Quotes from CONF_iture:

"If you increase speed, you're not at alpha max any more."

That's only true if you maintain the same Nz. If you need to flare, you have to increase Nz (load factor) for the duration of the flare manoeuvre. As you know very well, Valpha-max rises with the load factor.

"If you maintain full back stick you maintain alpha max, therefore you maintain Valphamax - No speed increase."

That's only true after the flare manoeuvre has been completed, and the new FPA has been established. Then the Nz can return to (roughly) 1g. During the flare (i.e., the period when the FPA is increasing), the increased "g" causes an increase in Valpha-max. (See above.)

"What makes you climb in this case is a sufficient thrust increase alone."

The increase in thrust does help a bit with the flare, because of its vertcal component (at these high pitch attitudes). But its contribution is minor, as HN39 has explained here.

"How would you want the pilot to go and get 'a small increase in pitch' as he's already full back stick in order to obtain and maintain alpha max ... ?"

If the EFCS does leave a small margin below alpha-max in 1g flight, as I am hypothesising, the quickest way to increase lift is to rotate the pitch and obtain (temporarily) a higher AoA.

This brings us back to your main problem - why was the published Flaps-3 alpha-max of 17.5 deg not achieved at Habsheim? I'm wondering if the EFCS only permits alpha-max when the a/c is in a turn, at a specific bank-angle/load-factor. If so, that might enable a sudden emergency turn without automatic de-rotation.

Hi Chris,

I think I answered this earlier.

From the report of the Aircraft Performance Group Chairman in the NTSB's investigation of the A320 ditching in the Hudson river:

(...) However, in α-protection mode, the flight control system incorporates a phugoid-damping feedback term in addition to side stick commands when computing the commanded elevator position (which in turn determines the pitch angle response). As described by Airbus,

"… the aircraft was in angle-of attack (AoA) protection from about 150 ft RA.
When in AoA protection law, stick command is AoA objective. Stick neutral commands alpha-prot and
full back stick commands alpha-max.
However, AoA protection shall take care of the A/C trajectory and, thus, looks after phugoid damping
as well as AoA control: there are feedbacks within the AoA protection law aiming at damping the
phugoid mode (low frequency mode). The feedbacks are CAS and pitch attitude variations. Without
these feedbacks, an aircraft upset from its stabilized flight point up to constant high AoA would enter
a phugoid (which is, by definition, a constant AoA oscillation) without possibility to stabilize the
trajectory. As a consequence, commanded AoA is modulated as a function of speed and attitude
variations: for instance, if A/C speed is decreasing and/or pitch attitude is increasing, pilot's
commanded AoA is lowered in order to avoid such a situation to degrade.
On the last 10 sec of the "Hudson" event, it is confirmed that pitch attitude is increasing and CAS decreasing. Then, the phugoid damping terms are non nul and are acting in the sense to decrease
the finally commanded AoA vs. the stick command, in order to prevent the aircraft from increasing the
phugoid features."

Based on this explanation, it appears that on the accident flight, the nose-up side stick commands from 15:30:36 to 15:30:43 were offset somewhat by the phugoid-damping feedback term, thereby limiting the pitch angle and α increase below 150 ft radio altitude.
(End of quote from the group chairman's report)

And let's not forget that the aircraft was still slowing down for several seconds after TOGA thrust was commanded while the engines were spooling up - the EFCS would likely behave more conservatively in such a scenario as I understand it.

I believe there may be some more concrete work on the EFCS forthcoming...

I thot I saw a reference to a 'bus driver that tried to duplicate the crash. At a safe altitude, of course, and with lottsa prior planning. Reference requested.

I must also point out that commanded "gee" ( Nz) may not result in actual gee. Once the jet gets slow enough, it just can't do it and the AoA laws kick in until you have max or prot or whatever all those "limits" are. You can reach some of the AoA limits at "speed" well above what we saw in the crash profile. You know, the basic "high speed stall" conditions.

I also continue to see comments that the jet maintains one gee. I don't think this is accurate, as all the manuals I have seen and the video that Chris referenced make it clear that the jet maintains a one gee Nz " command" corrected for attitude if the stick is in neutral ( hands off). So after takeoff at a 15 degree or so climb attitude, you won't have a one gee seat of the pants, but will be slightly less. The video also implies that in a steady 30 degree bank angle that hands-off gee command is more than one gee Nz.

