Official Aviation Thread...

[varg]

Oh, I'm just getting all this - the stick only operates the elevators. That trim wheel by your knee runs cables to the horizontal stabilizer. On some planes (maybe only little ones?) I thought that was stationary and the elevators did all the work. So if that big stabilizer is shoving the nose down, you need to know why and disable that pronto. If they didn't even know about this system, what could they do except keep trying to fight it? And what happened at the end where it just dropped?

So, "Trim" = horizontal stabilizer. Why would you ever change the trim in flight? For weight distribution? Isn't there an ideal attack angle that doesn't change? When you rotate the nose up at take-off, is that just elevators doing that? I always feel like that steep angle is asking for a stall, but I don't understand all the physics.

Think about what a horizontal stabilizer does and adjustable trim will make perfect sense from a physics standpoint. The trim must be adjusted based on configuration, speed, weight and balance, it is something that requires a wide degree of adjustment. Flaps and slats can change the center of lift/center of pressure of the wing, drag from landing gear can impart a moment on the airplane that must be countered, CG location can shift as the aircraft burns fuel, maintaining the higher angle of attack necessary at slower speeds, all of these things require trim changes.

Elevator trim is a universal thing in modern airplanes. Today, fixed trim tabs exist almost exclusively in the realm of ultralights and other very simple, very small airplanes. In the past it was something that existed more widely but that changed quickly because there is no single trim position that is optimal for the entire flight envelope of an airplane. Electric trim is widely used in commercial and even GA aircraft, and in any aircraft with electric trim dealing with a runaway trim situation is a necessary part of the training that familiarizes you with that aircraft in your transition.

[tttoon]

You need the trim because civil airplanes are designed to be speed stable. The tail actually produces downforce, and together with the lift of the wing, adds up to zero moment around the center of gravity of the airplane.

If you reduce airspeed, the change in wing lift and tailplane downforce will cause a pitch down, as the airplane tries to regain its trimmed speed. So you're actually only "in trim" for one airspeed and configuration. Airplanes without trim would require constant force on the elevators at all but one speed, if that makes sense.

We actually also have rudder and aileron trim, but those are not used that often. Some military planes (F16 springs to mind) are aerodynamically unstable, which makes them more manoeuvrable, but they have "artificial" stability due to their fly by wire system.

[LateFan]

...from an engineering standpoint, think of the surface area of the entire stab compared to the elevators.(

Oh, I'm just getting all this - the stick only operates the elevators. That trim wheel by your knee runs cables to the horizontal stabilizer. On some planes (maybe only little ones?) I thought that was stationary and the elevators did all the work. So if that big stabilizer is shoving the nose down, you need to know why and disable that pronto. If they didn't even know about this system, what could they do except keep trying to fight it? And what happened at the end where it just dropped?

So, "Trim" = horizontal stabilizer. Why would you ever change the trim in flight? For weight distribution? Isn't there an ideal attack angle that doesn't change? When you rotate the nose up at take-off, is that just elevators doing that? I always feel like that steep angle is asking for a stall, but I don't understand all the physics.

[varg]

Now that I've read more about it and seen that there was a problem with the aircraft on a previous flight I've been thinking about what caused the issue in the first place. Given the caliber of training, servicing, and personnel you'd expect from this airline with a less than stellar record in a developing nation, it would not be a surprise if the AoA sensors or the system itself was damaged in ground handling or maintenance and it was simply not reported, not known, or ignored.

[hoveringuy]

Unfortunately, the voice recorder is buried in the sea mud, it should answer those questions.

[flyboyx]

I'm not type rated on the 737 so I don't know first-hand what the relationship between trim and control effectiveness is (plus the elevator feel computer is adding forces), but the elevator trim is connected to the flight deck through cables, so when the trim is being commanded down the big wheel by your thigh is spinning. It's obvious.

It ends up being the exact same thing as runaway trim, which is taught and practiced in the simulators.

I mostly agree with you 100%. all I am saying is that full nose down trim of the entire horizontal stab can not be overcome with elevator input in flight. the article link latefan posted actually mentions this as well. from an engineering standpoint, think of the surface area of the entire stab compared to the elevators. perhaps like ttoon said, its possible that they had so many bad things going on in the cockpit, that the trim going bonkers was missed because they were busy with other things. maybe the trim slowly rolled to full nose down? maybe they tried to inhibit the trim but the airplane wouldn't allow it?

its possible that they lost the ship because they didn't have the proper training to overcome the problem and were not able to think outside the box. maybe they should have pulled a Denzel Washington and flew upside down? it is also possible that not even Sully could have saved this one. we will just have to wait and see.

I did read somewhere that they thought the airplane hit the water going about 600 miles an hour? I wonder if a dive at 5000' to sea level would attain such a speed without the engines producing full power? at the very least, why not turn off the auto thrust and bring the power to idle? in the end, I guess the only difference between hitting the water at 200 versus 600 is that the pieces will be a little bigger.:(

[tttoon]

Based on what we know so far, I think these guys got IAS disagree, ALT disagree and AoA flags pretty much after take-off. Throw in a trim runaway while you're doing the memory items for unreliable airspeed and things get interesting very quickly.

