A320 alternate law no protection

[IFLY_INDIGO]

Alternate law with reduced protection provides low speed and high speed stability.

few failures lead to alternate law without reduced protection - no low/high speed stability, only load factor protection.

How will pilot come to know if low/high speed stability is available or not?

I couldn't see any difference in ECAM indications. Did I miss something?

[Amadis of Gaul]

It would depend on the failure that threw you into Alternate Law in the first place. The QRH would tell you what protections are still available and so forth.

To me, though, the bigger picture issue here is this: when you see those amber Xs on the PFD and that ALTERNATE LAW ECAM message, the airplane is trying to tell you "look, don't do anything stupid because I may not be able to bail you out." My point is you shouldn't get yourself anywhere near a condition where you might need those speed stabilities.

[vilas]

FCOM mentions four multiple failures where protection are lost. They are Triple ADR, Double ADR second not self detected(Mach/CAS disagree), double SFCC slat channel and B + G Hyd fail. STATUS only tells you protections lost. QRH won't tell you that. Protections lost is good enough warning to fly carefully within the flight envelope.

[vilas]

DSC-27-20-20 P 2/8

[IFLY_INDIGO]

I read about those 4 failures. I was only interested to know if Airbus tells you the difference in some way or not. I have written to TECHREQUEST on airbus world about it. let's see what they reply.

[Zaphod Beblebrox]

I am studying for Recurrent, (what we call CQT), Continuing Qualification Training, and we now have everything on the ipad I don't like this method because a well worn manual with lot's of notes in the margins and highlights makes it easy to find stuff. They can slip in new stuff and you miss it if you don't really take the time to study every new update.

Anywho... over the summer they stuck in this little tidbit.

9.3.7. Erroneous Radar Altimeter Indications
Non-Normal Operations 9-31

Miscellaneous Non-Normal Procedures

Primary Flight Display (PFD).
— Frozen RA height indication (possibly negative) in either amber or green depending
on height
— Discrepancy between both PFDs (RA indications, FD orders, and if both AP engaged,
PFD FMAs)
System Display (SD).
— A pulsing Cabin Differential Pressure Advisory on the ECAM PRESS page (No
consequence on cabin pressure)
Warnings/Callouts.
— Untimely EGPWS alerts
— Untimely or absence of "RETARD" auto callout
— Untimely L/G GEAR NOT DOWN ECAM warning
— Absence or interruption of RA automatic callout (height announcement)
— Activation of AUTOLAND warning light in ILS approach with autopilot engaged in
LAND or FLARE mode when:
• one RA height goes below 200 ft and
• the difference between both RA height indications is greater than 15 ft.
• Note •
There is no ECAM message or audio warning in association
with the AUTOLAND warning light. The AUTOLAND
warning can be triggered even if AUTOLAND is not planned.
Autoflight System Mode Changes (Indications on FMA).
— NAV mode engagement not possible after takeoff
— During an ILS approach, and depending on the engaged Flight Guidance modes, the
consequences may be:
• untimely/early engagement of the LAND/FLARE/THR IDLE modes
• Note •
During ILS approach with AP and A/THR engaged, THR
IDLE mode untimely engagement will be associated with an
initial pitch attitude increase due to FLARE mode engagement.
• during AUTOLAND, the LAND/FLARE/THR IDLE modes may not engage
• in case of go-around and if the RA is still frozen at a very low height indication

Non-Normal Operations 9-32

Miscellaneous Non-Normal Procedures
— SRS and GA TRK modes engage
— NAV, HDG or TRK lateral modes cannot be selected
— LVR CLB will not be displayed on the FMA at thrust reduction altitude
— ALT* and ALT will not engage at the FCU target altitude
Disconnecting the autopilot and resetting both flight directors allows the engagement of
basic modes (HDG and V/S).
During an ILS Approach. During an ILS approach with autopilot engaged, if an
erroneous and very low RA height indication occurs, THR IDLE and FLARE modes may
engage early with the following consequences:
— Automatic autopilot disconnection due to high angle of attack is not available. If the flight crew does not immediately react, the angle-of-attack will increase and may reach the stall value.
— Loss of alpha floor.
— Loss of low energy audio warning, "SPEED, SPEED, SPEED"
— The auto-trim function may be inhibited. In this case, reducing thrust increases pitch down authority.
— In manual flight or after autopilot disconnections, with high thrust, significant
forward longitudinal sidestick input may be required.

