Habsheim

[DozyWannabe]

Originally Posted by rudderrudderrat

Maybe the system can rapidly achieve aMax during +ve delta g, because it is able to avoid exceeding aMax by simply reducing the rate of rotation.
Whereas at Habsheim, there was only the last 2.5° (if 17.5° is correct) of rotation available, and would only approach the limit gradually.

Plus on the Airbus video demo he was at low speed, but keeping the thrust on (i.e. the engines were not spooled down - unlike Habsheim). Also the demo airspeed was stable and not decelerating prior to full back-stick.

Originally Posted by CONF iture

Alpha max is at 15 deg in landing CONF FULL but 17.5 CONF 3

Do you have a reference handy for that, or is it a guess?

Being a programmer, I am naturally averse to numbers without knowing what they are and where they're from (aka "magic" numbers).

[Chris Scott]

Airbus contemporary FCOM "Operating Speeds Definition"

Dozy,
That's an interesting hypothesis, i.e., the EFCS might deliberately keep a margin to alpha-max in idle thrust. I wonder.
AFAIK, however, we haven't established what config/flap GC was using for that demo, and in any case I currently disagree with Confit's figure of 15 deg for alpha-max with config FULL (see my post).

HN39,
That's a good call, although 3.04.10 was re Flight Instruments, and not included in my issue. However, 3.04.01 "Operating Speeds Definition" may provide the best explanation of how the peculiarities of certification for the A320 in Normal Law - which (as you know) was unique in being incapable of meeting the contemporary norms of stall testing - were addressed, and a compromise reached to avoid it being unfairly disadvantaged, field-performance-wise, relative to types like the B737.

Airbus FCOM 3.04.01, REV01 SEQ001 (still current at SEP 1987, prior to formal type-certification) (my emphasis):
"For a conventional a/c the stall speed which is used for reference is VSmin based on a load factor lower than 1G, which therefore gives a stalling speed less than that obtained at 1G. All operating speeds are expressed in relation with this speed (for example Vref = 1.3 VSmin). Since the A320 icorporates a low speed protection feature (alpha limit) which cannot be overridden by the flight crew, the airworthiness authorities have reconsidered their position regarding the stall speed definition.
"All the operational speeds have to be referenced to a speed which can be demonstrated by flight test. This speed is VS1g for A320 and VSmin for previous a/c types. As a result the Authorities have agreed to allow A320 to have the following factors:
V2 = 1.2 x 0.94 = 1.13 VS1g
VREF = 1.3 x 0.94 = 1.23 VS1g
"It can be seen that these speeds are identical to those that would have been achieved, had the a/c been conventionally certificated to the 94% rule. Compared to a conventional a/c the A320 has exactly the same manoeuvre margin at its reference speeds.
"In the FCOM VS1g will be designed [designated] VS. " [*]

* VS = "Reference stalling speed (equal to VS1g)."

[HazelNuts39]

Originally Posted by Chris Scott

... the A320 in Normal Law - which (as you know) was unique in being incapable of meeting the contemporary norms of stall testing ...

I wouldn't know about that - under the 'contemporary norms' Vsmin would have been equal to Valpha-max.

All the operational speeds have to be referenced to a speed which can be demonstrated by flight test. This speed is VS1g for A320 ...

If "alpha-max = alpha (1.06 Vs)" and Vs=Vs1g, how was Vs1g demonstrated in flight test?

Also, if "alpha-prot >= alpha (1.13 Vs)" and "V2 = 1.2 x 0.94 = 1.13 VS1g", then V2>=V(alpha-prot). So V2 can be equal to V(alpha-prot)?

[rudderrudderrat]

Hi HazelNuts39,

So V2 can be equal to V(alpha-prot)?

Well spotted!

FCOM, Aircraft Systems, DSC-27-20-10, Protections:
"At Take off aProt is equal to aMAX for 5s."
By which time, the SRS will have commanded about V2+10.
During OEI, aFloor is disabled.

@Chris Scott
Hi Chris,

Thanks for those figures in post #213 because if we assume the Cl is proportional to AoA (straight line graph between afloor and aMax), then we can guesstimate aMax.

Lift = 1/2 rho * A*Cl*Vsquared.
Since the lift remains constant at 1g, and A is constant,
Cl(amax) * V(amax)squared = Cl(afloor) * V(afloor) squared.
Rearranging Cl(max) = Cl(afloor) * V(afloor)squared / V(amax)squared.
If we assume Cl is proportional to AoA then,

AoA(max) in conf3 = 15 * (1.1)squared/(1.06)squared = 16.15°.
Using the same assumptions, astall = 18.15°
Since the accuracy of the AoA probes is 2.5% (see post #46), then the error could be +/- 0.45°.
18.15 - 0.45 = 17.7

I wonder if that where the 17.5° came from?

