I don't think it is that "slow moving". I remember a figure from 1°/sec. up to 3°/sec in elecrical control, like for rudder trim, but I don't know where it is buried nor how much is the rate in manual control. Also, in this mode (back up), it could have a much higher rate than the one needed for precision trimming.

And again, I'm feeling now like the one having feed the (sleeping) Troll. You deliberatly use whatever is provided to make many assumptions based on nothing but your self ignorance. Your first assumption should be to give some credit to the engineers who designed those aircraft. You can be sure that there was a lot of thinking put in every design choice even if you don't quite understand the end result. Hence, study them in depth and, after that, come back with some constructive critisism about something you'll have really studied.

Hi takata. No problem. In 447's case, it was not THS authority that was desired, it was the desire (seemingly unaddressed, or ignored) to defeat it, to zero it. The Elevators were fine, yes? GY points out DIRECT LAW was not in. If TRIMMING can get the a/c into trouble, (fbw, or not :D), it should have a Zero channel, no?

This is what I need to know. It is said the THS follows the Pilot's ss input.
This isn't emergency; on the way up, it was doing NORMAL LAW stuff? Given the dire nature of the a/c assiete at top of climb, How could the lack of Power to the THS be the reason it stayed at ~13 NU? It appears at this point, that the climb was a problem (!), independent of conjecture re: the accident, With a need for STALL recovery, and the THS limit of 2degrees ND, no matter the reason, how did she abandon the THS so NU?

This is a big question for Pilotless flight, notwithstanding whatever 447' PF did or did not do?

Thanks again for your pics, and wow. (words of wisdom)

edit re: upper post. Manual Trim was required, isn't that .65 degree per "hand sweep"? Also, in active dynamic flight , (UA recovery), power and rate are not desired coupled together? Q/N, (Quick and Nasty?) Cruise flight wants those tender touch, Yes?

Of course yes.
What we are talking about is that the aircraft seems to have done, so far, nothing wrong by itself. The THS was trimmed here and left here. The obvious reason is that there was no real attempt to pitch down with elevators. Hence THS remained where it has been ordered to be and helped to maintain this aircraft nose-up, certainly added to further NU elevators imputs by pilots...

Then, everything is pointing at the pilots not understanding the situation at all: that they were stalled, and that they will crash without pitching down the aircraft. Consequently, what happened to them? What caused them to believe something else happened and lead them to make those imputs?
Maybe the full transcript will tell us more about that. What was their strategy? We can't recognise any procedure applied from their acts alone, be it UAS, stall recovery... or whatever else.

I must agree. The only hesitancy I have is I am nonplussed the crew would apparently leave out the THS in the recovery attempt.

And also that I remain convinced the actual problem began perhaps many seconds prior to a/p loss, and became critical alomost immediately. Alas, further data from BEA will be most welcome.

Puzzled by puzzlement
 

As a distant observer listening to all you experts being puzzled by the non-use of manual trim, I am in turn puzzled by your puzzlement.

We know that the training departments of the airlines had strenuously opposed any mention of the use of manual trim in training for upset recovery, even though the test pilots of both Boeing and Airbus had emphasized more than a decade ago that bringing the a/c into trim was in their opinion the first priority in a recovery.

So I assume (I don’t know) that if the crew had had upset recovery training (as distinct from training to recover from an approach to a stall) the use of manual trim would not have even been mentioned, and they may even had had warnings against its use (because of the danger of structural damage).

So why are you puzzled by the non-use of trim? They followed their training, which in part told them to forget what they were told in primary training, particularly WRT stalls. They were never trained to use manual trim. It wasn’t even mentioned.

QUESTION: Being a French crew for a French airline and knowing that any deviation from training and SOP might be investigated by a French Court, might they have been inhibited from deviating from training and/or SOP in case they might be blamed for any subsequent damage?