Oh well, think I'll go back to the lurk mode to see what all the "heavy" folks contribute.

It is not exactly clear why you guys need to introduce notions of "flare maneuver" or other "headwind component change" when the equation is simple.
As alpha max is maintained in level flight at Valphamax :

1. Any increase of thrust will produce a climb as alpha max is maintained at Valphamax
2. Any decrease of thrust will produce a descent as alpha max is maintained at Valphamax

• Thrust controls the V/S
• Thrust does not control the speed which stays at Valphamax

Why is that? Please explain.

This is what I think will happen if, starting from steady level flight at Valphamax, thrust is rapidly increased with the sidestick maintained at the aft stop:

The airplane will start a phugoid trajectory, accelerating, climbing, decelerating and (possibly) descending, always at (close to) alphamax. The phugoid damping feature will attenuate the phugoid oscillation and eventually the airplane will end up in a steady climb at Valphamax, provided it stays clear of the ground and of structural limits.

HN,

Isn't the phugoid period of order 10-20s, and so near the ground, at close to maximum angle of attack, and with a big change in thrust, would it be perceptible in light of other changes?

Am I right that the reason to damp the phugoid is for comfort when making subtle changes when cruising, and thus that the FBW system applying damping within 10s of the ground might be a complication that the pilot does not benefit from?

I understand that the A330/40 was later found to suffer from an unusually pronounced phugoid mode and needed reshaping during testing, so active control alone might anyway not be sufficient to arrest its development when flight conditions are changing quickly.

Gums,

I agree that 1g can't be right.

If the aircraft is accelerating, turning or slowing, then the seat-of-the-pants force will change in magnitude or direction from its value sitting on the ground.
It has to, as the aircraft needs to push your straps/trousers to change your velocity so that you move along with it.

In a steady climb, descent or turn the seat-of-pants force is constant, but different from 1g. In a steady climb or descent it's 1g directed a little forward or back; in a steady banked turn it's about 1/cos(bank angle) directly up out of the seat. At 30 degrees that would make you feel about 15% heavy.

Perhaps "maintains 1g" is shorthand for "Taking your hands off signals the FBW system to keep constant the seat-of-pants forces that are currently being experienced."

I really don't know that much about phugoids, and nothing about that of the A320. I merely stumbled on the phugoid when calculating cruise upset trajectories of the A330/340. In that case the period was of the order of 60 - 80s. The point I was trying to make is that increasing thrust doesn't instantly put the airplane in a steady climb at Valphamax.

The two accident/incident reports (Hudson and Bilbao) mention phugoid damping in the alpha protection mode only.

Phugoids
 

On the subject of phugoids and attempting to clarify them… firstly, I must stress that I am NOT an Airbus man and am only familiar with publicly available information on the a320 FCS.

My understanding is that the control laws, when in normal law, maintain an attitude compensated “g” (within a limited bank angle range). To a pilot, this means that at centre stick, the aeroplane keeps going where it was pointed. The phugoid mode is basically an exchange of potential and kinetic energy at constant(roughly) angle of attack. So, if the a320 is flying along at a given airspeed at centre stick, an increase in thrust will result in an increase in airspeed along the current flight path. To achieve this, the control laws will pitch the nose down (reducing angle of attack) as airspeed increases. Eventually, drag will balance the thrust and the result is a new trimmed speed along the original flight path. The phugoid motion is, effectively, removed.

In a conventional aeroplane, the increase in thrust will cause an increase in airspeed and, thus, an increase in lift (remember that a stable aeroplane likes to remain at its trimmed angle of attack and there is no control law to modify this). The increase in lift will cause the flight path angle to rise. The increase in flight path angle eventually causes a decrease in airspeed which results in a decrease in lift (still at constant angle of attack) and the aeroplane noses over and begins to increase speed and lift again. Depending on the stability of the phugoid mode, a cyclic variation in airspeed and flight path angle could develop. If the phugoid is stable, the aeroplane would stabilise at a new flight path angle and airspeed, at the original angle of attack.

