Cessna C208B down near Seattle, 4 deceased

[fdr]

Originally Posted by Pilot DAR

This accident, and the report are opening more questions for me than answering them....

I tend to agree P-DAR. As sad as a number of the findings are to read, I think that a number of them seem to be overstated in importance, and have not been conjugated for what was expected to occur.

Random musings:

• The modification to the gear is not relevant to a stall, overspeed/overstress outcome, other than increasing drag and being beneficial in reducing the speed buildup in the dive.
• The addition of stall fences on the wing themselves would not contribute appreciably to an aggravated stall roll off directly.
• Indirectly, addition of stall fences may increase rolling moment due to greater CL inboard in the propeller wake field. That still appears improbable, the fences are well outboard at part span, and the effect inboard would be minimal, but it is possible.
• The addition of gurney tabs inboard on the TE flaps can be a factor in increasing roll-off at the stall, whether flaps up or down. At any occasion that gurney flaps are added to the inboard TE area, the change in the CL/aoa needs to be accounted for. In simple terms, the gurney flap moves the CL/aoa slope by around 4 degrees +/- dependent on its h/c. The TE tab also alters the aoa that peak CL occurs, and that can be altered by design to end up with a very benign stall, by effectively increasing wash-out. OTOH, an aggressive TE tab with added VGs on the flap will result in a delayed stall inboard, and that results in an aggressive stall behavior.
• The C208B by memory has a set of ogive planform VGs on the flap at around 10%fc, and that would aggravate the stall roll-off for a straight addition of a TE tab to the flap.
• A constellation of low profile VGs outboard would result in a return to a conventional roll-off if the flap TE is modified. My own flight testing in this area used ramp, vane and similar VGs the latter in co-rotation-inboard, outboard and couter rotating geometry. I prefer counter rotating but that is not a large change in effectiveness, other than where yaw is involved in which case counter rotating is a much better proposition.
• Stall fences work well on swept wings, particularly to maintain aileron authority at moderate to high aoa. with a prop, and a straight wing, it wouldn't be my first choice for mitigation of the TE tab effect. (James Raisbeck unfortunately departed this earth well before his use by date, and that is a shame, he would have been familiar with what I am indicating here, as would Bob Desroche, from micro, who is nowadays feeding his horses and generally done with aerospace. Other than these guys, John C. Lin from NASA would be a good resource for anyone to talk to on this issue which is probably outside of the wheelhouse of the NTSB).
• Would like to see what the C208B roll off is with a normal un-accelerated turning stall. I have a suspicion that it will be a fair rate to the left. If I was the NTSB investigator, I would have asked for some background from Cessna for that. I am not interested in blame, I would think that it is worth knowing if a latent risk exists in the aircraft other than the configuration that the guys got into.

Back in 1991-97 I ran a series of tests on TE tabs on flaps on a long suffering PA23-250D that I had. The testing included a number of configurations, including flap TE, cove tabs (really cool) flaps with VGs, flaps with VGs and TE tabs, and outboard wings with VGs to offset the change in CL/aoa inboard. The effect of TE tabs is spectacular. To verify the effectiveness we deliberately set up for aggravated stall conditions, which have a validation of the effective change achieved by the various VGs, as well as the TE tab. The reference case stall speed of 52KIAS was reduced to 39KIAS, and that had to be calculated from GPS, the IAS was inaccurate at the very low speed case. The drag change was also notable, any TE tab on a flap will result in a lower drag at the full flap position, but when applied in conjunction with a transverse tab in the flap cove, the effect is spectacular. In tuft visualisation, fully attached flow exists over the flap upper surface, at F50, and at Vs for the associated stall speed. The flap cove tab results in a condition of off body recirculation, which gives a jet flow over the upper surface which is pretty cool to observe. The CFD shows that clearly, the tufting verifies that the CFD was not an artefact.

Originally Posted by hatstp

This highlights one of those significant challenges with flight testing: 'know your test article'. The challenge comes when aircraft that are used for STC type work are a 'Frankenstein' of mod state/STCs etc and the ability to fully understand the baseline standard and how that might affect testing, test outcome, mitigation etc is complex and challenging.

The other valuable lesson for all flight test professionals is the ability to be critical of unexpected results; to be able to pause, analyse and conclude why something didn't go as planned (failed). This also then needs any mitigation, risk reduction measures etc to be assessed for continued validity. Some similarities here with the G650 crash in 2011; unexpected results from previous testing (wing rock) that were not fully 'understood' and may have allowed a pause on testing to be instigated whilst a full analysis was carried out.

Yup.

Doing a rapid pitch rate rotate, at aft CG is an interesting test point; doing that on a wing that is swept, and has no LE device, is more than interesting. I understand why smart people at Gulftream have avoided having a LE Device on their wings, but I would prefer not to fly them due to the inherent sensitivity to contamination, and to non linear CL, and CM associated with LESB's. Wing rockin' should be a hint that things might be less than neat.

Originally Posted by BFSGrad

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

The pilot’s improper recovery following a departure from controlled flight after an intentional aerodynamic stall, which resulted in an exceedance of airspeed limitations, airframe overstress, and a subsequent inflight breakup.

NTSB Aviation Investigation Final Report - N2069B

thank you and the mod for the link correction. I would thank you but that would seem to be for the wrong reason. I am not sure I agree with the NTSB conclusions on this one.