The crew were probably unaware of the characteristics of a deep stall and had never experienced one. One of Gums’ early posts mentioned a deep stall as being “just like cruise flight”. So while on this subject, how many of you experts are happy with the recent upset recovery screed which lists as one of the indications of a stall as “Buffeting”? While this is true, there is still no mention of the possibility of a buffetless stable stall. Are you all happy with this omission?

An unrelated QUESTION (my apologies if this was already been answered 2,000+ posts back):
From where did the BEA get their 107 kts ground speed on impact with the sea?

Hi PickyPerkings,
Hopefully, you did not quoted me (it was Bearfoil's) as I'm neither an "expert", neither puzzled about the non-use of manual trim!!!
Nonetheless, you seem to be the one highly suffering from tunnel vision about a "manual" THS non-existant issue (related to AF447).
Quite simply, if you don't pitch down, autotrim won't certainly trim nose down, no? Then, why would you pitch up your nose and manually trim down your THS? Where would be the logic and what is the real issue?
Ask yourself!
Then maybe you'll understand why either "manual" nor "autotrim" are a NON issue from what is published so far from the investigation.

After this point, your comments seems a bit moot.

Not an expert. Also, far away. For me, TRIM is a verb. It implies through common usage the implementation of a small aerodynamic device to position a control surface in the airstream such that no continued force is necessary to keep it in place. It relieves the operator of a consistent and irritating need for pressure to be applied to the controls. As such it implies an adjustment in control pressure so that an attitude may be extended for a length of time. As used in large and flyable Horizontal surfaces, it takes on a somewhat different meaning. Rather than freezing a small device in place to relieve the pressure needed to maintain an attitude, THS essentially moves the whole kit and changes the "Angle of Incidence" of the Wings themselves.

So a THS is not strictly speaking a transitory attempt to relieve Pressure on the Controls, it changes the aspect of the airframe itself. It is in essence the 'Elevator'. It makes 'permanent' a flight attitude commanded by the FCS. It 'follows' elevator input, giving them a great deal more authority in Pitch.

So see, I am not an expert, as my clumsy explanation proves.

(Sorry for the typos above about your name)
Did you find any detailed informations about this crew training?
Just have a look at the the two or three pages above and you'll see that manual trim use is a mandatory training as for being able to fly this aircraft in particular flight control modes. Did they ever had the real oportunity to experience it? Who know, but I really doubt it. Nonetheless, they were all certified pilots for flying this aircraft in all configurations.

As for the manual, there was a very short part about stall at cruise :
So, how would you interpret "relax back pressure on the sidestick"?
Would not one use the trim wheel?
Don't they have retrieved the DFDR? It should have recorded the acceleration at impact (assuming all those accelerometers). Hence, a basic function will give the ground speed at impact, if not provided directly by the GPS.
PS: onset of buffet... buffet may be expected close to stall speed, but gums was also certainly talking about a completely developped stall, where 1 g stall is fully achieved, that may be quiet in such a large and basically very stable aircraft.

If you are getting a stall warning, it is because you are maneuvering (applying g).

Relax back pressure means do not pull so hard on the stick-period.

Using the trim wheel would have you trimming into a stall. Not a good idea.

Hi Machinbird,
What you say makes perfect sense. I interpreted that for cruise level flight while being trimmed a bit too high. Stall warning could trigger in turbulence (basically the same effect as maneuvering). It's certainly due to tunnel AF447 thinking about its first stall warning that I'm trying to explain. I don't know also how the EFCS would render this trim feedback with neutral sidestick (hence it was stupid).
I was certainly thinking about opting for the other way!

Gums' point is illustrated in this graph: cLalphaM06.gif

It should be noted that only the blue data points are 'fact'. The extrapolation of the trend line beyond the last datapoint should be treated as a 'guess'.
You will note in the graph that there is a substantial margin between the onset of buffet (alpha-max) and the 1 g stall. BEA says in its interim report no.2 that the stall is identified by 'vibrations' which is french for buffet, and I think they mean heavy buffet.

Hi takata,
I agree it seems to have performed as designed.

