Not so good news about flight 3407

[BirdCatcher]

However the stick shaker is probably run off some kind of AOA sensor which isn't adjusted for icing conditions...

The increase ref switch lowers the AOA required to activate the shaker/pusher.

 

[Kaman]

Agreed...Critical AoA is not a function of Airspeed. Aerodynamics 101...Logic would conclude that the engineers design the stall warning system to activate when the critical AoA is exceeded. On the SAAB that is the only function of the AoA vanes. Cannot speak to the "Q". But, I am thinking for the simplicity of certification that in a straight-wing turboprop the AoA vane is used only for stall warning.
I wish that the press would show more restraint and less sensationalism in this mishap.

 

[centralpilot]

I suppose a Q driver will chime in, but yeah, I'm thinking the switch for contamination simply reschedules IAS for the purposes of flying a faster speed.

The airplane will still stall at a particular AOA and the shaker and pusher will fire accordingly. You would not arbitrarily reschedule those types of safety devices.

Can't speak for the Q, but as atrdriver said one post before yours, the ATR did adjust the AOA margin when horn de-icing was selected on and was manually reset to normal when turning ice protection off. This wasn't arbitrary rescheduling, but rather intentionally increasing safety margins to account for the negative effects of ice accumulation.

 

[ACL65PILOT]

They are leaking this out slowly so that when it fully comes out it is not a bomb shell.

FWIW, it could be any of us, and if you think you are above it, you have not been aviation long enough.

 

[LJ45]

Agreed...Critical AoA is not a function of Airspeed. Aerodynamics 101...Logic would conclude that the engineers design the stall warning system to activate when the critical AoA is exceeded. On the SAAB that is the only function of the AoA vanes. Cannot speak to the "Q". But, I am thinking for the simplicity of certification that in a straight-wing turboprop the AoA vane is used only for stall warning.
I wish that the press would show more restraint and less sensationalism in this mishap.

a DHC 8 driver posted it does increase the stall warning speed (angle) and the Stall warning comes on BEFORE the critical AOA is reached...correct? That is why it is called a warning...to prevent a stall.

 

[q100]

They are leaking this out slowly so that when it fully comes out it is not a bomb shell.

FWIW, it could be any of us, and if you think you are above it, you have not been aviation long enough.

Amen to that last thought. Pretty common to hear in the cockpits I've been in: "that's why there's two of us."

 

[centralpilot]

a DHC 8 driver posted it does increase the stall warning speed (angle) and the Stall warning comes on BEFORE the critical AOA is reached...correct? That is why it is called a warning...to prevent a stall.

Exactly

 

[Captzaahlie]

As far as the 31 degrees of pitch up I want to know where the AP left the trim when it was disconnected...

I agree that would be very interesting to know.

 

[rickair7777]

Agreed...Critical AoA is not a function of Airspeed. Aerodynamics 101...Logic would conclude that the engineers design the stall warning system to activate when the critical AoA is exceeded. On the SAAB that is the only function of the AoA vanes. Cannot speak to the "Q". But, I am thinking for the simplicity of certification that in a straight-wing turboprop the AoA vane is used only for stall warning.
I wish that the press would show more restraint and less sensationalism in this mishap.

You are correct about critical AoA, in theoretical terms.

But I believe modern turbine aircraft use more than just AoA inputs for the SPS. IIRC they also look at Air Data and trends...if you are near critical AoA and AS is bleeding off fast, the SPS will need to lead the event rather than just wait for critical AoA...otherwise you could be in a deep stall before the SPS has a chance to help.

 

[belchfire]

The increase ref switch lowers the AOA required to activate the shaker/pusher.

Ok, that makes sense-except if their actual stall speed was down around 100kts and the AOA sensor got adjusted to go off at closer to 120kts how did the wing actually stall?

 

[Hugh Johnson]

Just to clarify, CAS is presented on the PFD airspeed tape, not IAS. The data goes through an ADC which makes those corrections before being displayed. A conventional airspeed indicator shows, well, indicated airspeed.

Indicated airspeed (IAS) is the airspeed read directly from the airspeed indicator on an aircraft, driven by the pitot-static system.

Calibrated airspeed (CAS) is the speed shown by a conventional airspeed indicator after correction for instrument error and position error.

Your J3 may read indicated airspeed. Modern airplanes read calibrated airspeed no matter what the indicator. Its part of the flight testing. You can check Part 25 and Part 23.

 

[centralpilot]

Ok, that makes sense-except if their actual stall speed was down around 100kts and the AOA sensor got adjusted to go off at closer to 120kts how did the wing actually stall?

In this specific case? We'll find out when the NTSB is done doing their job.

 

[ultrarunner]

Ok, that makes sense-except if their actual stall speed was down around 100kts and the AOA sensor got adjusted to go off at closer to 120kts how did the wing actually stall?

In reading the Q drivers explanation of how the AOA/Stall system is rescheduled in icing, the wing may not have actually aerodynamically stalled until it reached it's critical AOA during the abrupt pull-up maneuver.

There may not have actually been an aerodynamic stall when the pusher activated, since the pusher is biased by 20 knots as a function of being in icing conditions.

Put another way, with the 'icing switch' off, stall speed was shown to be computed at 90 kts or so.

What will be interesting to read is what was going on that resulted in the flight crew letting the airspeed decay?

Inattention? Preoccupation with icing conditions...running checklists, inexperience in type, etc...

Likely, as is typically the case, a combination. But it is clear the speed was allowed to decay to a very low value. After this, the abrupt pull-up sealed their fate at such a low altitude.

 

[ksu_aviator]

By all accounts, the stick shaker disconnected the autopilot. Stab trip would have been at whatever IAS was at that point...likely 100 kias or so...

Ok, so the stick shaker activates approaching the stall.

The aircraft was doing 134 before being configured. The adjusted Vref was 139. So, 139 represents 1.3Vso. Stall speed would then be "approximately 99 kias or so, depending naturally upon the amount of ice, bank angle, etc...

So, the plane likely slowed a hundred knots or so...

..stick shaker activated...
..AP disconnects...

Trim would have had the plane trimmed for 100 kts or what ever speed was indicated when the shaker disconnect the AP.

...speed likely continued to decay for another second or two during the "OH $HIT" phase...

...pusher fired
...Cpt yanked the control column to his chest...
...the resulting excessive ANU resulted in either an additional airframe stall, or an excessively slow IAS to make recovery impossible from the low altitude.

I was under the impression that the stall warning did not activate and the autopilot did not disconnect until the aircraft pitched up rapidly.

 

[belchfire]

I'm just wondering-if the warning angle of attack is indeed adjusted along with the ref speed the margin between the actual stall AOA and warning AOA would be much larger than usual unless there indeed was contamination on the wings and decreased the effective stall AOA.

Perhaps to the point that the normal spread between warning and actual stall did not exist...