Air France 447 - On topic only!

[TeeVee]

so yeah, this is from the article that Evan posted in response to my thread on airtran/Boeing AoA business. wonder if the same holds true for these AF yo-yo's

AOA backup indication following
pitot or static system failures. The
AOA instrument described in this article
is useful as a backup for unreliable
airspeed indication caused by pitot or
static source blockage because the calculation
of indicated AOA is not greatly
affected by pitot or static pressure
inputs for its calibration, and the displayed
value has not been normalized.
Pitot or static system failure
requires the flight crew to take several
fundamental steps to resolve the
problem (see “Erroneous Flight
Instrument Information,” Aero no. 8,
Oct. 1999):
Recognize an unusual or suspect
indication.
Keep control of the airplane
with basic pitch and power skills.
Take inventory of reliable
information.
Find or maintain favorable
flying conditions.
Get assistance from others.
Use checklists.
Recognition of a problem
will be accomplished by
instrument scanning and
cross-check practices or crew
alerts, depending on the
design of the system in the
airplane. In this respect, AOA
instruments can be useful as
an additional cross-check.
Present procedures for unreliable
airspeed call for flying
the airplane by reference to
pitch attitudes, and refer
the pilots to reference tables
showing pitch attitudes for
various configurations,
weights, and altitudes that
will result in safe angles of
attack and speeds. AOA could
be useful if the relevant data
is included in the pitch and
power tables that already
exist in the nonnormal
checklist procedures. AOA
would be most useful in flying
the airplane in multiple
failure conditions where all
pitot or static sources are
affected, making all airspeed indicators
unreliable.

you will note that the highlighted section (by me) kinda mimics what the real pilots here have been saying all along. now, unless the scarebus is such a piece of crap that it CANNOT be flow using basic pitch and power controls (which i doubt), then it very well may be that all three of these pilots were sucky. again, i doubt this to be the case.

i wonder if the automation-heavy scarebus literature ever recommends for the pilots to use basic "anything"

[Myndee]

Boys will be boys. Still, the pilot can pull up to 67° of bank in either direction at a roll rate of 15°/sec, and pitch up 30° or down 15° before protections limit him. I think that should be enough to throw an already unbalanced attacker off his feet without losing the wings in the process. But there is no hatchet-attack protection installed on any aircraft that I know of, aside from door locks.

The reason I said "boys will be boys" was to try to add a little levity to the bickering atmosphere that took shape around here this past couple of weeks. I have two sons and a husband, so I kind of was making a joke about how boys tend to act. I certainly hope my comment was not misconstrued as being directed to the Fedex crew? I am a little confused as to why you repeated it back to me.
Your answer makes sense to me with regard to the protections. I just can't see that a plane can be designed so that the pilots cannot override it when it is required to do so.
By the way, made it back to DFW. Flew home on a 319. I have to say that I find myself partial to the 320!

[Gabriel]

I just can't see that a plane can be designed so that the pilots cannot override it when it is required to do so.

Because then the pilots can alsoe override it when it is NOT required to do so. And apparently there are more accidents caused by the ability to do the last than by the negation to do the former.

I'm not supporting it. I'm just saying the arguments.

[guamainiac]

There is considerable water and low air traffic in that area of the Hudson.

Ferry, lighters with cargo, air ferry, sightseeing and you name it, that is one busy little place. Obviously he had room but they had a lot to look for. It has been the site of a few accidents and the FAA has considered closing it to pleasure flights.

[jrbeejay]

Because then the pilots can alsoe override it when it is NOT required to do so. And apparently there are more accidents caused by the ability to do the last than by the negation to do the former.

I'm not supporting it. I'm just saying the arguments.

All modes can be overridden by the pilot on any Airbus plane, don't believe all the propaganda on here by people with an axe to grind. The infamous crash of the first AF 320 was due to the pilots overriding all the safety features so they could do a very low unauthorised flyby.

jrbeejay

[MCM]

The modes can be overridden by unauthorised, and impractical overrides.

The display crash override was not a pilot in a normal operation, or something he could do as a matter of course. It isn't something he could do when he decided he needed more control inflight, nor could he do it consistently preflight without being put in jail.

For all reasonable purposes, they are not overridable.

[Evan]

The modes can be overridden by unauthorised, and impractical overrides.