One has to wonder at the wisdom of starting with an aircraft concept which is naturally longitudinally speed stable, and design ALT LAW handling characteristics which allow the aircraft to be flown (with UAS), to stalling Alpha and beyond in a trimmed condition.

gums, many thanks for the data.
I am aware that the Viper has a delta wing and strakes that generate powerful vortices at high AoA. The stabilators also form a delta-shaped plan (but without the strakes). Such wings tend to “stall” rather smoothly AFAIK, meaning there is no sharp drop in lift as AoA increases past Clmax AoA, thanks in part to vortex lift.

I tried to understand why the stabilators efficiency (measured as the distance between the neutral and full deflections curves of pitch moment vs AoA) starts decreasing at a lower AoA for ND than for NU deflection. I surmised this had to do with the downwash of the wing and since the stab airfoils are symmetric it is possible to evaluate the downwash and stabilators local Clmax AoA. This gives 21 deg and 15 deg respectively. I was puzzled by a 21 deg downwash angle until I remembered that the FCS adapts the wing curvature at high AoA, drooping slats and flaperons.

Of course, we miss pitch moment curves for the A330 to understand what ND or NU inputs could do. Owain Glyndwr posted a very interesting explanation but I am not convinced the THS was not stalled at 60 deg AoA. We know that the elevator was still working from BEA’s note but perhaps this was due to the elevator presenting a larger frontal area to the airflow in ND position than in NU. The effect of a stalled THS would be a positive slope of the Cm curve in the stalled area, meaning a decreasing downward moment with increasing AoA.

Another point where I doubt Owain’s conclusions is the estimation that, at high AoA, the mean aerodynamic center will move longitudinally to the wing center of area, hence behind the CG. Looking at the Viper curves that move apparently occurs at about 60 deg AoA when the curves turn south. Owain considered the wing only whilst he should consider the wing/body assembly. Adding the fuselage will generally move the mean aerodynamic center forward so I think it is unclear whether the wing/body adds up to or opposes the ND moment of the tail at high AoA.

The stall was very probably recoverable but imho it would take much more time and obstinate ND input to recover than the crew expected.

Yes DJ77, I wonder whether this 'lack of persistence' with ND inputs, was because no time in training (since ab initio) seems to have been allocated to 'stall recovery' - whereas 'stall avoidance' seems to have taken the allocated 'stall' training time... Without this recovery emphasis, few other than long time (old hands) would have any appreciation of what it takes to rotate such a beast back to a sensible AoA + time to accelerate back to a sensible speed.


That's I think what I was trying to get at a few posts back... I just wish I had the wisdom to chose those words - nicely put rrr :D

DJ77

Have you addressed the THS' AoA as an independent entity from the a/c?

What was 447's trimmed state nearing the top of climb? I ask because at cruise, the Tail would be loaded and would want some (minor?) value of a/cND to create its lift (which is subtracted from its cruise duty AoA?)

Assuming an input of steady ND, (after the initial NURL), would there be any chance of the Tailplane Stalling and dropping the tail before the Mainplane Stalled? Given slow speed and gentle (?) entry, would lack of buffet and a rapid NU cause alarm in the cockpit? Would the a/c have assumed a very high NU communicating an alarming (seat of the pants) reaction from PF to input max ND? Did the a/c "recover" (actually not a recovery, but a predictable "fall" off). This would set the stage for the PF's return to NU inputs for the duration? For that matter, What was 447's trimmed state at Handoff? One way to enter a remarkable climb is to experience a lack of effectiveness from the tailPlane? The initial SW, the PF NU, the roc, things went South quickly.

If of no value, disregard.........

To be fair, it would appear from the evidence we have to hand so far that it requires actions that would be pretty counter-intuitive from a trained pilot's perspective to do so though.

Hi DozyWannabe,

I don’t think that’s a fair comment.

From PJ2’s post #282 "The Effect of High Altitude and Center of Gravity on Handling Characteristics of Swept-wing commercial airplanes" -
“For a statically stable airplane the required column force, as speed varies from the trimmed condition, is less at an aft CG than it is at a forward CG.The minimum average gradient allowed by U.S. Federal Aviation Administration FAR Part 25 is one pound for each six knots.”

