FAA EAD for non-MAX 737s - engine failures
 

WASHINGTON — The Federal Aviation Administration (FAA) on Friday issued an emergency airworthiness directive for 2,000 U.S.-registered Boeing 737 NG and Classic aircraft that have been in storage, warning they could have corrosion that could lead to a dual-engine failure.
https://rgl.faa.gov/Regulatory_and_G..._Emergency.pdf

FAA EAD for non-MAX 737s - engine failures
 

The FAA has issued an Emergency Airworthiness Directive for 737-300 to -900ER aircraft that have been stored or in limited use:

The nut: possible corrosion and sticking of 5th-stage compressor bleed-air check valves due to lack of use. May cause unrecoverable compressor stalls on power reduction (e.g. TOD) and off-airport landings if both engines are affected.

https://www.documentcloud.org/docume...-for-737s.html

So to sum up, If it sticks your screwed.

I wonder what the implied wording would be if you turned off his voice and read his hands as signiing for the deaf

That youtube video has some inaccurate and confusing information. Any system or feature that bleeds air off the engine is an "engine bleed air" system or feature. There are stability bleed ports and valves, and there are aircraft service bleed ports and valves. The system affected by the AD is the aircraft service bleed air that comes from the engine from either the 9th stage or 5th stage of the HP compressor, depending on the source pressure. It's the system that feeds the air conditioning packs (pressurizing the airplane) and the wing anti-ice system. It is not an engine stability bleed or starting bleed valve. The 5th stage check valve on the affected airplanes is not spring-closed. It normally moves to the closed position due to gravity and air flow/pressure. When the check valve sticks open it's not the 5th through 9th stages that stall - it's the stages downstream of that area that stall initially, leading to compressor flow reversal (the "bangs" you hear about in the reports).

But yes, if it sticks open you will almost always end up with a shutdown, and if it sticks on both engines it's likely to be a forced landing. The AD was issued because the first few flights after a storage period have a risk of dual engine shutdown that is quite high.

Dave, do you know why this is 737 specific and doesn't apply to the CFM powered A320? That check valve is usually considered part of the engine and I'd expect if they used the same valve on both the CFM56-3 and -7, I would have expected them to use the valve on the CFM56-5 on the A320.

It does appear to, but it is also obvious that the guy is working, and trying to reverse-engineer, with incomplete information. There is a follow-up video which makes it a lot clearer, both to the viewer and to the presenter - because he was provided with (and shows) a more complete diagram and description of the system. Because the videos aren't edited much (if at all) you can see how he ends up on the wrong track, basically assuming that there is a control valve in addition to the check valve in the input from stage 5, whereas in fact (if I understand it right) the input from stage 5 is effectively controlled only by differential pressure with the check valve being the only valve (doing what check valves do, unless it sticks open...).

How did the AD come to be issued though? Is the stuck valve a known risk that was previously thought to be acceptable because 737s were not usually parked up for months, or has there actually been an incident due to this (maybe only on one engine)? Engines need to be regularly run-up on parked aircraft - is that enough to trip this failure mode, has it happened on the ground perhaps?

EDIT: now been pointed out, thanks, that I missed the reports of multiple single-engine events being the trigger, as noted in the AD. Must RTFAD slowly and carefully before posting, not skim read :-)

FYI: I believe there were 4 separate inflight single engine decels during letdown. The EAD was issued as there was a possibility that both engines could decel at the same time. There's an FAA doc out there and will post if I can find it.

Hi td. In this case the engine type design ends at the engine case bleed port orifice for the service bleeds. I'm pretty sure most or all Boeing models are similar - the only ones I'm not sure about are the RR installations where RR was supplying a built-up nacelle. The 5th stage check valve and the 9th stage "high stage valve" are part of the Boeing airplane type design for the 737-300 to -900ER. However I agree there is a good chance Airbus uses a very similar valve from the same vendor. I assume the concern was passed to EASA to see if the same part design features make the A320/CFM56 installations vulnerable. I have not yet heard of any similar shutdowns on A320's, but I don't normally see those reports for Airbus.

I saw reported four or so incidents. I can't post links but if you search Alaska N569AS 15th July is one example.

Happened on the aircraft's second flight after a three month spell on the ground. QED.

Benjamin

Ah, got it, thanks (and to others who have pointed it out). I figured single engine events with probability of dual event would be the trigger, but must have missed the reports on first read of the AD.

Why has this never affected a BBJ in the past? Or is it more to do with the consequences of parking them outside than just having an inactive aeroplane?

which airplanes
 

Hi, is the info out which airplanes had the 4 shutdowns ? I mean, were they parked near the ocean, in realtively salty, corrosion-prone air, before getting usee again ?
Thanks

The aforementiond Alaskan B738 had been parked at JFK.

So the valve that guy shows in the vid actually is sticky?

The valve in the video is a different part number valve from a different manufacturer. He says it has a spring, and it appears to be sticky. The valve on the 737s in question is a Honeywell valve, not Dunlop.

I heard today that Airbus uses a different valve from a different vendor on the CFM56-5 installations.

parking inof
 

Hi, thanks for the info. I guess that´s near the ocean, hm ? Couldn´t find any info on the other 737s, like which ones they were...
Thanks

Pretty much no suitable metal is safe forever around salt and water. Stainless steel is a marketing term. The technical term is corrosion resistant steel and the more resistant versions tend to gall in contact with similar or identical alloys. Aluminum parts requires surface treatments to protect them but damage to that surface treatment can leave the aluminum open to corrosion; I would guess that there are bits of grit in the air stream that would blast those surface treatments off eventually. Aluminum also expands more with temp than steels do, which would contribute to the next problem.

One problem in making these valves is that if the flaps have clearance in the hinges they will rattle and rub and wear against the hinge pin in the very high-frequency vibration environment, so the gaps need to be very small. But with small gaps any corrosion at all can bridge the gap and bind the hinge. In normal use any corrosion will be at some small location and operation will wear it down as if forms. Sitting stationary does not cause this wear. Unlike most locations this one cannot be lubricated; oil would be blown off and grease, which consists of oil+ thickener, would see depletion of the oil leaving the thickener to bind up the mechanism.

I hadn't heard that the sticky problem was deterioration of the metal (happy to hear if so) I was assuming the problem was foreign material coming out of solution and gumming up the valves when the engines were left not running for long periods of time