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FGD135
29th Mar 2016, 12:35
Hi all,

The accident to the FlyDubai B738 at Rostov-On-Don recently could have been due to "mistrimming". The vertical profile of this accident is almost identical to that of the Air Tatarstan B735 that crashed at Kazan, Russia in 2013.

In both cases, during a missed approach, the aircraft reached a height of several thousand feet above the aerodrome then rapidly pitched down, diving into the ground beside the runway. Both crashes were fatal. Neither appears to have stalled.

Is the pitch trim system of today's airliners poorly understood to the point that aircraft are crashing?

Please refer to this article on the subject:

Roger-Wilco | Do you really understand how your trim works? (http://www.roger-wilco.net/do-you-really-understand-how-your-trim-works/)

Here is an extract from it:

I have watched, in the simulator, a 737 go-around from a Cat lll fail passive approach (as described above) with its marked pitch up; HP kept his arms locked forward to contain the attitude whilst simultaneously running the trim forward with the thumb switch. I am sure he was expecting the trim to reduce push needed and he either didn’t know, or had forgotten, that it wouldn’t. We duly pitched straight back quickly into the ground as the tailplane incidence ‘bit’.

This account sounds a lot like what could have happened in the two accidents I referred to above.

So, what does actually happen when, in a type like the B737, you hold a particular attitude - against the feel forces - then run the trim. Do the forces reduce to zero, or do they stay the same?

Incredibly, the linked article doesn't actually say! Here is the first paragraph:

Picture yourself in a conventional airliner, say a 737 of any generation. You have to do a low level go-around, perhaps because your fail passive Cat lll has just failed, er, passively. You apply GA thrust, and the aircraft pitches up. If you are low enough, you may already have some extra helpful nose up trim applied thanks to the ‘design feature’ that ensures that in the event of AP failure at low level, the aircraft pitches up not down, and so a few units of nose up trim are applied late in the approach. Your speed is low, about Vapp and the thing is pitching firmly upward. You need ample forward stick/elevator to restrain it. You don’t want to carry this load for long so you retrim. Question: if you run the trim forward while maintaining forward pressure on the wheel, what happens? Hands up all those who think the load reduces to zero. I see a lot of hands. My unscientific polling to date suggests that just about everyone is convinced that this is what happens, but it doesn’t.

Frustratingly, the author doesn't clearly spell out what does actually happen, but reading between the lines, he seems to be saying that the forces DO NOT reduce to zero.

What happens on the B737? And, is it any different to all the other airliners that have trimmable horizontal stabilisers?

de facto
29th Mar 2016, 12:48
Ok ....pitch main control is the elevator(you use the control column).
Stabiliser is used by automatics or manual to relieve force on the elevator.
Stabiliser is used by the speed trim system to allow speed stability and elevator is used at higher mach number for speed stability and avoid mach tuck.
The stabiliser is the back up for elevator jam,elevator is back up for stabiliser jam.
Easy no?
Dont over trim is the main point,thrust can be used to reduce load,and avoid over trimming.

Some system helps the trimming (STS)and during every t/o ,a nose down trim,to force pilots to trim up at a certain height(about 400 feet or so)just in case an engine fails,so aircraft is a nose up trim.(system is part of mach trim).

Once again,go arounds require very little trimming and so do any other maneuver..flying manually needs also only very few clicks on the stabtrim here and there,normally while banking,1 or 2 clicks NU or when selecting flaps,intially would require nose down,(lift increase),but manul elevator enough,then when stable in speed,1 or 2 click of NU.
Selectionof flaps 40 for example on AP will command a significant trimming about 3 secs or so because the AP is reacting to drag and speed decay,pitching the ND, while on manual flight, not reducing thrust as much as the automatics would before/during selecting flaps 40,would require much less stab trim.
Pilots know in advance what will happen based on their input,,AP are only REACTING to an input.

nick14
29th Mar 2016, 12:53
The stabiliser moves therefore you are always attempting to achieve a control column CENTRAL position. As you trim you should relax pressure on the controls so that they gradually return to the central position without a significant change of attitude.

This is different to most light aircraft as you are trimming the pitch control to a new position NEURTRal.

The two are very different.

gums
29th Mar 2016, 13:29
Salute!

"TNX FG for bringing this over here from "rumor central".

For this old pilot, even tho he flew first operational fully FBW lite a few decades ago, I am fascinated by all the connections with the AP and such.

If I understand other posts, the AP trim "assistance" has two sets of gains and attempts to re-position the stab at different rates depending upon configuration. I am somewhat familiar, as our Viper changed gains, hence physical movement of the stabs, according to the force exerted. However, as with conventional flight control systems, we trimmed to reduce control stick pressure ( not movement).

Would really like to see this thread take a life.

de facto
29th Mar 2016, 14:57
Gums,
The 737 has an elevator feel computer which provides pilots a feel of their aircraft,via control force column,using the actual position of the stabiliser and aircraft speed.

fireflybob
29th Mar 2016, 16:44
I learned about this when I did my original conversion course on the B737-200 in 1980.

de facto
29th Mar 2016, 16:48
Arent we all very proud you did..

fireflybob
29th Mar 2016, 16:50
Arent we all very proud you did..

So is this not taught on conversion courses any more?

Basic a/c control which all pilots should be aware of on this type surely?

de facto
29th Mar 2016, 18:17
Obviously not everywhere..too busy teaching all failures as quickly as possible and get done with it.

hec7or
29th Mar 2016, 18:18
I posted this extract from the B737-800 FCOM in R & N but it was largely ignored,

During high AOA operations, the Stall Management/Yaw Damper (SMYD) reduces yaw damper commanded rudder movement. The EFS module increases hydraulic system A pressure to the elevator feel and centering unit during a stall. This increases forward control column force to approximately four times normal feel pressure. The EFS module is armed whenever an inhibit condition is not present. Inhibit conditions are: on the ground, radio altitude less than 100 feet and autopilot engaged. However, if EFS is active when descending through 100 feet RA, it remains active until AOA is reduced below approximately stickshaker threshold. There are no flight deck indications that the system is properly armed or activated.


the crucial part is the increase in control column force provided by the EFS Module by increasing system A pressure to the feel and centering unit which IMO could result in the PF over trimming. What is not clear to me, is if the feel force is increased, will the aircraft feel more out of trim than it actually is?

Sciolistes
29th Mar 2016, 18:18
So, what does actually happen when, in a type like the B737, you hold a particular attitude - against the feel forces - then run the trim. Do the forces reduce to zero, or do they stay the same?
It is quite simple and the linked article is I guess correct but kind of a storm in a teapot. If you hold the elevator forward in that go around example, say 2kg (just an example) of force is required, to hold 15 deg nose up, if you trim down and continue to hold about 2kg the aircraft will pitch down. If you trim down and reduce forward force commensurate with the trimming (allow the control column to return toward the centre) then the aircraft with remain at 15 deg nose up (all other things being constant of course).

This is because the Elevator Feel System will generate a simulated aerodynamic force for the commanded elevator position and airspeed (and stab position, but how much and in what way isn't documented). As you centralise the control column then the elevator is centralised but the pitch maintained by the stab position.

So if you hold a set pitch attitude and trim then you will need to relax the pressure on the control wheel - the article seems to be making the utterly intuitive reflex an issue.

