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Habsheim

Old 14th Feb 2014, 07:26
  #461 (permalink)  
 
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... with the power set correctly the A320 would be capable of safely maintaining Alpha Max indefinitely
Sure, but with the longitudinal control at the stop it doesn't have the controllability required to maintain 100 ft terrain clearance safely in anything but an absolutely 'dead' atmosphere.

Last edited by HazelNuts39; 14th Feb 2014 at 09:43.
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Old 14th Feb 2014, 11:17
  #462 (permalink)  
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Flight on an energy knife-edge

Quote from Dozy Wannabe (my emphasis):
"Theoretically speaking, at 100ft over a flat landscape and with the power set correctly the A320 would be capable of safely maintaining Alpha Max indefinitely until it ran out of fuel."

Theoretically, perhaps... Let's try to examine that in practice.

Quote from gums:
"...the nasal radiators [naval aviators] tend to use throttle for glide path rate of descent to land on a boat. They trim for an AoA/speed and control the flight path with power."

That's rather like most of us are taught to do before our first ever solo-flight on a light, propeller-driven aeroplane - albeit without an AoA gauge. We maintain the correct approach speed by adjusting the pitch-attitude, and adjust rate/angle of descent with throttle/power. (With a tractor-propeller, admittedly, the varying slipstream over the wing itself affects the wing lift slightly.)

Later, in jets, we are taught to adjust the rate/angle of descent with pitch-attitude, while maintaining the correct approach speed with power (thrust) - again without an AoA gauge. The two techniques are, however, essentially the same - achieving a suitable, stable AoA, while varying the flight-path angle (FPA) through the air. Although I've cited the approach case, it's worth pointing out that - in either technique - the FPA does not have to be negative.

How does this relate to Habsheim?

Quote from HN39:
"To achieve alpha-max you have to pull the sidestick to the rear stop and keep it there. To maintain level flight at alpha-max you have to slow down to Valphamax. The only way to 'pull up' in that situation is to increase thrust to accelerate to a speed greater than Valphamax."

So it's clear that the Habsheim scenario - once the stick is fully back, and the AoA is at alpha-max - is critically different from an approach at Vref. In fact, any change of the FPA through the air, as HN39 points out, becomes completely dependent on IAS (strictly speaking, CAS). Any change of FPA requires a change in "normal" acceleration (Nz). In a steady airstream, if that FPA change is to be upwards, the necessary increase in lift can only be achieved by an increase of IAS. That, in turn, can only be achieved by increasing the thrust. On the other hand, descent is not a problem...

Clearly, low flight at alpha-max is hazardous, for at least one reason: wind-shear, which is inevitable over and near trees and buildings, unless the wind at all levels is flat-calm (a rare event). Any loss of headwind or increase of tailwind leads to a loss of IAS. Recovery of IAS requires an increase in GS, i.e., kinetic energy. With no surplus of potential energy to convert, that increase in kinetic can only be supplied by an increase in thrust.

A jet engine will respond fairly quickly when already at a medium-to-high thrust setting (hopefully both engines, symmetrically...), but the pilot's reaction time also has to be taken into account. Arresting the inevitable descent caused by any loss of IAS would take time. There is little or no height to trade for IAS recovery.

To quote again from HN39:
"...with the longitudinal control at the stop it doesn't have the controllability required to maintain 100 ft terrain clearance safely."

Last edited by Chris Scott; 14th Feb 2014 at 16:16. Reason: Typos and syntax. Reference to drag curve deleted. Title.
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Old 14th Feb 2014, 21:45
  #463 (permalink)  
 
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C'mon, Chris.

The potential rate of climb is a simple function of power available versus power required for your configuration.

My light plane training was the pitch attitude and then power for speed. In later planes with constant speed props, this was even better. My first jets were straight wings and the same applied. Then I went to the high aspect ratio bent wings and things were different. No FBW, so trim was still AoA, or speed for those not used to the great indication of your actual lift capabilities and stall.

So the dude at the flyby needed more power to get the jet climbing, even at max AoA that the control laws allowed. A "conventional" system would have allowed him to pull further back on the stick and then get way behind the "power curve". You know, the area of reverse command.

Let's face it. Wasn't flare mode or Alpha protect or any of the limiters. It was low power setting and a poorly executed demo. And then the 6 "pees".
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Old 14th Feb 2014, 22:12
  #464 (permalink)  
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Hello gums,
"The potential rate of climb is a simple function of power available versus power required for your configuration."

Agreed! I was addressing the problems of flying level and VERY low, stabilised at alpha-max - as they intended, but never achieved. I think that's what HN39 also had in mind.

The TOGA go-around is another matter, although they didn't initiate it in time. Pity (the seventh P)...
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Old 14th Feb 2014, 23:50
  #465 (permalink)  
 
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Yeah, Chris, we are on the same page.

