Boeing 737 Max8 Aircraft Crashes and Investigations [Part 1] 20

[itsmoked]

Indonesian 737 Max 8 - Splashed

Most reports I've seen mention the aircraft had an 'undisclosed issue' before take-off. Does anyone know what that was?

Surely the crew was in contact and reporting something during the relatively long disaster.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[EdStainless]

The word out from Boeing to RTFM was real subtle.
Why they were still flying a plane with wonky sensors, well it is Lion.
In that part of the world there are not a lot of good options.
I flew Garuda once, it was an old 737 that they couldn't pressurize because of gaps in the skin panels.
It didn't matter we were island hopping, about 15 min in the air and never above 3,000ft.
The livestock on the plane was very interesting.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Alistair_Heaton]

Aft CofG is a killer, forward C of G is just a work out of your arms and a rather firm arrival.

On the Q400 None public transport we get a additional lump of performance graph to play with in the forward part. Rear is a hard limit and don't screw around with it. Thankfully its extremely hard to get into even with the pax cabin empty and the rear hold full. You would have to fill the rear hold and only seats behind the wing to get near it.

 

[MintJulep]

Isn't that big red knob the tow rope release?

 

[waross]

I was on a 45 minute flight on a King Air that had the CoG so far back that we were sitting on the tail prop with the nose in the air.
The pilot and co-pilot boarded and climbed uphill to their seats. As they were strapping in the nose slowly came down and the nose wheel bounced once on the ground. I guess three point contact meant good to go.
For 45 minutes we were at such an extreme AoA that I kept thinking;
"If the engines fail, we are going to slide backwards down into the ocean."
As you may have deduced, we made it.
Some of the King Airs had a radar set mounted low down on the center of the dash.
One plane had a full size colour picture of a radar set glued in the place where the radar set would normally be mounted.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[berkshire]

Emergency AD Issued On B737 Max






The FAA has issued an Emergency Airworthiness Directive (PDF) that directs the owners of all Boeing 737 Max aircraft to amend their operating manuals, to avoid a control problem like the one that apparently caused the fatal crash of a Max 8 last week. “Possible erroneous angle-of-attack inputs on Boeing 737 Max aircraft … can potentially make the horizontal stabilizers repeatedly pitch the nose of the airplane downward, making the aircraft difficult to control," the FAA says in an emergency AD dated Nov. 7. The airplanes are not grounded, and the owners have three days to comply with the AD, which requires a revision to the airplane flight manual.
“This emergency AD was prompted by analysis performed by the manufacturer showing that if an erroneously high single angle of attack (AOA) sensor input is received by the flight control system, there is a potential for repeated nose-down trim commands of the horizontal stabilizer,” the AD reads. “This condition, if not addressed, could cause the flight crew to have difficulty controlling the airplane, and lead to excessive nose-down attitude, significant altitude loss, and possible impact with terrain.” The AD requires revising certificate limitations and operating procedures to provide the flight crew with runaway horizontal stabilizer trim procedures to follow under certain conditions.
B.E.

You are judged not by what you know, but by what you can do.

 

[Sparweb]

How exactly the LionAir 737's AP could have gotten confused is not yet clear to me, even with the text of the AD to refer to. It seems a declaration that the AP system's functional hazard analysis has been found to be invalid, or relied on a backup source that doesn't do what it's supposed to. The AD says nothing about the configuration of the AP system, as if one faulty sensor should be expected to provoke a wild ride. It absolutely should not. The text of the AD makes mockery of a reliability requirement that has been standard practice in flight system design since the 1960's.

Part of the functional hazard analysis (FHA) for any critical flight system is the need to identify faulty readings, and disregard them. The resolutions become built into the design and programming of the system.

The Autopilot system must be programmed to compare, select, filter, or vote in some way that deals with bad sensors, otherwise the moment your anti-icing system fails you go into a spin. All of the sensors are backed up with at least a duplicate, and wherever possible a completely alternative source that can reconstruct the same information. AoA can be measured directly from AoA sensors (the 737 Max is typical, has two of these, one on each side of the cockpit) and a couple of inertial navigation units (INU) within (probably) more than one attitude and heading reference systems (AHRS). There may be other ways; I'm not a sparky. I do not know what system is on board the 737 max, so I do not know what it actually uses for backup data. But the requirements of FAR 25.1309 speak for themselves, and no single failure may pose a hazard to the aircraft.

I would like to know if sensor de-icing was turned off, or malfunctioning at the time. The flight crew would be aware of either conditions, if they were happening. Since the text of the AD doesn't demand examination of the de-ice, or admonish pilots to keep it turned on, this may have been checked already, and ruled out.

This AD sheds no light on what happened, to me. Raises more questions than it answers.

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[waross]

Could differential information collected from GPS receivers in the nose and the tail provide attitude information and/or AoA information?
Maybe not exact, but surely enough to identify a 20 degree error.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[VEBill]

...sensor de-icing was turned off, or malfunctioning...