If at FL 350 with UAS, the PF had pulled back with several pounds of force on the controls, and then manually selected several degrees of nose up stab trim – then I would agree with you and call that counter-intuitive.

I suspect that all he did was inadvertently pull back on the stick with a few ounces of force for a prolonged period of time (maybe the next report will clarify). The stall recovery action with the application of TOGA power was the start of the next chain of errors. (N.B, his action was i.a.w. the guidance in FCOM at the time – the new stall warning recovery procedure has removed that action because of the nose up couple it causes)

Bear,
AoA_tail = AoA_wing/body - downwash angle - THS trim angle.
No BEA info but close to 3 deg NU I think.
No.
As most people here, I cannot understand the PF insisting on NU inputs especially after the zoom climb. To me, it's akin to pilot incapacitation.

My point was that to maintain 60 deg AoA you need a lot of UP elevator, which takes the THS away from stall. Gums curves and the curves in that NASA report on civil aircraft upset recovery (cited in thread #4 IIRC) show that the THS can stall if DOWN elevator is applied at that sort of AoA but there is no sign of stall with up elevator in either case. To hold 60 deg AoA you need a DOWNLOAD on the THS. What you describe is a stall with a POSITIVE THS load. What I meant was that ND moment can be obtained by reducing the up elevator just as well as applying down elevator - just a different starting point.

As you note, the Viper is a delta wing, and the centre of pressure will be greatly affected by the strong LE vortices a la Concorde. Quite different flow patterns to a high aspect ratio swept wing.

Not that naive DJ - I did consider the wing body assembly - see my post of 12th July- and the net effect is the way I described it.

THS Rate:
 

I’ve got these figures of THS rate.

The THS itself (the ballscrew) is driven by 2 hydraulic (B & Y) motors.

Max. Operating Load and Max. Speed (2 motors)

18940 daN (42578.81 lbf) ----- 0.4°/s
16950 daN (38105.11 lbf) ----- 1.0°/s
6770 daN (15219.56 lbf) ---- 1.2°/s

Limit load (both compression as tension) is 32500 daN (73062.9 lbf).
I assume this limit load is mentioned as being the limit for the Hydr.
motors to drive the THS.

Half speed for 1 motor (1 hydr. B or Y system failure) operation.

The rate for the BEA mentioned 1 minute to go from 3 to 13 ANU was 10°/60 = 0.16°/s.


For going back to the 3° ANU position:
The hinges are on the trailing edge and therefore the aerodynamic
load on the drive spindle is in the AND direction it would take 12 sec. to travel from 13° back to 3° ANU.

The manual trim wheel THS displacement is ~0,65° a stroke, if one need
1s to complete the stroke and 1s to re-grab the wheel, the rate will be 0,325°/s.

--

For the PFD messages "MAN PITCH TRIM ONLY":




fall in re-centering mode.
The elevator becomes a 'fixed' part of the THS. (=MAN PITCH TRIM ONLY)

When autotrim in not available (e.g. Direct Law, Flare Law in case RA is
unavailable or NAV IR DISAGREE the PFD message "USE MAN PITCH TRIM" is shown.

Owain: do you reckon 447's stall was recoverable?
 

after your very thoughtful and helpful posts, I'm nevertheless still unclear about the bottom line as you see it.do you consider, on an aerodynamic basis, that the A330 is actually likely to be recoverable from even a ~60-degree AoA magnitude of stall upset, given some other set of control inputs [i.e. besides maintaining max NU on the SS until altitude is exhausted]?

Right, with the elevator in full up position almost masked to the airflow and counterbalanced by an assumed nose up wing/body moment. If the elevator goes down, the THS + elevator frontal area increases about 30% and could overcome the wing/body moment.