Now, if we go back to the a320 and consider the angle of attack protection control law. What this does is try to maintain the commanded angle of attack. I imagine that this (without phugoid damping) would return the phugoid behaviour of the a320 to more like that of a conventional aeroplane. Imagine slamming the stick to fully back with the thrust kept constant. The aeroplane would rapidly attain alpha max. Drag and flight path angle would increase, the airspeed would decay, the lift would reduce and the flight path angle would come back down again and off we could go into a potentially undamped phugoid.

What the phugoid damping terms in the alpha control laws seem to be designed to do is keep things nice and stable in terms of flight path, by looking at airspeed trend too. My guess is, at the expense of instant high angle of attack availability, flight path is considered (perhaps optimised) instead. In the case of a rapid pull from a low energy state, "firewalling" the throttles, the control laws make sure flight path doesn’t rapidly come back down again whilst the engines spool up. I also guess that they completely stabilise the phugoid to give the a320 characteristics that I have seen described, i.e. that it stabilises at airspeed for alpha max at full back stick (the eventual flightpath angle depending on the thrust set).

I would hazard a guess that Airbus engineers have done lots of simulation and modelling, with pilots too, to optimise flight path response in full back stick “avoidance” type manoeuvres.

Hi FCSoverride,

welcome to this thread and thanks for the great explanation!

However, I wonder if you have looked at the two documented alpha-prot encounters of A340's in cruise?

HN39,
Thanks for grasping the nettle and reminding me and others that the designers have to make provisions for the complications of phugoid oscillations, as confirmed by the Airbus input to the NTSB report on the Hudson River accident. That seems to be the explanation of the apparent shortfall of achieved AoA at Habsheim. My tentative suggestion that it might be a measure to allow sudden bank applications without the need to drop the nose can probably be discounted.

Hi FCSoverride,
That strikes me as a very elegant description of phugoid (some of us have been there in conventional a/c). :}
I think you've explained why it doesn't happen in Normal (C*) Law, and why provision has to be made for it in Alpha-Protection mode.

Quotes from gums

"I thot I saw a reference to a 'bus driver that tried to duplicate the crash. At a safe altitude, of course, and with lottsa prior planning. Reference requested."

Yes, see the BEA report, first part of Annexe X (text in French):
Habsheim F-GFKC
The flight test was performed at around 2000 ft agl, and the traces of the main parameters are superimposed on the equivalent Habsheim ones. A bit challenging to interpret.

"...the jet maintains a one gee Nz " command" corrected for attitude if the stick is in neutral ( hands off)."

In Normal Law, yes (also compensated for bank, BTW). But we are discussing Alpha-Protection mode here. The EFCS targets an AoA of alpha-prot with neutral stick, and alpha-max with the stick fully aft. Nz doesn't rule.

"So after takeoff at a 15 degree or so climb attitude, you won't have a one gee seat of the pants, but will be slightly less."

Correct, which is why, to avoid tripping over the trigonometry, I wrote "roughly 1g" in the following:
"...after the flare manoeuvre has been completed, and the new FPA has been established. Then the Nz can return to (roughly) 1g. During the flare (i.e., the period when the FPA is increasing), the increased "g" causes an increase in Valpha-max."

You can't change the trajectory of any moving object without changing the balance of forces - in this case, with a delta of lift, generating a delta of Nz.

TNX for the reference. I agree, hard to accurately interpret, but look like the duplication follows the actual parameters well. So the crash was inevitable, huh?

Secondly:
From looking at the FCOM stuff you folks provided two years ago, it looks like the "alpha prot" and such is simply the far right of the control law for pitch. In other words, AoA is blended into the Nz commands, and that resembles the law I had in the Viper. We could command 9 friggin' gees but the system would not let you get above a certain AoA versus gee. By the time you reached max AoA, all you could get was one gee positive Nz ( negative still available).

I don't see the alpha prot as an "alternate law" as such. Secondly, if not pulling, I can see why the system would continue to operate in an "alpha" mode unless speed was so slow and power reduced so much that the jet would stay there until thrust was increased ( and speed). Am I understanding that? Of course, with altitude, we could avoid that by rolling and getting the nose down while still holding full back stick ( not recommended for a large jet, heh heh).

Lastly, your description may explain some of what we saw with AF447. Once slow enough, pulling back on the stick would not help, although nose down authority was still available.

