADY there has now been a fifth engine forced shutdown!

See: Alaska Airlines Boeing 737-800 Suffers Engine Failure In Austin

Special note: The Emergency Air Directive is for planes that have not flown for 7 consecutive days. The fifth recent failure is for a plane that had been idle on the ground for six days and four hours before the engine failure .



Very worrying...

Hopefully, more immediate direct action will be taken before we see a failure of both engines whilst a 737 is in flight...


All in our only one greedy world...

Stay safe!
Martin

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More general news for Boeing developments:


Safety panel concerned about quality control on Boeing crew capsule
wrote:

Members of NASA’s independent panel of aerospace safety advisors raised concerns last week about quality control problems that “seemingly have plagued” Boeing’s Starliner crew capsule program, while urging NASA to closely monitor SpaceX’s plans to reuse Crew Dragon spaceships on astronaut flights...

... An unpiloted test flight of Boeing’s CST-100 Starliner spacecraft in December ended prematurely after a programming error in the capsule’s mission elapsed timer... The Starliner landed safely in New Mexico two days later, but ground teams identified another software problem ... Mission control uplinked a software patch shortly before re-entry, eliminating a risk that ... could have caused the jettisoned service module to ram into the crew module after separation. There were also problems with the Starliner’s communications system...

... An independent review team that investigated the problems ... issued 80 recommendations for Boeing and NASA engineers to address software issues, the communications problem, and management oversight shortfalls...

... Boeing is making progress toward resolving the technical problems...

... “However, despite this progress, which is definite and in fact measurable, the panel continues to be concerned about quality control problems that seemingly have plagued the Boeing commercial crew program,”...

... The panel recommended NASA’s Commercial Crew Program “maintain a balance” between setting and achieving schedule milestones and ensuring managers make appropriate technical decisions...

Aviation CEOs Warn of Europe-US Split on Boeing Max Return
wrote:

... European regulators have determined to conduct their own review, independent from the FAA, before allowing the plane to fly again in European airspace...

... Europe’s insistence on an independent review reflects an erosion of trust in the FAA after officials signed off on a software system that went haywire on the Max because of a [singular] faulty sensor...

... EASA has a favorable view of an extensive change Boeing has proposed for the flight-control computer architecture, including autopilot, that will compare readings for the plane’s two onboard computers, according to Ky’s presentation.

But the European agency still has concerns with the architecture Boeing has proposed for sensors that measure the angle of attack of a plane’s wings... “Still no appropriate response to angle of attack integrity issues,” said a slide in Ky’s presentation at the European Parliament...

My personal humble totally ignorant uneducated opinion from my reading of those two articles is that we need meaningful enforced push-back against the Boeing Management pushing to rush schedules and to cut all costs, all at the expense to others, as has been painfully demonstrated by multiple real world examples killing people.

Thankfully, EASA appear not to be politically bullied and cowed. Very good that they are insisting on appropriate redundant operation of the AoA sensors, and following that up with ensuring that the 737 MAX is fully flight tested for a scenario of a further MCAS failure.


Stay safe! (I ain't flying anything Boeing!)

All in our only one deadly greedy world,
Martin

---

See new freedom: Mageia Linux
Take a look for yourself: Linux Format
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Thankfully, EASA appear not to be politically bullied and cowed.

I wouldn't be so sure of their political purity. Remember they are suing Boeing over Washington State's tax deal with Boeing.

While we are focused on Boeing not everything is so sweet over at Airbus:
https://mentourpilot.com/indigo-expects-refurbishment-of-faulty-p-cfm-to-power-newer-aircraft/?fbclid=IwAR2Bn1V0CPpiTURasVNM_QAN6InAdXFr6ySZf9jHvVeuS7yDqlNpG4j3iHU

---

Bob Smith
Member of Seti PIPPS (Pluto is a Planet Protest Society)
Somewhere in the (un)known Universe?

https://www.businessinsider.com/boeing-737-max-faa-ad-changes-safety-crashes-2020-8

The FAA published its proposed changes to the Boeing 737 Max, outlining the fixes required for the plane to return to service — among the final steps before the plane can fly again.

The FAA also published a summary of its review of the plane, writing that the proposed changes will fix the problems that caused two of Boeing's flagship narrow-body jet to crash, killing a combined 346 people.

The proposed changes are subject to a 45-day public comment period, meaning the plane could return to service as soon as mid-September.

This links to the FAA documents, first the initial publication, which includes the "call for comment":
https://www.faa.gov/news/updates/?newsId=93206
(36 pages including a load of instructions and reasoning, but the required changes are outline on page 8 onwards)
One interesting thing is the table on page 20 which outlines the cost of compliance for each of the five physical changes (FCC OPS installation and verification; AFM revisions; MDS installation and verification, INOP marker removal; Stabilizer wiring change; AOA sensor system test) as being $14160 per aircraft, including changes to the manual, but excluding test flights and crew (re)training.

The (draft) Airworthiness Directive isn't accessible just now (typical of governmental websites, one page says "go and look for docket FAA-2020-0686" on this website "https://www.regulations.gov/" - only it isn't there (at the time of typing - it may be that it will be later in the day)

(Aside - The call for comment says there are only 73 US registered aircraft affected, but I thought there were far more B737MAX grounded by US airlines, never mind all those grounded by other airlines....)

---

Bob Smith
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Somewhere in the (un)known Universe?

https://en.wikipedia.org/wiki/List_of_Boeing_737_MAX_orders_and_deliveries
This page says 387 delivered.

Thanks - a quick glance down the list shows most are outside the USA. That's a pretty good export hit rate.

---

Bob Smith
Member of Seti PIPPS (Pluto is a Planet Protest Society)
Somewhere in the (un)known Universe?

Could it be that Boeing are, at the moment, concentrating only on the planes delivered to US companies, as they haven't yet convinced other Safety Agencies the 737MAX is safe.
Europe air safety regulator gives no firm date for 737 MAX to fly again

PARIS (Reuters) - Europe’s air safety watchdog has no firm date for Boeing’s (BA.N) grounded 737 MAX to resume flights, it said on Tuesday, adding that the U.S. planemaker had some more work to do before a 17-month-old safety ban could be lifted in Europe.

Maybe - but the FAA is the US civil aircraft certification authority so it is hardly surprising they are only looking at aircraft registered in the USA.
There is a sort of "mutual back scratching" when it comes to certifying civil aircraft, nobody will (re)certify the B737MAX until the FAA does, and they won't say when they will until after the FAA actually set a date. Remember just now the FAA has only released a proposed schedule for the consultation on what steps have to be taken before they will actually do the (re)certification. The FAA have given 45 days for public consultation, that leads us to mid September for the end of that phase. Then the FAA have to review the feedback from the feedback from the consultation - say a couple of weeks - and only once that is over can the FAA actually say "Yes, we will (re)certify the B737MAX provided these steps are taken" and this will give a date for the actual (re)certification. So I wouldn't expect to see that date being before October, which could well lead to the first "real" flights being late December or early January 2021. All this presupposes that nobody comes up with a substantial change to the currently published requirements, if there are then stop the clock and have a rethink.
And of course until the FAA publishes the real date for (re)certification EASA can sit on its hands and say "Wait a bit longer"

---

Bob Smith
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Yep, as previously stated - short term gain = long term pain.
Would have been much better & "cheaper" in the long term to have designed a new aircraft once the 737 reached 50 years old.