Sorry then, I thought you did not based this sentence in your post #295: “When it gets to this point I think the centre of lift/pressure will be fairly close to the centre of area of the exposed wing. For the A330, this is about 70% mac”.

Maybe because I was careful not to give a bottom line http://images.ibsrv.net/ibsrv/res/sr...lies/wink2.gif

Certainly not from the altitude at which they seem to have arrived at that AoA!

If they had had enough altitude then maybe yes, but they would have to have done it carefully. We know, or at least we think we know, that they were at something more like 40 deg AoA for a lot of the descent, so by my reckoning they could have got back that far just by relaxing on the stick. The problem with a more aggressive recovery from 60 deg is that if they tried to apply any significant nose down elevator when at that AoA they might have stalled the THS as DJ77 suggests. This would have screwed things up mightily.

If they had first got back to 40 deg, or indeed if they had started recovery from 40 deg or thereabouts in the first place then, again as DJ77 says, the THS AoA would be body AoA minus downwash minus THS setting - say about 40-20-13 = 7 deg. There wouldn't be any problem applying down elevator from that sort of AoA, so they should have been able to pitch the nose down far enough to get out of stalled conditions, accelerate a bit and initiate a pull up. Trouble is, that process would eat up several tens of thousands of feet in altitude, so unless they recognised the situation and started recovery early they were always going to be in danger of not being able to recover before they hit the sea.

Yes in principle, but I must admit that my remarks are biased by the curves for a 'typical twin engined aircraft' as shown in that NASA upset recovery report (plus some intuition) which seems to me to indicate that nose down capability would be strictly limited at this level of body AoA because of the risk of stalling the tail with down elevator applied (the nonlinearity of the published Cm~alpha curves in the NASA report shows this).

[quote] Sorry, I thought you did not based this sentence in your post #295: “When it gets to this point I think the centre of lift/pressure will be fairly close to the centre of area of the exposed wing. For the A330, this is about 70% mac”.['quote]

Sorry in turn; I didn't make it clear that this referred only to the wing contribution to PM.

in which height lays the CG over the wing (over the aerodynamic center) ? for the stability intresting is the horizontal distance, but with an pitch >15 deg the CG begins also to move back if it is high over the wing

Dozy:
Not at all. In fact, just the opposite. A properly trained pilot would know intuitively that he must reduce AoA to get out of a stall and to do that, he must push forward on the stick and trim nose-down if necessary to get the needed nose-down moment. That is why I've been saying for a couple of threads now that the autotrim should drop out with the autopilot so that he doesn't find himself with too much nose up trim. Autotrim with the autopilot engaged is what you need to ensure that when the autopilot is disengaged you don't get big attitude excursions. When hand-flying, trim should be a deliberate act by the pilot. I agree with the opinion stated by someone else here, there was a lack of situational awareness which allowed the trim to go to full NU unnoticed. I think these guys were blindsided due to inadequate training. This is a pilot's opinion.

Rudderradderrat says it all.

Hi Grity,

I don't think the height of the CG plays a significant role at any attitude.

recoverable, training, aero stuff
 

First - A33'a tidbit scares the hell outta me.

Deja vu one more time. Exactly what the Viper did if the confusers all failed ( in our case, it was total power loss to them, and the FCS folks were afraid the things would melt. We voted to keep them running if the main power supply had an overvoltage - let the suckers melt). Otherwise, the stabilators went to a neutral setting, and this meant 10 to 20 negative gees within a second!! Ask Wolfman, who survived one of those incidents back in 1981.

I do not fault the crew for training, or lack thereof. This was one in a thousand or more of "loss of pitot system" incidents. When I say that some jets enter the stall gracefully, I don't mean you can't feel SOMETHING. But with the storms and such, and the "bus great aero, the buffet could have been masked. Until the final report is out, I can't believe the pilots held back stick for 3 minutes. Shoot, just let go and watch for 30 seconds, then DO SOMETHING DIFFERENT. The OODA loop. From a pilot perspective flying at conditions unheard of in the heavies, I learned when a buffet meant "close to stall", or "prolly in a stall", or whoa!!! I had many students that could not "feel" the increase in buffet as I could. Bothered me, but some have "touch" and some don't.

