Tech Log The very best in practical technical discussion on the web

Habsheim

Old 17th Feb 2014, 12:03
  #481 (permalink)  
Thread Starter
 
Join Date: Jan 2008
Location: Blighty (Nth. Downs)
Age: 73
Posts: 2,076
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.
Chris Scott is offline  
Old 17th Feb 2014, 16:16
  #482 (permalink)  
 
Join Date: Jul 2009
Location: France - mostly
Age: 80
Posts: 1,689
Originally Posted by Chris Scott
This brings us back to your main problem - why was the published Flaps-3 alpha-max of 17.5 deg not achieved at Habsheim?
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)
HazelNuts39 is offline  
Old 17th Feb 2014, 18:08
  #483 (permalink)  
 
Join Date: Jul 2002
Location: UK
Posts: 3,182
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...
DozyWannabe is offline  
Old 17th Feb 2014, 22:28
  #484 (permalink)  
 
Join Date: Jun 2009
Location: florida
Age: 77
Posts: 1,128
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.
gums is offline  
Old 17th Feb 2014, 23:13
  #485 (permalink)  
 
Join Date: Jan 2005
Location: W of 30W
Posts: 1,939
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
CONF iture is offline  
Old 18th Feb 2014, 06:12
  #486 (permalink)  
 
Join Date: Jul 2009
Location: France - mostly
Age: 80
Posts: 1,689
Originally Posted by CONF iture
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.

Last edited by HazelNuts39; 18th Feb 2014 at 08:11.
HazelNuts39 is offline  
Old 18th Feb 2014, 09:06
  #487 (permalink)  
 
Join Date: Dec 2007
Location: Pasadena
Posts: 633
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.
awblain is offline  
Old 18th Feb 2014, 09:19
  #488 (permalink)  
 
Join Date: Dec 2007
Location: Pasadena
Posts: 633
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."
awblain is offline  
Old 18th Feb 2014, 10:10
  #489 (permalink)  
 
Join Date: Jul 2009
Location: France - mostly
Age: 80
Posts: 1,689
Originally Posted by awblain
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?
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.

Am I right that the reason to damp the phugoid is for comfort when making subtle changes when cruising
The two accident/incident reports (Hudson and Bilbao) mention phugoid damping in the alpha protection mode only.
HazelNuts39 is offline  
Old 18th Feb 2014, 12:52
  #490 (permalink)  
 
Join Date: Jul 2007
Location: Not here
Age: 41
Posts: 11
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.
FCSoverride is offline  
Old 18th Feb 2014, 13:55
  #491 (permalink)  
 
Join Date: Jul 2009
Location: France - mostly
Age: 80
Posts: 1,689
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?
HazelNuts39 is offline  
Old 18th Feb 2014, 14:05
  #492 (permalink)  
Thread Starter
 
Join Date: Jan 2008
Location: Blighty (Nth. Downs)
Age: 73
Posts: 2,076
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.
Chris Scott is offline  
Old 18th Feb 2014, 17:54
  #493 (permalink)  
 
Join Date: Jun 2009
Location: florida
Age: 77
Posts: 1,128
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:
"...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.
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.
gums is offline  
Old 18th Feb 2014, 21:14
  #494 (permalink)  
 
Join Date: Jul 2009
Location: France - mostly
Age: 80
Posts: 1,689
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?
HazelNuts39 is offline  
Old 18th Feb 2014, 21:17
  #495 (permalink)  
 
Join Date: Jul 2002
Location: UK
Posts: 3,182
Originally Posted by Chris Scott View Post
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.
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.

Last edited by DozyWannabe; 18th Feb 2014 at 21:38.
DozyWannabe is offline  
Old 18th Feb 2014, 21:43
  #496 (permalink)  
 
Join Date: Jul 2007
Location: Not here
Age: 41
Posts: 11
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.
FCSoverride is offline  
Old 18th Feb 2014, 21:57
  #497 (permalink)  
 
Join Date: Jul 2002
Location: UK
Posts: 3,182
Care with nomenclature...

Originally Posted by FCSoverride View Post
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.
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

Last edited by DozyWannabe; 18th Feb 2014 at 23:31.
DozyWannabe is offline  
Old 18th Feb 2014, 23:17
  #498 (permalink)  
 
Join Date: Jul 2009
Location: France - mostly
Age: 80
Posts: 1,689
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.

Last edited by HazelNuts39; 18th Feb 2014 at 23:33.
HazelNuts39 is offline  
Old 18th Feb 2014, 23:56
  #499 (permalink)  
 
Join Date: Jun 2009
Location: florida
Age: 77
Posts: 1,128
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?
gums is offline  
Old 18th Feb 2014, 23:59
  #500 (permalink)  
 
Join Date: Jan 2005
Location: W of 30W
Posts: 1,939
Originally Posted by HN39
Why is that? Please explain.
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.

The airplane will start a phugoid trajectory, accelerating, climbing, decelerating and (possibly) descending, always at (close to) alphamax.
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 is offline  

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.