There are these tempting switches on the overhead. Airbus doesn't provide anything in the FCOM describing their use*, but if you switch off all three PRIMS you get direct law, with SEC1/SEC2 doing flight control. You also lose a lot of non-essential systems and get a lot of ECAM entertainment. I think this would be a last-ditch move if the flight control really was preventing some kind of recovery.

(*There is mention of these switches in the FCOM for use when the associated FCC indicates FAULT on the switch, but no mention of using them without the FAULT indication to intentionally degrade control law).

[Gabriel]

The infamous crash of the first AF 320 was due to the pilots overriding all the safety features

That's not correct. In fact, the envelope protection prevented the pilot from stalling the plane despite his pulling back to the stops, thus probably saving quite a few lives that day.

[Black Ram]

While waiting for the new report (it's already summer), I found this at PPRUNE and it brings a new issue to the table - how effective would the THS be in near-freefall. It's not 100% correct, but still may be something to consider:

THS the stall
Lazerdog, quote:
"Would full forward side-stick have caused immediate stall recovery, or would the PF have to wait until the THS auto-trim caught up with nose-down trim?"

Short answer is: don't know. But I think we can expect that the AoA would have started to reduce immediately. Unless the THS motor stalled, which would be unlikely at such a low airspeed (low loading), the THS would have started moving immediately.

Again the question on what ground this believe on THS authority is founded.
The aircraft was in a stall with an AOA of around 60°. If this AOA applies to the aircraft cell, it applies as well to the two airfoils of the aircraft, the wings and the THS with minor offsets.

What AOA did the THS have during the fall from FL380? Aircraft AOA of 60° minus the 13.5° noseup trim of the THS, which gives us a THS AOA of 46.5, right? (I disregard the effect of curvature of the THS, dont know how that changes my assumption).
What kind of airfoil is working at that AOA and with what kind of outcome? If we trim the THS to full ND as necessary for recovery, the THS AOA even increases to 63.5° AOA plus the profile change due to elevator deflection.

See that nice picture in post 1817 from mm43. Just move the depicted angels to the THS!

What kind of miracle airfoil is there on the THS of the Bus? To bring the nose down its not enough to produce drag at the tail, you need a functioning (unstalled) airfoil producing lift (in this case downward ).

Remember also, the liftvector of an airfoil points basically perpendicular to its chordline, not to the relative wind. So even if the THS produced any measurable amount of lift, the main part of it would not have pointed in the desired direction.

IMHO Elevator ND or trimming the THS down, after this high AOA of 60° was established, only increased the drag of the tailplane but had no permanent effect to get the nose down to the horizon and to increase the speed as necessary.

Whereas stick NU decreased the AOA on the THS below 46.5° , reducing drag and causing some feedback to the crew (noseup input brings the nose somewhat up, but nosedown input doesn´t change anything). That would also explain the noseup input in the final moments.

[Taliesin]

I thought AoA was around 35-40°?

Be that as it may, I dont think there is any reliable data to answer this question. As far as I know, no one has ever test-flown a modern passenger jet at those AoA's. It's not part of the certification requirements and so it's not part of the flight test program.
Data of lesser AoA's is often extrapolated so the simulators have data to work with but as long as it's not testflight-verified data, it should be viewed with healthy skepticism.

So in short: No one knows.

[Black Ram]

As far as I remember we never thought about it. Maybe it's flawed logic, or maybe there is something to it, which could explain the crew's reluctance to push the stick down.
The BEA note is not very clear what the AOA was at all times, but it did say it was over 40deg or smth like that. Also not all details are correct in that pprune theory - for example, we know the THS didn't trim down.

[Gabriel]

While waiting for the new report (it's already summer), I found this at PPRUNE and it brings a new issue to the table - how effective would the THS be in near-freefall. It's not 100% correct, but still may be something to consider:

There are several major errors in that analyis (from least to most important):