Until I see a pitch moment chart of the 'bus with various THS angles of incidence, I shall continue to believe that a sustained nose down input could have helped at the outset. Once into a deep stall, all bets are off. The c.p. moves a lot, and as with the Viper, maybe a max up command could result in an attitude change, then put in max down and "rock" the thing outta the stall. DON'T TRY THIS AT HOME!

This CAA document may explain the PF's actions on receipt of the stall warning and his subsequent nose up inputs.

"STALL RECOVERY TECHNIQUE

1 Recent observations by CAA Training Inspectors have raised concerns that some instructors (both SFIs and TRIs) have been teaching inappropriate stall recovery techniques. It would appear that these instructors have been encouraging their trainees to maintain altitude during recovery from an approach to a stall. The technique that has been advised is to apply maximum power and allow the aircraft to accelerate out of this high alpha stall-warning regime. There is no mention of any requirement to reduce the angle of attack – indeed one trainee was briefed that “he may need to increase back pressure in order to maintain altitude”.

2 It could be argued that with all stall warning devices working correctly on an uncontaminated wing, such a recovery technique may well allow the aircraft to accelerate out of danger with no height loss at the lower to medium altitudes. The concern is that should a crew be faced with anything other than this idealised set of circumstances, they may apply this technique indiscriminately with potentially disastrous consequences.

3 The standard stall recovery technique should therefore always emphasise the requirement to reduce the angle of attack so as to ensure the prompt return of the wing to full controllability. The reduction in angle of attack (and consequential height loss) will be minimal when the approach to the stall is recognised early, and the correct recovery action is initiated without delay.

NOTE: Any manufacturer’s recommended stall recovery techniques must always be followed, and will take precedence over the technique described above should there be any conflicting advice."

Jan 2010

Just Let Go
 

Someone please correct me if I'm wrong but, as I understand it, letting go with this airplane wouldn't be enough because the THS would have trimmed in the new attitude and you would continue at that attitude rather than go back to the previously trimmed speed. You would also have to get your nose back to the original position.

Hi Gums,
MAN PITCH TRIM ONLY is a mechanical backup mode provided in case one lose its electrical elevator control (total electrical failure), which is displayed on PFDs, and A33Zab gave few more details about it.

But you certainly do understand that AF447, an no point during this event, ever came close to this situation, right?
In unlikely case that pitch trim was switched to manual mode (direct law, abnormal attitude,...), this backup would NOT be triggered. It is only in case of complete failure of electrical elevator control, and this never happened to AF447.

The question has to be : What is an improvement ?
If the B or others were crashing at a far greater rate than the A, I would say that the protections were absolutely justified and needed ... but is it the case ?

It is actually what I do and don't see any justification for such phrasing :
"In the previous generation jet you'd have had no control other than thrust, but in this case you have thrust, pitch trim and rudder."
unless you consider that electronics will prevent the Airbus to suffer from total hydraulic failure ... ?

Hi Smilin Ed,

Correct.
AB ALT LAW is a very peculiar manual flight mode. It has a mixture of Roll Direct (conventional aileron behaviour) and auto-trimmed pitch stable mode.
They require different ways to fly them satisfactorily. The ailerons would require constant adjustment, but the pitch only needs a nudge and then let go (else the input gets continuously integrated).

I don't understand why it was deemed necessary to design such an awkward combination.

Airbus cg
 

I have found the document I was talking about few pages ago about the CG.
Airbus A330 Instructor Support - Normal Operations (DEC 2000)
________________________
SOME CONSIDERATIONS ABOUT THE CG
The location of the CG has significant influence on Performance, on Loading flexibility, on structure and on handling characteristics when in Direct Law.
All those factors contribute to define the CG envelope.