The article then criticises this:
To recover from a stall, angle of attack must be reduced below the stalling angle. Nose down pitch control must be applied and maintained until the wings are unstalled. Application of forward control column (as much as full forward may be required) and the use of some nose-down stabilizer trim should provide sufficient elevator control to produce a nose-down pitch rate. It may be difficult to know how much stabilizer trim to use, and care must be taken to avoid using too much trim. Pilots should not fly the airplane using stabilizer trim, and should stop trimming nose down when they feel the g force on the airplane lessen or the required elevator force lessen. (my emphasis)
It is absolutely correct. To recover from a nose high condition, the elevator may not be sufficiently effective. It says "the required elevator force lessen". In other words the amount of deflection (i.e force) required to reduce the angle of attack sufficiently to break the stall or recover.

For example, if you try the Turkish AMS event in the sim, recovering from the stall requires immense amounts of forward pressure to prevent the nose from rising with go around thrust. To go from full aft trim to something like say 7 units takes about 14 seconds, which feels like a long time. It takes several seconds of trimming to actually feel the elevator start to have a reasonable amount authority and the amount of required force to reduce.

de facto
29th Mar 2016, 18:28
The feel of the aircraft is 99% of the time as expected.
In some occasions the feel is wrong and maintenance is advised.
The aircraft control column could feel "over sensitive" or "heavy".

gums
29th Mar 2016, 20:54
Salute!

Lemme see if I have this straight, Betwixt ( Sciol xxxx), Gotta love those cryptic/cute locations, huh?

So the 737 flcs ( flight control system) tries to provide feedback like the old, really old planes ( not fully hydraulic or hybrid systems, but more like a Moth, Chipmunk, Champ, Camel, Warhawk, etc) did with mechanical connections and you could actually "feel" the aero forces or lack thereof. Is this all the time or with only AP engaged? Does it use actual dynamic pressure and static pressure? Are pitch rates integrated? and so forth.

I have a hard time understanding why we are not using a straightforward procedure that requires one to maintain a notional attitude regardless of control pressures or seat of the pants cues. I like trim. I do not like holding "x" pounds of pressure or deflection more than I have to. I also get a good "feel" for the jet when it is trimmed and I note how far off of "normal" the surfaces are positioned.

Looking fwd to more explanations of trim implementations here.

de facto
29th Mar 2016, 21:30
General
The elevator feel computer receives pneumatic, hydraulic, and mechanical in- puts. With these inputs, the elevator feel computer sends metered pressure output to the elevator feel and centering mechanism.
Physical Description
The elevator feel computer has these components:
- Stabilizer actuated cam (2)
- Q diaphragm (2)
- Droop spring (2)
- Relief valve (2)
- Force balance valve (2)
- Differential pressure switch.
The elevator feel computer has these ports:
- Pneumatic pitot port (2)
- Pneumatic static port (2)
- Hydraulic pressure port (2)
- Hydraulic return port (2)
- Hydraulic computed pressure port (2).
Functional Description
Hydraulic pressure from the system A and B flight control modules goes to the pressure ports.
As the airspeed increases, pitot pressure goes through the pitot port and pushes on the Q diaphragm.
The Q diaphragm moves and pushes the relief valve. This movement is in pro- portion to the airspeed. The stabilizer actuated cam position affects the range that the Q diaphragm can move due to pitot pressure. When the relief valve moves, it pushes the force balance valve. The force balance valve controls the pressure that goes though to the dual feel actuator. The dual feel actuator is part of the feel and centering unit.

gums
29th Mar 2016, 23:45
Thanks for the reply, 'facto"

Seems like a lotta stuff to provide basic "feel", but I have never flown a heavy.,

I shall wait until some heavy pilots comment that have a few thousand hours in the 737.

Capn Bloggs
29th Mar 2016, 23:58
I shall wait until some heavy pilots comment that have a few thousand hours in the 737.
Careful, Defacto's forgotten more about the 737 than you'll ever know...

With respect, Gums, the 737 was designed before your F16 was a twinkle in someone's eye. Companies don't just start completely redesigning FCSs just for stamps when the latest gadget comes along. The 737 works. Why change it?

The message that is reinforced from this discussion, for me, is "don't fly with the trim". Push and pull all you like but only use the trim to remove sustained pressure on the stick after you've been holding the same attitude. I hadn't really appreciated the stick position after trimming the stab, but yes, it's always in the same position when trimmed. I looked at it yesterday! :ok:

FGD135
30th Mar 2016, 02:37
... it's always in the same position when trimmed. I looked at it yesterday!Good info, Bloggsy, very good info.

And the artificial feel? That is a function of the relative positions of the elevator/stab?

What I understand, and would like you to confirm, is that when the elevator and stab are "aligned" (or "neutral") there are no feedback forces. That is, the feedback forces only start when you deflect the elevator away from its "neutral" position with respect to the stab.

This would seem highly logical, and what I'm really after is this confirmation. If confirmed, that would confirm a number of other highly significant things, in relation to trim.

_Phoenix
30th Mar 2016, 04:45
Salute!
Does it use actual dynamic pressure and static pressure?

Yes it does.
So, if speed data goes wild. As example, an amazing speed due Pitot tube clogged with ice. Are automatically deactivated the "feel" computer and the STS (speed trim system)?

Jwscud
30th Mar 2016, 09:26
The FCOM is delightfully vague on this matter. Any sustained differential pressure at the elevator feel unit (as caused by a hydraulic or elevator pitot failure) illuminates the FEEL DIFF PRESSURE light and associated master caution. However, the QRH is a one liner saying "continue normal operation". However, a quick look in the MEL tells us this is a NOGO on the ground.

Hopefully somebody with access to the AMM (de facto?) can enlighten me more. There is no indication that the Speed Trim System is deactivated. However, if you are manually trimming, the speed trim is deactivated and remains so for 6 seconds after you stop trimming.

I do find the complete lack of joined up info in the FCOMs and QRH very frustrating...

gums
30th Mar 2016, 13:30
Great stuff, Bloggs, and I am "in the zone" with you and defacto.

I did not intend to diminish the experience and knowledge of defacto, and should have said "waiting for MORE heavy pilots to add to the thread".

Interesting to find use of a "bellows" equivalent to our older fighter flight controls that used them to keep us from ripping the wings off when snatching on the stick.

As far as flying using the trim, I think I understand your position Bloggs. Our problem is that some of the FCS trim implementations move the stab so much that we have almost no elevator authority. The bloggers on the AF447 thread that had sim time and tried to duplicate the deeply stalled, and trimmed stab used very positive manual trim and then 15,000 feet to regain control!!

Back to my question: the trim gains are a function of BOTH the autopilot mode AND the configuration, right?

And BTW, are there indicators that let the pilot know the current trim position of the stab/ailerons/rudder? I am assuming the trim implentation moves the stab to reduce yoke/stick forces required to maintain an attitude or AoA and does not "position" the elevator.

Denti
30th Mar 2016, 14:10
Back to my question: the trim gains are a function of BOTH the autopilot mode AND the configuration, right?

Well, yes in a way. It is actually a function mainly of configuration. Both the autopilot stabilizer trim circuit and the main electric trim stabilizer circuit have two speeds, slow and fast. Fast is used with the flaps extended, slow with flaps retracted. The main electric trim (manual trim) is faster than the autopilot trim iirc. And of course, the fallback mode, manual trim, is as fast as you can turn that trim wheel.

xollob
30th Mar 2016, 14:46
hec7or - your post is really interesting.