Hi OK465,

Thanks for your interesting observations in what I assume was a simulator exercise. When I wrote about the phugoid I had no idea of the amplitude of the oscillations, and I understand they were so small that most pilots would not recognize it as a phugoid.

Just to complete my picture, since the amplitude may be expected to be a function of the magnitude of the 'disturbance' that initiated it, do you recall the gradient or rate of climb achieved in the stabilized climb?

Not necessarily. If I understand the concept correctly, the design will carry a certain amount of leeway for any potential change in dynamic loads (or "all of the above"). The graph posted earlier provides a qualitative view of how the system should behave, but that graph is not annotated with specific values - an omission which I'm convinced is deliberate.

I can't say for certain, but if I were designing such a system, I'd be inclined to make the behaviour consistent - but make specific attainable values dependent on the prevailing conditions. In particular, if the aircraft has airspeed or thrust in reserve, then reaching the optimum values should happen sooner than if the aircraft is already on the edge of the envelope.

While the deviation below 100ft RA made Alpha Floor moot in this case, its existence indicates an implicit understanding at the design stage that AoA protection can only get you so far. Without sufficient airspeed and/or thrust in reserve, attaining optimum AoA is likely to be doubtful.

Phugoid
 

Hi HazelNuts39. I recall reading about those two events a while ago, though I have lost familiarity with them now. The impression I got (I might be wrong), from one of those events, was that the alpha protection control law in the Airbus seems like a separate “bolt on”feature on top of the C* law. I was surprised to learn that the alpha protection law can “latch” on.

The control law approach I am most familiar with is to blend smoothly between the “auto trim” and “alpha demand” modes based on pitch stick position and sensed alpha. It wouldn’t run into the problem of an alpha spike latching it in a mode in which the computed and commanded centre stick alpha value wasn’t appropriate.

An interesting point to note is that we have no phugoid damping features in the alpha demand path on our aeroplane. Full back stick gives unmodified maximum alpha within moments (unless g limited). Our phugoid is more damped naturally.

Care with nomenclature...
 

In Airbus speak, "latching" is a case whereby a failure degrades the flight control law to the point where it cannot regain a higher-level control law. AoA Protection law/mode can be exited as soon as the conditions which triggered it no longer apply.

EDIT : Therefore, because AoA Protection mode is triggered by a commanded condition rather than a systems failure (as in Alternate/Direct/Abnormal Attitude laws), it does not "latch" in that sense

Care with nomenclature (2) ...
 

Alpha-protection is considered "Normal Law", distinct from Alternate and Direct laws. Normal law changes from C* mode to alpha-command mode when the phase-advanced angle-of-attack exceeds alpha-prot.

Great inputs and insight from Nuts and Doze.

As far as Okie's comments about flying at the edge of the envelope.....

It seems to me that a functional 'bus could be flown to the "limiters" at a safe altitude to demonstrate the characteristics of the jet and the FCS. If I were flying one of the things, then I would like to do it, not in the sim. I would probably notice the "feel" of the jet and any burble or other indication that I was approaching stall.

Any other pilots here agree with that?

That's how the alpha protection has been designed.
At full back stick the elevators are all for the AoA control to maintain it at alpha max.

If you have seen those Airbus alpha max demo during air show, you probably have noticed the engine thrust variations, aimed at maintaining the vertical speed at zero. With full back stick the FCS is keeping the AoA at alpha max, and at the end of the pass the pilot applies full thrust still maintaining full back stick and the airplanes climbs away at alpha max. I haven't seen anyone 'descending' ... even temporarily.

"The only way to 'pull up' in that situation is to increase thrust to accelerate to a speed greater than Valphamax."
Your statement is erroneous.

@CONF iture:

On what evidence are you basing these assertions? To achieve an Alpha Max flypast at or above 100ft RA without Alpha Floor activating and causing the aircraft to climb away, A/THR must be disabled. Therefore there should be no automatic "hunting" of thrust during the manoeuvre, because thrust setting is manual. Any variation of thrust needs to be manually commanded.

As Chris Scott says:
The EFCS *targets* Alpha Max with the stick fully aft - it doesn't *guarantee* it.