- Performance considerations
The weight and lift forces do create a pitching moment which is counteracted by the THS setting.
When the CG is located forward, the resulting pitching down moment is counteracted by a large THS nose
down setting which induces a lift decrease and a drag increase.

http://takata1940.free.fr/cgfig1.jpg

At Take-Off and landing, it affects:
* The Stall speeds: typically on A330/A340, the stall speed increases by 1.5 kts when CG varies from 26% to full forward CG. This affects Take-Off and landing speeds thus associated distances.

* The rotation maneuver: it is “heavier”, thus longer at forward CG. This affects the Take-Off distance. For example, on an A340 at 250 t, the TOD increases from 3,165 m to 3,241 m, when CG varies from 26% to full forward CG, which represents a 2.42% TOD increase (T/O, FLAP3, PACK: OFF, ISA, ALT 0).

* The climb performance itself: for example, if a climb gradient of 5% is required (e.g. due to obstacles) in the previous Take-Off conditions, the MTOW is reduced from 257.6 t down to 256.2 t when CG varies from 26% to full forward CG.

This is why on A320 and A340 Take-Off Performance charts are provided at forward CG (which, in most cases, is penalizing) and at 26%; these last charts may be used provided the actual aircraft CG is at least 28%.


In cruise, an AFT CG minimizes the THS induced drag, thus improves fuel consumption. For example, the fuel increase on a 1,000 nm cruise segment is as follows, considering a heavy aircraft in high altitude and CG 20% or 35%:
http://takata1940.free.fr/cgfig2.jpg

- Handling Characteristics considerations
On Fly By Wire aircraft, in Direct Law, the handling characteristics of the aircraft are affected by the location of the CG as a mechanically controlled aircraft:


Stability Issue
- Aerodynamic Centre or Neutral Point
The aircraft is considered as stable, if in case of a perturbation or gust, the aircraft tends to react back towards its previous status. The aerodynamic centre, also called neutral point, is the location where an increase (or decrease) of lift is applied when the aircraft angle of attack varies.

http://takata1940.free.fr/cgfig3.jpg

Maneuvering criteria – Maneuver point.
Depending upon the CG location, a given deflection of the elevator causes a more or less sharp aircraft maneuver. In other words, the CG has a direct influence on the maneuverability of the aircraft. If a very small deflection of the elevator causes “a lot of g”, the efficiency of the elevator is very high; the aircraft is considered as very touchy to maneuver.

The maneuver point is the location of the CG for which the efficiency of the elevator is infinite. The CG must obviously be forward of the maneuver point by a lot. This lot is defined by a maneuverability criteria which states that “at least 1° of elevator deflection is required to pull 1g load factor”. This condition defines the AFT CG limit maneuverwise.

But the CG must not be too far forward: indeed, the maximum elevator deflection must allow to pull at least the maximum acceptable load factor (e.g. 2.5 g). This condition defines a FWD CG limit maneuverwise.

http://takata1940.free.fr/cgfig4.jpg

Ground handling characteristics
Essentially at high GW (thus at Take-Off), the CG is limited AFT so as to ensure enough Nose Gear adherence to allow an efficient aircraft steering on the ground.


Take-Off rotation characteristics
The CG must be limited so as to allow:
- enough maneuverability during rotation -> FWD CG limit.
- enough margin versus potential tailstrike -> AFT CG limit.

Obviously the the THS is preset nose up in case of forward CG or nose down in case of AFT CG, in order to get homogeneous aircraft rotation. But certification maneuvers require to demonstrate “abuse cases” such as taking-off with FWD CG limit while THS is set nose down.


THS stall potential
- in approach with flaps extended, there is a nose down moment counteracted by a THS nose up setting. The more CG is forward, the more THS nose up setting is required. This may lead to a THS stall, more particularly in cases of push over where the pilot pushes hard on the stick when he notices a significant speed decrease. This limits the FWD CG in approach.

- in Go-Around or Alpha Floor, the thrust increase to TOGA, more particularly at low speeds, induces a significant pitch up moment which increases when CG is more AFT. The elevator efficiency must allow to counteract this pitch up moment, even at very low speed. This limits the AFT CG in Go-Around and Alpha Floor.