I posted this extract from the B737-800 FCOM in R & N but it was largely ignored,

During high AOA operations, the Stall Management/Yaw Damper (SMYD) reduces yaw damper commanded rudder movement. The EFS module increases hydraulic system A pressure to the elevator feel and centering unit during a stall. This increases forward control column force to approximately four times normal feel pressure. The EFS module is armed whenever an inhibit condition is not present. Inhibit conditions are: on the ground, radio altitude less than 100 feet and autopilot engaged. However, if EFS is active when descending through 100 feet RA, it remains active until AOA is reduced below approximately stickshaker threshold. There are no flight deck indications that the system is properly armed or activated.the crucial part is the increase in control column force provided by the EFS Module by increasing system A pressure to the feel and centering unit which IMO could result in the PF over trimming. What is not clear to me, is if the feel force is increased, will the aircraft feel more out of trim than it actually is?
I would love to know the answer to that one, potentially in the low speed high AOA option the pilots recovering from it and maybe the effects of those quick blips on the trim having a magnified effect..... especially if it is combined with any possible thought process of use the trim if not getting full authority using elevators only !

Sciolistes
30th Mar 2016, 16:11
hec7or - your post is really interesting.




I would love to know the answer to that one, potentially in the low speed high AOA option the pilots recovering from it and maybe the effects of those quick blips on the trim having a magnified effect..... especially if it is combined with any possible thought process of use the trim if not getting full authority using elevators only !
The opposite xollob. The amount of required force to push the control column forward during a AoA event is significantly reduced making it far easier to reduce the angle of attack and so less trimming will be required.

Just don't stall upside down!

xollob
30th Mar 2016, 17:10
The opposite xollob. The amount of required force to push the control column forward during a AoA event is significantly reduced making it far easier to reduce the angle of attack and so less trimming will be required.

Just don't stall upside down!

You see, every day is a learning day, rather learn that one away from a live environment :)

I can see myself asking to experience that in my next sim to see if there is any noticeable difference.

nick14
30th Mar 2016, 18:30
The elevator feel shift module will (as far as I understand) INCREASE the force required to move the control column further aft in order to prevent a worsening situation. Never heard of it reducing forward force.

hec7or
30th Mar 2016, 18:45
sciol

your explanation would make sense only if there was a reduction in hydraulic pressure to the feel and centring actuator during forward control column displacement.

the FCOM says that the EFS module commands higher system pressure to the elevator feel and centring unit during a stall resulting in higher forward control column force.

In normal flight, feel is provided at higher airspeeds by the feel control computer which increases hydraulic pressure to the feel pressure hydraulic actuator which increases the centring force on the control column.

It appears from the FCOM, that the EFS module commands higher "forward control column force" at low airspeeds by increasing hydraulic system pressure to the feel and centring unit.

Could it be poorly worded or do they really mean that centring force is increased at high AoA or are we actually talking about an increase in forward control column force?

I agree with nick14, i think what they mean by the term "forward control column force" is in fact "aft column displacement feel force" but as with everything in the manuals, it is too condensed to make any sense except to the person who wrote it.

Sciolistes
31st Mar 2016, 03:42
My apologies fellas,

I think I am correct in what I wrote, but I think I got two separate elements of the system confused. Yes, I think you are correct, the EFS will make it more difficult to move the stick aft. The crucial bit is that the aircraft is however automatically trimmed nose down by the speed trim system.

I interpret therefore that the net result would seem to be that, fortunately as I wrote, in terms of the required force, it is easier to lower the nose and that less trim would be required. Do you guys concur with that explanation?

safelife
31st Mar 2016, 04:17
Flying the fly-by-wire Airbus in direct law indeed makes this much clearer.
The situation is the same, artificial feel on the control, and a completely separate trimming of the horizontal stabilizer.
But the sidestick is just centered by a spring so it is obvious that it's load will not change until you release it (and it centers). Just running the trim can't and won't change the load while holding it deflected.
With a conventional yoke somehow the expectation remains that the trim would relief the load.

ECAM_Actions
31st Mar 2016, 06:42
I think the main issue being missed/not appreciated, is that the artificial feel unit is present exactly because without it there would be no feedback whatsoever! It is there to provide a dynamic load against which the controls are moved, but a critical point is it generally does not reflect trim condition (ignoring aircraft response, the controls feel the same whether trimmed or out of trim, for a given airspeed).

The hint that the aircraft is not trimmed is in the fact you must hold a pressure against the artificial feel unit in order to maintain a desired pitch.

Centaurus
31st Mar 2016, 14:18
I have done many go-arounds in a 737-300 full flight simulator and that includes from well below Vref. Have also simulated the Turkish Airlines 737 coupled approach at Amsterdam where the radio altimeter defect caused the thrust levers to close allowing the Vref to reduce to Vref minus 30. On that occasion the stab trim winds back a long way as the AP attempts to hold the ILS glide slope.
Providing the stab trim is toggled forward for a few seconds at the same time the GA commences and the aircraft pitches up because of the effect of GA thrust, the GA is relatively straight forward.

But if you are not up to scratch with a manual GA on instruments in IMC, especially if you are slow to operate the stab trim in conjunction with forward elevator to ease the stick forces as the pitch up occurs, then the pitch angle can quickly increase to a dangerous angle.

If the GA starts at an already slow airspeed well below VREF, then the trick is not to call for Flap 15 until passing VREF in the climb. Calling for Flap 15 at Vref minus 30 from a Flap 30/40 landing approach is most unwise as the aircraft will almost certainly stall. Treat the GA like a Windshear escape manoeuvre and don't touch the gear or flaps until the climb is safely under control and speed is beyond VREF and increasing slowly.

Capn Bloggs
31st Mar 2016, 14:18
are there indicators that let the pilot know the current trim position of the stab/ailerons/rudder?
Here're my indicators:

http://s17.postimg.org/nzrkkjutr/trim.jpg (http://postimage.org/)
This is on one of the centre displays. Whether one would have the wherewithal to look at the stab position if fighting the beast on a dark and stormy night is debateable....

In the cruise the stab is around 0-1°; on approach 5-9°.

nick14
31st Mar 2016, 17:55
It's a fundamental procedure that you do not retract flaps until above Vref surely?

Approach to stall recovery first then when above Vref "return to desired flight path" IE start the missed approach procedure.

gums
31st Mar 2016, 18:17
Thnx, Bloggs, what I had hoped.

My thot was having a way to tell if the A/P had run in a gob of trim due to fuel balance, icing, whatever. Then, upon disconnect you are confronted with an unusually large amount of required stick/yoke force.

Agreed about prolly not noticing when very BZ. But you might take a peek if holding in or close to icing conditions.

Denti
31st Mar 2016, 19:12
Haven't seen that kind of display on any 737 though (yes, i'm cautious, there are more options than anybody knows). Usually it is just the stab trim scale next to the big clunky trim wheels, and a small white pointer moving mechanically above it. Not at all in the primary field of view, but usually not needed either.

actus reus
1st Apr 2016, 00:31
I am unfamiliar with the aircraft and the control systems of which you speak, but the system you describe OK465 sounds like it may be in place to assist the pilot to avoid the 'super stall' regime common to T tail aircraft?