[HazelNuts39]

Originally Posted by RRR

"At Take off aProt is equal to aMAX for 5s."
By which time, the SRS will have commanded about V2+10.

With one engine inoperative?

If we assume Cl is proportional to AoA then, ...

Is that a valid assumption? In the landing configuration AoA for Cl=0 is almost certainly some significant negative value. Also Cl-alpha is non-linear near Clmax.

[rudderrudderrat]

Hi HazelNuts39,

With one engine inoperative?

OEI - SRS commands V2, but aFloor is disabled so there is no problem.

Also Cl-alpha is non-linear near Clmax.

Correct.
That is why I've only considered "straight line graph between afloor and aMax" (see post #211 for the picture).

[Chris Scott]

Inconsistencies in contemporary FCOM

Quotes from HN39:
If "alpha-max = alpha (1.06 Vs)" and Vs=Vs1g, how was Vs1g demonstrated in flight test?

It could not have been (in Normal Law), which is inconsistent with the text I quoted. The plot thickens... One possible explanation is that the "Operating Speeds Definition" comes from REV01, whereas the factors you mention are from REV03 in a different volume. All three volumes were issued to us at Blagnac in early January 1988, but we were warned that amendments were already pending.

Also, if "alpha-prot >= alpha (1.13 Vs)" and "V2 = 1.2 x 0.94 = 1.13 VS1g", then V2>=V(alpha-prot). So V2 can be equal to V(alpha-prot)?

Presumably not!

[rudderrudderrat]

Hi Chris,

Vs1g was demonstrated in flight test in ALT LAW.
It was demonstrated at 1g, whereas in a conventional aircraft, there is a tendency for the nose to drop and the demonstrated value is around 94% of 1g (allegedly).

So V2 can be equal to V(alpha-prot)

- apparently it can be.

[CONF iture]

Originally Posted by RRR

Maybe the system can rapidly achieve aMax during +ve delta g, because it is able to avoid exceeding aMax by simply reducing the rate of rotation.
Whereas at Habsheim, there was only the last 2.5° (if 17.5° is correct) of rotation available, and would only approach the limit gradually.

If the system is fast enough to efficiently control the elevators to rapidly command achieve or control alpha max in one case I don't see how it would not in another case.
Also, an excursion over alpha max is not an issue, as demonstrated in the video.

[Owain Glyndwr]

rrr

OK, but in Conf 3 the zero lift AOA Would be -6.5 deg

In earlier stall demonstrations Vsmin was taken as the minimum speed measured in the stall and stall recovery - this usually corresponded to about 0.94g

[HazelNuts39]

Originally Posted by RRR

Vs1g was demonstrated in flight test in ALT LAW.

Vs1g=reference stall speed is the current norm for conventional aircraft. In ALT LAW the A320 can be stalled as a conventional aircraft, so why would it need a different treatment of stall speed? The explanation given in the FCOM makes no sense if Vs1g was demonstrated in ALT LAW.

[rudderrudderrat]

Hi HazelNuts39,

why would it need a different treatment of stall speed?

A conventional aircraft stall speed is demonstrated by trimming the stab only as far as Vref, then decelerating at 1 kt/sec whilst applying back elevator until the nose drop. It was found that this always produced slightly less than 1g (94% ish) at the measured stall speed value.
In Alt Law, the Airbus continues to trim and level flight was achieved right up until the speed where you will lose control - all apparently done at 1g.

See page 36:http://www.skybrary.aero/bookshelf/books/2263.pdf

"Airworthiness Authorities have agreed that a factor of 0.94 represents the relationship between VS1g for aircraft of the A320 family and VSmin for conventional aircraft types."

[DozyWannabe]

Originally Posted by Chris Scott

Dozy,
That's an interesting hypothesis, i.e., the EFCS might deliberately keep a margin to alpha-max in idle thrust. I wonder.

Not so much that it keeps a margin for idle thrust (though it may do so) as it takes into account whether the aircraft is accelerating/decelerating and by how much, then uses the trends to stay ahead of the aircraft.

I tried to illustrate this earlier:

Originally Posted by me

For example, it would do no good to set pitch attitude to match an AoA of 17.5 degrees based on the current airspeed if, by the time that pitch attitude was reached, the aircraft was flying a few knots slower.

Capt. Corps alludes to the idea that the overriding ethos behind the EFCS in this particular mode is retaining stability and keeping the aircraft flying.

[EDIT : In fact, if we go back to Chris Scott's earlier post:

Originally Posted by Chris Scott

In case it helps anyone, this is my translation into received English:
This flight law provides a special automatic protection preventing the aeroplane from reaching an incidence [angle of attack] greater than 17.5 degrees, to conserve a sufficient margin with respect to the stall, even if the pilot maintains a full climb [pitch-up] demand.