Structural Considerations
The CG cannot be too much forward due to Nose Gear structural limits; it cannot be too much AFT due to wing and main landing gear strut limit.


Loading Considerations
All the previous criterias allow to determine limits which, for example, would favor AFT CG configurations for obvious performance efficiency. However, the CG envelope must also take into account loading flexibility constraints.

Passenger movement
The CG envelope must also allow passenger to move in the cabin. This is the reason why once the Take-Off CG envelope has been determined, as well as the landing one (which is less constraining), then the inflight envelope is defined usually providing at least a 2% margin with the previous envelopes.

http://takata1940.free.fr/cgfig5.jpg

1) Performance / loading compromise at Take-Off and Landing
2) Nose gear strength structural limit
3) Main gear strength structural limit
4) Alpha Floor limit
5) Nose gear adherence limit
6) Alpha Floor limit (landing)

The inflight limit is deduced from the Take-Off / Landing envelope by adding a typical 2% margin, provided all handling characteristics criteria are fullfilled.

http://takata1940.free.fr/cg.jpg

Cof G
 

Takata #393

Thank you for the information.

In an earlier post I had mentioned that when hand flying a York as F/O in the 1950s, that when the Captain went back to the toilet, I was able to fly 1 or 2 kts faster and when he returned the speed reverted. He had altered the C of G slightly.
In Autumn 1970 I asked "our" Boeing rep. what was the optimum C. of G. for a 707. I had a telex the following day saying that aft loading would save 1/2 % fuel.
( Perhaps not really measureable on a flight. But a year... ? For a fleet...? And that was with cheap fuel, too, £20/tonne.)
Boeing's Airliner of January 1974 put the figures at 0.5% per 4% of aft shift of M.A.C. on 707, 727 and 737, but 0.2% for the 747.
The last type of aircraft that I had flown ( also made by a B.) with a full load of SLF (usually), baggage was normally loaded 2/3 forward and 1/3 aft. With hindsight it ought to have been the other way round.
Another manufacturer,( yet a different B.) refused to give any information. ( It may have been a coincidence that the airline ordered more aircraft from the B. who wanted the airline to make a profit.)

No fuel trim tanks were fitted.

ok, it is not realy significant with pitch 15 deg, but also for the calculation for the pich-up moment for the engins (toga...) you need the different in height between the engins and CG...

Hi Linktrained,
If one takes Airbus average FH per a/c and figures (above) for its fleet, Mach .82 at cruise :

A340 = 5,000 FH/year => ~ 2400 (1000 NM)*380 kg = 912 tonnes saved per a/c.
-> 912*367 (a/c operational) => 334,704 tonnes of fuel.

A330 = 3,000 FH/year => ~ 1425 (1000 NM)*220 kg = 313 tonnes saved per a/c.
-> 313*791 (a/c operational) => 247,583 tonnes of fuel.

Total = 582,287 tonnes saved per year for the fleet.
Quite a few bucks then at today prices.

Well, there could be some debate here.. its often said you only need to do the sums about a (any) fixed point (usually the c.g) and resolve them all out as a single force vector together with a moment (torque)

However, instantaneously, the engine thrust moment will rotationally accelerate around the c.g. but in resolved steady state flight the nett lift and drag centres are where the thrust will be opposed from. So only in accelerated conditions will the c.g. be the obvious force resolution centre.

Agree?

takata,
Your Airbus cg post is great reading.
That aft fuel transfer for fuel economy is a great concept, but your figures are a bit on the optimized side. MEL mentions only 1% penalty if trim tank is disabled (no aft xfer)
Nevertheless, one A330 at 6000 kg/H for 3000 FH/year is still 180 tonnes saved.

Multiplication
 

Takata

Thank you for working it out... It was beyond both my Jeppeson and my Dalton. Both of them overheated.
Each aircraft might have a Working Life of ( x ) Years, too.