OK465
1st Apr 2016, 02:47
Possibly, but more specifically the special system on the MD-90 was directed at providing handling qualities that would more closely match the previous iterations of the DC-9 (-80s included) and allow for a common type rating from the DC-9-10 all the way through the MD-90.

actus reus
1st Apr 2016, 03:06
OK465,

Hmm, for cross crew qualification requirements the appropriate advisory material is the
AC 120-53.

Common handling, i.e. recovery technique required, in the stall regime is one thing but if the system of which you speak is only active in that regime, I am not sure it would be required to make the advisory requirements.

I will have to think about that.

gonebutnotforgotten
1st Apr 2016, 09:36
I own up to being the author of the original article quoted by FGD135 at the start of the thread. I apologise to him if I didn't make myself totally clear about what happens when the pitch trim is moved while still holding the column forward or aft at a constant force; the forces do not change (though as has been pointed out by others in extreme situations when as hec7or says the Stall management/Yaw damper system may intervene, so everything may well feel different - I've never been there). I had hoped the message was clear enough without inelegant repetition. I am pleased that this little article has caused some head scratching, which is exactly what it was intended to do. The Big Lesson was meant to be 'Know your control system'. Sadly far too many people, including especially the trainers who should know better, don't think it through thoroughly. I hope people read the whole article and reflect on the other situations I quote and think what they would do. I note Sciolistes' view that this is all a storm in a teacup... I wish it were, but I continue to see examples of crew actions that show understanding could be improved.

framer
1st Apr 2016, 12:53
It certainly got me thinking gonebutnotforgotten.
I took off this morning in a light 737-800 with plenty of thrust and paid close attention to what I was doing with the trim and yoke as I cleaned up.
A fair amount of fwd trim was required, but I noticed that I was automatically ( without thought) reducing the pressure on the yoke so that it came back towards neutral as the trim was going in.
It seemed it is a natural thing to do even though I confess my understanding of what was happening was low prior to reading your article.
Do you think, or is it your experience, that in an extreme ANU situation I would have kept the yoke forward and then as the nose came down towards the horizon had to apply large back pressure to prevent an extreme AND?
Is that what you see in the sim?

FGD135
1st Apr 2016, 13:06
gonebutnotforgotten,

Thank you so much for joining us in this thread and thank you so much for that original article.

Just a few minutes ago, I read an update on the FlyDubai crash. That update came from a short report, in Russian, issued today by the MAK. It said that "on reaching 900 m, there was recorded stab travel to the nosedive position, resulting in aircraft beginning descent and hitting runway".

That must be a sadly familiar scenario to you, as it sounds exactly like the simulator account you gave in your article.

gonebutnotforgotten
1st Apr 2016, 22:53
FGD

The incident I quoted was (probably) a little different to the FlyDubai in that what I saw was a simulated Cat 3a GA from the flare following an Autopilot failure. In that case the speed was very low, so trim is more nose-up + there was the nose up bias that's fed in at low level (nominally at 400ft Radio), and the control deflection to counteract the nose up pitch from the engines is large. As I understand it, the the flyDubai went around from a higher altitude , possibly above the height at which the auto pitch trim is applied, and logically at a much higher speed than in the flare, both of which should have minimised the difficulty of trimming for the go-around. There are also reports that the airline did all approaches manually using a HUD, in which case I would assume no auto pitch trim would have been applied. Finally, the incident I quoted happened very quickly, very close to the ground, whileFlyDubai seems to have climbed a few thousand feet before losing it. All in all quite a different scenario. I wouldn't exclude the possibility of confusion over the functioning of the pitch control system as I described in the Roger-Wilco article, because as I've said, so few people seem to understand it, and that understanding matters most when large pitch changes occur (as in any GA). But if it did play a part, I suspect there were other errors too. We'll just have to wait for the report.

Thanks for the interest from you and framer

JammedStab
1st Apr 2016, 22:57
It seems to me that a big part of the problem is the large amount of thrust which is why the aircraft is pitching up so much to begin with. Some of the bigger Boeings which are typically on automatics are designed to only add enough thrust for a 2000 fpm climb if the TOGA switch is pushed once. I am not sure about the 737-800.

I remember in the old 727 being empty on a cold day with a 6000 fpm climb with a level off coming up. All was solved by bringing the power back about halfway at least 1500 feet before level off resulting in a nice reduction toward a 1000 fpm climb and level off for the last 1000 feet of climb.

That being said, the flaps had been retracted at this point. But on a manually flown go-around with a high climb rate and a fairly low altitude level off, there can be some conflicting limits. The missed approach altitude is nearing quickly with a high climb rate resulting in a desire to push forward on the controls to avoid passing through it, but there is also a speed limit on the flap setting. Things start happening very fast which can perhaps easily be slowed down significantly by disconnecting the autothrottle and reducing thrust. Perhaps some button pushing can be done with the autothrottle connected but there seems to be a time lag doing this.

Centaurus
2nd Apr 2016, 00:43
But if you are not up to scratch with a manual GA on instruments in IMC, especially if you are slow to operate the stab trim in conjunction with forward elevator to ease the stick forces as the pitch up occurs, then the pitch angle can quickly increase to a dangerous angle.

What I should have added to the highlight above (but I forgot) is that even if the pitch up is getting out of control due to slow reaction by the pilot, the Boeing advice is to:

"If normal pitch control inputs do not stop an increasing pitch rate, rolling the airplane to a bank angle that starts the nose down should work. Bank angles of about 45 degrees, up to a maximum of 60 degrees could be needed. Unloading the wing by maintaining continuous nose-down elevator pressure keeps the wing angle of attack as low as possible making the normal controls as effective as possible.

With airspeed as low as stick shaker onset, normal roll controls - up to full deflection of ailerons and spoilers - may be used. The rolling maneuver changes the pitch rate into a turning maneuver, allowing the pitch to decrease. Finally, if normal pitch control then roll control is ineffective, careful rudder input in the direction of the desired roll may be required to induce a rolling maneuver. for recovery.

Only a small amount of rudder is needed. Too much rudder applied too quickly or held too long may result in loss of lateral and directional control. Because of the low energy condition, pilots should exercise caution when applying rudder. The reduced pitch attitude allows airspeed to increase, thereby improving elevator and aileron effectiveness. After the pitch attitude and airspeed return to the desired range the pilot can reduce angle of bank with normal lateral flight controls and return the airplane to normal flight". Unquote
............................................................ ...............................................

That advice is gold standard and should be learned by heart for all pilots as it applies in general to most types of aircraft from Cessna 150 to Boeing 737.
Because of the relative complexity of this recovery action from high pitch angles, it is should be practiced regularly in the simulator under simulated IMC. That is because when unusual attitude situations occur such as those described above, it is most likely at night or in IMC.

From the above, and that includes the Boeing FCTM advice, it is easy to understand the vital importance for pilots to maintain their basic instrument flying skills on raw data.

LOMCEVAK
2nd Apr 2016, 10:50
I have flown several types that have pitch trimming effected by a variable incidence tailplane/horizontal stabiliser but have never come across one that has the problems alluded to for the B737. This has set me thinking ....

I think that the first issue is predictably following an AP disconnect. If an aircraft is in trim, the pilot will control the aircraft in pitch intuitively. However, if it is sometimes in trim, sometimes not and by differing amounts then it will be very unpredictable and there will be a finite reaction time before the pilot starts to make any required inputs. I have flown one aircraft that was longitudinally statically stable and did not have any follow-up trim post engagement. Therefore, if a disconnect occurred and there had been a speed change since engagement then the aircraft would be out of trim. However, it was still predictable because knowledge of the speed variation from selection fitted a mental model of the aircraft's flying qualities and it was consistent.