I did not mean to challenge techniques and such, just point out that "power" was the problem at the flyby, and not the flight control laws and such.
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Old 15th Feb 2014, 01:01
  #466 (permalink)  
 
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Staying out of the coffin corner near the ground.

Back at the dawn of the commercial jet age, when piston pilots were transitioning to jets, there was a rather horrific accident caused by a poorly planned approach and failure to allow sufficient time to spool the engines up. United Airlines Flight 227 - Wikipedia, the free encyclopedia The Captain had 334 hours of jet time at the time of the accident having transitioned from the DC-6. Again, this was early in the jet age, and many of the safety initiatives that you might consider standard had not yet been developed.

Since I was then undergoing basic jet training and was soon to go to the boat, I understood the reasons for the accident and it made an impression on me that still remains. You do not ever want to be caught near the ground on the wrong side of the jet engine acceleration curve. Too bad that lesson was forgotten/not known by the Habsheim crew.

More recently we have had a couple of accidents that resulted from pilots expecting A/T to manage their thrust while they flew the nose of their aircraft. When the A/T did something unexpected, they were caught with insufficient time to spool up before smiting the ground.

By this point in the jet age, pilots should universally understand that jet engines have a significant spool up time from idle (although it is much better now than when I was carrier qualifying in aircraft powered by a centrifugal flow jet engines and early axial flow engines.)

Since we seem to design modern cockpits where you must stare at a PFD to get your critical flight data, do we need some simple thrust indications on the PFD? Do we need a "bitching Betty" when your thrust vs altitude doesn't make sense? (Something that would not be disabled by the Radalt locking on zero as happened in Amsterdam.) I think we still have room for improvements in thrust awareness.
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Old 16th Feb 2014, 03:30
  #467 (permalink)  
 
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Originally Posted by HN39
The only way to 'pull up' in that situation is to increase thrust to accelerate to a speed greater than Valphamax.
No need for acceleration, sufficient thrust alone will do.

Originally Posted by Dozy
That the action to do so (holding down A/THR disconnect) was erroneously omitted
It was not erroneously omitted, it was simply not necessary.

Originally Posted by Chris Scott
Regarding the flypast as executed, you will have noted that it was flown well below 100R, and (as Dozy Wannabe writes above) it is not entirely clear whether the captain ever carried out his briefed manual inhibition of Alpha Floor.
If he had, I do suspect the BEA would not have kept quiet about it.

Originally Posted by gums
In any other plane at that time, he would likely have stalled and crashed right in front of everybody there.
Or he could have got what was still in the aerodynamics to attempt to survive the tree tops and head back to BSL ...
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Old 16th Feb 2014, 10:56
  #468 (permalink)  
 
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Thrust alone will do?

Yes, if you're flying a fighter, yes it will, although it might not be the fastest way to achieve the desired height gain.

With an A320 full of people, weighing something like 130,000lb and with about 50,000lb of thrust, it'd be hard to climb without getting more speed to add some lift to help.
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Old 16th Feb 2014, 12:16
  #469 (permalink)  
 
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Originally Posted by CONF iture
No need for acceleration, sufficient thrust alone will do.
Some basic aerodynamics and flight mechanics: Changing the direction of the flightpath upwards requires that the lift force plus the vertical component of thrust is greater than the weight. The only way to increase lift at alpha-max is to increase airspeed. At alpha-max of 17.5 degrees roughly 30% of the gross thrust force contributes to the lift, while 95% contributes to acceleration. So yes, increasing thrust sufficiently to cause the airplane to accelerate will change the flight path upwards.
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Old 16th Feb 2014, 17:34
  #470 (permalink)  
 
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Power required versus power available

Guess I had a different aero course and pilot training courses than those here.

I can't find a single reference to the effect that we must increase lift to climb, with the notable exception of being so far behind the "power curve" that we are virtually stalling.

See: http://www.dept.aoe.vt.edu/~lutze/AOE3104/climb.pdf

See also discussions right here on PPRuNe forums:

BEST ANGLE vs BEST RATE of climb [Archive] - PPRuNe Forums

So I stand behind the Vermont aero department statement referenced above:

13. Climbing Flight
In order to increase altitude, we must add energy to the aircraft. We can do this by increasing the thrust or power available. If we do that, one of three things can happen:
1. We will increase kinetic energy (accelerate).
2. We will increase potential energy (climb).
3. We will do both, accelerate and climb.
If we desire to climb, we should hold the airspeed constant and use all excess power to increase our potential energy.
Further, as Nuts pointed out, at an attitude of 17.5 degrees +/-, a significant amount of thrust should contribute to a climb. No need to increase the trimmed AoA or even the negligible amount of gee command to the 'bus computers.

