Lion Air 737-Max missing, presumed down in the sea near CGK (Jakarta)

[Chris K]

Thanks everyone for your answers! I guess it brings me to the rather obvious question which is whether an incident the day prior to this accident should have necessitated a test flight. Unless I've got Capt. Sulley or Boeing's test pilot version of Neil Armstrong flying, I'd sure much rather have been the 2nd flight out after the maintenance than the first for the simple reason that, say what you will, unless the plane is airborne I'm not sure you could convince me you're 100% sure this problem has been fixed.

The general consensus that every professional pilot should be able to handle this dealt hand doesn't really cut it with me for the simple reason that A. Whether or not this turns out to be what actually caused the accident remains to be seen, but it is certainly true that this type of issue has been known to throw pilots a big curve ball they don't always handle, and B. Just as in every other profession there are great, good, average, and bad levels of skill and the flight after a maintenance event that corrected an issue with critical data inputs is the day I'd rather have seen someone take the bird up for a spin before loading up with pax and crew. I would bring the maintenance person and their supervisor along for the check ride - extra motivation that they get the repair right the first time.

[Gabriel]

There are many failure modes so it is not a simple answer. What I can say is that whether a flight test is required or not is not based only on the severity of the issue but also on the ability to test the affected system on the ground and on the confidence that what was fixed was the real cause of the issue.

For example, something big like a full hydraulic system failing in flight may not require a flight test if, for example, it was identified that the hydro pump was not working and it was replaced and the hydro system is tested on ground.

Even an engine replacement doesn't necessarily require a flight test.

Most of the times a test flight is not required for individual issues and fixes.

Major scheduled maintenance events, like a level D (where the plane is pretty much disassembled a re-assembled), always require tests flight.

[3WE]

I can't wait to read the replies! Experts, I await the diatribes.

I am pretty sure there are somewhat complex and robust procedures and “thresholds” as to exactly how you test a repair.

Am I correct?

[BoeingBobby]

I imagine the threshold would be: is this a problem that cannot be detected on the ground? For example, replacing a pressure bulkhead would require a pressure differential to determine if the work was done correctly, so that would warrant a test flight. Most things don't. For example, a malfunctioning probe can be replaced with one that has been pre-tested in production to withstand its environment. It can be inspected and mechanically tested on the ground. The same goes for a malfunctioning avionics unit. These are called LRU's (line replaceable units). They are designed to be replaced without the need for the aircraft to be ferried to a major maintenance base and flight tested. I think, if the problem persists, if the problem defies standard line maintenance, the aircraft would be taken out of service and more extensive testing, including any flight testing, would be done.

But this is the most important thing to understand: aircraft components are expected to fail in flight. The aircraft are expected to be fail-operational. Some of that is by designed redundancy; some of that is by pilot preparedness and abnormal operations procedure.

For example, a complete loss of airspeed data and autoflight should not result in a fatal loss of control. If it does, the problem lies in the quality of the pilots and the quality of pilot training. If there is a failure in that regard, then yes, they definitely need to be flight tested before re-entering service.

Now explain DMI to the gentleman!

[Evan]

I'm in no way part of the aviation industry (other than a passenger) but I'm obsessed with crashes (probably because my dad was a small plane pilot who almost killed me and the rest of my family a couple of times and I'm not exaggerating) so I beg your indulgence with a question that I've always wondered about:

When maintenance is performed on a plane, what is the threshold and/or necessity for some type of testing (as in an actual test flight) to take place prior to the aircraft being returned to service? Obviously my thought with this example is something along the lines of maintenance supposedly fixed a pretty major issue the day before - as a passenger should I feel safe that the fix was really done correctly? We've all had times we've taken the car in for service and the mechanic didn't get it quite right!

My guess as to my answer is that I've no doubt the carriers loathe this type of testing ($$$$) and that there are plenty of safeguards that would make this almost always unnecessary. And yet I still have to believe there *are* maintenance events performed that would suggest a test flight be conducted.

I imagine the threshold would be: is this a problem that cannot be detected on the ground? For example, replacing a pressure bulkhead would require a pressure differential to determine if the work was done correctly, so that would warrant a test flight. Most things don't. For example, a malfunctioning probe can be replaced with one that has been pre-tested in production to withstand its environment. It can be inspected and mechanically tested on the ground. The same goes for a malfunctioning avionics unit. These are called LRU's (line replaceable units). They are designed to be replaced without the need for the aircraft to be ferried to a major maintenance base and flight tested. I think, if the problem persists, if the problem defies standard line maintenance, the aircraft would be taken out of service and more extensive testing, including any flight testing, would be done.

But this is the most important thing to understand: aircraft components are expected to fail in flight. The aircraft are expected to be fail-operational. Some of that is by designed redundancy; some of that is by pilot preparedness and abnormal operations procedure.

For example, a complete loss of airspeed data and autoflight should not result in a fatal loss of control. If it does, the problem lies in the quality of the pilots and the quality of pilot training. If there is a failure in that regard, then yes, they definitely need to be flight tested before re-entering service.

[BoeingBobby]

I'm in no way part of the aviation industry (other than a passenger) but I'm obsessed with crashes (probably because my dad was a small plane pilot who almost killed me and the rest of my family a couple of times and I'm not exaggerating) so I beg your indulgence with a question that I've always wondered about:

When maintenance is performed on a plane, what is the threshold and/or necessity for some type of testing (as in an actual test flight) to take place prior to the aircraft being returned to service? Obviously my thought with this example is something along the lines of maintenance supposedly fixed a pretty major issue the day before - as a passenger should I feel safe that the fix was really done correctly? We've all had times we've taken the car in for service and the mechanic didn't get it quite right!