The second consideration relating to out of trim conditions is whether the pilot is attempting to maintain a constant pitch attitude or to generate a pitch rate. If he/she is attempting to maintain an attitude and has the elevator authority to do so (i.e, requires less than full elevator), the force can be held initially and then stabiliser trim inputs made. The force will reduce and the control column will need to be moved back towards the trim position. However, whilst retrimming the pilot will subconsciously be correcting any errors in the desired pitch attitude via yoke displacements rather than forces and so the aircraft's response will still be predictable.

I believe that the problem aspect is when the pilot is trying to generate a pitch rate but full elevator deflection does not give him the required rate. In the case of a low manual go-around with an AP disconnect following an AP nose up trim input and a nose up pitching moment due to power, a nose up pitch rate may exist even if the pilot applies full forward yoke. If nose down stab trim is applied simultaneously, a nose down pitch rate should eventually be generated, and due to the resulting steep nose up attitude a nose down pitch rate is now required to regain the desired pitch attitude. This pitch rate will be generated by both the elevator and the stabiliser but the pilot has no sense of the relative contributions of the two controls. When the desired pitch attitude is achieved, aft movement of the yoke may not stop the nose down pitch rate if too much stabiliser trim has been applied and even with full aft yoke the nose may continue to pitch nose down. I suspect that this is why the Boeing FCTM advice posted by Centaurus advocates the use of bank and not simultaneous elevator inputs and stabiliser trim; the key is in the first line when it refers to "pitch rate".

john_tullamarine
2nd Apr 2016, 12:18
One of the sideline issues is training and exposure to such problems .. unfortunately not always addressed adequately in these modern fiscally driven days.

I recall (quite fondly) my first tailwheel endorsement on the SuperCub. A very thorough endorsement, it included a full flap, min approach speed, in trim final approach and then the miss.

The stick load was like nothing I had experienced to that time in my flying career (probably around 100 hours or so total).

One of those experiences which progressively convinced me of the need for pilot exposure to out of the routine flight events in the simulator in later years. The time not to get a surprise first time around is in the heat of the real moment ..

flyingchanges
2nd Apr 2016, 14:08
This pitch rate will be generated by both the elevator and the stabiliser but the pilot has no sense of the relative contributions of the two controls. When the desired pitch attitude is achieved, aft movement of the yoke may not stop the nose down pitch rate if too much stabiliser trim has been applied and even with full aft yoke the nose may continue to pitch nose down.


15 seconds of trim, flaps up, will change the in trim speed by 100 knots (737-800). It would be more speed / less time with flaps out.

I wonder what the perception would be if the other pilot were applying some opposite force to the yoke.

LOMCEVAK
2nd Apr 2016, 14:57
flyingchanges,

In this situation force only becomes an issue if it is so high that full elevator deflection cannot be achieved, which could occur with manual, unpowered controls. The problem here is one of elevator authority, which is a function of displacement rather than force.

alldaysushi
2nd Apr 2016, 16:49
Centaurus, Boeing definitely recommends unloading the wings in nose high attitude upset, great technique, however, probably not in a low altitude GA in the soup.

Capn Bloggs offers the most relevant lesson, IMHO,

1. Pitch to attitude.
2. Trim out stick forces.

Safer Journies,

Sushi

Otto Throttle
3rd Apr 2016, 06:34
Gentlemen, thank you for not only an informed discussion, but one refreshingly free of hysteria and childish one upmanship.

I don't want to drive the discussion too far away from its intended purpose, but turning briefly for a moment to the specifics of the FZ accident, I would like to clarify some points.

FZ do not authorise their crew to fly dual AP approaches, so in this instance, talk of nose up trim at the point of GA would not apply. The latter stages of the approach (typically from around 1000' AGL), are hand flown, and it's reasonable to expect that this means an in trim aircraft at the point of GA. Autopilot and autothrottle are typically disengaged together.

The FZ airframe involved was fitted with 27K rated engines. My understanding is that standard GA thrust, from a single push on the TOGA button, gives 26K thrust. This is more than adequate to achieve a relatively unexciting rate of climb (between 1000-2000fpm) as previously mentioned. As the aircraft was relatively light, I would expect closer to the higher rate.

In relation to the approach prior to the accident, it is not clear exactly what the configuration of the aircraft was (flap 15 or 30), or whether the AP was still engaged or not. These could have a significant bearing on the response to the GA. We also don't know the reason for the GA, except that it took place somewhere around 1400-1500' AGL. The weather conditions were a strong, quartering headwind, with a significant increase in the lower levels above the point of GA - from some 25kts on the field to 60kts at the final altitude reached.

Purely for the purposes of discussion, I would be very interested to know the likely effect on the aircraft trim system in the following circumstances:

Autopilot engaged, final landing configuration of gear down, flap 30. At approx 1500', a hard windshear warning ("windshear, windshear, windshear" from the EGPWS). As the autopilot and autothrottle are both still engaged, the PF pushes TOGA and activates an automatic, AP coupled windshear escape manoeuvre. This will be with the full 27K thrust available.

Assuming no change of configuration, and a rapidly increasing headwind, what is the trim system going to do? How long would the STS run before potentially reaching the limit of its full authority?

If (and I mean IF), the crew misidentify an excessively long STS trim, coupled with unusually high pitch, as a trim runaway, what is the immediate aircraft response going to be on AP disconnect? How much more difficult does the situation become if the Stab Trim cutout switches are activated?

I'm embarrassed to admit, it's way beyond my current understanding. I have an idea, but that is the whole point if this thread. People have ideas, but usually grounded in an incomplete or inadequate knowledge of the system.

Centaurus
3rd Apr 2016, 06:47
Boeing definitely recommends unloading the wings in nose high attitude upset, great technique, however, probably not in a low altitude GA in the soup.


Think about it. The pilot has stuffed up big time if during a go-around in IMC he has failed to prevent or recover from a dangerously high nose attitude accompanied by rapid airspeed bleeding back to onset of stick shaker. What are his options under these circumstances?

1. Watch the aircraft stall since his current control movements have failed to stop the speed bleed and in a few seconds he and his passengers are dead.

2. Take only some of the Boeing FCTM advice and roll the aircraft to drop the nose to permit speed gain; disregard the advice to unload to better aid recovery and instead pull back on the stick to prevent height loss near the ground.

3 Take the full Boeing FCTM advice which includes unloading the wings during the roll manoeuvre regardless of proximity of nearby terrain and hope you have acted soon enough.

Your choice. I sure know which option I would choose.:ok:

FullWings
3rd Apr 2016, 07:59
The mechanics of trimming a stabiliser/elevator combo are very interesting and one hopes that all who fly such things have a working understanding, especially on aircraft that have a pronounced pitch-power couple.

One thing that strikes me is that if you are aware of your attitude, there is enough feedback that control movements should be instinctive. Pitch too high? Push forward until desired attitude reached then trim out force while holding attitude. Unable to reduce pitch with full forward movement? Trim forward until you can. The inverse applies with low pitch.

All of the above requires an instrument scan in IMC, at least of the blue and brown bits of the PFD on a 737 or looking out of the window if you have visual references. It’s the same in a Supercub. If you’ve lost SA in attitude then bad things might happen, regardless of the trimming situation...