Where did you see these sorties being performed?

Quotes from CONF_iture:

"It is not exactly clear why you guys need to introduce notions of "flare maneuver" ..."

Simply because it takes a force to change the trajectory of any vehicle - in this case the FPA. The vertical component of increased thrust alone is not sufficient - extra lift from the wing ** is required. Just for the moment, let's forget about any measures to avoid phugoid oscillations, or whatever. In the case that you are arguing, where the AoA is already at alpha-max, the only way to increase wing lift is to increase the speed. As I have previously reminded you, Valpha-max increases as the load factor (Nz) increases. To put it the other way round, increased speed at alpha-max generates an increase of Nz - which gives an increase of FPA.

In fact, even after the climb has been established, the wing lift requirement will be greater than in level flight, because its vertical component still has to balance the weight. So, at a steady AoA of alpha-max, Valpha-max has to be slightly higher in a climb than in level flight.

"Thrust controls the V/S"

In effect, that's correct, and for the PF to expect the PNF to do that for him would be impracticable.

"Thrust does not control the speed which stays at Valphamax"

For the reasons I have explained, at any given weight, Valpha-max is a variable speed.

** [EDIT]
However, see the reference to "TOTAL aerodynamic lift" in the fourth paragraph of Owain Glyndwr's post (below).

If I may throw in a few thoughts:

The natural phugoid period is approximately 0.23*Vkts TAS, that is about 28 seconds at 120 kts. If you are looking at a transition period of say 3 seconds that is only going to be one tenth of the phugoid cycle and I doubt it you would see much variation in that time. The terms AI have put into alphaprot to stabilise the trajectory also have a significant short term effect, basically by limiting the allowable AOA if the speed is decreasing but also there may be (probably is) a pitch rate term to add damping to whatever short period motion is associated with alphaprot.

WRT the discussion on what happens transiently if you increase thrust at alphamax I can offer two partial explanations; the reality is probably some combination thereof.

If you add thrust you will, of course, get a nose up pitching moment. In steady state conditions to maintain AOA constant this additional pitch has to be balanced by a nose down pitch from the tail. This means a reduction in tail download so there will be an increase in TOTAL aerodynamic lift even if AOA stays constant. In addition of course you will get an increase in total vertical force from the vertical component of the increased thrust.This is why Vs1g is established with idle thrust.

In the transient the increased thrust will cause the aircraft to pitch upwards and the EFCS will react to this changed pitch rate by applying down elevator (or more accurately will reduce the amount of up elevator) and the mechanism described above will come into play. So the total vertical force will be increased a little by increasing thrust even at constant AOA and airspeed.

The other explanation, which I think would be more powerful, rests on the fact that alphamax is not a 'hard' limit. The generation of wings we are discussing does not have an abrupt 'stall' [I cannot answer for the latest generation with sharklets or curved up winglets/wing tips; they may well have different characteristics]. The likes of the A320 start to have flow breakdown somewhere near midspan and this spreads outboard and inboard as AOA is increased above this point. This is true for both the clean aircraft and with flaps deflected. It is the buffet produced by these separations that becomes the limit to useable lift. In effect alphamax is almost a subjective limit, although formally it may be defined by the level of buffet 'g' at the pilot's station reaching a set level.

Now the aircraft is not going to fall out of the sky, or the simulator come off its mountings if the buffet temporarily goes up a little - assuming that is that the simulator has realistic buffet reproduced anyway. Consequently, there will be a margin of AOA available, but not available for general use, beyond alphamax.

Any closed loop control system will need to preserve some margin between the nominal system maximum and any genuine physical limit. The margin will depend on the nature and consequences of that physical limit. If it is a potentially catastrophic 'cliff edge' limit then the margins will need to be carefully set but if it is, as here, a 'soft' limit then one can be a little more relaxed.

To judge by the Gordon Corps video, AI are quite happy to see AOA go as high as 17 deg with full flaps even though the maximum usable AOA is set at 15 deg.

That is correct - it is not designed to maintain airspeed. But never mind, OK465 has settled that aspect. Apparently the speed variation is only a few knots for the magnitude of the thrust increase and the rate of change involved.

P.S.
With the natural phugoid period being approximately 28 seconds the speed variation of 'a couple of knots' in that period would be quite slow so it would easily go unnoticed.