My guess as to my answer is that I've no doubt the carriers loathe this type of testing ($$$$) and that there are plenty of safeguards that would make this almost always unnecessary. And yet I still have to believe there *are* maintenance events performed that would suggest a test flight be conducted.

I can't wait to read the replies! Experts, I await the diatribes.

[Chris K]

I'm in no way part of the aviation industry (other than a passenger) but I'm obsessed with crashes (probably because my dad was a small plane pilot who almost killed me and the rest of my family a couple of times and I'm not exaggerating) so I beg your indulgence with a question that I've always wondered about:

When maintenance is performed on a plane, what is the threshold and/or necessity for some type of testing (as in an actual test flight) to take place prior to the aircraft being returned to service? Obviously my thought with this example is something along the lines of maintenance supposedly fixed a pretty major issue the day before - as a passenger should I feel safe that the fix was really done correctly? We've all had times we've taken the car in for service and the mechanic didn't get it quite right!

My guess as to my answer is that I've no doubt the carriers loathe this type of testing ($$$$) and that there are plenty of safeguards that would make this almost always unnecessary. And yet I still have to believe there *are* maintenance events performed that would suggest a test flight be conducted.

[3WE]

Hi X-speedy. We see your questions. The shortest answer all of them is "yes" there are various procedures for loss of instruments and for conveying maintenance stuff.

The other thing to consider is "Most plane crashes should not occur, but when they do occur, something went wrong with the procedures that are designed so that nothing bad almost never happens".

It's pretty remarkable, and often hard-to-believe circumstances.

Fortunately, plane crashes almost never occur, except when they do occur.

[xspeedy]

Was this a different crew from the prior flight of this craft? You would think that crew has been consulted about what issues they may have experienced, as is claimed by pax of that flight.

https://www.yahoo.com/news/previous-flight-crashed-lion-air-023155494.html

[xspeedy]

From the BBC News site:

The pilot is reported to have radioed air traffic control in Jakarta asking for permission to turn back, shortly after taking off.

Now it has emerged that the plane had some technical problems on Sunday on its penultimate flight.

"A technical log obtained by the BBC for that flight - from Denpasar airport in Bali to Jakarta - suggests that the airspeed reading on the captain's instrument was unreliable, and the altitude readings differed on the captain's and first officer's instruments.

As a result of the problem, the captain handed over control of the plane to the first officer, the crew continued their flight and they landed safely at Jakarta."

it is a bit crazy that a malfunctioning speedometer will result in something like this. I know pilots are taught to trust their instruments, but what is the SOP if you suspect fault? Can’t pilots determine something is wrong when the dials indicate high airspeed with low throttle and corresponding engine speeds?

[pierpp]

From the BBC News site:

The pilot is reported to have radioed air traffic control in Jakarta asking for permission to turn back, shortly after taking off.

Now it has emerged that the plane had some technical problems on Sunday on its penultimate flight.

"A technical log obtained by the BBC for that flight - from Denpasar airport in Bali to Jakarta - suggests that the airspeed reading on the captain's instrument was unreliable, and the altitude readings differed on the captain's and first officer's instruments.

As a result of the problem, the captain handed over control of the plane to the first officer, the crew continued their flight and they landed safely at Jakarta."

[Evan]

How do you make a 737 go into alternate law / what's it doing now?

The main difference in terms of flight control between the NG and the MAX is the FBW spoiler set-up. No primary flight controls on the MAX are governed by FBW 'laws', and the spoiler system is reportedly a relatively simple one. It has features such as an elevator jam assist mode that you wouldn't want malfunctioning, but I still can't find much detail about the system architecture.

Another strange thing is that apparently they called ATC with the return request just a couple of minutes after take off but then kept flying away 8 more minutes.

To me, that suggests a crew working checklists to deal with a problem that isn't requiring an immediate return.

The ADS-B looks like there was either a significantly erratic autoflight behavior (ADIRU issue?) or it was being hand-flown either with difficulty or distraction (or both).

AFAIK the MAX uses the same air data system as the NG.

[Gabriel]

BUT, there's so many backups? Look it's all on the table (including meteor strikes), but so strange for it to be "out of control". (Out of control = relative, not much stable flight there- albeit nothing too awfully wild until the end).

And now you've taken spatial disorientation out of the equation.

I agree, it also called my attention the instability of the speed and altitude, followed by what seems to be a total loss of control. Obviously the problems to control the plane started way before the total loss of control. The fact that they seem to have been in VMC daylight makes it even stranger. Another strange thing is that apparently they called ATC with the return request just a couple of minutes after take off but then kept flying away 8 more minutes.

I don't want to wait for the final report. We will probably have good (and bad) information coming since we have the previous crew that wrote the report, witnesses of the crash in a boat, the wreckage sie easily accessible, and the black boxes have already been located.

[3WE]

So many ways...

BUT, there's so many backups? Look it's all on the table (including meteor strikes), but so strange for it to be "out of control". (Out of control = relative, not much stable flight there- albeit nothing too awfully wild until the end).

And now you've taken spatial disorientation out of the equation.

Yes, wait for a formal report.

[Gabriel]

How do you make a 737 go into alternate law / what's it doing now? [Edit, maybe you turn off the autopilot and have folks who haven't hand flown in true IMC on little tiny backup gauges].

So many ways... Other than the obvious rudder hard-over (to which the 737 MAX should not be susceptible), you have hydraulic failures, engine failures, stalls (why is it not going up if I have been pulling up all the time) and a vast list of etc. Now you mention IMC, but it looks like it was quite VMC, I mean, legal for VFR.