Jwscud
3rd Apr 2016, 09:10
Otto, you have one key misconception regarding the 737 systems. In any single channel approach a press of TOGA for any reason immediately disconnects the autopilot.

For a hard windshear warning (two tone and WINDSHEAR), the drill is to push TOGA then disconnect everything and apply maximum (full forward stop) thrust.

For an activation of the Predictive Windshear system on the radar a normal go around or the Windshear escape manoeuvre is at the captain's discretion.

We do not know what was attempted but in either cause, the autopilot would be disconnected the moment TOGA was pushed.

A single push of TOGA with the AT engaged annunciates GA in the FMA and the AT commands thrust to provide a rate of climb of 1-2000fpm (typically around 85-90% N1)

A second push of TOGA annunciates N1 in the FMA and you will get all 26k available at which point you will be climbing like a homesick angel at light weights.

actus reus
3rd Apr 2016, 09:12
Two of the most alarming accidents that involved pre-programmed back trim (system input) during a coupled approach and a subsequent missed approach, both involved in a very short space of time, either the A300 or the A310.

I am going back to the 90's and when I get some time I will find the references (apologies that I am not able to do that at the moment) but it should be relatively easy to find the accident reports.

First one: Nagoya.
Second one: Taipei (Chang Kei Shek as it was at the time).

PIC unable to overcome the pitch trim input following a Manual AP disconnect during the Missed Approach before the aircraft(s) stalled.

I remember seeing both unfortunate wreckages. Plan form on the ground.

I will give you more when I have the time; apologies.

Otto Throttle
3rd Apr 2016, 09:47
Jwscud

Boeing would disagree regarding the automatic coupled windshear escape manoeuvre, but as I have no real world, first hand experience, I can only accept their word for it. Please check the QRH Manoeuvres and please don't shoot the messenger.

For the other aspects, we are in full agreement regarding the use of TOGA buttons.

RAT 5
3rd Apr 2016, 11:23
Capn Bloggs offers the most relevant lesson, IMHO,

1. Pitch to attitude.
2. Trim out stick forces.

Being a KISS person, and in stressful situations, I do not have any more mental capacity than to follow this advice. I wonder if some are over complicating it.
What I do notice in the sim is pilots (cadets) who get themselves in odd spots and keep their thumb on the trim wheel firing a full magazine. I encourage 3 shot bursts and then reassess the target (control forces). Another 3 shot burst if necessary. It is not hard to imagine a very high nose attitude, perhaps due to max thrust and an overzealous pull up; full forward elevator control (PFD can be nearly invisible in this scenario) then a full burst of nose down trim, an over speed (flap) and a thrust reduction. The nose will pitch down with much gusto and the required elevator pull force to arrest this plummet is considerable.
Over trimming and agricultural use of thrust is not a healthy combination in B737's.

Smilin_Ed
3rd Apr 2016, 14:52
Full Wings: One thing that strikes me is that if you are aware of your attitude, there is enough feedback that control movements should be instinctive.

That is if you have a qualified pilot in the cockpit. (Remember AF447?)

framer
4th Apr 2016, 00:21
After reading all the input here I have come to the conclusion that in all operations where full forward control column is not needed, the system is intuitive. I monitored my own actions more closely than usual a few days ago in a climb that required several seconds of AND trim, and it was intuitive, the force required initially on the control column was quite high but nowhere near full forward, as the stabiliser started to have an effect it was natural to allow the control column to move aft towards the neutral position.
I have never needed full forward control column in the seven or so years that I have been flying the NG, so if tomorrow I did.....would it be intuitive how much AND trim to put in?
I think that if a pilot ends up with nose high, full forward control column, and trimming forward, that the trick would be to stop trimming forward as soon as the nose does start to move in the right direction rather than keep trimming with a determination to get the nose down. It would be quite natural to keep trimming when the attitude slowly drops from 35 nose up to 34...33...32... because the pilot really wants it to hurry up and get to 10 degrees where they feel comfortable, but by the time the nose is coming through 15 degrees the stabiliser is trimmed to require significant back pressure to maintain level flight.
I'd welcome any thoughts on the above, that is my take.
Framer

actus reus
4th Apr 2016, 03:04
Reference the two accidents I mentioned, here is the wiki link.
I am not saying Wiki is the best source of anything let alone aviation accidents but the links to the reports are there.

https://en.wikipedia.org/wiki/China_Airlines_Flight_140

See also China Airlines flight 676 mentioned and linked at the bottom of the article.

I will say again, these entries are by no means enough for professionals to form a view.

Derfred
4th Apr 2016, 03:36
framer,

Thank you for that summary of the thread... this is how I read it too, and you have weeded out the distractions.

I've been flying this thing for decades, and this scenario has certainly made me think, and wonder how I would behave in this situation.

What I'm having trouble with, however, is how does a normal go-around (or windshear escape) end up running out of elevator? I've done plenty of go-arounds in the Sim and aircraft, and plenty of windshear escapes (in the Sim only, lucky for me), and I've never come close to full forward elevator.

The only time I can recall getting to full forward elevator was practicing stall recovery after having allowed the autopilot to trim back to stick-shaker, so commencing the recovery with the stab considerably aft.

If full forward elevator happened in a go-around, I would simply reduce thrust (after all, the aircraft was in-trim at lower thrust just prior to the go-around). I certainly wouldn't be flinging it into a 60 degree bank at low level.

If it happened during a windshear escape, which is more likely due to application of maximum thrust, then that would be more interesting, because one wouldn't want to be reducing thrust in windshear. But if a few seconds of forward trim doesn't solve the problem, then I would think another emergency exists (some form of flight control non-normal).

P.S. Otto, Jwscud is correct. Auto-coupled windshear escape is only available for dual channel approach.

Otto Throttle
4th Apr 2016, 09:10
Derfred, thanks for that. Mr Boeing, like many things, does not make that clear.

de facto
4th Apr 2016, 13:04
Seriously Otto?,
Boeing could not make it clearer by including such fact in the QRH under "windshear escape maneuver" AUTOMATIC.
Obviously as a pilot you would know that 1AP ON+TOGA=AP OFF and 2 AP ON +TOGA=AP ON.

Derfred,
forward elevator happened in a go-around, I would simply reduce thrust (after all, the aircraft was in-trim at lower thrust just prior to the go-around). I certainly wouldn't be flinging it into a 60 degree bank at low level.


Refreshing,thank you.

FullWings
4th Apr 2016, 18:41
I think that although there are in-extremis recovery techniques for most sort of LoC scenarios, the very fact that the departure from the normal flight regime has happened while the pilot was on the controls and maybe because they were on the controls, it means that they are unlikely to be able to take much appropriate action in the near future as they have basically lost SA. Near the ground that’s often fatal.

It’s been a while since I was on the 737 but I do remember doing a lot of GAs during sim and base training in various configurations. There were certainly issues with thrust and trim but not unsurmountable ones and after a few goes you got the general idea. The nastiest one was probably a CAT III auto approach with an engine failure on the GA: the aircraft was out-of-trim in all three axes and required some deft handling to get it back on an even keel...

FGD135
5th Apr 2016, 12:35
Ok, so for aircraft with trimmable horizontal stabilisers (THS) you must never hold the trim switch engaged for extended periods, but must release the switch frequently, so as to allow the column to return to neutral - and allow the forces to diminish.