And the problem is ... ?

A properly flown alpha max demo will deliver alpha max, what make you think the FCS would not comply ?

I disagree on that as well. For the PF to maintain full back stick for a while is not of all comfort. If he wants to concentrate outside, I find it practicable for the PNF to stay inside and manage thrust versus v/s.

That's why it is surprising in the Habsheim case, as alpha max was set at 17.5 deg and thrust reached 83% N1, the elevator had no intention to deliver anything closer than 2.5 deg short of alpha max ...

@Confiture

Well of course the elevator doesn't have an intention to deliver anything; it is merely the servant of the control laws. So far as those are concerned the behaviour is not really surprising - you should read the Bilboa report where it says:

IIRC in the Habsheim accident, the aircraft was still losing speed up until one second before collision with the trees. Although higher power had been commanded, it was simply set too late to arrest the deceleration and remove the damping correction in time for pilots demand for higher AOA to be satisfied safely and in time.

Hi OK465,
Thanks for your detailed, hands-on sim reports. Just to pick up on one:

"The jam engine accels or decels seem to cause the largest AOA variations (both below and above depending) from the stabilized alphamax value, but still resulting in no more than a couple knots above Valphamax on the thrust increases or dipping a couple of knots into the red band on rapid thrust reductions before the available FCS actuated elevator (and AOA control) gets things back under control at Valphamax."

You describe the IAS as varying above or below Valpha-max by "no more than a couple of knots". But did you notice whether the Valpha-max itself remains constant, or if it varies slightly with load factor and/or FPA?

Quote from CONF_iture:
"For the PF to maintain full back stick for a while is not of all comfort. If he wants to concentrate outside, I find it practicable for the PNF to stay inside and manage thrust versus v/s."

So, starting from straight-and-level at alpha-max, let's look at the control of the flight path:
the PF controls bank and can initiate a descent (intentionally or accidentally), but not reverse it;
the PNF controls climb and descent, provided the PF doesn't interfere.

Interesting work-share arrangement, 100ft above the ground. I suppose it might have worked.

Quote from OK465:
"The first obvious change in Valphamax occurs when your wrist gets sore and push forward on the SS resulting in Valphamax noticeably decreasing as the aircraft is unloaded. http://images.ibsrv.net/ibsrv/res/sr...s/badteeth.gif "

Yes, perhaps you need to alternate between seats! (Being right-handed, I tended to over-control initially on my occasional visits to the R/H seat.) Any further info on variations in Valpha-max (or lack of) would be most helpful.

Returning to the planned Habsheim flypast (at alpha-max, with thrust effectively controlling VS/FPA while the EFCS adjusts pitch to maintain Valpha-max), I've had another look at the degree of control available to the "PF". As you say, if his wrist gets sore and he releases the stick, the a/c pitches down to target the new AoA that the stick has commanded. If the PNF leaves the thrust constant, the IAS will increase to a figure higher than the 1g-Valpha-max. So, as the PF pulls the stick fully back again, there will be a slight excess of IAS available to flare the a/c as the alpha-max is restored. Until the speed bleeds off again, the lift will be higher than before the disturbance, enabling a climb. In other words, it's the old story of potential energy being converted to kinetic, and then back to potential again.

However, the above scenario illustrates the potential for dis-coordination between the two pilots. As Confit says, the PNF's task is to adjust thrust to keep the VS zero. The PF has initiated a descent by allowing the stick to move towards neutral, but the PNF doesn't know that. Therefore, observing the undesired descent, he is likely to increase the thrust just as the PF is flaring the a/c. The thrust increase will have to be reversed immediately if the a/c is to be prevented from overshooting the 100 ft target height because of the increase in total energy.

Had this pilot-duo ever practised the manoeuvre ensemble?

In #410 I did suppose ( with no evidence )


" This may have been a display which had been practiced a number of times successfully, perhaps along a standard R/W...
If there were no trees."


To add dual control, with only the PNF able to increase altitude... And at 100ft ( or could we allow 46ft R ?) might be overstressing the co-operation of the two pilots... without their having had a lot of practice.