Note that this BEA description does not specify that an alpha of 17.5 deg will be achieved if the pilot maintains full back-stick.

What if the hypothesis that 17.5 degrees should be consistently held with full back-stick in High AoA mode is incorrect?

Is it not possible that 17.5 degrees AoA represents the absolute "do-not-exceed" maximum alpha from a normal limit of around 15 degrees AoA? If Chris's translation is correct, the wording of the BEA report is certainly consistent with that scenario. This interpretation would also be consistent with Capt. Corps' narration on the video.

]

[CONF iture]

Originally Posted by OK465

or is the max AOA available strictly a function of actual slat/flap position being detected by the FMGEC regardless of the MCDU CONF prompt selection?

I would go for that one - I did not find anything in the FCOM to state otherwise.
As you mentioned, CONF 3 selection on the MCDU will modify VLS and VAPP, but it is also a signal for the GPWS in its FLAP MODE operation.

Alpha max at 17.5 deg CONF 3 as per BEA
Alpha max at 15 deg in landing configuration as per video
CONF FULL is the usual landing configuration as per FCOM.

Originally Posted by Dozy

For example, it would do no good to set pitch attitude to match an AoA of 17.5 degrees based on the current airspeed if, by the time that pitch attitude was reached, the aircraft was flying a few knots slower.

To lose 1kt/sec is very soft and normal and certainly nothing to restrain the AoA at 15 deg.

[DozyWannabe]

Originally Posted by CONF iture

I would go for that one - I did not find anything in the FCOM to state otherwise.

And nothing in the FCTM states that 17.5 degrees AoA is guaranteed in such a scenario, but you still seem to take it as gospel.

To lose 1kt/sec is very soft and normal and certainly nothing to restrain the AoA at 15 deg.

The deceleration delta was nothing like stable at -1kt/sec throughout the sequence - try not to think about it as "restraining" AoA as anticipating where it will be if the aircraft continues to slow (which it would do until the engines have spooled back up).

[CONF iture]

Originally Posted by Dozy

The deceleration delta was nothing like stable at -1kt/sec throughout the sequence

How much was the deceleration in the last 10 sec as the airplane was level ?

try not to think about it as "restraining" AoA as anticipating where it will be if the aircraft continues to slow (which it would do until the engines have spooled back up).

When the elevators movement oppose the AoA increase, it is a clear restriction.
But nice theory of your own making, too bad it is not detailed either in the documentation or the BEA report.

What if the hypothesis that 17.5 degrees should be consistently held with full back-stick in High AoA mode is incorrect?

What if the Airbus documentation is wrong ?

Is it not possible that 17.5 degrees AoA represents the absolute "do-not-exceed" maximum alpha from a normal limit of around 15 degrees AoA? If Chris's translation is correct, the wording of the BEA report is certainly consistent with that scenario. This interpretation would also be consistent with Capt. Corps' narration on the video.

17.5 deg of AoA for CONF 3 is the target to maximize the performance, that's why Airbus chose to call it Alpha Max.
The video demonstrates how rapidly Alpha Max (15 deg for CONF FULL) is obtained.
But again ... nice theory of your own making.

[rudderrudderrat]

Hi CONF iture,

The video demonstrates how rapidly Alpha Max (15 deg for CONF FULL) is obtained.

The video also shows (at time 10:50 ish) an AoA of 16° after rolling on about 15° of bank. I would guess from that demo that Alpha Max is >15°, but the system remains on the conservative side of Alpha Max to allow for rapid changes in bank.

[CONF iture]

Originally Posted by RRR

The video also shows (at time 10:50 ish) an AoA of 16° after rolling on about 15° of bank.

I can even read 17 deg for a very temporary excursion over alpha max.

I would guess from that demo that Alpha Max is >15°, but the system remains on the conservative side of Alpha Max to allow for rapid changes in bank.

Then it would be publicized as such ... Any reference ?

[rudderrudderrat]

Hi CONF iture,

Any reference ?

It simply does this rather well. No promise of achieving aMax - only of not exceeding it.
FCOM 27-20-10 Protections.
"HIGH ANGLE OF ATTACK PROTECTION

Under normal law, when the angle-of-attack becomes greater than αprot, the system switches elevator control from normal mode to a protection mode, in which the angle-of-attack is proportional to sidestick deflection. That is, in the αprot range, from α prot to αMAX, the sidestick commands α directly. However, the angle-of-attack will not exceed αMAX, even if the pilot gently pulls the sidestick all the way back. If the pilot releases the sidestick, the angle-of-attack returns to αprot and stays there."

[HazelNuts39]

Originally Posted by RRR

No promise of achieving aMax - only of not exceeding it.

You and I must be reading this differently:

the angle-of-attack is proportional to sidestick deflection. That is, in the αprot range, from α prot to αMAX, the sidestick commands α directly.