So, that process would be:

1. "Blip" the thumb-switch for a second or two, then release;
2. Relax grip on the wheel, allowing it to move toward neutral;
3. Assess the remaining forces;

Then repeat. Blip, relax, assess. Blip, relax, assess.

But this is NOT how pilots are taught to trim an aircraft. When pilots first learn to fly, one of the first things they learn is to hold the attitude, then trim until the control forces go to zero.

But it seems that trimming a large, THS aircraft that way can be fatal.

Do pilots undergo retraining on how to apply trim when they transition onto these larger aircraft? It appears not.

Of course, when the trim change is small, pilots will instinctively employ and repeat the "blip, relax, assess". This is something they have learnt, by themselves, to do - as it is perfectly appropriate. But things are much more interesting when the trim change is large, it would appear.

Drawing on gonebutnotforgotten's excellent trimming article once again, I quote this passage from Boeing, relating to nose-high upsets:

... the use of some nose-down stabilizer trim should provide sufficient elevator control to produce a nose-down pitch rate. It may be difficult to know how much stabilizer trim to use, and care must be taken to avoid using too much trim.
Are pilots trained to know how much "some" nose-down trim is? And how to "avoid using too much trim"?

de facto
5th Apr 2016, 15:11
2. Relax grip on the wheel, allowing it to move toward neutral;
3. Assess the remaining forces;

Then repeat. Blip, relax, assess. Blip, relax, assess.


Relax indeed,a pause i use to zip my coffee but yes thats how it works...

fireflybob
5th Apr 2016, 16:37
Do pilots undergo retraining on how to apply trim when they transition onto these larger aircraft? It appears not.

FGD135, when I was training captain on B737 in the 1980s and 1990s this was part of the syllabus on the conversion course and was well covered.

Centaurus
6th Apr 2016, 02:29
Unable to reduce pitch with full forward movement? Trim forward until you can.

Be very careful about that statement. It could lead to an ever increasing dive angle due to over-trimming.

Far safer to follow the Boeing 737 FCTM advice on Upset Recovery that states: "If normal pitch control inputs do not stop an increasing nose attitude, rolling the airplane to a bank angle that starts the nose down should work. Bank angles of about 45 degrees, up to a maximum of 60 degrees, could be needed."

Nothing about trimming forward if unable to reduce pitch with full forward elevator

framer
6th Apr 2016, 07:26
Centaurus, there is something about applying trim. The QRH tells you to " apply appropriate nose down stabiliser trim". There is an asterisk beside the statement that takes you to a warning note at the bottom of the page which tells you that excessive trim may lead to a loss of control.
The QRH trumps the FCTM.
I'm not saying it is well written or understood but to say that there is nothing about applying trim if unable to reduce attitude with elevator is incorrect and may mislead some readers.

framer
6th Apr 2016, 07:32
Centaurus:
Further to my last post, I have just read the latest version of the FCTM which also recommends using trim if in a wings level nose high upset ( greater than 25 degrees nose up).
It states In this situation the pilot should trade altitude for airspeed, and maneuver the airplane's flight path back toward the horizon. This is accomplished by the input of up to full nose-down elevator and the use of some nose-down stabilizer trim.
What version number of the FCTM are you preaching from?

FullWings
6th Apr 2016, 07:35
I think the previous paragraph needs a mention as well:

"In this situation the pilot should trade altitude for airspeed, and maneuver the airplane's flight path back toward the horizon. This is accomplished by the input of up to full nose-down elevator and the use of some nose-down stabilizer trim. These actions should provide sufficient elevator control power to produce a nose-down pitch rate. It may be difficult to know how much stabilizer trim to use, and care must be taken to avoid using too much trim. Pilots should not fly the airplane using stabilizer trim, and should stop trimming nose down when they feel the g force on the airplane lessen or the required elevator force lessen. This use of stabilizer trim may correct an out-of-trim airplane and solve a less-critical problem before the pilot must apply further recovery measures. Because a large nose-down pitch rate results in a condition of less than 1 g, at this point the pitch rate should be controlled by modifying control inputs to maintain between 0 g and 1 g. If altitude permits, flight tests have determined that an effective way to achieve a nose-down pitch rate is to reduce some thrust.”

Also, whacking on loads of bank near the ground I would regard as the ultimate, last-ditch, do-or-die manoeuvre, probably initiated because of a fault with the flying controls or some issue with the CofG.

If you are way out of trim on the stabiliser, at some point you’re going to have to trim it otherwise you’re going to spend the rest of the flight going around in small circles until the fuel runs out.

I think I stand by my statement. You need to regain elevator authority and as soon as you have it, i.e. you get the ability to reduce pitch back, you can stop trimming. At the point you realise that you can’t get the nose down are you in an upset or quickly approaching one? I would have said the latter, so decisive corrective action will get the attitude under control and thus the flightpath before any more extreme measures are necessary.

Jwscud
6th Apr 2016, 09:50
As a secondary point to the above, I have always been slightly concerned about my airline's teaching that the moment you are in a go around (or indeed takeoff) both hands should be on the control column and away from the thrust levers. In a situation like this action in all controls is necessary and if your hands are not in the right place your life may be harder than necessary.

Coming from a manual thrust aircraft, the only time hands were off the thrust levers were between V1 and reducing to climb thrust. In many normal go arounds, action on the thrust levers is required reasonably promptly and having a hand there is an important tactile handling cue.

Centaurus
6th Apr 2016, 15:08
If you are way out of trim on the stabiliser, at some point you’re going to have to trim it otherwise you’re going to spend the rest of the flight going around in small circles until the fuel runs out.
If I recall correctly, some early Canberra bombers were lost due to runaway stabilizer (or was it elevator?) trim. The immediate action if the runaway was upward was to go into a steep turn to get the nose back to the horizon and appropriate power to maintain a safe speed. If you were then stuck in the steep turns forever because of inability to counter the runaway if you tried to straighten out, the only other option was to eject. The point being it was vital to combat the runaway trim by acting instantly to lower the nose from extreme high attitude into a steep turn rather than allow the aircraft to stall.

If the nose of (say) a Boeing has been allowed to pitch up excessively (45 degrees or more) during a go-around because of delayed reaction by the pilot (poor instrument scan in IMC), then such will be the rapidity of airspeed reduction, that seconds may count to prevent the aircraft from stalling.
At slow speeds, elevator effectiveness is less. In addition, trimming the stab trim forward to a guesstimate appropriate position, takes a few seconds of time, keeping in mind the 737 QRH warning to: "Use pitch trim sparingly." The angle of change from 45 degrees or more nose up, to just below the horizon, takes a few valuable seconds but is needed to get airspeed increasing again.

Depending upon the rate and angle of pitch up, as well as pilot competency, there may not be time to wait and see if elevator and stab trim combined is working quickly enough before control is lost. The QRH continues: "Roll (adjust bank angle) to obtain a nose down pitch rate"

Clearly the pilot is racing against time to get the pitch down to safe manageable levels. Unloading and simultaneously rolling to get the nose dropping is more or less guaranteed to get the nose going down immediately. On the other hand, leaving those corrective actions too late by first waiting for full forward elevator and appropriate forward stab trim to take positive effect before resorting to a roll, might be the straw that broke the camel's back.

Sciolistes
6th Apr 2016, 16:02
Nothing about trimming forward if unable to reduce pitch with full forward elevator
Look at the nose high UA non-normal manoeuvres memory items in the QRH. From memory it says 'as much as full nose down elevator may be required. Use electric pitch trim as required.' It says something about excessive use of pitch trim can cause more controllability issues.