1- AoA was around 40°, not 60.
2- "Remember also, the lift vector of an airfoil points basically perpendicular to its chordline, not to the relative wind. So even if the THS produced any measurable amount of lift, the main part of it would not have pointed in the desired direction." 100% false. Lift is always perpendicular to the relative wind. That's the definition of lift: The component of the aerodynamic force perpendicular to the relative wind.
3- The implicit statement that there is no lift after stall, or that lift doesn't grow with AoA after stall. Again false. A wing (including the stabilizer) doesn't stop producing lift at stall. The stall AoA is where no further lift can be obtained by increasing the AoA. In the close post-stall AoA zone, lift diminishes very quickly with AoA, to about half of the max lift just at the stall AoA. But in the far post-stall AoA zone, lift starts to grow again with AoA, reaching its maximum at about 45° (that maximum being typically lower than the max lift at stall AoA).
4- "To bring the nose down its not enough to produce drag at the tail" Says who? At AoA's below stall, lift is many times the drag. At stall however, the lift has a big step down and the drag has a big step up. The max drag (at 90° AoA) can be even higher than the max lift (at stall AoA). The contribution of the tail's drag to the pitching moment is always neglected at tail AoAs below the tail's stall, for two reasons: a) The tail's drag is very little compared to the tail's lift, and b) the arm of the tail's drag (the vertical distance between the CG and the tail) is very small at say 16° of AoA compared to the arm of the tail's lift (the horizontal distance between the CG and the tail). These two things are opposite at very high AoA: a) The tail's drag can be comparable and even greater than the tail's lift and b) the arm of the tail's drag is comparable to the arm of the tail's lift. Stalled airplanes, and in particular at very high AoA, is the domain of the drag rather than the domain of the lift.

So you've got the plane at some 10° nose-up, and falling at some 30°, making for an AoA of 40°. The THS is some 16° nose up, and hence it's AoA is some 24°. And the elevator is yet another 15° above the THS (making for a convex surface as seen from the incoming air). No doubt that getting the THS to say 0° and the elevator to say 5° down (concave) will immensely increase both the lift and drag of the tail, providing for a very effective nose-down pitching moment.

But an image is worth a thousand words, or so they say.
Click the thumbnail below.

[Black Ram]

Thanks Gabriel, you even drew diagrams. I think for #4 they meant it's not enough to produce drag and that you also need lift on the THS, but you made the point there should have been enough lift and drag would have been in the desired direction, when the THS would have been trimmed down.

[3WE]

Big iron stall review.

So Gabriel- I need you to review some stuff:

You state that "lift does not end when you stall"; however, can we not say that with typical stalls, there is a sudden and significant reduction in lift. In the "Cessna World" the bottom falls out and you feel your stomach coming up.

Of course, this is a swept wing plane where the big iron boys have regularly chastised us that their is a stall has is very little "break" but a gradual building of sink rate....still, the plane is losing a lot of lift...right?

Number 2- is that in the small plane world, you can often find a big loss of stability/control and the tendency to spin, etc. However, it would appear that the cheap, composite crackerbox was relatively well behaved as it mushed down.

Edit- I guess the sub title should be big fiberglass...

[Gabriel]

So Gabriel- I need you to review some stuff:

You state that "lift does not end when you stall"; however, can we not say that with typical stalls, there is a sudden and significant reduction in lift. In the "Cessna World" the bottom falls out and you feel your stomach coming up.

Correct. It's sort of like the "normal" region of AoAs where a wing "normally" operates is an "abnormality" zone (compared to most of the AoAs between stall and 180°), where with a little increase in AoA and very little drag you get a lot of lift. That sweet region ends at the stall AoA. At that point the lift tops, then a few degrees more and it goes very steeply down, but once the airfoil is fully stalled a further increase in AoA increases the lift again, because while the flow is totally separated from the upper surface, you are still deflecting the air that hits the lower surface. From that point on the airfoil, any airfoil, behaves like a flat plate, with the lift having another local max at 45° (typically lower than the first local max at the stall AoA).

However, planes are not flyable in that region, because the drag is of the same order of magnitude than the lift (and of the weight), so you'd need a 1:1 thrust-to-weight ratio, and if you had it then you would not bother in lift, drag and stall because you could keep hanging from the engine. It's called a helicopter.

Of course, this is a swept wing plane where the big iron boys have regularly chastised us that their is a stall has is very little "break" but a gradual building of sink rate....still, the plane is losing a lot of lift...right?

Yes. Immediately after the stall a lot of lift is lost. Beyond that, lift starts to increase again, as said.

Number 2- is that in the small plane world, you can often find a big loss of stability/control and the tendency to spin, etc. However, it would appear that the cheap, composite crackerbox was relatively well behaved as it mushed down.

That's an interesting point for which I don't have a final answer. But I bet that there was some significant banking to one side and the other during the stall. However, again, once the "lift increases with AoA" rule is restored at very high post-stall AoA, the control should be better that at stall and slightly beyond (where the lift diminishes with AoA).

Have you seen the Cl-Alpha diagram above? I think it's easier to understand than this, hmm, explanation?