Can icing occur 5000 feet over Indonesia? Also, within such a few minutes?

 

[Sparweb]

GPS is too slow (IIRC) to act as an attitude reference.

With a ground temperature of ~27C, and a dew point ~24C, you are right to be skeptical of icing. However, the atmosphere does not always obey the standard temperature lapse rate.
Since I haven't seen anything yet explicitly ruling out this problem, it remains "on the board" despite the low probability. Especially since it was a contributing factor in the AF447 crash, which bears some similarities.



No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[GregLocock]

"Could differential information collected from GPS receivers in the nose and the tail provide attitude information and/or AoA information?
Maybe not exact, but surely enough to identify a 20 degree error."

We use GPS sensors at the front and back of the car to measure yaw velocity pitch and accelerations on cars for vehicle dynamics work.

These have replaced conventional accelerometers and gyroscopes so far as we're concerned. Bandwidth >10Hz

Differential GPS should be able to measure nose to tail pitch distance to within 800mm, which, unless you are flying very short aeroplanes, would comfortably indicate to a degree or so.

Not too sure where SparWeb is coming from on this.

Cheers

Greg Locock


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[Sparweb]

It was phrased as a guess, about the GPS for attitude. If you can get 10Hz and <1m resolution from a GPS board then sure, it can do that if you want it to. And want to add antennas to the tail. May not be necessary if you already have multiple AHRS units with IMU in the aircraft, but a backup to a backup... I didn't mean to get drawn into a comparison of IMU / GPS resolution and data rates. This is far from my specialty, so if you can prove me wrong, touché.

I did feel comfortable asking questions about the seeming lack of redundancy in the AP system. This is more in my wheelhouse. Something there doesn't add up. One faulty AoA sensor should not render the aircraft difficult to control or uncontrollable. The message in the AD (see attached) speaks of a very difficult condition to recover from:

Runaway Stabilizer
Disengage autopilot and control airplane pitch attitude with control column
and main electric trim as required. If relaxing the column causes the trim to
move, set stabilizer trim switches to CUTOUT. If runaway continues, hold
the stabilizer trim wheel against rotation and trim the airplane manually.
Note: The 737-8/-9 uses a Flight Control Computer command of pitch
trim to improve longitudinal handling characteristics. In the event of
erroneous Angle of Attack (AOA) input, the pitch trim system can trim
the stabilizer nose down in increments lasting up to 10 seconds.
In the event an uncommanded nose down stabilizer trim is experienced
on the 737-8/-9, in conjunction with one or more of the indications or
effects listed below, do the existing AFM Runaway Stabilizer
procedure above, ensuring that the STAB TRIM CUTOUT switches
are set to CUTOUT and stay in the CUTOUT position for the
remainder of the flight.
An erroneous AOA input can cause some or all of the following
indications and effects:
• Continuous or intermittent stick shaker on the affected side only.
• Minimum speed bar (red and black) on the affected side only.
• Increasing nose down control forces.
• IAS DISAGREE alert.
• ALT DISAGREE alert.
• AOA DISAGREE alert (if the option is installed).
• FEEL DIFF PRESS light.
• Autopilot may disengage.
• Inability to engage autopilot.
Initially, higher control forces may be needed to overcome any
stabilizer nose down trim already applied. Electric stabilizer trim can be
used to neutralize control column pitch forces before moving the STAB
TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be
used before and after the STAB TRIM CUTOUT switches are moved
to CUTOUT.

The previous AFM instructions were a common "abnormal procedure" for when the elevator trim system suffers a runaway. That kind of procedure is known to pilots. Even light aircraft flight manuals will advise how to respond to failure of the trim system.

This new procedure is more dire, describing the crew's response to "higher control forces" and "relaxing the column causes the trim to move". Makes it sound like an AP gone crazy, not a trim runaway.

 

[GregLocock]

The trick is to use several GPS at the extremity of the vehicle. Most of the noise in a GPS output is due to atmospherics, so if you have more than one they seem the same errors, and so when you subtract the outputs (hence the name differential GPS) the noise disappears.

Cheers

Greg Locock


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[Alistair_Heaton]

this is a bit of a funny one because the trim system that's effected is only functional when the plane is being flown manually.

• Continuous or intermittent stick shaker on the affected side only.
• Minimum speed bar (red and black) on the affected side only.
• Increasing nose down control forces.
• IAS DISAGREE alert.
• ALT DISAGREE alert.
• AOA DISAGREE alert (if the option is installed).
• FEEL DIFF PRESS light.
• Autopilot may disengage.
• Inability to engage autopilot.

Now thats some list of items to go wrong and apart from the AoA mismatch possibly Feel diff none of them we would immediately associate with having problems with the Angle of Attack sensor.

With my engineers hat on instead of pilots if the AoA data is getting linked into so many systems i would say it needs to have a third backup vane to allow 2 out of three agreeing for fault tolerance just like we do with the other primary data sources.