Doggone it, the guy was behind "the curve" at his power setting - no climb capability without more thrust, and it appears he had plenty but got it applied way too late.
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Old 16th Feb 2014, 18:14
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Hi gums,

strange that you never felt the 'gee' when you pulled up to increase your rate of climb. The point is that with the stick at the stop and the airplane at alpha-max in level flight you can't pull up.
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Old 16th Feb 2014, 18:34
  #472 (permalink)  
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gums,

To repeat myself, the tricky scenario HN39 and I were discussing was not the go-around, but the planned fly-past at an AoA of alpha-max, using manual thrust to maintain height by keeping the IAS at Valpha-max. This would involve a delicate balancing act of thrust that doesn't apply in the case of the go-around.

As HN39 first pointed out: with the stick already fully back, the only way to arrest a descent (perhaps due to a tailwind shear) would be to add thrust. You are wrong to say that no extra lift is required to initiate recovery from a descent (or, for that matter, start a climb from level flight). As I stated in a previous post, any increase of FPA requires a temporary increase in Nz (normal acceleration), which can only be achieved by increasing the lift. With the AoA already at the maximum permitted, the extra lift is dependent on extra IAS or an increased vertical component of thrust.

Once the new FPA is established, the lift requirement reduces somewhat, but the higher pitch-attitude reduces the vertical component of the wing lift.

Hope that makes sense! When compared with what we airline guys are used to, it's worth remembering that in this case we are dealing with the very edge of the flight anvelope.
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Old 16th Feb 2014, 18:51
  #473 (permalink)  
 
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Gums,

Are you thinking of having rather more thrust to weight in hand to gain height?

I think you're right in general, you can use increased thrust to climb without changing speed, but perhaps not usefully in the special situation that the A320 was holding itself at near the maximum AoA, and lift, that it could achieve at that speed. I think that adding height from more thrust in that state would be possible, but gradual.
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Old 16th Feb 2014, 18:57
  #474 (permalink)  
 
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Potatoes are better with salt, tea is worse with salt !
So is Any SOP's design, depending on the case, better or worse with automation.

Designers of a new aircraft must know the limit between the two domains in SOPs of that aircraft.

Human activities use mostly closed loops and wait the feedback .
Go around is nearly the only SOP that pilots learn to do in direct loops. That is taught before the first solo flight. Instructors and pilots know that it ALWAYS works. But automation and systems never claim probability=1.

Reading Machinbird's, gums' and others' experience, Go Around are much safer, better understood, better realized when it is strictly handflown rather with magic A/T TOGA button, despite we must learn the lesson from late thrust decision from Capt Gale C. Kehmeier and Michel Asseline , in United Airlines flight 227 and Habsheim.

Pilots must respect what their old and wise instructors taught hundreds times.
Then the SOP will be safe and not confusing. Designers must respect pilots' brain training.

TOGA button provides royalties on every sold aircraft l, would it crash or not.

Another issue for Asseline was that he has been surprised TWO times by the landing gear warnings. He was not waiting these alarms, he was behind his aircraft already at that moment. He was totaly in trouble, did not undestand the systems, the runways, the landing gear, the warning, and surely was no more able to resolve the energy, drag and
lift reckoning, angle and Vs climb, he was living in a schyzophrenic -Classic and Airbus - system. So was Mazières too : during the trial 9 years after Habsheim, he still said to the Court he thougt the plane could not stall, and seemed to still think it. Pierre Baud laughed and said the energy decided.
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Old 16th Feb 2014, 21:32
  #475 (permalink)  
 
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Thank you, Okie.

From Okie's posts previously, I assume he had experience in a FBW system before the AB320 came on line. The Block 10 comment is the clue, and I flew Bk1,5,10 and 15.

The problem I have seen in the 'bus laws was the implementation of an AoA input, primarily with the gear down. In short, the only way to establish a constant AoA gear up or down is to use the Okie technique and pull full back, achieve whatever AoA the FCS allows and go from there. Don't try this at home, but if you have sim time, then go for it.

Our primitive FBW law with the gear down had an AoA bias mixed with the gee command. And recall that ours used a gee command like the 'bus, but we could trim for a certain gee if we wished. So with gear down we had the "feel" of what all of us were used to - speed stability due to AoA. On my leading edge flap failure approach, I was able to control flight path using power versus increasing or decreasing AoA/speed. This was important because the guy before me pranged by increasing the AoA in a "flare".

@ Chris , et al.....As far as feeling a Nz change when using power to increase or decrease rate of descent, it's negligible. 100 or 200 feet per minute is about 0.07 gee for a second or two until stabilized. up or down. Even my sensitive butt could not feel that.