FullWings
6th Apr 2016, 19:50
I think some of the problem with discussions like these (and very useful they are too) is that we all have a scenario in the back of our minds when we post and it isn’t necessarily exactly the same as the ones others are considering.

For the sake of argument, let’s assume that:

a) The aircraft (737) is in normal working order
b) SOPs have been followed up to this point
c) A GA in landing config. is now initiated

Yes, the aircraft has a tendency to pitch up (not actually a bad thing in some circumstances) and this is enhanced on a coupled approach at low level. Is it uncontrollable? No. Would it become less controllable if pitch were allowed to increase to the point airspeed was being lost? Yes. If you still didn’t do anything would it carry on to the point of stalling as the elevator lost effectiveness? Possibly.

If a crew has got to this juncture without any action or intervention, then they are out of the loop. It’s too much to expect a timely recovery from a self-induced scenario: they were unable to diagnose a nose-high reducing-airspeed situation while it was happening, which often has a simple remedy, so what are the chances of correct recovery action being taken? Not good. We’re not talking about test pilots exploring the envelope, it’s line guys well out of their depth, for whatever reason (distraction, fatigue, gradient. etc.)

Possibly better to deal with the incipient low speed event with elevator and trim if required, maybe with an “I have control” in there somewhere. Unless something has actually broken then rolling to contain the pitch shouldn’t be necessary; you have it as a last resort but it’s going to look awfully odd to the guy in the other seat unless you’re very quick with the explanations. Plus, if you have the skills and gonads to throw in 60degs of bank at low level why the *&%^ didn’t you do something less extreme earlier!?

Centaurus
8th Apr 2016, 02:15
but it’s going to look awfully odd to the guy in the other seat unless you’re very quick with the explanations

Not trying to tell a perceived "war story" but believe me there are situations that have happened where the captain (assuming he is PF or he is forced to take control from the PF) has to take instant action without the normal luxury of a CRM/TEM round table agreement. The fact the action is "going to look awfully odd to the guy in the other seat" should be of no concern. I had two of those instances in my flying career of long ago.

One was a runaway propeller within seconds after liftoff in a Convair 440. The the other was a training flight in the circuit when we experienced a sudden full flap runaway to down while on three engines in a four engine aircraft. On both occasions things happened so fast that the copilots concerned had absolutely no idea what had happened until shortly after control was gained and I got my own breath back.

Plus, if you have the skills and gonads to throw in 60degs of bank at low level why the *&%^ didn’t you do something less extreme earlier!?

I don't have a simple answer to that question.

Centaurus
8th Apr 2016, 13:34
If a crew has got to this juncture without any action or intervention, then they are out of the loop. It’s too much to expect a timely recovery from a self-induced scenario: they were unable to diagnose a nose-high reducing-airspeed situation while it was happening, which often has a simple remedy, so what are the chances of correct recovery action being taken? Not good. We’re not talking about test pilots exploring the envelope, it’s line guys well out of their depth, for whatever reason (distraction, fatigue, gradient. etc.)

Possibly better to deal with the incipient low speed event with elevator and trim if required, maybe with an “I have control” in there somewhere. Unless something has actually broken then rolling to contain the pitch shouldn’t be necessary; you have it as a last resort but it’s going to look awfully odd to the guy in the other seat unless you’re very quick with the explanations. Plus, if you have the skills and gonads to throw in 60degs of bank at low level why the *&%^ didn’t you do something less extreme earlier!?




Report following is remarkably similar to the FlyDubai 737crash sequence.

On 24 December 2015, the Interstate Aviation Committee released their final report stating that the crash was caused by an under-qualified crew who lacked the skills to recover from an excessive nose up attitude during a go-around procedure. The go-around was necessitated by a positional error in the navigation system, a map drift. The pilots’ deficiencies were caused by lack of airline safety management and lack of regulators’ oversight.[24][25]

According to the final report, during the final approach the crew initiated a go-around, but being under high workload, which possibly caused a "tunnel vision effect", they did not perceive warning messages related to auto-pilot disconnection. When the plane climbed to 700 m, its pitch angle reached 25 degrees and the airspeed dropped to 230 km/h.

At that moment the captain, who never performed a go-around before, apart from the training, moved the yoke, pitching nose down, which led to stopping climb and started descent and increase of the aircraft's airspeed. After reaching the altitude of 700 m, the aircraft started a steep nosedive, with the pitch angle reaching −75° when the aircraft impacted the ground. The plane crashed on the airport's runway with a speed exceeding 450 km/h. The time from the start of the go-around maneuver until the impact was about 45 seconds, including 20 seconds of aircraft descent

FullWings
8th Apr 2016, 14:49
There have been some interesting observations in the gliding world, showing that a small proportion of people learning to fly are very sensitive to reduced or negative g and associate it with falling (stalling), giving an instinctive reaction to push their hands out in front of them to cushion the fall, so the theory goes. As they are holding the stick, this reduces the g even more, leading to more sensation, pushing, etc.

There have been quite a few unexplained dives into the ground in aircraft that have been found to have been serviceable pre-impact. I’ve been witness to two.

It is possible that someone could train as a commercial pilot without being exposed to this kind of scenario, especially in IMC. I’m not saying this is what happened in many of these crashes, just putting it forward for consideration...

Centaurus
12th Apr 2016, 01:54
Far safer to follow the Boeing 737 FCTM advice on Upset Recovery that states: "If normal pitch control inputs do not stop an increasing nose attitude, rolling the airplane to a bank angle that starts the nose down should work. Bank angles of about 45 degrees, up to a maximum of 60 degrees, could be needed."


Have just read with great interest the latest Flight International report on the Dassault Falcon 7X that had a trim runaway on 24 May 2011. Edited for brevity (self) :

French investigators have revealed the first officer of a Falcon 7X used an escape technique from bombing exercises to regain control of the jet during a trim runaway event on 24 May 2011.

The aircraft trimmable horizontal stabiliser moved from its neutral position to 12 degrees down in 15 seconds as the 7X descended through 13,000 feet on approach to Kuala Lumpur's Subang Airport. Although the autopilot acted to counter the resulting pitch up, it disengaged after 8 seconds and the aircraft pitched up to 25 degrees.

The French speaking F/O who was flying realised his attempts to correct the pitch were ineffective but struggled to explain the situation to the English speaking captain.

He resorted to a bombing manoeuvre learned during his military career. He banked to 90 degrees to reduce pitch while the captain initially opposed the extreme 98 degree right bank. The F/O took priority control and maintained the bank at 40-80 degrees for 20 seconds.

This reflexive correction reduced the pitch from 41 degrees nose-up to 10 degrees and brought the angle of attack down to five degrees. The jet's airspeed fell to 150 knots from 297 knots during the upset which lasted 2 minutes and 30 seconds and subjected the aircraft to loads of up to 4.6g.

The BEA has credited the F/O for his rapid improvisation in recovering the Swiss registered aircraft (HB-JFN)"
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Quick thinking indeed and it showed the difficulty of trying to make a fast explanation to the other crew member of what's going on, when immediate action is the priority. Rolling into a steep turn to get the nose down from a rapidly increasing pitch angle seems not to be part of airline simulator training. The first officer's military training certainly paid off in this incident.