Procedure. Monitor and crosscheck all primary flight parameters and the FMA during
all phases of flight.
During ILS approaches with autopilot engaged, and in the event of an unexpected, early
"THR IDLE" and "FLARE" engagement, immediately:
— perform an automatic go around applying TOGA thrust or
— disconnect the autopilot, then:
• continue the landing using raw data or visual references (FDs set to OFF) or
• perform a manual go around applying TOGA thrust.
Caution
Significant longitudinal sidestick input may be required.
Go Around. In case of go around (automatic or manual), when climb gradient is
established, and at or above the thrust reduction altitude, reduce thrust without delay
(thrust levers set to CLB detent).

GOOD GRAVY, ALL THAT'S POSSIBLE FOR A FAILED RADAR ALTIMETER SYSTEM? Keep this all in mind if you dispatch with one inop!

[kikwon]

"The system also injects bank-angle compensation, so that operation effectively maintains a constant angle of attack"

this is stated under low speed stability, what do you guys think this means?

[vilas]

It may possibly mean the aircraft compensates for loss of lift fue to lift vector tilt by maintaining angle of attack. Pitch may automatically increase to do that.

[Vessbot]

Pitch would have to be decreased, not increased, to maintain AOA and speed stability. That means that altitude is sacrificed.

If you want to maintain altitude in a bank, total lift is increased (to maintain a constant vertical lift component) therefore AOA is increased.

If you want to bank, you have to choose between giving up altitude or stall margin. You can't keep both.

[vilas]

Airbus is not speed stable. It is true that in a turn AOA needs to be increased to maintain altitude. Bank angle compensation my be exactly doing that by maintaining a constant required AOA and not necessarily the straight and level AOA. The stall margin will reduce but not an issue at normal bank angles.

[IFLY_INDIGO]

Here is the Answer that I got from Airbus:

ANSWER:
As you correctly stated, in some specific failures (such as Triple ADR failure, double hydraulic failures
G+B,...), this leads to the 'Alternate law, without protection'.
We agree there is no direct mean to inform flight crew that the reduced protection (low-speed stability and
high-speed stability) are lost in case of Alternated law.
Airbus considered there was not necessary to provide this level of information to flight crew.
When the aircraft go to alternate law, in any case, flight crew has to follow the ECAM action provided by
"ALTN (PROT LOST) procedure".

[Vessbot]

Originally Posted by vilas

Airbus is not speed stable. It is true that in a turn AOA needs to be increased to maintain altitude. Bank angle compensation my be exactly doing that by maintaining a constant required AOA and not necessarily the straight and level AOA. The stall margin will reduce but not an issue at normal bank angles.

You're surmising that it compensates to maintain altitude by increasing AOA, but the quote you're responding to says the opposite: "The system also injects bank-angle compensation, so that operation effectively maintains a constant angle of attack."

In the context of the ongoing discussion about which safety protections are kept vs. lost in various equipment-failure based downmodes, surely the "compensation" is referring to the maintenance of stall margin, and not its sacrifice in return for the maintenance of altitude, a relatively unimportant goal.

(This stall margin does become an ever-increasingly important issue at slower speeds/higher AOA's no matter how small the bank angle. And a high AOA scenerio is of course the whole point of the "protection" under discussion here. And, it is also a speed stability that very much exists at the edge of the envelope, while not in the middle.)

[vilas]

Vessbot

And, it is also a speed stability that very much exists at the edge of the envelope, while not in the middle.)

The low speed and high speed stability at the end of envelope is in form of protection where it resists the speed excursion beyond the envelope. That doesn't make airbus speed stable. Speed stable aircraft when disturbed pitch up or down to acquire in trim speed. To give an example, take the without protection case. If airbus is left nose high and thrust levers brought to idle, as the speed drops it will not pitch down but try to maintain the flight path even when below VLS and trim itself into a stall. As far as constant angle of attack is concerned in absence of information regarding the purpose of it and clear explanation yes I was trying to guess. I used the words "may". Again I am guessing it may be that at normal speeds left to itself in a turn, as it maintains AOA it will descend and the pilot will have to increase the pitch like a conventional aircraft, to maintain altitude.The exact mechanics can only be found out from airbus through Tech request.