So one more time: The 'bus FCS control law is for a one gee command corrected for pitch attitude. Without being at one of the AoA limits, you won't see any change in rate of climb with a power change. You just keep going faster or slower on the existing flight path. But once at an aero limit like AoA, you perform just like all the books say.
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Old 16th Feb 2014, 21:46
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Originally Posted by OK465
Having the SS full aft doesn't necessarily equate to full ANU elevator or any specific elevator position at all.
That is entirely correct but not relevant. With the SS full aft the pilot has no control over the elevator position, the FCS in alpha-protection mode controls it. Holding the SS full aft sufficiently long results in AoA equal to alpha-max. With the thrust correctly set the airplane can fly around all day at Valphamax with the SS on the aft stop, provided it stays clear of terra firma. To stay clear of the ground it is not sufficient to 'set the correct thrust'. The airplane must be actively controlled to counteract disturbances. It requires a certain 'room to maneuver', depending on the magnitude of disturbances and how easily and accurately it can be controlled.

Sure, thrust changes will produce changes in vertical speed, but in my judgement this mode of flight path control is less responsive, less accurate, and less intuitive than pitch control.

As to the magnitude of disturbances, the accident occurred on a sunny summer afternoon. The wind was northerly at 5 - 10 knots. The presence of 1/8 cumulus at 780 m indicates that the lower atmosphere was slightly unstable, and surface wind therefore likely to be variable in magnitude and direction.

A change in headwind component results in a change of IAS. A loss of airspeed results in a loss of lift at constant AoA. But we do not know how accurately the FCS maintains a constant AoA because there is also the phugoid damping feature to consider.

So given these factors, what would have been a safe height above terrain to conduct the planned demonstration?

Last edited by HazelNuts39; 16th Feb 2014 at 22:56.
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Old 17th Feb 2014, 00:34
  #477 (permalink)  
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Quotes from OK465:

"Having the SS full aft doesn't necessarily equate to full ANU elevator or any specific elevator position at all."

I don't recall anyone arguing here that it does.

"Though I don't have the luxury of an AOA gage, as long as there exists some residual elevator authority (i.e. elevator not pegged full ANU), the FCS can 'move' the nose resulting in a flight path change commensurate with thrust applied."

To "move the nose" up = instantaneous increase of AoA.

"The range of AOA that's available to the FCS to generate a pitch rate, I can't see on a cockpit gage unfortunately..."

Yes. BTW, is there a read-out on the sim-instructor panel?

"...but observed on FFS flight package diagnostics, ostensibly from flight test data, overshoots and undershoots from the stabilized Alphamax value appear to occur during vertical transitions due to thrust changes....but no more than a couple knots IAS variation depending on rate of thrust application or reduction."

Now that is interesting... And reminiscent of the Gordon Corps video, in which the AoA does vary noticeably - despite his steady application of full back-stick (as discussed on this thread from January 2nd).

It suggests that the EFCS may keep a degree or two in hand - short of alpha-max - to allow the pilot a small increase in pitch to deliver an increase in AoA. That would provide an increase in lift to deliver the increase in Nz involved in any flare manoeuvre.
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Old 17th Feb 2014, 01:08
  #478 (permalink)  
 
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Originally Posted by HN39
The point is that with the stick at the stop and the airplane at alpha-max in level flight you can't pull up.
If by pulling up you mean climbing, why couldn't you ?
Or are you already full thrust to barely maintain the altitude ?

The only way to increase lift at alpha-max is to increase airspeed.
If you increase speed, you're not at alpha max any more.
If you maintain full back stick you maintain alpha max, therefore you maintain Valphamax - No speed increase.
What makes you climb in this case is a sufficient thrust increase alone.

Originally Posted by Chris Scott
It suggests that the EFCS may keep a degree or two in hand - short of alpha-max - to allow the pilot a small increase in pitch to deliver an increase in AoA. That would provide an increase in lift to deliver the increase in Nz involved in any flare manoeuvre.
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 ... ?
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Old 17th Feb 2014, 03:27
  #479 (permalink)  
 
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CONF iture
With full backstick flight path change due to elevators is taken out of equation so you can only climb due to thrust vector but it has to develop first then overcome the momentum so there will be noticeable lag. If you are doing this at 100ft or 50ft above ground I don't think you have a chance. If thrust was idle then it takes almost 3 to 4 seconds for the thrust to come out of idle.

Last edited by vilas; 17th Feb 2014 at 07:17.
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Old 17th Feb 2014, 07:44
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Originally Posted by CONF iture
If you increase speed, you're not at alpha max any more. If you maintain full back stick you maintain alpha max, therefore you maintain Valphamax - No speed increase. What makes you climb in this case is a sufficient thrust increase alone.
" If you maintain full back stick you maintain alpha max" - correct. The lift at alpha max is then proportional to airspeed squared. If the speed drops(*) you get nz < 1 and start to descend, if the speed increases you get nz > 1 and start to climb.

(*) Due to a change in headwind component, for example.

(To keep it simple I left out the thrust vector and phugoid damping).

Last edited by HazelNuts39; 17th Feb 2014 at 08:02.
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