 

[Spartan5]

The trick is to use several GPS at the extremity of the vehicle. Most of the noise in a GPS output is due to atmospherics, so if you have more than one they seem the same errors, and so when you subtract the outputs (hence the name differential GPS) the noise disappears.

I think that some of the issues here regarding the viability of GPS for use in determining the real-time positioning of aircraft and their orientation are due to the conflation of a few different concepts. Not being an expert, I will be speaking in generalities. But I think I have some idea of what is happening here.

The first is your explanation of the name differential GPS. It's true that comparing the difference between local celestial GPS outputs will be more accurate than looking at a single celestial signal, but only marginally so. That's comparing the absolute location (where am in the world) vs. the relative accuracy (where am I relative to this other local sensor experience the same atmospheric interference).

Differential GPS relies on terrestrial base stations transmitting from fixed, precisely-known locations to broadcast a signal correction to the rovers allowing for very precise real-time location. These fixed terrestrial base stations need to be located within several miles of the rovers for the system to function properly.

Am I correct in my assumption that where you are using the differential GPS to measure vehicle dynamics is at a fixed location like a test track and not out and about anywhere in the world at any given time?

 

[Alistair_Heaton]

Aircraft don't use dgps. We use the SBAS system.

Galileo if it ever comes online will also give much better resolution.

Even dgps is a bit pants at vertical position.

 

[IRstuff]

While GPS attitude/heading sensors use the differential signals and phase between antennas to determine angles, they are not technically what is(was) called dGPS. dGPS traditionally refers to using a receiver with surveyed, absolute, GPS location with the same receiving environment to "correct" the GPS at an unknown location. This removes the atmospherics as well as the inherent errors in the GPS satellite outputs to achieve, in the best cases, sub-centimeter positional accuracies.

An attitude GPS with 10-m antenna separation can achieve heading accuracies better than 0.02° rms, which is better than the heading performance of an eTalin from Honeywell. However, during a bad enough war, the US GPS likely will turn back on its Selective Availability, which would possibly cripple a GPS-based attitude/heading sensor. However, modern GPS receivers can receive at least the GLONASS signals, and may still have decent performance.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[waross]

It was my understanding that the various flavours of GPS reception based on differential signals were developed to cancel out the effects of selective availability.

There was an urban legend concerning Selective Availability during the Gulf war.
Legend had it that during the Gulf war, the army had a demand for quantities of military grade GPS receivers that could not be met quickly enough. (Apparently there is second set of GPS signals that is available only to the US military).
According to the legend, selective availability was turned off so that civilian GPS receivers could be used by the US military.
Some of us felt that the GPS accuracy was improved during and after the war.
Is there any correlation for this legend?

My concern is that the GPS system may be turned off completely during a war, or at least the civilian service.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Alistair_Heaton]

The sa has been turned off for years.

And it is that easy to jam GPS that they have said they won't turn it on again. With Galileo transmitting low Res as well these days it's pretty pointless turning it on again.

 

[Sparweb]

All this talk of using GPS for attitude reference is getting off topic. I am duly impressed by growing capabilities of GPS. It is not used for this function in commercial aircraft. GPS has other functions in commercial aircraft, most especially as a navigation reference. You can even couple your approach to runway landing to a suitable GPS reference. However, attitude is not navigation. Nose pitch up and pitch down is attitude, not navigation.

This is what commercial aircraft use for AHRS (Attitude & Heading Reference System).
Collins
Universal
Honeywell

They contain MEMS gyros. Lots of them. There is no co-ax connector on these boxes. They don't use GPS.

With this context clarified, my question bcomes "How did the autopilot system let the airspeed sensors out-vote the AHRS sensors?"

Now that I'm on this rant, I would be remiss if I didn't spill some more acronyms, such as Air Data and Inertial Reference Unit (ADIRU) and Standby Attitude and Air Data Reference Unit (SAARU), which are similar to the AHRS, except they actually DO HAVE airspeed data inputs. Each of these also depend on inertial references for attitude. The 737 MAX may have multiples or none, depending on the system design. I don't have a 737 MAX Flight Crew Operation Manual here, so I can't tell you exactly which boxes it uses. If it does have these, then inside these specific boxes is an algorithm that evaluates the integrity of all data sources and chooses the best for the attitude and condition of the aircraft. I really want to know if something is going wrong inside these boxes, causing the confusion in the flight director system, because that confusion is supposed to be designed out of them. At a very fundamental level, the design goal in these systems is to minimize this specific hazard.

Here's a summary of the Boeing 777's Fly-By-Wire System for reference. The new 737-Max could be similar.

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[Alistair_Heaton]

just been speaking to a mate that's done the differences training on it.

Apparently there was no mention of this trimming system feature during the training.

BTW this system is not linked to the Autopilot system its linked to the trim system with an input from the stall protection system apparently. They are getting almost daily updates on the subject. Every time they check in there is updates.

They suspect there will be an airworthiness directive with a mod on the subject.

The 737 max is not fly by wire flight controls. It has fly by wire spoilers and bleed air.

the triple is digital flight controls. ie no direct control runs to the flight controls.