[hoveringuy]

Yes, but in a jet airliner, the trim moves the entire horizontal stabilizer as in the entire tail wing surface. There is no way in hell that full up elevator input will overcome a fully pitched down horizontal stabilator.

I'm not type rated on the 737 so I don't know first-hand what the relationship between trim and control effectiveness is (plus the elevator feel computer is adding forces), but the elevator trim is connected to the flight deck through cables, so when the trim is being commanded down the big wheel by your thigh is spinning. It's obvious.

It ends up being the exact same thing as runaway trim, which is taught and practiced in the simulators.

[varg]

I'm no commercial pilot or fighter pilot, just an engineering student with private pilot training. But it seems to me like an significant oversight to neglect to mention an automatic stall mitigation system that has full authority over stabilator angle in training or the manuals/handbooks. I've never been through it, so I'm not sure what training for a new variant of an aircraft under the same type certificate is like, but from what I've read since I found out about this it is a system that did not exist in any capacity in the older 737 variants and seems worth mentioning.

[flyboyx]

I've always believed that, if the plane is trimming down and trying to fly you into the water, don't let it! Pilot controls (at least in Boeing) overcomes automation. Pull back hard, fly the plane, and figure-out why it was doing that later.

Yes, but in a jet airliner, the trim moves the entire horizontal stabilizer as in the entire tail wing surface. There is no way in hell that full up elevator input will overcome a fully pitched down horizontal stabilator.

[LateFan]

You guys who know what you're looking at in a cockpit will get something out of this piece.....
https://www.nytimes.com/interactive/...h-cockpit.html




Possible first indication of trouble


Pilot appears to be struggling with manual control


Plane plummets

[hoveringuy]

I've always believed that, if the plane is trimming down and trying to fly you into the water, don't let it! Pilot controls (at least in Boeing) overcomes automation. Pull back hard, fly the plane, and figure-out why it was doing that later.

[LateFan]

A rare screw-up for Boeing? (which I hate to see)

Or a training lapse and poor communication?

https://www.reuters.com/article/us-indonesia-crash-boeing-explainer/explainer-unraveling-the-boeing-737-max-lion-air-crash-idUSKCN1NK2BE

"...as a result of the larger engines, which are placed higher and further forward of the wing, the jet’s balance changed. To address that, Boeing put in place more anti-stall protections, Leeham Co analyst Bjorn Fehrm said in an online post."

"An automated protection system called the Maneuvering Characteristics Augmentation System (MCAS) kicks in when the angle of attack is too high, when the plane’s nose is too elevated, threatening a stall."

"One of two angle of attack sensors on the Lion Air jet was faulty, according to Indonesian investigators."

"The U.S. Federal Aviation Administration (FAA) last week warned airlines that erroneous inputs from those sensors could lead the jet automatically to pitch its nose down even when autopilot is turned off, making it difficult for pilots to control."

"Lion Air’s flight manual did not contain information about the new anti-stall system, according to investigators and an airplane flight manual seen by Reuters. U.S. pilots were also not made aware in training courses, pilot unions say."

"Boeing Chief Executive Dennis Muilenburg told Fox Business Network on Tuesday that Boeing provides “all of the information that’s needed to safely fly our airplanes”."

"Pilots can stop the automated response by pressing two buttons if the system behaves unexpectedly, the FAA says.

That action is set out in a checklist used by Lion Air pilots for in-air troubleshooting, an instructor said. It is also required to be committed to memory by pilots."

"Pilots on a flight from Jakarta to Bali the day before the crash experienced a similar sensor issue but managed to land safely by turning off the system, the New York Times reported."

[tttoon]

Nice, Will!

My understanding of the systems is that in the NG, a high AoA would 1) activate stick shaker 2)inhibit rudder yaw damping and 3) use Sys A hydraulics to re-bias the elevator feel to give more control pressure, but it wouldn't move the controls directly.

The Max has the additional feature of trimming the nose down which is the same as a control input. I also need to research a rumor that the high AoA also triggers a thrust increase in autothrottle.

So, a false high AoA signal could pitch the nose down and increase thrust...

The (unadvertised) new MCAS (Manual Control Augmentation System) in the MAX apparently does that. If a high AoA is sensed (error or not) the system commands nose down trim. The trimming is also much faster than the STS on the NG. Boeing's argument for not having the system in the manual is that a failure of the system just needs to be treated as a trim runaway. I'm hearing lots of airlines do not agree with that. We just got the bulletin today, stating basically that they will incorporate the system into the FCOM.

[BlackbirdM3]

So this happened... I shot this as the owner was leaving Oshkosh.


Not to get too far off topic, here is 184 seconds over Oshkosh, at night. This is a single exposure shot of one of the night airshow performers.



This shot was kinda cool. That straight, bright blue beam of light was a laser that prompted a TFR to 99K feet. (Yes you read that correctly, 99000 feet.) The performer was in a Yak 52.


Will