adrianrf

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]?

DJ77

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”.

Owain Glyndwr

Maybe because I was careful not to give a bottom line

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.

Owain Glyndwr

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.

grity

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

Smilin_Ed

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.

DJ77

Hi Grity,

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

gums

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!

rudderrudderrat

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

Smilin_Ed

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.

takata

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.

CONF iture

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 ... ?

rudderrudderrat

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.

takata

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.



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%:


- 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.



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.



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.



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.

Linktrained

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.

grity

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...

takata

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.

HarryMann

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?

CONF iture

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.

Linktrained

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.