And their awareness of the trees.
LT

Interesting you mention Bilbao as the part you did highlight is applicable to pretty turbulent conditions on final approach phase below 200 ft RA when the aircraft encountered strong and changing vertical and horizontal gusts while descending at a rate of around 1,200 ft/min ...

How smooth was Habsheim ...

In Habsheim, as the airplane was never slow enough to get to alpha max for the flypast, the altitude control through thrust management by the PNF did not come to the point it was applicable.
Thrust management by the PNF for vertical speed control during a more conventional alpha max presentation by Airbus is surely a good idea and I'm not too sure what you find 'impracticable' about it ... ?

Phugoid
 

Just to illustrate what I had in mind when I wrote about a phugoid in post #493.

The following graph shows the variation of speed and height that would result in the following conditions:
- the airplane is initially in level flight at a constant speed of 110 kts TAS
- at t=0 thrust is instantaneously increased to that required for stabilized climb at a flight path angle of 8.5 degrees
- after t=0 thrust and drag are constant and both act in the direction of the flight path, i.e. no thrust component normal to the flight path
- angle of attack is constant, i.e. no damping
- the variation of air density is negligible

This is of course entirely theoretical, but it illustrates the effectivity of the artificial damping in OK465's simulator exercise, combined with the effect of increasing thrust in 2-3 seconds instead of instantaneously and of the thrust component normal to the flight path.

It also illustrates the fundamental difficulty of controlling the flight path with thrust alone, without the ability to control angle of attack.

P.S.
The graph illustrates the point I made in the discussion: the acceleration starts immediately, the flight path changes after some speed increase.

http://i.imgur.com/27eDzJY.gif?1

I am really unsure what point you are trying to make here. AFAIK the same alphaprot law applies no matter what the atmospheric conditions.

Could you please explain what relevance the conditions pertaining to Bilbao have to the behaviour of the system in Habsheim?

Apparently Airbus thought otherwise as following Bilbao they developed a new standard for the ELAC, standard L81, to modify the logic in the AOA protection in case of turbulent conditions.

The post-Bilbao changes related to the phugoid-damping logic, not Alpha Protection. There were no "turbulent conditions" at Habsheim, nor did the EFCS pitch commands at Habsheim reflect what happened at Bilbao.

True, but if you have read the Bilbao report you will also have been aware that the modification was a deletion of the phase advanced AOA term that was part of the logic that triggered entry into the alphaprot mode [and a change to the logic of alphaprot deselection, but that is not relevant here], not a change to the basic alphaprot laws themselves.

You may also know that this post Bilbao change was actually a reversion to the standard that was applicable at Habsheim.

Despite what Dozy has written I have seen nothing to suggest that the basic phugoid damping terms in alphaprot have ever been changed, (and anyway the phugoid damping is an intrinsic part of alphaprot so his remarks make no sense). Consequently I see no reason why the point I was emphasising:

should not be a valid explanation of the reason why alphamax was not developed at Habsheim. I repeat - the necessary thrust increase was applied too late.

Owain, you and Dozy are more or less saying the same thing in different words.


A rate factor in the feedback loop provides a damping function - think "D" in a PID controller.


Fully agree with your conclusion though, and glad to see some consensus emerging through all the smoke put up about this affair. Great thread people and thanks from a mostly-lurker for all the info and discussion.

Fizz,

I didn't think Dozy and I were saying the same thing, because he was suggesting that the phugoid damping logic was changed and I am suggesting that it wasn't.
Nice to know that you agree with the conclusions though.
I think that rate of change of AOA signal was in the forward loop not the feedback. The feedback loop I believe still includes pitch rate as a damper.

Owain Glyndwr,


" Necessary thrust increase was applied too late..."


Or too low ? Or both ? The planned height was supposed to be 100 ft.

(I do not recall the actual height of the trees.)

Linktrained

A fine distinction!

As a non-pilot I would have said he was flying too low and left it too late. so I suppose my answer is "both".

The BEA report gives the average tree height as 12m (39ft)

Interesting graph, Nuts

Although the magnitude of the phugoid seems a bit large, we still see a very rapid increase in FPA and climb rate.

I bet that the pilot would have donated vital parts of his anatomy to gain 100 feet in about 1300 feet of horizontal travel, ya think? 7 seconds at 115 knots +/-